U.S. patent application number 11/255482 was filed with the patent office on 2006-03-30 for method, tool and device for the production of threads.
This patent application is currently assigned to Joerg Guehring. Invention is credited to Juergen Mann.
Application Number | 20060068926 11/255482 |
Document ID | / |
Family ID | 33154332 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060068926 |
Kind Code |
A1 |
Mann; Juergen |
March 30, 2006 |
Method, tool and device for the production of threads
Abstract
A method for the production of threads, especially internal
threads, by means of a rotationally driven thread former, with
which the thread pitches are driven in and out of the surface of
the workpiece in a chipless fashion by pressure forming, in
particular in and out of the inner surface of a workpiece bore. In
order to be able to produce threads of different nominal diameter
especially economically and with improved joining quality in the
area of the thread, the thread is formed such that a shank tool in
the fashion of a thread miller, equipped with at least one profile
projection, preferably two profile projections located at a
constant distance from one another, where the profile projections
at its forming head are constructed as continuous over the
circumference and with radial extension varying over the
circumference, is driven into the workpiece initially at a
circumferential point of the workpiece bore, preferably is brought
to the total thread pressing depth, and while substantially
retaining the eccentricity set with respect to the axis of the
workpiece bore, executes a relative circular movement running
through 360.degree. (circular movement) relative to the axis of the
tool bore, while the forming head simultaneously executes a
constant axial relative feed movement by the extent of the thread
pitch to be produced.
Inventors: |
Mann; Juergen;
(Albstadt-Laufen, DE) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Assignee: |
Joerg Guehring
Albstadt
DE
|
Family ID: |
33154332 |
Appl. No.: |
11/255482 |
Filed: |
October 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/EP04/04150 |
Apr 19, 2004 |
|
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11255482 |
Oct 21, 2005 |
|
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Current U.S.
Class: |
470/204 |
Current CPC
Class: |
B23G 2225/08 20130101;
B23G 7/00 20130101; B21H 3/08 20130101; B23G 5/186 20130101; B23G
2210/08 20130101; B23G 2200/26 20130101; B23G 2200/16 20130101;
B23G 2240/36 20130101; B23G 2225/28 20130101; B23G 7/02 20130101;
B23G 5/18 20130101; B23G 2200/18 20130101 |
Class at
Publication: |
470/204 |
International
Class: |
B21J 13/02 20060101
B21J013/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2003 |
DE |
DE 103 18 203.9 |
Claims
1. A method for the production of threads by means of a
rotationally driven thread tool, with which the thread pitches are
driven in and out of the surface of the workpiece in a chipless
fashion by pressure forming, in particular in and out of the inner
surface of a workpiece bore, wherein the thread is formed such that
a shank tool in the fashion of a thread miller, equipped with at
least one profile projection where each profile projection at its
forming head is constructed as continuous over the circumference
and polygonal with radial extension varying over the circumference,
inserted in a workpiece bore having a larger diameter and moved
along the inner surface of the workpiece bore with a pre-determined
axial feed while turning, wherein after insertion into the
workpiece bore, the shank tool is driven radially into the
workpiece initially at a circumferential point of the workpiece
bore, and while substantially retaining the eccentricity set with
respect to the axis of the workpiece bore, executes a relative
circular movement running through 360.degree. relative to the axis
of the tool bore, while the forming head synchronously executes a
constant axial relative feed movement by the extent of the thread
pitch to be produced.
2. The method according to claim 1, wherein the forming head has an
axial extension which corresponds to the length of the thread to be
produced.
3. The method according to claim 1, wherein the forming head is
driven into the workpiece bore substantially centrally to the
extent of the thread depth, is then driven radially outward while
retaining the axial relative position to the workpiece bore, until
a thread ridge is fully formed at a circumferential point between
adjacent profile projections, then executes the circular movement
extending over 360.degree. with simultaneous axial feed and finally
is driven radially inward so that the profile projections of the
forming head come out of engagement with the internal thread
produced.
4. The method according to claim 3, wherein the radially outwardly
directed movement of the forming head takes place along an
arc-shaped curve.
5. The method according to claim 4, wherein on entry of the profile
projections into the workpiece, the arc-shaped curve has a motion
component in the direction of the subsequent circular movement.
6. The method according to claim 1, wherein the profile projections
with the radial extension varying over the circumference each form
a plurality of pressing lands over the circumference.
7. The method according to claim 6, wherein the processing lands
are nonuniformly distributed over the circumference.
8. The method according to claim 6, wherein the pressing lands of
adjacent profile projections are offset with respect to one another
in the circumferential direction.
9. The method according to claim 8, wherein the axially adjacent
pressing lands of the forming head each lie along a helix.
10. The method according to claim 1, wherein the area of engagement
with the workpiece the circumferential speed of the forming head is
synchronized with the circular movement.
11. The method according to claim 1, wherein the area of engagement
with the workpiece the circumferential speed of the forming head is
oppositely directed to the circular movement.
12. A rotationally drivable tool for the production of threads by
means of chipless pressure forming of the inner surface of a
workpiece bore, comprising a forming head comprising at least two
profile projections embodied in the fashion of a thread miller and
located at a constant distance from one another, which are
constructed as continuous over the circumference and with radial
extension varying over the circumference, so that in the area of
each profile projection, at least one pressing land is formed over
the circumference, wherein the profile projections each form a
plurality of pressing lands over the circumference with radially
varying axial extension over the circumference, wherein the
pressing lands of neighboring profile projections are offset with
respect to one another in the circumferential direction.
13. The tool according to claim 12, wherein the profile projections
are axially offset with respect to one another by the extent of the
thread pitch to be produced.
14. The tool according to claim 12, wherein the forming head has an
axial extension which substantially corresponds to the length of
the thread to be produced.
15. The tool according to claim 12, wherein the pressing lands are
nonuniformly distributed over the circumference.
16. The tool according to claim 12, wherein the axially adjacent
pressing lands of the forming head each lie along a helix.
17. The tool according to claim 12, wherein the depth of the
grooves between neighboring profile projections varies over the
circumference.
18. The tool according to claim 12, wherein the depth of the
grooves between neighboring profile projections remains
substantially the same over the circumference.
19. The tool according to claim 12, wherein said tool consists of a
high-strength material.
20. The tool according to claim 12, further comprising a tool
carrier made of a support material which receives at least one tool
strip of another material.
21. The tool according to claim 12, wherein at least in the area of
the pressing lands, the forming head is provided with a
coating.
22. The tool according to claim 20, wherein the coating is a hard
material layer which consists of nitrides comprising the metal
components Cr, Ti and Al and comprising the component C, wherein
the Cr fraction is 30 to 65%, the Al fraction is 15 to 80%, and the
Ti fraction is 16 to 40%, each relating to all the metal atoms in
the entire layer.
23. The tool according to claim 21, wherein the structure of the
entire layer consists of a homogeneous mixed phase.
24. The tool according to claim 21, wherein the structure of the
entire layer consist of a plurality of homogeneous individual
layers per se, which alternately consist on the one hand of
(Ti.sub.xAl.sub.yY.sub.z)N with x=0.38 to 0.5 and y=0.48 to 0.6 and
z=0 to 0.04 and on the other hand consist of CrN wherein the
uppermost layer of the wear-protective layer is formed by the CrN
layer.
25. A device for carrying out the method according to claim 1,
comprising a drive spindle for the rotationally driven thread
former and a triaxial control system with which the feed movement
along the tool axis, the driving-in and driving-out movements of
thread formed into and out of engagement with the workpiece and the
circular movement are executed in a synchronized fashion.
26. The device according to claim 25, wherein the triaxial control
system is provided by a 3D CNC machine tool.
Description
[0001] The invention relates to a method and a tool for the
production of threads, especially internal threads, according to
the preamble of claim 1 or 12. The invention further relates to a
device for carrying out the method.
[0002] Various methods for producing so-called female threads,
i.e., internal threads, are known. Commonly used is thread cutting
where a tapping tool equipped with cutting lands and cutting
grooves is driven into a workpiece bore having a pre-determined
core size with the feed predetermined by the pitch of the tool and
synchronized speed. In this case, cutting machining of the
workpiece takes place.
[0003] In order to achieve a higher strength especially in the
bearing thread flanks, the so-called thread forming method is used
where a thread former or thread ridging machine, also known as
thread bulging machine, is used and with which the material is cold
formed without the so-called "fiber course" in the material being
interrupted as in thread cutting. An important advantage compared
with thread cutting is furthermore that no chips are formed during
thread forming.
[0004] During thread forming a helical thread portion provided with
a polygon for the formation of pressing lands is screwed into the
pre-bored workpiece with a uniform feed corresponding to the pitch
of the thread. In this case, the thread profile presses stepwise
into the material of the workpiece via the start of the thread
portion whereby the stress in the compression zone is so high that
the compression limit is exceeded and the material is plastically
deformed. The material yields radially, flows along the thread
profile into the free gullet and thus forms the core diameter of
the female thread. The degree of deformation of the thread can be
controlled by means of the pre-bore diameter.
[0005] The particular advantage is that the improvement in the
structure that can be achieved has the result that the loading
capacity of the thread is still sufficient even at 50% bearing
depth. Since the lubrication is of decisive importance for this
method, lubricating grooves must be formed in the tool between the
pressing lands.
[0006] A disadvantage of this method is that a separate forming
tool corresponding to the thread diameter is required for each
thread, the geometry of the start for controlling the performance
of the thread former is relatively difficult to optimize, and the
cutting speeds and feed values cannot be selected independently of
one another.
[0007] Finally it is known to mill internal threads. In this case,
the thread is produced by juxtaposing the cutting lines of the
thread milling cutter. The thread pitch is produced by the machine
and corresponds to the so-called "pitch" of the thread milling
cutter, i.e., the axial spacing of the rows of milling teeth.
[0008] An important advantage of this method is that one and the
same milling cutter can produce threads having different diameters
and the same pitch. Compared with cut threads, there is the further
advantage that the milled thread is fully formed over almost the
entire tool length used. A disadvantage with this method of
production is that as a result of the cutting treatment, the
structure is quasi-disturbed in the area of the cut teeth as a
result of which the bearing force of the thread remains
limited.
[0009] It is thus the object of the invention to provide a method,
a tool, and a device for the production of threads, especially
internal threads with which threads of different nominal diameter
can be produced particularly economically and with improved
structure quality in the area of the thread.
[0010] This object is solved with regard to the method by the
features of claim 1, with regard to the device by the features of
claim 27 and with regard to the tool by the features of claim
12.
[0011] According to the invention, the thread is as it were
"hammered" into the inner surface of the workpiece bore by means of
a newly shaped tool. The tool has a forming head which combines the
shaping features of a thread milling cutter with those of a thread
former and specifically such that the forming head is equipped with
at least one, but preferably at least two profile projections or
teeth constructed in the fashion of a thread milling cutter and
positioned at a constant distance from one another, which are
constructed as continuous over the circumference but with radial
extension varying over the circumference so that at least one
pressing land is formed in the area of each profile projection over
the circumference as in the case of a thread milling cutter.
[0012] It has surprisingly been found that the radial forces acting
on the shank of the tool in this type of chipless machining with
milling kinematics can easily be absorbed without excessive radial
deflection of the forming head and even if the forming head has a
plurality of profile projections and thus is capable of itself
producing relatively long threads with a circular movement. This is
because the forming work to be undertaken by a pressing land can
simply be controlled merely by means of the speed of the forming
head and the speed of the circular movement of the tool which is
independent thereof and thus kept sufficiently small so that the
shank of the tool need not be excessively stressed. Further
possibilities for limiting the radial forces acting on the forming
head involve varying the number of pressing lands per profile
projection and/or displacing the pressing lands of adjacent profile
projections in the circumferential direction.
[0013] In addition to ensuring an improved bearing force of the
thread as a result of the better structures, the concept for
producing threads according to the invention has the additional
advantage that the machine control provides the possibility of
influencing the diameter tolerances in the range of the nominal
and/or the flank diameter during production. A further important
advantage of the concept for producing threads according to the
invention is that no chips are produced.
[0014] Advantageous further developments are the subject matter of
the dependent claims.
[0015] The further development of claim 2 has the advantage that
the traveling-in movement can be used for centering the tool and
that the forming of the thread can be completed using a singular
movement.
[0016] If the radially outwardly directed movement of the forming
head takes place along an arc-shaped curve, preferably in the form
of a 180.degree. run-in loop, so that on entry of the profile
projections into the workpiece, the arc-shaped curve has a motion
component in the direction of the following circulation motion, the
tool loading during immersion into the workpiece will be the
lowest.
[0017] If the profile projections i.e., the geometrical design of
the polygon, each form a plurality of pressing lands over the
circumference with the radial extension varying over the
circumference, the efficiency of the tool is increased.
[0018] The pressing lands can be distributed uniformly over the
circumference but also nonuniformly which has the advantage of
reducing the tendency to vibration.
[0019] If, in accordance with claim 8, the pressing lands of
adjacent profile projections are offset with respect to one another
in the circumferential direction, for example, in the form of a
helix, the total radial force which acts on the forming head during
the pressing process of the internal thread can be further
reduced.
[0020] In this case, it is advantageous from the production
technology point of view if, according to claim 9, the axially
adjacent pressing lands of the forming head each lie along a
helix.
[0021] The process parameters can be optimized within broad limits
according to the material to be processed. As a result of the
kinematics of claims 10 and 11, in conjunction with alternating
axial feed movement left- or right-hand threads can be produced
using the same tool.
[0022] Advantageous embodiments of the tool are the subject matter
of claims 12 to 26.
[0023] If the depth of the grooves between neighboring profile
projections varies over the circumference, the flow behavior of the
plastic material can be advantageously influenced.
[0024] For certain materials it can be advantageous to keep the
depth of the grooves between adjacent profile projections
substantially the same over the circumference.
[0025] If the tool, in accordance with claims 21, consists overall
of a high-strength material, preferably of a hard material,
especially a hard-metal material or another high-strength sintered
material, the stability of the material is particularly high which
has a particularly favorable influence on the bending deformation
as a result of the overall radial acting force.
[0026] These tools could likewise be made of a combination of
different materials. For example, strips made of a different
material, can be inserted in suitable receptacles in a support made
of a support material, for example, hardened or soft steel (or
heavy metal), high-speed steel HSS or HSSE, aluminum alloys, hard
metal or other sintered materials. For example, mention is merely
made here of hard metals, cermets, PkD, CBN, ceramic etc. The
pressing lands on the finished tool are formed using these
materials.
[0027] The inserted strips can be executed as different shapes,
e.g. cylindrical, conical, round etc. The strips can be joined to
the support in different fashions, such as, for example, soldering,
screwing, clamping, gluing or welding.
[0028] The forming head is preferably provided with a coating at
least in the area of the pressing lands. All commonly used coatings
which can achieve a reduction in friction and/or a reduction in
wear can be achieved. Especially preferred is a hard material
layer, such as of diamond, for example, preferably nanocrystalline
diamond, TiN, TiAlN or TiCN or a multilayer coating. A lubricating
layer, for example of MoS2 known under the name "MOLYGLIDE" would
also be feasible.
[0029] Particularly good lifetimes and processing parameters are
obtained using a coating according to claims 24 to 26.
[0030] A device for carrying out the method according to the
invention requires the functions specified in claim 27 which can be
realized for example using a triaxial control system of a 3D CNC
machine tool.
[0031] In detail, hard materials, especially sintered materials can
be used particularly economically in the tool according to the
invention, solid hard metal but also so-called cermet materials
being especially preferred. In this case, a particularly economical
method of production is ensured. This is because the polygonal
shape of the cross-section of the forming head can already be
formed in the sintered blank so that subsequent machining can
substantially be restricted to the area of the thread ridges.
[0032] Cermets can also be used, i.e. sintered materials which have
titanium carbide and nitride (TiC, TiN) as essential hardness
carriers and where nickel is predominantly used as the binding
phase. In this material the criteria such as low chemical affinity
to steel alloys, low heat conduction coefficient, higher hot
hardness and fine-grained structure have a particularly
advantageous influence of the field of application. The structural
makeup of cermets has now been studied to such an extent that very
fine-grained structures with high toughness can be prepared by
suitably controlling the process parameters. It can be advantageous
to incorporate titanium nitride into the material which has a low
solubility in iron as a result of its high thermodynamic stability
and thus positively influences the diffusion and friction
behavior.
[0033] An independent object of the invention is furthermore a
sintered blank for the forming head of a thread former which has
profile projections with pre-formed pressing lands located at
uniform axial spacing with respect to one another so that the end
processing of the forming head after the finish-sintering, usually
the grinding to the final dimension, can be restricted to a
minimum. These forming heads formed from sintered blanks can be
obtained from the manufacturer as semifinished products. These
forming heads are then advantageously ground with allowances of the
order of magnitude of only 0.5 mm relative to the nominal
dimension.
[0034] Further advantageous embodiments of the invention are the
subject matter of the remaining dependent claims.
[0035] Exemplary embodiments of the invention are explained in
detail hereinafter with reference to schematic drawings. In the
figures:
[0036] FIG. 1 is a schematic side view of a tool for forming an
internal thread according to the method according to the
invention;
[0037] FIG. 2 is the schematic section along II-II in FIG. 1;
[0038] FIG. 3 is a schematic perspective view of a variant of the
forming head used for the tool as a blank before incorporating
thread profile projections;
[0039] FIG. 4 is a diagram to illustrate how the individual work
steps for executing the method according to the invention are
successively arranged; and
[0040] FIG. 5 is a schematic view to illustrate a preferred run-in
loop for the forming tool.
[0041] Shown schematically in FIGS. 1 and 2 is a tool which can be
used to carry out the method according to the invention for
producing threads, especially internal threads.
[0042] This comprises a thread former 10 which can be rotatably
driven about a shank axis 14 by means of which the thread pitches
can be driven in a chipless fashion by pressure forming from the
surface of the workpiece, namely from the inner surface of a
workpiece bore which is indicated by the dot-dash line 16 in FIG.
1.
[0043] The tool 10 has a shank 12 and a forming head 18.
[0044] The forming head 18 has a plurality of profile projections
20 constructed in the fashion of a thread milling cutter and
located at a constant axial spacing T (pitch) with respect to one
another, which are continuous over the circumference as can be seen
from FIG. 2. As can likewise be seen from FIG. 2, they have a
radial extension which varies over the circumference, which lies
between ERMAX and ERMIN so that in the area of each profile
projection 20 at least one pressing land 22 is formed over the
circumference (in the embodiment according to FIGS. 1 and 2 four
pressing lands 22 are formed).
[0045] The profile projections 20 are axially offset with respect
to one another by the dimension of the thread pitch to be produced,
i.e. the dimension T corresponds to the thread pitch of the thread
to be produced.
[0046] It can be seen from the diagram in FIG. 2 that the core
cross-section of the forming head 18 has a polygonal shape and that
the radial depth RT of the profile projections 20 remains
substantially the same over the entire circumference. However, it
should already be emphasized at this point that this detail is not
essential. The circumferential contour of the core cross-section
and/or the contour of the envelopes of the profile projections 20
can easily be varied so long as the function of the pressing lands
22 is maintained.
[0047] The forming head 18 has an axial extension EA which
substantially corresponds to the length TG (see position 4.5 in
FIG. 4) of the thread to be produced.
[0048] It can be seen from FIG. 2 that the pressing lands 22 are
uniformly distributed around the circumference. However, they can
also be nonuniformly distributed to reduce the tendency of the tool
to vibrate.
[0049] The production process using the thread forming tool
according to the invention is explained in detail with reference to
FIG. 4:
[0050] The thread is formed by setting in rotational motion the
shank tool 10 equipped in the fashion of a thread milling cutter
with at least two profile projections 20 located at a constant
spacing T with respect to one another and, as is shown in FIG. 4A
on the left-hand side under position 4.1 positioning centrally
above the workpiece bore 16. The tool 10 can be constructed such
that a cutter 24 for application of a chamfer 26 is provided in the
transition zone to the shank 12, as is indicated in position
4.2.
[0051] The tool 10 is then driven axially to such a distance from
the center parallel to itself until the tool tip 28 has reached the
desired dimension TG of the thread depth, as is indicated in
position 4.3. The tool 10 is then fed radially and specifically in
such a fashion that, as is shown in position 4.4, at a
circumferential point SU it first reaches its radial position
required for pressing the thread to the desired dimension of the
flank and/or nominal diameter by plastic deformation of the
workpiece. This dimension is usually determined empirically and
depends, among other things, on the material parameters (flow
behavior) of the workpiece and the parameters of the forming
process.
[0052] The path on which the tool 10 is brought from the center of
the workpiece bore 16 into the position SU is designated by the
arrow ES. This curve as it were describes a type of run-in loop
which will be discussed in detail at a later point.
[0053] After the tool 10 has thus been brought to the full
processing or thread depth, while maintaining the eccentricity EX
of the axis of rotation 14 set with respect to the axis 30 of the
workpiece bore 16, it executes a circular movement running through
360.degree. (circular movement BZ, see arrow in position 4.5) about
the axis of the workpiece bore 16 while the forming head 18
simultaneously executes a constant axial feed movement BV by the
dimension of the thread pitch P to be produced. In this case, the
material of the workpiece is, as it were, subjected to a continuous
pressure forming in the fashion of a hammer treatment by the
pressing lands successively coming into engagement, which causes
the material to flow so that it can penetrate into the grooves 42
between the profile projections 20 in a controlled fashion. In this
way, the thread acquires a similar quality and geometry in the area
of the thread ridges as in thread forming.
[0054] When this circular movement (BZ) is completed, the thread is
ready-formed. The tolerance of the flank and the nominal diameter
can be specifically and continuously influenced during the forming
process by correction interventions in the area of the control
system for the circular movement, which benefits the service life
of the tool 10.
[0055] When the tool 10 has again reached the position SU, it is
driven radially inward on a curve AS, as shown in position 4.6, so
that the profile projections 20 of the forming head 18 come out of
engagement with the internal thread 34 which has been produced.
[0056] The method for "chipless thread milling" by means of a tool
10 which has an unchanged cross-section in the axial direction has
been described with reference to FIGS. 1 to 4. The pressing lands
22 of all the profile projections 20 thus each simultaneously come
into contact with the material of the workpiece to be expelled. To
overcome the forces thus produced and to be absorbed by the shank
12 of the tool 10, the feed in the circular direction is matched to
the speed of the tool so that the deformation work of the pressing
land 22 located in engagement is as small as possible.
[0057] In order that these forces can be applied even more
uniformly to the tool 10, in accordance with the further
development from FIG. 3, the pressing lands 122 of neighboring
profile projections can be offset with respect to one another in
the circumferential direction. Advantageously the axially adjacent
pressing lands 122 of the forming head 118 each lie along a helix
which is indicated by the dashed line W in FIG. 3. In other words,
FIG. 3 shows a pre-finished or pre-machined blank for a forming
head 118 in which the profile grooves 142 are then incorporated to
finish the forming head.
[0058] FIG. 5 finally shows a detail of the construction of the
run-in loop ES on a slightly enlarged scale. It can be seen that
the motion takes place along a circular movement guided through
180.degree. which has a positive influence on the tool loading.
However, especially if there is a fairly large difference in
diameter between tool and thread, it is equally advantageous to
select a quadrant run-in loop which is easier to program.
[0059] The tool described hereinbefore can equally well be used to
produce right and left-hand threads where only the feed direction
BV and, optionally the direction of the rotation of the tool need
to be reversed in this connection so that deformation can take
place concurrently and/or countercurrently.
[0060] For additional improvement of the processing quality the
forming head can be provided, at least in the area of the most
stressed sections, i.e., in the area of the pressing lands 22, 122,
with a coating which is preferably constructed as a hard material
coating. This hard material layer can, for example, be diamond,
preferably nanocrystalline diamond, titanium nitride or titanium
aluminum nitride. Particularly suitable among other things are a
titanium aluminum nitride layer and a so-called multi-ply layer
which is marketed under the name "Fire I" from Guhring oHG. This
comprises a TiN/(Ti, Al)N multi-ply layer.
[0061] This can particularly preferably comprise a wear-protection
layer which substantially consists of nitrides having the metal
components Cr, Ti and Al and preferably a small fraction of
elements for grain refining wherein the Cr fraction is 30 to 65%,
preferably 30 to 60%, especially preferably 40 to 60%, the Al
fraction is 15 to 80%, for example 15 to 35% or 17 to 25% and the
Ti fraction is 16 to 40%, preferably 16 to 35%, especially
preferably 24 to 35%, and specially each related to all the metal
atoms in the entire layer. In this case, the layer structure can be
single-layer with a homogeneous mixed phase or it can consist of a
plurality of homogeneous layers per se which alternately consist on
the one hand of (Ti.sub.xAl.sub.yY.sub.z)N with x=0.38 to 0.5 and
y=0.48 to 0.6 and z=0 to 0.04 and on the other hand consist of CrN
wherein the uppermost layer of the wear-protective layer is formed
by the CrN layer.
[0062] The forming head 18, 118 and/or the shank or the entire tool
10 of the exemplary embodiments show can consist of a wide range of
materials, but a material which exhibits high stability and wear
resistance is especially advantageous. The shank of the tool should
have a particularly high bending strength so that the forces acting
radially on the forming head 18, 118 can be effectively intercepted
which benefits the manufacturing precision of the thread.
[0063] In the stressing profile here the criteria abrasive wear and
hot hardness can also be of decisive importance so that cermet
types such as, for example a cermet of the type "HTX" marketed by
Kennametal-Hertel can well be used. Good results are also achieved
using the types "SC30" from the manufacturer Cerasiv GmbH
(Feldmuhle) and "Tungaly NS530" from Toshiba Europa GmbH.
[0064] It is especially preferable to use as material for the tool
a hard material such as, for example, a carbide, a nitride, a
boride or a nonmetallic hard materials or a hard material system
such as has become known for example in the form of mixed carbides,
carbon nitrides, carbide boride combinations or mixed ceramics and
nitride ceramics. In this case, those hard material which can be
manufactured as sintered moldings can also be used.
[0065] In the embodiment according to FIGS. 1 to 5, the forming
head 18, 118 consists of solid hard metal but can also be made of
another hard material, that is for example also from a cermet
sintered molding into the geometry of the forming head, i.e., the
polygonal shape of the core and/or the forming grooves are already
incorporated before the sintering. This can be accomplished for
example by suitable forming during the pressing process or also by
suitable aftertreatment of the sintered blank before the sintering
process. Thus, the sintered blank can be pre-fabricated with a
small allowance relative to the nominal diameter. In this case, it
is sufficient, for example to restrict the allowance to the range
of about 0.5 mm relative to the final dimensions. It is even
possible to preform the polygonal shape of the core section of the
forming head in the area between the polygon edges more or less to
the final dimension in the original molding process so that
grinding treatment to the final dimension in this area can even be
omitted after the sintering process. Grinding to the final
dimension is thus merely required in the area of the forming teeth
. . . 18.
[0066] As a result of the very high values of the compressive
strength, bending strength and elastic modulus of hard materials,
especially hard metal or cermet, the shank, when made of such
materials, is deformed only very slightly even under very large
radial forces which act over the entire length of the forming head
so that the forming precision can be maintained at a very good
level even when processing materials which are particularly
difficult to deform.
[0067] It has been shown that the material cermet is particularly
suitable for the forming head 18, 118. The material has a low
chemical affinity to steel alloys, a lower heat conduction
coefficient, a higher hot hardness and a relatively fine-grained
structure so that small forming edge radii can be achieved. The
heat conduction coefficient of cermet, which is a factor of 7 lower
than that of hard metal has the effect that the heat of deformation
produced does not flow directly into the forming head and thus
lower operating temperatures are obtained at the tool which in turn
makes it possible to achieve higher dimensional constancy over the
entire forming process.
[0068] However, the invention is not restricted to a specific
material for the forming head and/or shank or to a specific hard
material such as, for example, cermet quality which the average
person skilled in the art selects from the now widely
compartmentalized range of hard materials depending on the area of
application of the tool.
[0069] In particular, the type "HTX" has particularly long
lifetimes, i.e. a very low volume wear compared with other types of
cermet during the forming process, especially during forming
without lubrication. The sintered material can however also be
selected according to other criteria, such as, for example, the
desired bending strength or according to the respective material to
be processed. Thus, for example the material hard metal in the
so-called K quality has proved to be particularly favorable with
regard to the attainable useful life for the processing of cast
iron and aluminum.
[0070] The material for the shank can be selected according to one
variant so as to produce a low tendency to vibration. Thus,
conventional heat-treatable steels and tool steels can also be
used, for example the heat-treatable steel 42CrMo4 with tensile
strengths 1000 N/mm.sup.2<sigma<1500 N/mm.sup.2.
[0071] Naturally, deviations for the variants of the tool and the
method described hereinbefore are also possible without departing
from the basic idea of the invention.
[0072] Thus, for example the kinematics of the tool with respect to
the workpiece can naturally be reversed so that for example, the
workpiece executes the movement of the run-in loop and/or the
circular movement.
[0073] The polygonal shape in the forming head need not necessarily
be incorporated before a sintering process. It is equally possible
to incorporate the blank mold of the head with its polygonal
cross-sectional shape and/or the profile projections into the
cutting head before the sintering process.
[0074] The forming head or the entire tool can also consist of
other metals, advantageously from a high-strength material such as,
hard metal for example, high-speed steel such as, for example, HSS,
HSSE or HSSEBM, ceramic, cermet or another sintered metal
material.
[0075] It is further possible to provide a coolant and lubricant
supply, including internally.
[0076] The invention can also be used if the axial length of the
thread to be formed exceeds the length of the forming head 18, 118.
In this case, the motion kinetics of FIG. 4 is executed several
times in succession, i.e. with axial staggering of the thread
forming.
[0077] In principle, the forming principle can also be applied to
the production of external threads.
[0078] The invention accordingly provides a method for the
production of threads, especially internal threads, by means of a
rotationally driven thread former, with which the thread pitches
are driven out of the surface of the workpiece in a chipless
fashion by pressure forming, in particular out of the inner surface
of a workpiece bore. In order to be able to produce threads of
different nominal diameter especially economically and with
improved joining quality in the area of the thread, the thread is
formed such that a shank tool in the fashion of a thread miller,
equipped with at least two profile projections located at a
constant distance from one another, where the profile projections
at its forming head are constructed as continuous over the
circumference and with radial extension varying over the
circumference, is driven into the workpiece initially at a
circumferential point of the workpiece bore, preferably is brought
to the total thread pressing depth, and while substantially
retaining the eccentricity set with respect to the axis of the
workpiece bore, executes a relative circular movement running
through 360.degree. (circular movement) relative to the axis of the
tool bore, while the forming head simultaneously executes a
constant axial relative feed movement by the extent of the thread
pitch to be produced.
* * * * *