U.S. patent application number 09/951195 was filed with the patent office on 2003-06-19 for apparatus and method of cryogenic cooling for high-energy cutting operations.
Invention is credited to Frey, John Herbert, Harriott, George Matthew, Swan, Robert Bruce, Zhang, Xiaoguang, Zurecki, Zbigniew.
Application Number | 20030110781 09/951195 |
Document ID | / |
Family ID | 25491401 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030110781 |
Kind Code |
A1 |
Zurecki, Zbigniew ; et
al. |
June 19, 2003 |
Apparatus and method of cryogenic cooling for high-energy cutting
operations
Abstract
A cryogenic fluid jet is used in an apparatus and a method for
remote cooling of a cutting tool engaged in machining a workpiece
under high-energy conditions, such as high-speed machining,
hard-turning, cutting of difficult to machine materials, and
combinations thereof. The apparatus and method use a stabilized,
free-expanding cryogenic fluid jet having a pulse cycle time less
than or equal to about 10 seconds. The apparatus and method
increase the cleanliness of machined parts and chips and machining
productivity of hard but brittle tools, including but not limited
to tools which should not be cooled with conventional cooling
fluids.
Inventors: |
Zurecki, Zbigniew;
(Macungie, PA) ; Frey, John Herbert; (Allentown,
PA) ; Swan, Robert Bruce; (Lehighton, PA) ;
Harriott, George Matthew; (Allentown, PA) ; Zhang,
Xiaoguang; (Macungie, PA) |
Correspondence
Address: |
AIR PRODUCTS AND CHEMICALS, INC.
PATENT DEPARTMENT
7201 HAMILTON BOULEVARD
ALLENTOWN
PA
181951501
|
Family ID: |
25491401 |
Appl. No.: |
09/951195 |
Filed: |
September 13, 2001 |
Current U.S.
Class: |
62/64 ;
62/373 |
Current CPC
Class: |
Y10T 407/14 20150115;
B23Q 17/09 20130101; F17C 2250/0443 20130101; Y10T 82/16065
20150115; F17C 2221/014 20130101; B23Q 11/1053 20130101; Y10T 82/10
20150115; F17C 2223/0161 20130101; F17C 2221/016 20130101; B23Q
17/24 20130101 |
Class at
Publication: |
62/64 ;
62/373 |
International
Class: |
F25D 017/02 |
Claims
1. A method for cooling a cutting tool, comprising the steps of:
providing a supply of a cryogenic fluid; and delivering a
free-expanding stabilized jet of the cryogenic fluid to the cutting
tool.
2. A method as in claim 1, wherein the cutting tool has a cutting
edge and wherein a means for delivering the free-expanding
stabilized jet of the cryogenic fluid to the cutting tool has at
least one discharge point spaced apart from the cutting edge by a
distance greater than or equal to about 0.1 inches and less than
about 3.0 inches.
3. A method as in claim 1, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
temperature below about minus 150 degrees Celsius (-150.degree.
C.).
4. A method as in claim 2, wherein at least a portion of the
cryogenic fluid has a pressure greater than or equal to about 25
psig and less than or equal to about 250 psig during or immediately
prior to discharge from the at least one discharge point.
5. A method as in claim 1, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate greater than or equal to about
0.5 lbs/minute and less than or equal to about 5.0 lbs/minute.
6. A method as in claim 1, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate having a flow pulse cycle time
less than or equal to about 10 seconds.
7. A method as in claim 1, wherein the cutting tool has a rake
surface and at least a portion of the free-expanding stabilized jet
of the cryogenic fluid impinges on at least a portion of the rake
surface.
8. A method as in claim 1, wherein at least a portion of the
cryogenic fluid is selected from a group consisting of liquid
nitrogen, gaseous nitrogen, liquid argon, gaseous argon and
mixtures thereof.
9. A method as in claim 1, wherein at least a portion of the
cutting tool has a traverse rupture strength (TRS) value of less
than about 3000 MPa.
10. A method as in claim 1, wherein the cutting tool is engaged in
a high-energy chip-forming and workpiece-cutting operation.
11. A method for machining a workpiece with a cutting tool using a
method for cooling the cutting tool as in claim 1.
12. A workpiece machined by a method as in claim 11 and
characterized by an improved surface.
13. Recyclable chips obtained as a byproduct of a method as in
claim 11 and characterized by an improved purity.
14. A method for cooling a workpiece, comprising the steps of:
providing a supply of a cryogenic fluid; and delivering a
free-expanding stabilized jet of the cryogenic fluid to the
workpiece.
15. A method for controlling cooling of a cutting tool during a
cutting operation, comprising the steps of: providing a supply of a
cryogenic fluid; delivering a flow of the cryogenic fluid to the
cutting tool; and regulating the flow of the cryogenic fluid to the
cutting tool at a substantially uniform mass flowrate, whereby a
frost coating is maintained on at least a portion of the cutting
tool during substantially all of the cutting operation in an
atmosphere having an ambient relative humidity in a range of about
30% to about 75% and an ambient temperature in a range of about
10.degree. C. to about 25.degree. C.
16. A method as in claim 15, wherein the cutting tool is engaged in
a high-energy chip-forming and workpiece-cutting operation.
17. A method for machining a workpiece with a cutting tool using a
method for controlling cooling of the cutting tool as in claim
15.
18. A workpiece machined by a method as in claim 17 and
characterized by an improved surface.
19. Recyclable chips obtained as a byproduct of a method as in
claim 17 and characterized by an improved purity.
20. A method for cooling a cutting tool having a cutting edge,
comprising the steps of: providing a supply of a cryogenic fluid;
providing a nozzle adapted to discharge a jet of the cryogenic
fluid, said nozzle having at least one discharge point spaced apart
from the cutting edge by a distance greater than or equal to about
0.1 inches and less than about 3.0 inches; and delivering a
free-expanding stabilized jet of the cryogenic fluid from the
discharge point to the cutting tool, wherein the cryogenic fluid
has a temperature of about minus 150 degrees Celsius (-150.degree.
C.) at the discharge point.
21. A method for controlling cooling of a cutting tool during a
cutting operation, comprising the steps of: providing a supply of a
cryogenic fluid; providing a nozzle adapted to discharge a flow of
the cryogenic fluid, said nozzle having at least one discharge
point spaced apart from the cutting tool; delivering a flow of the
cryogenic fluid from the discharge point to the cutting tool; and
regulating the flow of the cryogenic fluid to the cutting tool at a
substantially uniform mass flowrate greater than or equal to about
0.5 lbs/minute and less than or equal to about 5.0 lbs/minute
having a flow pulse cycle time less than or equal to about 10
seconds, whereby a frost coating is maintained on at least a
portion of the cutting tool during substantially all of the cutting
operation in an atmosphere having an ambient relative humidity in a
range of about 30% to about 75% and an ambient temperature in a
range of about 10.degree. C. to about 25.degree. C.
22. An apparatus for cooling a cutting tool, comprising: a supply
of a cryogenic fluid; and means for delivering a free-expanding
stabilized jet of the cryogenic fluid to the cutting tool.
23. An apparatus as in claim 22, wherein the cutting tool has a
cutting edge and wherein the means for delivering the
free-expanding stabilized jet of the cryogenic fluid to the cutting
tool has at least one discharge point spaced apart from the cutting
edge by a distance greater than or equal to about 0.1 inches and
less than about 3.0 inches.
24. An apparatus as in claim 22, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
temperature below about minus 150 degrees Celsius (-150.degree.
C.).
25. An apparatus as in claim 23, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a pressure
greater than or equal to about 25 psig and less than or equal to
about 250 psig during or immediately prior to discharge from the at
least one discharge point.
26. An apparatus as in claim 22, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate greater than or equal to about
0.5 lbs/minute and less than or equal to about 5.0 lbs/minute.
27. An apparatus as in claim 22, wherein at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate having a flow pulse cycle time
less than or equal to about 10 seconds.
28. An apparatus as in claim 22, wherein the cutting tool has a
rake surface and at least a portion of the free-expanding
stabilized jet of the cryogenic fluid impinges on at least a
portion of the rake surface.
29. An apparatus as in claim 22, wherein at least a portion of the
cryogenic fluid is selected from a group consisting of liquid
nitrogen, gaseous nitrogen, liquid argon, gaseous argon and
mixtures thereof.
30. An apparatus as in claim 22, wherein at least a portion of the
cutting tool has a traverse rupture strength (TRS) value of less
than about 3000 MPa.
31. An apparatus as in claim 22, wherein the cutting tool is
engaged in a high-energy chip-forming and workpiece-cutting
operation.
32. An apparatus for machining a workpiece with a cutting tool
using an apparatus for cooling the cutting tool as in claim 22.
33. A workpiece machined by an apparatus as in claim 32 and
characterized by an improved surface.
34. Recyclable chips removed from a workpiece by an apparatus as in
claim 32 and characterized by an improved purity.
35. An apparatus for cooling a workpiece, comprising: a supply of a
cryogenic fluid; and means for delivering a free-expanding
stabilized jet of the cryogenic fluid to the workpiece.
36. An apparatus for controlling cooling of a cutting tool during a
cutting operation, comprising: a supply of a cryogenic fluid; means
for delivering a flow of the cryogenic fluid to the cutting tool;
and means for regulating the flow of the cryogenic fluid to the
cutting tool at a substantially uniform mass flowrate, whereby a
frost coating is maintained on at least a portion of the cutting
tool during substantially all of the cutting operation in an
atmosphere having an ambient relative humidity in a range of about
30% to about 75% and an ambient temperature in a range of about
10.degree. C. to about 25.degree. C.
37. An apparatus as in claim 36, wherein the cutting tool is
engaged in a high-energy chip-forming and workpiece-cutting
operation.
38. An apparatus for machining a workpiece with a cutting tool
using a method for controlling cooling of the cutting tool as in
claim 36.
39. A workpiece machined by an apparatus as in claim 38 and
characterized by an improved surface.
40. Recyclable chips removed from a workpiece by an apparatus as in
claim 38 and characterized by an improved purity.
41. An apparatus for cooling a cutting tool having a cutting edge,
comprising: a supply of a cryogenic fluid; a nozzle adapted to
discharge a jet of the cryogenic fluid, said nozzle having at least
one discharge point spaced apart from the cutting edge by a
distance greater than or equal to about 0.1 inches and less than
about 3.0 inches; and means for delivering a free-expanding
stabilized jet of the cryogenic fluid from the discharge point to
the cutting tool, wherein the cryogenic fluid has a temperature of
about minus 150 degrees Celsius (-150.degree. C.) at the discharge
point.
42. An apparatus for controlling cooling of a cutting tool during a
cutting operation, comprising: a supply of a cryogenic fluid; a
nozzle adapted to discharge a flow of the cryogenic fluid, said
nozzle having at least one discharge point spaced apart from the
cutting tool; means for delivering a flow of the cryogenic fluid
from the discharge point to the cutting tool; and means for
regulating the flow of the cryogenic fluid to the cutting tool at a
substantially uniform mass flowrate greater than or equal to about
0.5 lbs/minute and less than or equal to about 5.0 lbs/minute
having a flow pulse cycle time less than or equal to about 10
seconds, whereby a frost coating is maintained on at least a
portion of the cutting tool during substantially all of the cutting
operation in an atmosphere having an ambient relative humidity in a
range of about 30% to about 75% and an ambient temperature in a
range of about 10.degree. C. to about 25.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the field of machining of
material by cutting (e.g., shaping parts by removing excess
material in the form of chips), and more particularly machining of
materials by cutting with cryogenically cooled cutting tools.
[0002] Numerous references are cited throughout this application,
including the endnotes which appear after the Detailed Description
of the Invention. Each of those references are incorporated herein
by reference with regard to the pertinent portions of the
references cited herein.
[0003] As used herein, the term "cutting" includes but is not
limited to the following operations: turning, boring, parting,
grooving, facing, planing, milling, drilling, and other operations
which generate continuous chips or fragmented or segmented chips.
The term cutting does not include: grinding, electro-discharge
machining, ultrasonic cutting, or high-pressure jet erosion
cutting, i.e., operations generating very fine chips that are not
well defined in shape, e.g., dust or powder.
[0004] Cutting hard or difficult to machine materials, as well as
high-speed cutting of materials from all groups except the
low-melting point group including zinc or polymers, leads to very
high levels of energy dissipated at the cutting tool. Table 1 below
presents examples of easy and difficult to machine ferrous and
non-ferrous metals with their machining responses modified by both
composition and thermo-mechanical condition.
[0005] Materials characterized by the unit power (P.sub.c) of more
than 1 hp/in.sup.3/minute, unit energy (E.sub.c) of more than 2.7
J/mm.sup.3, and/or hardness of more than 30 HRC are considered
difficult to machine. In the case of steels and other metals
melting above 1400.degree. C., high-speed machining proves
difficult even if the hardness level is only 25 HRC.
1TABLE 1 Examples of Hardness, Power, Energy and Temperature
Encountered in Cutting.sup.(1) Nominal Increase Assumed in Work
Specific Assumed Material/Chip Unit Power Unit Energy Density
Specific Heat Temperature Materials: Hardness: [hp/in.sup.3/minute]
[Joules/mm.sup.3] [grams/cm.sup.3] [cal./(gram * K)] [deg. K. or
C.] Magnesium 40-90 HB 0.13-0.17 0.36-0.46 Low strength aluminum
alloys 30-150 HB 0.20 0.55 2.7 0.21 230 6061 - T6 aluminum alloy
0.35 0.96 2.7 0.21 400 2024 - T4 aluminum alloy 0.46 1.26 2.7 0.21
520 Soft copper alloys 10-80 HRB 0.50 1.37 8.9 0.09 400 70Cu-30Zn
brass 0.59 1.61 Copper and harder copper 80-100 HB 0.70-0.80
1.91-2.18 8.9 0.09 580 alloys Steels: AISI 1020 carbon steel
150-175 HB 0.58 1.58 7.8 0.11 440 AISI 1020 carbon steel 176-200 HB
0.67 1.83 7.8 0.11 500 Carbon, alloy, and tool 35-40 HRC 1.15 3.14
7.8 0.11 870 steels, 40-50 HRC 1.20 3.28 7.8 0.11 900 various
hardness 50-55 HRC 1.60 4.37 7.8 0.11 .about.1200 levels . . .
55-58 HRC 2.75 7.51 7.8 0.11 >1500 Stainless steels, wrought and
135-275 HB 1.05 2.87 cast, 30-45 HRC 1.12 3.06 various hardness
150-450 HB 1.12 3.06 levels: . . . Precipitation hardening
stainless steels Soft grades of cast irons 110-190 HB 0.55 1.50
Gray, ductile, and malleable 190-320 HB 1.12 3.06 grades Titanium
alloys 250-375 HB 1.0-1.9 2.73-5.18 4.4 0.12 1186->1600 Nickel
based superalloys 200-360 HB 2.0 5.46 8.9 0.11 >1350 Niobium
alloys 217 HB 1.4 3.82 Molybdenum 230 HB 1.6 4.37 10.2 0.06 1710
Tantalum 210 HB 2.25 6.14 Tungsten 320 HB 2.3 6.28 19.2 0.03 2440
Notes: .sup.(1)Unit power - power at cutting tool required to
remove work material at the rate of 1 in.sup.3/minute. .sup.(2)Unit
energy - total energy dissipated by cutting tool removing 1
mm.sup.3 of material. 1.0 hp/in.sup.3/min = 2.73 J/mm.sup.3.
[0006] 3. Listed above, average values of unit power required in
turning are valid for sharp high-speed steel (HSS) and carbide
(WC-Co) tools cutting within the feedrate range of 0.005 to 0.020
inches per revolution and exclude spindle efficiency factor.
Average values of unit power required in milling may vary by
+/-10%.
[0007] 4. Values of unit power should be multiplied by a factor of
about 1.25 in the case of cutting with dull tools or tools
characterized by a negative rake geometry.
[0008] 5. Calculated above, nominal increase in chip temperature is
an estimate assuming: (1) constant specific heat of work material
across the entire temperature range, (2) no energy losses to work
material and tool, and (3) a uniform temperature distribution
across chip thickness including the chip/tool contact interface
within so-called secondary shear zone.
[0009] Table 1 also shows how the unit power and energy translate
into high temperatures of a machined chip staying in contact with
the cutting tool. It is clear that the high-energy materials and
cutting conditions require tool grades retaining hardness at the
highest temperatures--hard but brittle grades of cemented carbides
(WC-Co) and, ideally, advanced non-metallic tool materials that
offer an ultimate level of hardness at the cost of low rupture
strength and fracture toughness.
[0010] Table 2 below outlines the typical values of traverse
rupture strength (TRS) and fracture toughness (K.sub.1c) of the
major groups of tool materials.
2TABLE 2 Selected Properties of HSS, Carbide and Advanced Tool
Materials - Cermets, Ceramics and Diamond.sup.(2) Traverse rupture
Fracture strength toughness Tool material (M Pa) (K.sub.1c) M Pa
m.sup.1/2 Al.sub.2O.sub.3 500-700 2.5-4.5 Al.sub.2O.sub.3-TiC
600-850 3.5-4.5 Al.sub.2O.sub.3-1%ZrO.sub.2 700-900 5-8
Al.sub.2O.sub.3--SiC 550-750 4.5-8 SiAlON 700-900 4.5-6.5
Si.sub.3N.sub.4 100-1000 1.5-8 SiC 550-860 4.6 Polycryst. CBN
(PCBN) 800-1100 4-6.5 Polycryst. Diamond (PCD) 390-1550 6-8
TiC--TiN--WC--TaC--Ni--Co--Mo 1360 8.5 (C7-C8/C3-C4 class) 97WC-3Co
(with alloying additions) 1590 9 71WC-12.5TiC-12TaC-4.5Co 1380
84WC-16Co (straight cemented 3380 10-13.5 carbide grades) High
speed steel M42 (CPM grade) 4000
[0011] Comparing to the traditional high-speed steel (HSS) and
tougher grades of cemented carbides containing more cobalt binder,
the advanced, non-metallic tool materials are significantly more
brittle, i.e., sensitive to irregularities in stress loading,
irregularities in thermal loading or cooling and thermal stress
shocking. Tools with a TRS value of less than 3 GPa (3000 MPa) and
a K.sub.1c value of less than 10 MPa m.sup.1/2 are considered
brittle and prone to rapid fracturing under high-energy cutting
conditions. Thus, the machining community is aware of the necessity
of either avoiding the use of conventional cutting fluids when
machining with these brittle tool materials or, if it is possible
and practical in a given cutting operation, using the brittle tool
materials with extreme care by a complete and uniform flooding of
the tool, chip, and contact zone.
[0012] For example, numerous publications and tool manufacturer
recommendations alert machining operators to the problem of reduced
life of ceramic tools on contact with conventional cutting fluids.
Despite the inherent deficiencies, e.g., overheated workpiece,
reduced dimensional accuracy, or risk of chip fires, dry machining
is recommended if hard but brittle tools are used. P. K. Mehrotra
of Kennametal teaches in "Applications of Ceramic Cutting Tools",
Key Engineering Materials, Vol. 138-140 (1998), Chapter 1, pp.
1-24: "the use of coolants is not recommended when these tools are
used to machine steels due to their low thermal shock resistance".
R. C. Dewes and D. K. Aspinwall ("The Use of High Speed Machining
for the Manufacture of Hardened Steel Dies", Trans. of NAMRI/SME,
Vol. XXIV, 1996, pp.21-26) tested a range of oxide and nitride
tools including: 71% Al.sub.2O.sub.3-TiC, 75%
Al.sub.2O.sub.3-SiC.sub.w, 50% CBN-AlB.sub.2-AlN,
50%-TiC-WC-AlN-AlB.sub.- 2, 80% CBN-TiC-WC, as well as 95%
CBN-Ni/Co. They found that the use of a conventional cooling fluid
applied by flooding or spraying resulted in the reduction of tool
life by more than 95% except for the whisker reinforced alumina,
for which the life was shortened by about 88%. Similar test results
showing a dramatic tool failure by brittle chipping on contact with
cooling fluid have been published for PCBN cutting inserts by T. J.
Broskea et al. of GE Superabrasives at MMS Online
(www.mmsonline.com/articles) and by others elsewhere.
[0013] Table 3 below represents typical machining conditions
recommended in the prior art for a range of work materials and tool
materials. While different combinations of depth of cut (DOC),
feedrate (F), cutting speed (Vc), and unit power (Pc), lead to high
or low total power levels (P), the most important value
characterizing high-energy cutting and critical to tool life is the
power flux (P.sub.f), which is calculated by dividing P by the
cross-sectional area of an undeformed chip (a product of DOC and
F).
3TABLE 3 EXAMPLES OF MACHINING CONDITIONS RECOMMENDED IN PRIOR ART
FOR A RANGE OF CUTTING, VARIABLES, INCLUDING WORK MATERIALS, WORK
HARDNESS LEVELS, AND TOOL MATERIALS Recommended Assumed Depth
Cutting Speed, Work Material Unit Total Work of cut, Feedrate,
Medium Value, Removal Rate, Power in Power, Power Flux, Material
Tool Type and DOC F Vc MRR Cutting, Pc P Pf Work Material Hardness
Material [inches] [inch/rev] [feet/min] [in3/min] [hp/in3/min] [hp]
[kW/mm2] Carbon Steel, 150 HB indexable carbide, 0.150 0.020 490
17.6 0.6 10.2 3.9 1020 grade C-6 (P20) Carbon Steel, 150 HB HSS,
M2-M3 0.150 0.015 120 3.2 0.6 1.9 1.0 1020 grade H13 Tool Steels,
Q&T 48-50 indexable carbide, 0.150 0.010 150 2.7 1.2 3.2 2.5
HRC C-8 (P01) H13 Tool Steels, Q&T 48-50 indexable carbide,
0.300 0.015 120 6.5 1.2 7.8 2.0 HRC C-8 (P01) High-carbon Alloy
52-54 indexable carbide, 0.150 0.005 115 1.0 1.6 1.7 2.6 or Tool
Steels HRC C-8 (P01) Cold Work Too 58-60 PCBN (DBC50) 0.012 0.004
490 0.3 3.0 0.8 20.4 Steel HRC Austenitic St. 135-185 indexable
carbide, 0.150 0.020 350 12.6 0.8 10.1 3.9 Steels HB C-2 (K10/M10)
Austenitic St. 135-185 Cold-pressed 0.150 0.010 900 16.2 0.8 13.0
10.0 Steels HB Alumina, ceramic Austenitic St. cold drawn indexable
carbide, 0.150 0.015 300 8.1 0.9 7.3 3.7 Steels to 275 HB C-3
Austenitic St. cold drawn HSS, T15-M42 0.150 0.015 80 2.2 0.9 1.9
1.0 Steels to 275 HB Ti-6Al-4V ELI 310-350 indexable carbide, 0.150
0.008 195 2.8 1.4 3.9 3.8 HB C-2 (K10, M10) Ti-6Al-4V ELI 310-350
HSS, T15-M42 0.150 0.010 60 1.1 1.4 1.5 1.2 HB NOTES: CUTTING
POWER, POWER FLUX, AND VELOCITY INDEX ARE ESTIMATED FROM DATA IN
TABLE 1. REFERENCES FOR MACHINING CONDITIONS - IAMS AND ASM LISTED
IN TABLE 1. Power Flux = Total Power/DOC/F 1 hp/in2 = 1.15
W/mm2
[0014] The representative examples in Table 3 are not intended to
be an exhaustive list. Persons skilled in the art will recognize
that numerous other conditions are possible that would result in
similar patterns.
[0015] High values of power flux indicate the magnitude of
potential upset in thermo-mechanical tool loading or irregularity
in tool cooling. Only the HSS tools and certain cemented carbide
tools operate under the range of cutting conditions where these
process irregularities can be neglected. Being a product of cutting
speed and unit power, power flux indicates whether a given set of
machining conditions leads to a high-energy cutting situation. If a
cutting speed is selected for a given tool, depth of cut, and
feedrate, which is higher than the cutting speed recommended by the
tool manufacturer, and/or the work material requires unit cutting
power exceeding 1 hp/in3/minute, the resultant power flux value
exceeds the conventional power flux value and the operation may be
classified as high-energy cutting.
[0016] Although the machining industry has strong economic
incentives to enhance cutting operations within the high-energy
range, it is limited by tool overheating, high power flux values,
and inability of removing cutting energy from tools in a uniform
manner required to prevent rapid failures. All tool materials,
including HSS, carbides, and refractory ceramics, have one thing in
common--as the temperature of the tool material increases, the tool
material softens and may develop localized, internal stresses (due
to thermal expansion, especially if compounded with limited
conductivity), as described by E. M. Trent and P. K. Wright in
"Metal Cutting", 4.sup.th Ed., Butterworth, Boston, Oxford, 2000,
and the ASM Handbook on "Machining, Ceramic Materials". This poses
limits on workpiece hardness, cutting speed, and power flux during
machining. With conventional machining methods, the industry is
unable to cope with the cooling problem while satisfying the other
needs enumerated above. Other problems facing the machining
industry include significant environmental and health related
problems associated with the conventional cutting fluids and
coolants presently used in the industry. For example, carbon
dioxide (CO.sub.2), a commonly used industrial coolant, is a
greenhouse generator. Also, since CO.sub.2 is denser than air it
presents a potential asphyxiation concern. In addition, CO.sub.2
also has the potential to cause acid corrosion, since it is soluble
in water. Freons and freon substitutes, some other commonly used
coolants, also are greenhouse generators and ozone depleters. These
substances also are explosive and/or toxic when heated on contact
with red-hot solids. Other coolants which can be explosive include
hydrocarbon gases and liquified ammonia. Coolants such as
cryogenic/liquified air with oxygen in it can result in chip
fires.
[0017] There exists a relatively large body of prior art
publications pertaining to cryogenic cooling of tools, including:
WO 99/60079 (Hong) and U.S. Pat. Nos.: 5,761,974 (Wang, et al.),
5,901,623 (Hong), 3,971,114 (Dudley), 5,103,701 (Lundin, et al.),
6,200,198 (Ukai, et al.), 5,509,335 (Emerson), and 4,829,859
(Yankoff). However, none these publications nor the other prior art
references discussed herein solve the problems discussed above or
satisfy the needs set forth below.
[0018] U.S. Pat. No. 5,761,974 (Wang et al.) discloses a
cryogenically cooled cap-like reservoir placed at the top of a
cutting tool, as shown in FIG. 1A herein (corresponding to FIG. 1
of Wang et al.). Wang's method and apparatus provides for uniform
and stable cooling, except that the reservoir requires dedicated
tooling and repositioning if depth of cut and/or feedrate are
changed during cutting operations. Such requirements and
limitations are cost-prohibitive and unacceptable in the industrial
machining environment.
[0019] U.S. Pat. No. 5,901,623 (Hong) discloses a cryogenic fluid
spraying chip-breaker which is positioned adjacent the rake face
for lifting a chip from the rake face after the chip is cut from
the workpiece. See FIGS. 1B and 1C herein (corresponding to FIGS. 3
and 7B of Hong). Hong's method does not provide for uniform cooling
of the entire cutting tool, which is desired in the case of hard
but brittle tools used in high-energy cutting operations. Moreover,
Hong's chip-breaking nozzle requires dedicated tooling and
repositioning if depth of cut and/or feedrate are changed during
cutting. Such requirements and limitations are cost-prohibitive and
unacceptable in the industrial machining environment.
[0020] U.S. Pat. No. 3,971,114 (Dudley) discloses a cryogenic
coolant tool apparatus and method in which the tool is internally
routed, the internal passage is thermally insulated, and the
coolant stream is jetted at a precise angle at the interface
between the tool edge and the workpiece so that the chip cutting
from the workpiece does not interfere with the stream. See FIGS. 1D
and 1E herein (corresponding to FIGS. 2 and 3A of Dudley). This
method also does not provide the desired uniform cooling of hard
but brittle cutting tools used in high-energy cutting operations.
Moreover, it requires an involved, dedicated tooling. This
requirement is cost-prohibitive and unacceptable in the industrial
machining environment.
[0021] U.S. Pat. No. 5,103,701 (Lundin, et al.) discloses a method
and apparatus for the diamond machining of materials which
detrimentally react with diamond cutting tools in which both the
cutting tool and the workpiece are chilled to cryogenic
temperatures. The method and apparatus require a fundamental
modification of the entire machine tool and workpiece handling,
something cost-prohibitive and unacceptable in the industrial
machining environment.
[0022] U.S. Pat. No. 6,200,198 (Ukai, etal.) discloses a method of
cutting metallic and non-metallic materials in a non-combustible
gas atmosphere where a lightly chilled mixture of nitrogen and
oxygen gases, or nitrogen gas and air, are blown to the contact
area between the tool and the work material. The effectiveness of
this method in high-energy cutting operations is questionable in
view of the fact that the gases used are not cryogenically cold.
Gas jets are known to quickly aspirate warm air from the
surroundings, which means that the jet temperature at the tool
surface is nearly the same as the temperature of the surrounding
air.
[0023] U.S. Pat. No. 5,509,335 (Emerson) discloses a cryogenic
machining system including a hermetically sealed atmosphere chamber
enclosing the entire machine tool and the material handling
mechanism and directing cryogenic fluid toward a workpiece retained
by a workpiece holder. The effectiveness of this method in
high-energy cutting operations is doubtful since the cryogen cools
the workpiece material thereby making it even harder to cut the
workpiece. The method cannot be practiced without a fundamental
modification of the entire machine tool and workpiece handling,
which is cost-prohibitive and unacceptable in the industrial
machining industry.
[0024] U.S. Pat. No. 4,829,859 (Yankoff) discloses a very
high-pressure system, pulse-mixing and jetting a conventional fluid
and cold CO.sub.2 at the tool, workpiece and chips. While very
effective in breaking long chips, the system generates a mist of
toxic fluid which is not acceptable for environmental and health
reasons. The capital and operating costs of the high-pressure
system are prohibitive and would be unacceptable in the machining
industry. Its very high-pressure jet pulsing, frequently combining
solid particles, may affect the life of hard but brittle tools
operating in the high-energy cutting mode.
[0025] WO 99/60079A2 (Hong) discloses a cryogenic milling cutter
including rotating transfer tubes and nozzles positioned next to
the cutting edges and continuously exposed to the abrasive chips
evolving from the work. Apart from the question regarding the life
and maintenance costs of this system, its application in the
machining industry requires an expensive retooling of existing
machining centers, something undesired and unacceptable in the
production environment.
[0026] It is desired to have an apparatus and a method for cooling
cutting tools, including hard but brittle tools, which improves
tool life in cutting operations characterized by power flux values
exceeding the common values recommended for conventional machining
processes by tool manufacturers, tool suppliers, and technical
authorities recognized within the machining industry.
[0027] It is further desired to have an apparatus and a method for
cooling such cutting tools that increases work material cutting
speeds and/or productivity, both of which are limited by the
lifetime (and costs) of cutting tools, inserts, and tips.
[0028] It is still further desired to have an apparatus and a
method for machining a workpiece which improves safety and
environmental conditions at workplaces by using a cryogenic coolant
to cool cutting tools, thereby eliminating conventional, emulsified
cutting fluids and/or oil mists.
[0029] It is still further desired to have an apparatus and a
method for machining a workpiece which improves safety and
environmental conditions at workplaces by minimizing the risks of
chip fires, burns and/or chip vapor emissions while using an
environmentally acceptable, safe, non-toxic and clean method of
cooling cutting tools.
[0030] It is still further desired to have an apparatus and a
method for machining which reduces production costs by elimination
of workpart, workplace, and/or machine cleaning necessitated by the
use of conventional, emulsified cutting fluids and/or oil
mists.
[0031] It is still further desired to have an apparatus and a
method for machining which provides for effective cutting of work
materials that cannot tolerate conventional, emulsified cutting
fluids and/or oil mists, such as medical products or
powder-metallurgy parts characterized by open porosity.
[0032] It is still further desired to have an apparatus and a
method for cooling cutting tools, an apparatus and a method for
controlling cooling of cutting tools during cutting operations, and
an apparatus and a method for machining a workpiece, which overcome
the difficulties and disadvantages of the prior art to provide
better and more advantageous results.
BRIEF SUMMARY OF THE INVENTION
[0033] Applicants' invention is an apparatus and a method for
cooling a cutting tool, an apparatus and a method for controlling
cooling of a cutting tool during a cutting operation, and an
apparatus and a method for cooling a workpiece. Another aspect of
the invention is an apparatus and a method for machining a
workpiece with a cutting tool using the apparatus and method for
cooling the cutting tool and/or the apparatus and method for
controlling cooling of the cutting tool. Other aspects are a
workpiece machined by the apparatus and method for machining, and
the recyclable chips removed from the workpiece as a byproduct of
the apparatus and method for machining.
[0034] A first embodiment of the method for cooling a cutting tool
includes multiple steps. The first step is to provide a supply of a
cryogenic fluid. The second step is to deliver a free-expanding
stabilized jet of the cryogenic fluid to the cutting tool. ("A
free-expanding stabilized jet" is defined and discussed in the
Detailed Description of the Invention section below.)
[0035] There are several variations of the first embodiment of the
method for cooling. In one variation, the cutting tool is engaged
in a high-energy chip-forming and workpiece-cutting operation.
Preferably, at least a portion of the cryogenic fluid is selected
from a group consisting of liquid nitrogen, gaseous nitrogen,
liquid argon, gaseous argon and mixtures thereof. In another
variation, at least a portion of the free-expanding stabilized jet
of the cryogenic fluid has a temperature below about minus 150
degrees Celsius (-150.degree. C.). In another variation, at least a
portion of the free-expanding stabilized jet of the cryogenic fluid
has a substantially uniform mass flowrate greater than or equal to
about 0.5 lbs/minute and less than or equal to about 5.0
lbs/minute. In another variation, at least a portion of the
free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate having a flow pulse cycle time
less than or equal to about 10 seconds. In another variation, the
cutting tool has a rake surface and at least a portion of the
free-expanding stabilized jet of the cryogenic fluid impinges on at
least a portion of the rake surface. In another variation, at least
a portion of the cutting tool has a traverse rupture strength (TRS)
value of less than about 3000 MPA. In another variation, the
cutting tool has a cutting edge and a means for delivering the
free-expanding stabilized jet of the cryogenic fluid to the cutting
tool has at least one discharge point spaced apart from the cutting
edge by a distance greater than or equal to about 0.1 inches and
less than about 3.0 inches. In a variant of this variation, at
least a portion of the cryogenic fluid has a pressure greater than
or equal to about 25 psig and less than or equal to about 250 psig
during or immediately prior to discharge from the at least one
discharge point.
[0036] In another embodiment of the method for cooling a cutting
tool, in which the cutting tool has a cutting edge, there are
multiple steps. The first step is to provide a supply of a
cryogenic fluid. The second step is to provide a nozzle adapted to
discharge a jet of the cryogenic fluid. The nozzle has at least one
discharge point spaced apart from the cutting edge by a distance
greater than or equal to about 0.1 inches and less than about 3.0
inches. The third step is to deliver a free-expanding stabilized
jet of the cryogenic fluid from the discharge point to the cutting
tool, wherein the cryogenic fluid has a temperature of about minus
150 degrees Celsius (-150.degree. C.) at the discharge point.
[0037] Another aspect of the invention is a method for machining a
workpiece with a cutting tool using a method for cooling the
cutting tool as in the first embodiment of the method for cooling.
Other aspects are a workpiece machined by such a method for
machining and characterized by an improved surface, and recyclable
chips removed from the workpiece as a byproduct of the method for
machining the workpiece, the recyclable chips being characterized
by an improved purity.
[0038] The method for cooling a workpiece involves multiple steps.
The first step is to provide a supply of a cryogenic fluid. The
second step is to deliver a free-expanding stabilized jet of the
cryogenic fluid to the workpiece.
[0039] A first embodiment of the method for controlling cooling of
a cutting tool during a cutting operation includes multiple steps.
The first step is to provide a supply of a cryogenic fluid. The
second step is to deliver a flow of the cryogenic fluid to the
cutting tool. The third step is to regulate the flow of the
cryogenic fluid to the cutting tool at a substantially uniform mass
flowrate, whereby a frost coating is maintained on at least a
portion of the cutting tool during substantially all of the cutting
operation in an atmosphere having an ambient relative humidity in a
range of about 30% to about 75% and an ambient temperature in a
range of about 10.degree. C. to about 25.degree. C. In one
variation of this embodiment, the cutting tool is engaged in a
high-energy chip-forming and workpiece-cutting operation.
[0040] Another embodiment of the method for controlling cooling of
a cutting tool during a cutting operation includes multiple steps.
The first step is to provide a supply of a cryogenic fluid. The
second step is to provide a nozzle adapted to discharge a flow of
the cryogenic fluid, the nozzle having at least one discharge point
spaced apart from the cutting tool. A third step is to deliver a
flow of the cryogenic fluid from the discharge point to the cutting
tool. The fourth step is to regulate the flow of the cryogenic
fluid to the cutting tool at a substantially uniform mass flowrate
greater than or equal to about 0.5 lbs/minute and less than or
equal to about 5.0 lbs/minute having a flow pulse cycle time less
than or equal to about 10 seconds, whereby a frost coating is
maintained on at least a portion of the cutting tool during
substantially all of the cutting operation in an atmosphere having
an ambient relative humidity in a range of about 30% to about 75%
and an ambient temperature in a range of about 10.degree. C. to
about 25.degree. C.
[0041] Another aspect of the invention is a method for machining a
workpiece with a cutting tool using a method for controlling
cooling of the cutting tool as in the first embodiment of the
method for controlling cooling. Other aspects are a workpiece
machined by this method for machining and characterized by an
improved surface, and the recyclable chips removed from the
workpiece as a byproduct of this method for machining, which chips
are characterized by an improved purity.
[0042] A first embodiment of the apparatus for cooling a cutting
tool includes: a supply of a cryogenic fluid; and means for
delivering a free-expanding stabilized jet of the cryogenic fluid
to the cutting tool.
[0043] There are several variations of the first embodiment of the
apparatus for cooling. In one variation, the cutting tool is
engaged in a high-energy chip-forming and workpiece-cutting
operation. Preferably, at least a portion of the cryogenic fluid is
selected from a group consisting of liquid nitrogen, gaseous
nitrogen, liquid argon, gaseous argon and mixtures thereof. In
another variation, at least a portion of the free-expanding
stabilized jet of the cryogenic fluid has a temperature below about
minus 150 degrees Celsius (-150.degree. C.). In another variation,
at least a portion of the free-expanding stabilized jet of the
cryogenic fluid has a substantially uniform mass flowrate greater
than or equal to about 0.5 lbs/minute and less than or equal to
about 5.0 lbs/minute. In another variation, at least a portion of
the free-expanding stabilized jet of the cryogenic fluid has a
substantially uniform mass flowrate having a flow pulse cycle time
less than or equal to about 10 seconds. In another variation, the
cutting tool has a rake surface and at least a portion of the
free-expanding stabilized jet of the cryogenic fluid impinges on at
least a portion of the rake surface. In another variation, at least
a portion of the cutting tool has a traverse rupture strength (TRS)
value of less than about 3000 MPa. In another variation, the
cutting tool has a cutting edge and a means for delivering the
free-expanding stabilized jet of the cryogenic fluid to the cutting
tool has at least one discharge point spaced apart from the cutting
edge by a distance greater than or equal to about 0.1 inches and
less than about 3.0 inches. In a variant of this variation, at
least a portion of the free-expanding stabilized jet of the
cryogenic fluid has a pressure greater than or equal to about 25
psig and less than or equal to about 250 psig during or immediately
prior to discharge from the at least one discharge point.
[0044] In another embodiment of the apparatus for cooling a cutting
tool, in which the cutting tool has a cutting edge, there are
several elements. The first element is a supply of a cryogenic
fluid. The second element is a nozzle adapted to discharge a jet of
the cryogenic fluid. The nozzle has at least one discharge point
spaced apart from the cutting edge by a distance greater than or
equal to about 0.1 inches and less than about 3.0 inches. The third
element is a means for delivering a free-expanding stabilized jet
of the cryogenic fluid from the discharge point to the cutting
tool, wherein the cryogenic fluid has a temperature of about minus
150 degrees Celsius (-150.degree. C.) at the discharge point.
[0045] Another aspect of the invention is an apparatus for
machining a workpiece with a cutting tool using an apparatus for
cooling the cutting tool as in the first embodiment of the
apparatus. Other aspects are a workpiece machined by an apparatus
for machining and characterized by an improved surface, and
recyclable chips removed from the workpiece as a byproduct, the
recyclable chips being characterized by an improved purity.
[0046] The apparatus for cooling a workpiece includes: a supply of
a cryogenic fluid; and a means for delivering a free-expanding
stabilized jet of the cryogenic fluid to the workpiece.
[0047] A first embodiment of the apparatus for controlling cooling
of a cutting tool during a cutting operation includes several
elements. The first element is a supply of a cryogenic fluid. The
second element is a means for delivering a flow of the cryogenic
fluid to the cutting tool. The third element is a means for
regulating the flow of the cryogenic fluid to the cutting tool at a
substantially uniform mass flow rate, whereby a frost coating is
maintained on at least a portion of the cutting tool during
substantially all of the cutting operation in an atmosphere having
an ambient relative humidity in a range of about 30% to about 75%
and an ambient temperature in a range of about 10.degree. C. to
about 25.degree. C. In one variation of this embodiment, the
cutting tool is engaged in a high-energy chip-forming and
workpiece-cutting operation.
[0048] Another embodiment of the apparatus for controlling cooling
of a cutting tool during a cutting operation includes several
elements. The first element is a supply of a cryogenic fluid. The
second element is a nozzle adapted to discharge a flow of the
cryogenic fluid. The nozzle has at least one discharge point spaced
apart from the cutting tool. The third element is a means for
delivering a flow of the cryogenic fluid from the discharge point
to the cutting tool. The fourth element is a means for regulating
the flow of the cryogenic fluid to the cutting tool at a
substantially uniform mass flowrate greater than or equal to about
0.5 lbs/minute and less than or equal to about 5.0 lbs/minute
having a flow pulse cycle time less than or equal to about 10
seconds, whereby a frost coating is maintained on at least a
portion of the cutting tool during substantially all of the cutting
operation in an atmosphere having an ambient relative humidity in a
range of about 30% to about 75% and an ambient temperature in a
range of about 10.degree. C. to about 25.degree. C.
[0049] Another aspect of the invention is an apparatus for
machining a workpiece with a cutting tool using a method for
controlling cooling of the cutting tool as in the first embodiment
of the apparatus for controlling cooling. Other aspects are a
workpiece machined by this apparatus for machining and
characterized by an improved surface, and the recyclable chips
removed from the workpiece as a byproduct, which chips are
characterized by an improved impurity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] The invention will be described by way of example with
reference to the accompanying drawings, in which:
[0051] FIGS. 1A to 1E illustrate various prior art devices used for
cryogenic cooling in cutting or machining operations;
[0052] FIG. 2A is a schematic illustration of one embodiment of the
invention;
[0053] FIG. 2B is a schematic illustration of an embodiment of the
invention used in a turning operation;
[0054] FIG. 2C is a schematic illustration of an embodiment of the
invention used in a milling operation;
[0055] FIG. 3A is a graph illustrating the tool nose temperature
over time during high-energy turning of stainless steel 440.degree.
C.;
[0056] FIG. 3B is a graph illustrating the temperature over time of
a tool nose and a tool back corner during high-energy turning of
Ti-6Al-4V ELl;
[0057] FIG. 3C is a graph illustrating the correlation between
cryo-fluid pulse cycle and tool nose temperature during high-energy
turning of stainless steel 440.degree. C.;
[0058] FIG. 3D is a graph illustrating the effect of the RPM of a
cutter on impact flowrate of pulsing cryo-fluid reaching a cutting
insert;
[0059] FIG. 4 is a graph illustrating tool life and temperature in
high-energy cutting of Ti-6Al-4V;
[0060] FIG. 5A is a graph illustrating the life of a ceramic
composite tool used in a high-energy cutting operation at the speed
of 300 ft/minute;
[0061] FIG. 5B is a graph illustrating the life of a ceramic
composite tool used in a high-energy cutting operation at the
speeds of 300 ft/minute and 400 ft/minute;
[0062] FIG. 6 is a graph illustrating the effect of pulsing
cryo-fluid jet on the life of cubic boron nitride under certain
conditions; and
[0063] FIG. 7 is a graph illustrating the effect of the invention
on the chemistry of chips collected for a Ti-6Al-4V work
material.
DETAILED DESCRIPTION OF THE INVENTION
[0064] The invention addresses fundamental, unresolved needs of the
machining industry--to produce cleaner parts faster and at less
cost, and to improve environmental and health conditions in
manufacturing operations. An important factor in reducing
manufacturing costs is to replace slow grinding operations on hard
to machine parts with more cost-effective cutting operations. The
machining industry needs improved methods for hard-turning. Another
important but frequently overlooked factor is the cost of tooling
and conventional process modifications. The machining industry
needs machining process improvements that also minimize the extent
of the modifications required to existing equipment and
processes.
[0065] The invention is an apparatus and a method for cooling a
cutting tool, an insert, a tip, an edge, a blade, or a bit, any of
which may be either stationary or moving (e.g., rotating, with
respect to a workpiece), by using a free-expanding (unconstrained)
stabilized jet of cryogenic fluid. The jet of cryogenic fluid,
which may be a single phase gas, a single phase liquid, or a
two-phase combination, preferably is liquid nitrogen, gaseous
nitrogen, liquid argon, gaseous argon, and/or mixtures thereof.
However, persons skilled in the art will recognize that other
cryogenic mixtures of liquids, gases, and solid particles could be
used as the cryogenic fluid.
[0066] The free-expanding or unconstrained jet is a stream of
cryogenic fluid expanded from a higher pressure via a nozzle into
an unconfined surrounding or a space. Due to differences in
velocity, density, and temperature, the resultant shearing forces
and mixing eddies lead to the aspiration of surrounding gas(es),
such as ambient air. A jet expanding from a nozzle located at or
above a flat plane, such as rake surface, is free-expanding, but a
jet expanding between two or more fixed planes is not
free-expanding, because the boundary film attachment effect is
significantly enhanced and aspiration of the surrounding gas
atmosphere is significantly reduced. (Rake surface is the cutting
tool surface adjacent the cutting edge which directs the flow of
the chip away from the workpiece. In the embodiment shown in FIG.
2A, rake surface is the cutting tool surface adjacent the cutting
edge which directs the flow of the chip 86. The rake surface may be
completely flat, chamfered or may have a more complex,
three-dimensional topography produced by molding or an addition of
a plate in order to provide an enhanced control chip flow and/or
chip breaking.)
[0067] The nozzle for issuing a free-expanding jet may be made of
tubing terminating behind, above, or at the rake surface.
Alternatively, the nozzle also may be made in the form of a channel
drilled in an insert-holding clamp 80 holding a cutting tool on the
back end within a toolholder 82 as shown in FIG. 2A. The nozzle may
be formed by any combination of fixed or adjustable mechanical
components attached to an insert-holding clamp or a toolholder
which have channels drilled for the discharge of the cryogenic
fluid from the desired distance at a rake surface and toward a
cutting edge of the rake surface. FIG. 2B illustrates an example of
an adjustable-angle nozzle attached to a toolholder. The nozzle
exit may be round or flat vertically or horizontally, converging,
straight or diverging. There are no particular limitations on the
nozzle, as long as the nozzle jets a free-expanding jet of
cryogenic fluid at the tool from the desired distance in the
desired direction while positioned away from the chip.
[0068] FIG. 2A illustrates a preferred embodiment of an apparatus
70 taught by the invention in which a free-expanding jet of
cryogenic fluid 72 is directed at the surface of a tip 74 of a
cutting tool. Cryogenic fluid passes through a delivery tube 76 and
through bore 78 which is drilled throughout a clamp 80 to form a
nozzle. The clamp is attached to a toolholder 82 by a fastening
mechanism (not shown). The jet of cryogenic fluid expands from the
nozzle onto the tip 74 of a cutting insert 84. In a most preferred
mode of operation, the free-expanding jet terminates at the surface
of the tip of the cutting insert. Alternatively, the free-expanding
jet may be allowed to expand further away to reach the chip 86
evolving from the workpiece as well as the surface of the workpiece
around the chip and the tool/workpiece contact zone.
[0069] The embodiment shown in FIG. 2A minimizes the extent of
modifications needed on a standard machining tool set-up to
practice the present invention. The cryogenic fluid jetting nozzle
is incorporated into a metal clamp 80 commonly used for holding the
cutting inserts 84 in work position, which cutting inserts may be
made of a brittle material. The exit of the nozzle and the front
part of the clamp are located away from the chip 86 evolving from
the workpiece 88 during cutting, and are never in continuous
contact with the chip and do not participate in the chip breaking
operation.
[0070] The illustration in FIG. 2A shows the direction 90 of
rotation (measured in RPM) of the workpiece 88, the depth of cut
(DOC) 92, the feed rate (F=undeformed chip thickness) 94, and the
cutting power 96.
[0071] FIGS. 2B and 2C illustrate a preferred mode of jet
application in turning and milling operations. The jet of cryogenic
fluid 72 impinges directly at the target tool. For the turning
application (FIG. 2B), the cryogenic fluid enters delivery tube 76
and is discharged from the nozzle assembly 98, which is an
adjustable-angle nozzle. A free-expanding stabilized jet of
cryogenic fluid is transmitted from the nozzle assembly to the tool
nose of the cutting tool insert 84. The axial length 100 of the jet
from the nozzle exit to the tool nose is a critical feature of the
invention, as discussed herein. The arrows 102 indicate entrainment
of ambient air by the jet.
[0072] In the milling operation shown in FIG. 2C, the
free-expanding stabilized jet of cryogenic fluid 72 is transmitted
from a nozzle at the end of the delivery tube 76. The distance
between the nozzle and the tool 104 must be less than three inches.
The arrows 102 indicate entrained ambient air. The tool rotates in
the direction shown by the arrow 106 as the workpiece 88 moves in
the direction shown by the arrow 108. The figure illustrates a
depth of cut 92, a width of cut 110, and the chips 86' formed by
the milling process.
[0073] Cryogenic nitrogen and/or argon fluids (in liquid or gaseous
phase) are preferred because these fluids are inert, non-toxic,
non-corrosive, acceptable environmentally, and can be made
sufficiently cold at the exit of the nozzle to refrigerate a remote
target, such as a cutting tool, if jetted at the target from a
distance. The boiling points of liquid nitrogen, liquid argon, and
several other cryogenic fluids scale with their delivery pressure
to reach the following minimum if expanded into a 1 atmosphere
pressure environment:
[0074] liquid N.sub.2=-196.degree. C.=-320.degree. F.
[0075] liquid Ar=-186.degree. C.=-302.degree. F.
[0076] liquid CO.sub.2=-79.degree. C.=-110.degree. F. (sublimation
point)
[0077] chlorofluorocarbon Freon-12 CCl.sub.2F.sub.2=-30.degree.
C.=-22.degree. F.
[0078] An expanding jet tends to entrain a large quantity of
ambient gas, such as room temperature air in typical machining
operations. The entrainment of room temperature air results in a
drastic reduction of refrigeration capacity of a cryogenic jet
within a relatively short distance from a nozzle exit. U.S. Pat.
No. 5,738,281 (Zurecki et al.) discloses a method of minimizing
this entrainment in the case of isothermal or preheated gas jets.
However, that patent does not teach about free-expanding, cryogenic
jets which tend to expand both radially and axially on mixing with
warmer surroundings.
[0079] Applicants discovered that if a cryogenic fluid is jetted
from a distance of 0.1 to 3.0 inches at a target tool surface, has
an initial temperature at the nozzle exit less than minus
150.degree. C. (-150.degree. C.), and has a flowrate of at least
0.5 lbs/minute, then the jet of cryogenic fluid arriving at the
tool surface is sufficiently cold and can, potentially, enhance the
life of the tool under high-energy cutting conditions. Applicants
also discovered that if the flowrate of the cryogenic fluid jet
exceeds 5.0 lbs/minute (37.8 grams/second), excessive spreading of
the jet of cryogenic fluid within the cutting area results in a
detrimental pre-cooling of workpiece material, a transient effect
of hardening the workpiece just upstream of the cutting edge,
leading to a drop in tool life. Applicants also determined that the
minimum discharge pressure required for effective tool cooling is
25 psig (1.7 atm). The maximum pressure (250 psig) is established
by the large-scale economics of storing and handling cryogenic
nitrogen and argon--the most common and cost-effective large tanks
holding these cryogens are rated up to 230 psig and rapidly vent a
thermally compressed and expanding cryogen if the cryogen pressure
exceeds 250 psig (17 atm). Applicants recognized that in order to
meet the economic necessities of the machining industry, the
cryogenic cooling of tools engaged in high-energy cutting
operations should be performed using a cryo-fluid stream sourced
from a large, "bulk" tank under its own cryogenic vapor pressure.
Thus, Applicants optimized their tool cooling procedure for a
maximum discharge pressure of no more than 250 psig. The discharge
pressure is the pressure measured at the inlet side of the
cryogenic fluid jetting nozzle.
[0080] The free-expanding jet of cryogenic fluid should be aimed
toward the rake, nose, and cutting edge of the cutting tool to
maximize the cooling effect. If the use of multiple cryo-jets is
desired in a given cutting operation due to work material or tool
geometry considerations, the primary cryo-jet characterized by the
highest flowrate should be aimed toward the rake, nose, and cutting
edge. Applicants found it surprising and unexpected that the
cryogenic fluid jet impinged at the rake surface in such a way that
the jet does not induce fractures, chipping, or cleavage of hard
but brittle tool materials preferred in high-energy cutting
operations. The advanced, non-metallic tools, as well as other hard
but brittle tools (characterized by a traverse rupture strength of
less than 3 GPa or a fracture toughness of less than 10 MPa m.sup.0
5) cooled according to Applicants' method lasted longer than the
same type of tools operated dry under high-energy cutting
conditions. This finding is contrary to the teachings of the prior
art
[0081] While the exact reasons for the surprising and unexpected
results (which provide a substantial improvement over the prior
art) are not clear, it appears that these results may be due to a
combination of factors. Without wishing to be bound by any
particular theory, Applicants believe that these factors include
but are not limited to: (1) cryogenic hardening of the entire
cutting tool material, (2) reduction in thermal expansion-driven
stresses within the entire tool, and (3) reduction in thermal
gradients at tool surfaces due to the boundary film effect and the
Leidenfrost phenomenon. The boundary film is a jetting
condition-controlled, semi-stagnant, transient film which "softens"
the cryogenic chilling effect and "smoothens" thermal profiles at
the impingement-cooled surface. The Leidenfrost phenomenon occurs
to a larger or smaller degree with all liquids sprayed at a target
surface that is hotter than the boiling point of the liquid. Liquid
drops boil above a hot surface and, thus, the hot surface is
screened by a layer of vapor. In the case of cryogenic liquids,
especially if colder than -150.degree. C., all tool surfaces are
hot, which means that a typical cryo-liquid jet slides on a
boundary film of its vapor without directly wetting the tool. This
makes the thermal profile of the impingement-cooled tool surface
smoother and may explain why Applicants' free-expanding cryo-fluid
jet is effective in enhancing the life of brittle tools. In the
case of an oil or water-based cutting fluid, with its boiling point
significantly higher than room temperature, boiling occurs only at
a very close distance from the perimeter of a chip contact zone at
a tool surface. When the chip changes direction during cutting, or
the cutting tool encounters a sudden cutting interruption, such a
conventional fluid spreads over a suddenly exposed, hottest tool
surface area where it boils explosively, releasing vapor,
microdroplets, and pressure waves. The boundary film thickness,
Leidenfrost phenomenon, sudden changes in boiling behavior with a
change in temperature difference between jetted liquid and target
surface (hydrodynamic instabilities), as well as the importance of
nozzle orientation and flow conditions, have been taught in many
references..sup.(3) Applicants believe that their method, practiced
within the above-described range of cryo-fluid jetting conditions,
promotes the desired, thin boundary film and/or Leidenfrost effects
which, in turn, prevents fracturing of brittle tools while cooling
and enhances tool life during high-energy cutting operations.
[0082] As shown in FIGS. 2B and 2C, Applicants' method and
conditions for free-expanding jet cooling overcome the fundamental
shortcomings of the prior art pertaining to cryogenic machining.
Since Applicants' nozzle is located well behind the area of chip
formation and the work-tool contact zone, the feed-rate, depth of
cut, and other machining conditions could be easily changed during
a given operation without a need for readjusting the nozzle
placement or risk of nozzle damage. Thus, the machining industry
may practice the invention with minimum costs, no disruptions,
enjoying the operational flexibility that arises from the fact that
the nozzle is not attached to a cutting insert or dependant on any
particular insert geometry. A key to an effective tool cooling with
the free-expanding jet is the adjustment of the cryo-fluid flowrate
within the range of 0.5 to 5.0 lbs/minute and the supply pressure
within the range of 25 to 250 psig in order to deliver its
refrigeration capacity from the exit of the remote nozzle to the
rake surface.
[0083] Applicants found that a time-average cryo-fluid flowrate
becomes sufficient only when the walls of the cutting tool are
frosted during the entire cutting operation in spite of the fact
that a significant amount of cutting energy, i.e., heat, enters the
tool through the hot chip contact area. If the frost line forms
during cutting near the cutting edge and contact zone on the side
walls and rake which moves back toward the other end of the tool,
the cryogenic cooling effect is diminished, indicating the need for
an increase in the time averaged flowrate and/or pressure of the
cryo-fluid. Note that under the preferred conditions, no frost
coating is expected to develop inside the spot of the direct
impingement of the cryogenic fluid, a moisture-free product of
N.sub.2 and/or Ar. Thus, a part of the rake and/or side-wall
surface may be free of frost coating because of a continuous
washing by a rapidly expanding and moisture-free cryo-fluid.
[0084] An exception to the tool frost-coating rule would occur if
cutting operations are carried out under very low humidity
conditions, e.g., in a controlled atmosphere chamber or in a vacuum
where the benefits of the invention could be achieved without
producing a frost coating. The normal atmospheric conditions for
the tool frosting control are 55% relative humidity (RH) plus or
minus 20% and 20.degree. C. temperature plus or minus 5.degree. C.
The minimum moisture content for the frosting control is 30%
relative humidity at a temperature of at least 10.degree. C.
[0085] Applicants also developed a diagnostic technique for
controlling the high-energy cutting operation carried out according
to the invention and involving observation of dynamic effects at
the tool-workpiece interface which may change during any particular
operation as the tool wears or cutting conditions are changed.
First, if the chip or work surface just below the cutting edge is
bright red, or appears to melt, or burn, the flowrate and/or
pressure of the cryo-fluid should be increased. Second, if the tool
nose or the perimeter of the chip contact area on the rake surface
is cherry-red, there is no need to increase the flowrate and/or
pressure of the cryo-fluid unless the frosted coating on the tool
starts to shrink. Third, if the tool nose or the perimeter of the
chip contact area on the rake is intensely bright red, the flowrate
and/or pressure of the cryogenic fluid should be increased
regardless of the condition of the frosted coating on tool surface.
An occasional local temperature increase at the work/tool contact
area may indicate geometric or compositional inhomogeneities of the
work material, and can be easily quenched by increasing the
flowrate of the cryogenic fluid within the prescribed range of 0.5
to 5.0 lbs/minute to the point at which the whole contact zone, not
just the tool surface, is cooled in a direct cryogenic fluid
impingement mode.
[0086] A cutting tool cryo-cooling operation carried out according
to the above guidelines will provide for improved results. It was
surprising and unexpected to Applicants that their cryogenic fluid
cooling method resulted in an improved fracture resistance of
brittle cutting tools during cutting, an improved life of tools
engaged in high-energy cutting, and improved surface of machined
work material, mirror-clean chips, and a practical, low-cost
process control method based on visual observation of the frost
coating and the tool nose during cutting. These improved results
were surprising and unexpected to Applicants and would be
surprising and unexpected to other persons skilled in the art.
[0087] One of the basic technical problems with the transfer of
compressed cryogenic fluids and discharging of free-expanding jets
of cryogenic fluid is a tendency for pulsing and boiling flow
instabilities, especially if flowrates fall below 1.1 lbs/minute,
which overlaps the lower range of flowrates required by Applicants'
method. Since the pulsing flow problem would significantly limit
industrial applications of cryogenic fluids, a number of more or
less effective flow-stabilizing systems have been developed which
include a combination of cryogenic subcooling below the temperature
of equilibrium vapor and venting vapor formed in transfer
lines.
[0088] Some more recent examples of such flow-stabilizing systems
are disclosed in U.S. Pat. Nos. 5,392,608 (Lee), 5,123,250 (Maric),
4,716,738 (Tatge), 4,510,760 (Wieland), and 4,296,610 (Davis). A
method of stabilizing a low-flowrate cryogenic fluid flow in
industrial machining and cutting applications was presented by
Zurecki and Harriott, "Industrial Systems for Cost Effective
Machining of Metals Using an Environmentally Friendly Liquid
Nitrogen Coolant", Aerospace Manufacturing Technology Conference,
Jun. 2-4, 1998, Long Beach, Calif., Session MP5C, Machining and
Machining Processes--Coolants and Process Safety, Paper No.
981,865, and by Zurecki et al., "Dry Machining of Metals With
Liquid Nitrogen", the 3.sup.rd International Machining &
Grinding '99 Conference and Exposition, Oct. 4-7, Cincinnati, Ohio,
1999. Since the described systems vary in cost and complexity, it
is important to identify the key features determining the
effectiveness of a given cryo-fluid flow stabilizing system in
high-energy cutting operations.
[0089] Applicants discovered that the cycle time of pulsing flow is
critical for an effective free-expanding of a cryogenic fluid jet
and an effective tool cooling under high-energy conditions.
[0090] FIG. 3A shows a change in the temperature of a hard WC-Co
cutting insert during high-energy turning of stainless steel (grade
440C) without cooling, i.e., dry, with conventional emulsion flood
cooling, and with liquid nitrogen cooling applied according to the
present invention. The high-energy turning was at a depth of cut of
0.025 inches and a feedrate of 0.010 in/rev. using a carbide insert
tool described as follows: CNMG-432, PVD-coated, ISO: M01 -M20
(K01-K20), Industry Code: C-3. The cryogenic fluid jet is turned on
a few seconds before the cutting tool begins cutting, i.e.,
contacting the workpiece and making chips. Such a "cooldown" time
is sufficient to pre-quench the most typical tools or inserts to
cryogenic temperatures required to practice the invention. The
cryogenic stream used in this test was stabilized using a slight
subcooling, and the resulting jet was steady, with no perceptible
pulsation intervals or flowrate amplitude changes.
[0091] Based on experiments with cryo-fluid cooling of cutting
tools in high-energy cutting operations, a jet pulsation amplitude
of more than 25% of the time-averaged flowrate is both easily
detectable and significant for the outcome of cooling. A jet
pulsing with an amplitude of less than 25% of its time-averaged
flowrate can be considered a stable jet for all practical purposes.
Temperatures shown in FIG. 3A were recorded with a
micro-thermocouple tip located 1.41 mm behind the cutting edge and
1.41 mm below the rake surface, inside the insert nose next to the
cutting edge. In all three machining operations noted in FIG. 3A,
the time delay between the start of the cut and the wave of the
heat diffusing from the edge and arriving at the thermocouple tip
was from one to two seconds. After this delay, the temperature
stabilized at its own characteristic level reflecting the
effectiveness of tool cooling: minus 200.degree. F. (-200.degree.
F.=-129.degree. C.) for the liquid nitrogen jet cooling, plus
150.degree. F. (+150.degree. F.=+65.degree. C.) for the
conventional flood cooling, and more than plus 300.degree. F.
(+300.degree. F.=+149.degree. C.) for the dry cutting. The
continuous climbing of the temperature in the case of dry cutting
reflected a progressive heat accumulation and wearing of the
cutting tool leading to an increasing cutting power flux entering
the tool.
[0092] FIG. 3B shows the change over time in the temperature of the
nose of the front-end of an insert (next to the cutting edge) and
in the temperature of the back corner of the insert during
high-energy turning of Ti-6Al-4V ELl. The type of insert used in
FIG. 3B is the same as in FIG. 3A, but the cutting conditions are
much heavier, and the flowrate of liquid nitrogen applied for
cooling is less than required in Applicants' method. The
high-energy turning in this case (FIG. 3B) was at a depth of cut of
0.120 inches, a feed rate of 0.010 in/rev., and a cutting speed of
230 ft./minute. After a few seconds from the start of cutting, the
frost coating on the insert shrinks and starts to completely
disappear from its walls, while the nose of the front-end heats up
to the point of emitting red light. The deficient cooling and
thermal imbalance result in a rapid wear of this insert.
[0093] FIG. 3C shows a magnification of the initial,
non-steady-state portion of the temperature plots from FIG. 3A.
More specifically, FIG. 3C shows the correlation between cryo-fluid
pulse cycle and tool nose temperature in high-energy turning of
stainless steel (grade 440C). Two different pulsing flow profiles
are superimposed on this graph to show the effect of frequency and
phase shift on insert cooling during the first seconds of cutting.
If the cryogenic fluid pulse cycle time is short compared to the
1-2 second delay in the heating of the nose of the frond-end, the
insert material is "unable to sense" the pulsation and behaves as
if the insert material was cooled by a steady jet impacting the
tool with a time-averaged flowrate. If a cryogenic fluid pulse
cycle time is long compared to the 1-2 second time delay, the
insert material may be temporarily undercooled or overcooled
depending on the phase shift between the jet amplitude and the
start-up delay interval. The former results in a dangerous
overshooting of the temperature of the nose of the front-end
leading to a steep temperature excursion, as shown in FIG. 3B, and
to a rapid tool wear. Since the synchronization of the jet pulse
phase with the delay interval is impractical and difficult under
industrial cutting conditions, the best practical solution is to
use a cryo-fluid jet that does not pulse at all or has been
stabilized enough to pulse with the cycle time shorter than the
start-up delay of a given tool.
[0094] Table 4 below details the high-energy cutting conditions
used during the tests plotted in FIGS. 3A, 3B and 3C.
4TABLE 4 EVALUATION OF MACHINING CONDITIONS IN TEST EXAMPLES
PRESENTED Cutting Speed Recommended Work Assumed: for Selected
Actual Cutting Material Unit Power Work Depth of Feedrate, Med
Speed Used in Removal Rate, in Cutting, Unit Energy Total Power
Work Material Tool Type and cut, DOC Feedrate, F Value, Vc-r Test,
Vc MRR Pc in Cutting, Ec Power, P Flux, Pf Material Hardness
Material [inches] [inches/rev] [feet/min] [feet/min] [m3/mm]
[hp/in3/min] [Joules/mm3] [hp] [kW/mm2] Stainless steel, 25 HRC
indexable carbide, 0.025 0.010 410 -- 1.2 1.0 2.7 1.2 5.7 440C
grade C-3, Sandvik GC1015-1025 Stainless steel, 25 HRC indexable
carbide, 0.025 0.010 -- 625 1.9 1.0 2.7 1.9 8.7 440C grade C-3,
Sandvik GC1015-1025 Stainless steel, 25 HRC indexable carbide,
0.025 0.010 -- 1015 3.0 1.0 2.7 3.0 14.1 440C grade C-3, Sandvik
GC1015-1025 Ti-6Al-4V 32 HRC indexable carbide, 0.120 0.010 165 --
2.4 1.8 4.9 4.3 4.1 ELI alloy C-3, Sandvik GC1015-1025 Ti-6Al-4V 32
HRC indexable carbide, 0.120 0.010 -- 230 3.3 1.8 4.9 6.0 5.7 ELI
alloy C-3, Sandvik GC1015-1025 Ti-6Al-4V 32 HRC indexable carbide,
0.030 0.008 150 -- 0.4 1.8 4.9 0.8 3.7 ELI alloy C-3, Sandvik
GC1015-1025 Ti-6Al-4V 32 HRC indexable carbide, 0.030 0.008 -- 750
2.2 1.8 4.9 3.9 18.7 ELI alloy C-3, Sandvik GC1015-1025 Hardened
tool 62 HRC indexable ceramic 0.020 0.005 365 -- 0.4 3.8 10.2 1.6
19.0 steel, A2- composite Al2O3- grade SiCw, Sandvik CC670 Hardened
tool 62 HRC indexable ceramic 0.020 0.005 -- 300 0.4 3.8 10.2 1.4
15.6 steel, A2- composite Al2O3- grade SiCw, Sandvik CC670 Hardened
tool 62 HRC indexable ceramic 0.020 0.005 -- 400 0.5 3.8 10.2 1.8
20.8 steel, A2- composite Al2O3- grade SiCw, Sandvik CC670 Hardened
tool 62 HRC indexable PCBN 0.020 0.004 325 -- 0.3 3.8 10.2 1.2 16.9
steel, A2- (low-content grade CBN), Sumutomo BN300 Hardened tool 62
HRC indexable PCBN 0.020 0.004 -- 500 0.5 3.8 10.2 1.8 26.0 steel,
A2- (low-content grade CBN), Sumutomo BN300 NOTES: CUTTING POWER,
POWER FLUX, AND VELOCITY INDEX ARE ESTIMANTED FROM DATA IN TABLE 1.
REFERENCES FOR MACHINING CONDITIONS - SANDVIK COROMANT AND SUMITOMO
Power Flux = Total Power/DOC/F
[0095] FIG. 3D illustrates the effect of pulse jet cooling in the
case of a rotating tool like the milling cutter illustrated in FIG.
3B. More specifically, FIG. 3D shows the effect of the RPM of a
cutter on impact flowrate of the pulsing cryogenic fluid reaching
the cutting insert under the following conditions: 8.0 seconds
cryo-fluid jet cycle time and 60 RPM cutter, 0.4 radian phase shift
between jet and cutter, average jet flowrate--3.0 lbs/min., jet
flowrate deviation=+/-50%, and average impact flowrate=1.5
lbs/minute. Since industrial mill cutters operate at high
rotational speeds (rpm) with the rotational frequency typically
ranging from 1 Hz to 700 Hz, the pulsing jet flow cycle time of
about 8 seconds (0.125 Hz) is sliced into short sections which are
"invisible" to the rotating cutting edge. In effect, the tool
behaves as if it was cooled by the cryo-fluid jet that pulses at
its original, "low" frequency but impacts the tool with the
flowrate reduced by the effect of rpm superimposed on the lower,
time-averaged flowrate of the pulsing jet. This drop in the impact
flowrate of the cryo-fluid could be compensated for by increasing
the average discharge flowrate of the jet at the nozzle. The
practical significance of this example is that no flowrate
adjustment could compensate for an excessively long jet pulsing
cycle time. Based on the available data, Applicants believe that
there exists a limiting value for the jet pulse cycle time (or
frequency) and that a non-steady cryogenic fluid jet which pulses
slower than the limiting value would be an ineffective coolant for
high-energy cutting operations regardless of the time-average
flowrate.
[0096] The time interval of one to two seconds required to reach a
steady-state condition within the front cutting portion of the hard
WC-6Co carbide insert, as shown in FIGS. 3A and 3B, is in line with
the experimental and numerical determination of J. Lin et al.,
"Estimation of Cutting Temperature in High Speed Machining", Trans.
of the ASME, Vol. 114, July 1992, pp.289-296. Its value is the
limiting pulse cycle time value required for an effective cryogenic
fluid cooling of, specifically, harder grades of carbide tools
engaged in high-energy cutting operations. Since its value scales
with the thermal diffusivity of tool material, Applicants evaluated
it for a range of hard but brittle tools which are preferred in
high-energy cutting operations. See Table 5 below.
5TABLE 5 TIME REQUIRED TO REACH STEADY- STATE TEMPERATURE ON TOOL
RAKE SURFACE BASED ON TEST DATA FOR WC-Co INSERT.sup.(4) t =
L.sup.2 .multidot. .rho. .multidot. C.sub.p .multidot.
.lambda..sup.-1 where: t = time to reach steady-state temperature
at the distance L from the undeformed chip imprint at the rake
surface of a cutting tool .rho. = specific density of tool material
C.sub.p = specific heat of tool material .lambda. = thermal
conductivity of tool material source data: calculated: Tool time to
steady-state Specific Thermal Thermal relative to WC-6Co tool Tool
Density heat (Cp) conductivity diffusivity t (tool material)/t
(WC-6Co) Material g/cm{circumflex over ( )}3 J/(kg K) W/(m K)
m{circumflex over ( )}2/sec. where: L (material) = L (WC-Co) WC-6Co
14.7 230 100 3.0E-05 1.0 90% PCBN 3.4 810 100 3.6E-05 0.8 50% PCBN
4.3 810 44 1.3E-05 2.3 SI3N4 3.4 170 40 6.9E-05 0.4 Al2O3 3.9 770
18 6.0E-06 4.9 assumed constant values at room temperature
[0097] Due to a relatively low diffusivity as compared to the
carbide tool, Al.sub.2O.sub.3-based and low-content PCBN tools were
found to carry out the heat from the cutting edge about 2.5 to 5
times slower. Thus, for the one to two second-long time delay
recorded in FIG. 3A, the Al.sub.2O.sub.3 time delay will range from
about 5 to 10 seconds. This is in line with the numerical
estimation of A. Kabala for ceramic inserts, "Heat Transfer in
Cutting Inserts", Experimental Stress Analysis 2001, the 39.sup.th
International Conf., Jun. 4-6, 2001, Tabor, Czech Republic, and
sets the limiting value for the maximum pulse cycle time of 10
seconds. Because of very high power fluxes (P.sub.f) entering
cutting tools through the contact zone in high-energy cutting
operations, as shown in Tables 3 and 4, a fluctuation in tool
cooling exceeding the limiting pulse cycle time of 10 seconds would
lead to a premature tool failure. Consequently, the free-expanding
cryo-fluid jets used in high-energy cutting should be sufficiently
stabilized during the transfer from the source tank to the nozzle
to pulse at a cycle time of less than 10 seconds whenever the pulse
amplitude exceeds 25% of the time-averaged flowrate.
EXAMPLES
[0098] FIG. 4 shows an evolution of insert temperature and flank
wear during a high-energy finish-turning test cutting Ti-6Al-4V
with a hard grade of WC-Co insert. The depth of cut was 0.030
inches at a cutting speed of 750 ft./min. and a feed rate of 0.008
in./rev. using the same type of insert as that used in FIG. 4A. The
life of a tool cooled with a cryogenic nitrogen jet applied
according to the invention was more than four (4) times longer than
the life of a tool cooled using a conventional (emulsified) flood
coolant.
[0099] FIG. 5A shows the life of a ceramic composite tool
(Al.sub.2O.sub.3-SiC.sub.w) in a high-energy cutting operation on
A2-steel at a speed of 300 ft/minute, a depth of cut of 0.020
inches, a feedrate of 0.005 inches/rev., and a removal rate of 0.36
in..sup.3/min. The tool life was evaluated using three criteria:
the maximum flank wear, V.sub.b max=0.6 mm; the maximum flank (or
DOC notch) wear, V.sub.b max=0.7 mm; and the dimensional cutting
error producing parts 0.004 inches (0.1 mm) larger than required.
Four different cutting methods were used: (1) a conventional
emulsion flood, (2) a conventional dry, (3) a cryogenic gas-jet
applied according to the invention, and (4) a cryogenic liquid-jet,
also applied according to the invention. The conditions for (1),
(3) and (4) were as follows: (1) a uniformly flowing and completely
flooding conventional cutting fluid, 10% concentration; (3) a
stable, non-pulsing gas-phase cryogenic jet flowing at 1.8
lbs/minute, nozzle discharge temperature of minus 150.degree. C. at
7.8 atm (115 psig) pressure; and (4) a stable, non-pulsing
liquid-phase cryogenic jet containing a minute fraction of vapor,
total flowrate of 0.9 lbs/minute, nozzle discharge temperature of
minus 172.degree. C. at 8.1 atm (120 psig). The results point out
that the cryo-fluid cooling applied according to the invention
extended tool life over the two conventional methods.
[0100] FIG. 5B shows the life of the same type of tool in the same
type of test (as in FIG. 5A) at two cutting speeds of 300 ft/min.
(with a material removal rate of 0.36 in.sup.3/min) and 400 ft/min.
(with a material removal rate of 0.48 in.sup.3/min.), where the
life in minutes is a composite, averaged from the life measurements
according to the same three criteria as above for FIG. 5A. Again,
the cryo-fluid cooling applied according to the invention enhanced
tool life during this hardturning test under both cutting
speeds.
[0101] FIG. 6 shows an evolution of flank wear and cutting edge
chipping during hardturning of A2-steel with low-PCBN content
(BN-300), brazed-tip insert tools at the speed of 500 ft./minute, a
depth of cut of 0.020 inches, a feedrate of 0.004 inches/rev., and
a removal rate of 0.48 in..sup.3/min. Three cutting conditions were
compared: (1) dry turning, (2) cryogenic liquid jet cooled turning,
where the jet was insufficiently stabilized and pulsed at the
frequency of 6 seconds, and (3) cryogenic liquid jet cooled
turning, where the jet was completely stabilized and showed no
pulses or flow instabilities. The conditions for (2) and (3) were
as follows: (2) liquid-phase cryogenic jet containing a significant
volumetric fraction of vapor, total flowrate of 2.0 lbs/minute,
nozzle discharge temperature of minus 169.degree. C. at 10.2 atm
(150 psig) pressure, 6-sec. cycle; and (3) liquid-phase cryogenic
jet containing an insignificant volumetric fraction of vapor, total
flowrate of 2.0 lbs/minute, nozzle discharge temperature of minus
169.degree. C. at 10.2 atm (150 psig) pressure. The ISO tool life
criterion of maximum flank wear (V.sub.bmax) to 0.6 mm was adopted
in this test. The shortest cutting edge life was noted for the
conventional dry cutting. The life with the pulsing jet was longer,
but the longest tool life was noted for the stable, non-pulsing
jet.
[0102] Table 4, which was discussed earlier, details the
high-energy cutting conditions used during the tests plotted in
FIGS. 4 to 6 and compares the cutting speeds and power fluxes to
the respective values recommended by the manufacturers of the
tested inserts.
[0103] An additional milling test was carried out to correlate tool
frosting and jet pulsing with tool performance in high-energy
cutting. The milling cutter used in this test was a 3/4 inch (19.05
mm) diameter, 45.degree. helix, 5-flute, high-performance carbide
(WC-Co) end-mill, S545-type, made by Niagara Cutter
(http://www.niagaracutter.com/techinfo) for maximum metal removal
rates during machining of Ti-alloys and other difficult to machine
materials. The recommended speeds and feeds for this tool were 90
to 160 ft/minute (27.4 to 48.8 m/minute) and 0.002 inches/tooth
(0.05 mm/tooth), respectively. The following accelerated cutting
conditions were selected for the conventional milling operation
with this cutter using an emulsified cutting fluid (water with
"soluble" lubricant): cutting speed--178 ft/minute, rotational
speed--907 rpm, feed per tooth--0.003 inches, table feed--13.6
inches/minute, width of cut--0.080 inches, axial depth of
cut--1.000 inches, material removal rate--1.09 in.sup.3/minute.
Under these cutting conditions, all 5 cutter edges were terminally
worn after removing of 1-3.1 in.sup.3 of a Ti-6Al-4V workpiece
characterized by a hardness of 36 HRC.
[0104] In a comparative test, a liquid nitrogen jet discharged from
a pressure of 80 psig at the time-averaged flowrate of 2 lbs/minute
was directed at the cutter from the distance of 0.5 inches between
the exit of a remote nozzle and the corners of the flutes of the
end-mill as shown in FIG. 3B. As a result, the jet impinged on all
five flutes and rake surfaces of the cutter. Initially, the jet
flow was delivered via an insulated line from a saturated liquid
nitrogen cylinder in an unstabilized condition, and the jet pulse
cycle was found to be about 15 seconds. During the pulse cycle, the
low flowrate was estimated at 0.75 lbs/minute, and the high
flowrate at 3.25 lbs/minute. It was observed that the cutter could
not develop a white frost coating at the surfaces which were
unwetted by the impacting cryo-fluid jet for at least a minute
after start-up, and once that coating was established, it was
unstable, appearing and disappearing, following the jet pulse cycle
with some delay. The life of the tool cooled with this non-stable
jet and tested with the conditions used above was comparable to
that of the conventionally cooled tool.
[0105] In another comparative test, the liquid nitrogen flowrate
was stabilized using an upstream, liquid nitrogen subcooling
system, so that no jet pulsation could be visually detected. The
milling operation was repeated using progressively increasing
cutting speeds. It was observed that the frost coating was stable
throughout the entire operation. When the cutting speed, table
feedrate, rpm, and material removal rate were increased by 60% over
the values used with the conventional (emulsified) flood-cooling
cutting fluid, the tool did not experience any detectable wear
after removing the same volume of 13.1 in.sup.3. The cutting speed
(V.sub.c) used in this stabilized jet cutting test was 2.3 times
higher than the middle-point cutting speed recommended by the tool
manufacturer. The power flux used was 1.5 times higher than the
power flux calculated from the middle-point cutting speed and the
maximum feedrate recommended by the tool manufacturer. The
resultant material removal rate or productivity with the stabilized
cryo-fluid flow was 3.4 times higher than the maximum material
removal rate anticipated from the manufacturer's recommended
conditions.
[0106] Applicants observed that the application of the
free-expanding, stable cryo-fluid jet cooling to the tool and
tool-workpiece contact zone, as outlined in the examples above
(FIGS. 3-6 and Table 4), always resulted in very clean and shiny
surfaces of the workpiece and the chips produced during cutting.
The surfaces of the workpiece and chips were cleaner than in the
case of the conventional (emulsion) flood cooling and, quite
unexpectedly, the surfaces appeared much cleaner than in the case
of the conventional dry cutting.
[0107] An elemental chemical analysis was performed on metal chips
produced during high-energy cutting of Ti-6Al-4V, and the results
are shown in FIG. 7. The turning conditions for this high-energy
cutting were: cutting speed, V.sub.c=260 ft/min; feedrate, F=0.010
inches/rev.; and depth of cut, DOC=0.100 inches. In addition to dry
turning, results were collected for emulsion flood--cooled turning
using an emulsion flood coolant (Hangsterfers S-506CF, 13%), and
liquid nitrogen cooled turning using a free-expanding stabilized
jet impacting a cutting insert at a flow of 2.6 lbs/minute. The
results are tabulated below and shown in FIG. 7.
6 Work Material Tl-6Al-4V Nitrogen Oxygen Carbon Hydrogen Dry
turning 0.041% 0.180% 0.039% 0.0037% Liquid nitrogen cooled turning
0.037% 0.170% 0.033% 0.0049% Emulsion flood-cooled turning 0.040%
0.190% 0.063% 0.0096%
[0108] The chips produced during the cryo-fluid cooled cutting
absorbed the least amount of nitrogen, oxygen, carbon, and hydrogen
impurities; the dry cutting operation was second; and the
conventional (emulsion) flood cutting was third, and also is the
most polluting method. Interestingly, the nitrogen pick-up in the
case of liquid nitrogen cooled cutting was less than for dry and
conventional (emulsion) flood cutting, which can be explained by
the fact that the tool-workpiece contact zone was much cooler.
[0109] The results show that the chips produced during the
cryo-fluid cutting can be more easily recycled than in the case of
the conventional cutting methods. This is a significant economic
benefit in the machining industry, especially in the case of
expensive and reactive titanium, tantalum and superalloy work
parts, since the purification of these materials is extremely
difficult and expensive. More importantly, the lower contamination
of the chips collected indicates a correspondingly lower
contamination of the work material, which is desired from the
standpoint of (1) part stress distribution, (2) corrosion
resistance, and (3) post-machining processability. It is known that
the surface of metallic parts characterized by reduced oxygen,
carbon, and hydrogen contamination would be more resistant to
fatigue cracking in service, less brittle, and more corrosion
resistant. Thus, the use of Applicants' free-expanding stabilized
cryo-fluid jet cooling method brings about two additional economic
benefits to the machining industry--improved properties of parts
produced and more valuable, recyclable chips.
[0110] Applicants discovered that if a cutting tool insert is
cooled with a free-expanding, cryogenic fluid jet discharged from a
remote nozzle located away from the cutting zone, the inherent flow
instabilities or pulsation of such a jet may unexpectedly interact
with the cutting process, affect insert cooling during operation,
and reduce its life. Applicants established and optimized
cryo-fluid jetting flowrate and stabilizing conditions in order to
minimize this problem. None of these findings and inventive
techniques could be anticipated from the prior art.
[0111] With the free-expanding cryo-fluid jet, stabilized according
to the method outlined above, Applicants tried to use the
stabilized jet for cooling of hard but brittle tools preferred in
high-energy cutting operations, such as a high-speed machining,
hardturning, or cutting of difficult to machine materials in order
to enhance tool life under demanding machining conditions.
Unexpectedly, the remote and stabilized jet cooling resulted in the
enhancement of tool life even in the case of those tools which,
according to prior art and machining publications, should not be
cooled with conventional coolants in order to prevent brittle
fracturing.
[0112] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the spirit
of the invention.
ENDNOTES:
[0113] (1) Data compiled and calculated from: "Machining Data
Handbook,"3.sup.rd Edition, Vol. 2, Machinability Data Center,
Institute of Advanced Manufacturing Sciences, Inc., 1980, p.17-10,
ASM Handbook, 9.sup.th Ed., Vol. 16, "Machining,"1995, p. 15,
"Application of Metal Cutting Theory,"F. E. Gorczyca, Industrial
Press, New York, 1987, and "Analysis of Material Removal
Processes," W. R. DeVries, Springer Texts in Mechanical Eng.,
Springer-Verlag, 1992.
[0114] (2) Data compiled from: "Ceramics and Glasses, Engineered
Materials Handbooks," Vol. 4, ASM Int., The Materials Information
Soc., 1991, ASM Specialty Handbook, "Tool Materials," Ed. J. R.
Davis, 1998, "Microstructural Effects in Precision Hard Turning,"
Y. K. Chou and C. J. Evans, MED-Vol. 4, Mfg. Sci. and Engr., ASME
1996; and "Temperature and wear of cutting in high-speed machining
of Inconel 710 and Ti6Al-6V-2Sn," T. Kitagawa, et al., Wear 202
(1997), Elsevier, pp.142-148.
[0115] (3) "The Leidenfrost phenomenon", F. L. Curzon, Am. J.
Phys., 46 (8), August 1978, pp. 825-828, "A boiling heat transfer
paradox", G. G. Lavalle et al., Am. J. Phys., vol. 60, No. 7, July
1992, pp.593-597, "Cooling by immersion in liquid nitrogen", T. W.
Listerman et al., Am. J. Phys., 54 (6), June 1986, pp.554-558, "An
Analytical Method to Determine the Liquid Film Thickness Produced
by Gas Atomized Sprays", J. Yang et al., J. of Heat Transfer,
February 1996, Vol. 118, pp. 255-258, "Optimizing and Predicting
Critical Heat Flux in Spray Cooling of a Square Surface", I.
Mudawar and K. A. Estes, J. of Heat Transfer, August 1996, vol.118,
pp. 672-679, and "Film Boiling Under an Impinging Cryogenic Jet",
R. F. Barron and R. S. Stanley, Advances in Cryogenic Engineering,
Vol. 39, Ed. P. Kittel, Plenum Press, New York, 1994, pp.
1769-1777.
[0116] (4) Data compiled from: "Ceramics and Glasses, Engineered
Materials Handbook," Vol. 4, ASM Int., The Materials Information
Soc., 1991, "CRC Materials Sci. & Engineering Handbook,"
2.sup.nd Edition, CRC Press, 1994, Edited by J. F. Shackelford et
al., "Analysis of Material Removal Process," W. R. DeVries,
Springer Tests in Mechanical Engineering, Springer-Verlag 1992,.
"Transport Phenomena," R. R. Bird et al., John Wiley & Sons,
1960, "Numerical and Experimental Simulation for Cutting
Temperature Estimation using 3-dimensional Inverse Heat Conduction
Technique.
* * * * *
References