U.S. patent application number 09/838328 was filed with the patent office on 2002-03-21 for method and a device for vibration control.
Invention is credited to Claesson, Ingvar, Hakansson, Lars, Lago, Thomas.
Application Number | 20020033083 09/838328 |
Document ID | / |
Family ID | 20413036 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020033083 |
Kind Code |
A1 |
Claesson, Ingvar ; et
al. |
March 21, 2002 |
Method and a device for vibration control
Abstract
The invention relates to a device for vibration control in a
machine for cutting, said machine comprising a cutting tool
supported by a tool holder. The device comprises a control unit and
converting means which are connectible to the control unit and
comprise a vibration sensor and an actuator. The actuator comprises
an active element which converts an A.C. voltage supplied by the
control unit to the actuator into dimensional changes. Said active
element is adapted to be embedded in the body of the tool holder
and in such manner that said dimensional changes impart turning
moments to the body of the tool holder. The invention further
relates to a method for vibration control in cutting. The invention
also relates to a tool holder.
Inventors: |
Claesson, Ingvar; (Dalby,
SE) ; Lago, Thomas; (Aldine, UT) ; Hakansson,
Lars; (Helsingborg, SE) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
20413036 |
Appl. No.: |
09/838328 |
Filed: |
April 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09838328 |
Apr 20, 2001 |
|
|
|
PCT/SE99/01883 |
Oct 19, 1999 |
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Current U.S.
Class: |
82/118 ;
82/133 |
Current CPC
Class: |
Y10S 82/904 20130101;
Y10T 82/10 20150115; Y10T 82/2595 20150115; Y10T 82/2533 20150115;
B23Q 11/0032 20130101; Y10T 408/76 20150115; Y10T 82/2502 20150115;
B23B 27/002 20130101; Y10T 409/304312 20150115; Y10T 82/2585
20150115 |
Class at
Publication: |
82/118 ;
82/133 |
International
Class: |
B23B 007/00; B23B
029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 1998 |
SE |
9803605-6 |
Claims
1. A device for vibration control in a machine for cutting, said
machine comprising a cutting tool supported by a tool holder, the
device comprising a control unit and converting means which are
connectible to the control unit and comprise a vibration sensor and
an actuator, and the actuator comprising an active element, which
converts an A.C. voltage supplied by the control unit to the
actuator into dimensional changes, wherein said active element is
adapted to be embedded in the body of the tool holder, and wherein
said active element is adapted to be embedded in such manner that
said dimensional changes impart turning moments to the body of the
tool holder.
2. A device as claimed in claim 1, wherein said active element is
adapted to be embedded with its centre axis spaced from the centre
axis of the tool holder.
3. A device as claimed in claim 1, wherein said active element is
adapted to be embedded close to the surface of the tool holder.
4. A device as claimed in claim 1, said tool holder being elongated
and having an end portion which is adapted to be received in a
mounting recess of the machine, wherein said active element is
positioned along the tool holder such that, when the tool holder is
held in said recess, a portion of said active element is within
said recess.
5. A device as claimed in claim 4, wherein said portion of said
active element consists of approximately half of said active
element.
6. A device as claimed claim 1, wherein said active element is
plate-shaped.
7. A device as claimed in claim 1, wherein said actuator comprises
a double element which consists of two active elements which are
attached to each other.
8. A device as claimed in claim 1, wherein said active element is a
piezoceramic element.
9. A method for vibration control in cutting, comprising the steps
of detecting the vibrations of a tool holder during cutting, and
generating control vibrations in the tool holder, by means of at
least one active element which is electrically controllable to
dimensional changes, the method further comprising the steps of
embedding said active element in the body of the tool holder and,
for generating the control vibrations, imparting turning moments to
the body of the tool holder by generating at least one control
voltage and applying the control voltage across said active
element, and by varying the control voltage according to the
detected vibrations.
10. A method as claimed in claim 9, wherein said step of detecting
is performed by carrying out the detection of vibrations
piezoelectrically.
11. A tool holder which is adapted to support a tool for cutting,
the tool holder comprising an actuator, said actuator comprising an
active element, which is electrically controllable to dimensional
changes, wherein said active element is embedded in the body of the
tool holder and is adapted to impart, through said dimensional
changes, turning moments to the body of the tool holder.
12. A tool holder as claimed in claim 11, wherein said active
element is embedded with its centre axis spaced from the centre
axis of the tool holder.
13. A tool holder as claimed in claim 11, wherein said active
element is embedded close to the surface of the tool holder.
14. A tool holder as claimed in claim 11, wherein at least one pair
of active elements is arranged in such manner that the active
elements included in the pair are oppositely arranged on each side
of the centre axis of the tool holder.
15. A tool holder as claimed in claim 11, said tool holder being
arranged to be mounted in a machine for boring, said tool holder
being elongated and having an end portion which is adapted to be
received in a mounting recess of the machine, wherein said active
element is positioned along the tool holder such that, when the
tool holder is held in said recess, a portion of said active
element is within said recess.
16. A device as claimed in claim 15, wherein said portion of said
active element consists of approximately half of said active
element.
17. A tool holder as claimed in claim 11, wherein said active
element is arranged in a recess in the tool holder and is connected
with the tool holder via a glue joint which transfers at least part
of said dimensional change to the tool holder, and that the recess
is sealed.
18. A tool holder as claimed in claim 11, wherein said active
element is arranged in a recess in the tool holder and has two
opposite power transmitting surfaces, said power transmitting
surfaces being engaged with surfaces of the body of the tool holder
and said dimensional changes changing the distance between the
power transmitting surfaces, and that the recess is sealed.
19. A tool holder as claimed in claim 11, the tool holder
consisting of an insert holder for a turning lathe.
20. A tool holder as claimed in claim 11, the tool holder
consisting of a teeth holder for a milling machine, wherein the
teeth holder comprises active elements, which are helically
arranged round the centre axis of the teeth holder.
21. A tool holder as claimed in claim 11, the tool holder
consisting of a teeth holder for a drilling machine, wherein the
teeth holder comprises active elements which are helically arranged
round the centre axis of the teeth holder.
22. A tool holder as claimed in claim 11, the tool holder
comprising an embedded, piezoelectric sensor element.
23. A tool holder as claimed in claim 11, wherein said embedded
elements are cast into the body of the tool holder.
24. A tool holder as claimed in claim 11, wherein said active
element is a piezoceramic element.
25. Use of a device as claimed in claim 1 in a machine, the machine
being one of a machine for turning, a machine for milling or a
machine for drilling.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and a device for
vibration control, and more specifically a method, a device and a
tool holder for vibration control in cutting.
BACKGROUND ART
[0002] In cutting, such as turning, drilling, milling or planing,
dynamic motion arises between the tool and the workpiece. The
motion is largely due to the fact that the chip-forming process,
i.e. the removal of the generally relatively hard material from the
workpiece, results in dynamic excitation of the tool, especially
the tool holder. The dynamic excitation results in a dynamic
motion, in the form of, for instance, elastic bending or torsion,
of the tool and the tool holder. The chip-forming process is
largely stochastic and the excitation appears in the form of tool
vibrations and noise. In addition to thus causing problems in the
working environment, the dynamic motion also affects the evenness
of the surface of the workpiece and the service life of the
tool.
[0003] It is therefore important to reduce the dynamic motion as
far as possible. It is known that the vibration problem is closely
connected with the dynamic stiffness in the construction of the
machine and the material of the workpiece. It has therefore been
possible to reduce the problem to some extent by designing the
construction of the machine in a manner that increases the dynamic
stiffness.
[0004] An important part of the construction is the actual tool
holder. The cutting tool, for instance turning insert (or tooth),
milling teeth or drilling teeth, is rigidly supported by the tool
holder. Consequently the vibrations arising between the cutting
edge and the workpiece are transferred almost completely to the
tool holder. In many cases, it is the lack of dynamic stiffness of
the tool holder that is a main problem.
[0005] Efforts have therefore recently been made to increase the
dynamic stiffness of the actual tool holder by means of active
technique in order to control the response of the tool. This means
that active control of the tool vibrations is applied.
[0006] The active control comprises the introduction of secondary
vibrations, or countervibrations, in the tool by means of a
secondary source which is often called actuator. The actuator is
operated in such manner that the countervibrations interfere
destructively with the tool vibrations.
[0007] U.S. Pat. No. 4,409,659 discloses an example of such a
control unit. An ultrasonic actuator is arranged on the tool holder
and produces countervibrations in the tool. The operating current
of the actuator is controlled according to physical parameters that
are measured and by means of the work of the actuator are kept
within defined limits. This construction is unwieldy since the
actuator is a comparatively large component which must be mounted
on a suitable surface of the tool holder. Moreover, the directive
efficiency is not quite distinct.
[0008] JP-63,180,401 discloses a very different solution where the
actuator is built into the tool holder which holds a turning
insert. A laterally extending through hole which is rectangular in
cross-section is formed in the tool holder. A piezoelectric
actuator, in series with a load detector, is fixed between the
walls that define the hole in the longitudinal direction of the
tool holder. The load detector detects the vibrations and is used
by a control unit to generate, via the actuator, countervibrations
which reduce the dynamic motion. This construction necessitates a
considerable modification of the tool holder and indicates at the
same time that the designer has not been aware of the essence of
the excitation process. In fact, the modification counteracts the
purpose of the construction by reducing the stiffness of the tool
holder in the most important directions, above all vertically,
which in itself causes a greater vibration problem, or
alternatively means that the dimensions of the tool holder must be
increased significantly in order to maintain the stiffness. During
turning, the rotating workpiece produces a downwardly directed
force on the cutting edge. When the cutting edge offers resistance,
material is broken away from the workpiece. In this context, most
of the vibrations arise. In JP-63,180,401, one imagines that the
surface of the workpiece is uneven (wave-like) and thus mainly
excites the tool holder in its longitudinal direction. Via the
actuator, one generates an oscillation in opposition towards the
wave pattern and thus obtains a constant cutting depth.
[0009] There is thus a need for a solution which controls the most
essential vibrations in cutting, such as turning, milling, drilling
or planing, and which causes a minimum of negative effects, such as
bulky projections of dynamically weakening modifications, and still
has a good effect.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a device
and a method for controlling of tool vibrations, said device and
said method having no or at least a negligible negative effect on
the dimensions of the tool.
[0011] Another object of the present invention is to provide a
device and a method for controlling of tool vibrations, said device
and said method having no or at least a negligible negative effect
on the mechanical properties of the tool.
[0012] A further object of the invention is to provide a device and
a method for controlling of tool vibrations, said device and said
method producing a directed and direct control of the tool
vibrations.
[0013] One more object of the invention is to provide a device and
a method for controlling of tool vibrations, said device and said
method enabling control of tool vibrations in an optional
direction.
[0014] The objects with regard to a device are achieved by a device
for vibration control in a machine for cutting, said machine
comprising a cutting tool supported by a tool holder, the device
comprising a control unit and converting means which are
connectible to the control unit and comprise a vibration sensor and
an actuator, and the actuator comprising an active element which
converts an A.C. voltage supplied by the control unit across the
actuator into dimensional changes. Said active element is adapted
to be embedded in the body of the tool holder, and it is adapted to
be embedded in such manner that said dimensional changes impart
turning moments to the body of the tool holder.
[0015] The objects with regard to a method are achieved by a method
for vibration control in cutting, comprising the steps of detecting
the vibrations of a tool holder during cutting, and generating
control vibrations in the tool holder, by means of at least one
active element which is electrically controllable to dimensional
changes. The method is characterised by the steps of embedding said
active element in the body of the tool holder and, for generating
the control vibrations, imparting turning moments to the body of
the tool holder by generating at least one control voltage and
applying the control voltage across said active element, and by
varying the control voltage according to the detected
vibrations.
[0016] The idea of embedding, according to the invention, at least
one active element in the tool holder implies a minimal
modification of the tool holder and at the same time uses the
rapidity and the capability of changing dimensions of the active
element in an optimal manner. The embedding makes it possible to
transfer more efficiently the dimensional change direct to the body
of the tool holder and with maximum efficiency. The prior-art
technique according to JP-63,180,401 where the actuator element is
arranged freely except for the end walls gives space for outwards
bending of the actuator element, whereby power is lost. The
embedding is also advantageous by the device being useable in
practice since it is protected against cutting fluids and chips.
The known devices are possibly useable for laboratories, but not in
the industry.
[0017] The device is adapted to impart a turning moment to the tool
holder through the arrangement of the active element/elements. The
corresponding actuator element in JP-63,180,401 is deliberately
arranged so that the dimensional change occurs along the
longitudinal axis of the tool holder, which does not result in a
turning moment. This depends on the above-mentioned lack of
knowledge of what primarily causes the vibration problems. Thus one
has not realised that the most important excitation forces have any
other direction but parallel with said longitudinal axis. Even with
this knowledge, the construction according to JP-63,180,401,
however, is not easily adjustable to any other kind of mounting
than the one shown.
[0018] The active element according to the invention can be made
small. This makes it easy to build the active element into the tool
holder when manufacturing the same without any detrimental effect
on the mechanical properties of the tool holder. Besides it will be
possible later to mount the element in existing tool holders.
[0019] Moreover, the mounting will be flexible since the active
element may be mounted with an optional orientation. Consequently
it will be possible to achieve maximum controllability for
vibrations of practically any direction whatever.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will now be described in more detail with
reference to the accompanying drawings, in which
[0021] FIG. 1 exemplifies in a perspective view the application of
forces on a cutting tool;
[0022] FIG. 2 is a schematic cross-sectional view of an embodiment
of the invention applied to a tool for turning;
[0023] FIG. 3 is a block diagram of controlling according to the
embodiment in FIG. 2;
[0024] FIG. 4 illustrates a different embodiment of the invention
applied to a tool for milling; and
[0025] FIG. 5 is a schematic view of yet another embodiment of the
tool holder according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0026] A basic object of the invention is to counteract the arising
of vibrations causing noise, wear and uneven surfaces in connection
with cutting of a workpiece. The casual relation for the arising of
vibrations in cutting has been described above. A correctly
performed vibration control according to the invention obviates the
problems and results in an excellent surface finish.
[0027] FIG. 1 shows an example of forces to which a tool 1, in this
case a turning insert, is exposed owing to the working of a
workpiece 2. The tool 1 is supported by a tool holder 3, with which
the tool 1 is rigidly connected. The workpiece 2 rotates in the
direction of arrow A. The tool holder 3 moves in a direction of
feed indicated by arrow B. The rotation of the workpiece 2 and the
motion of the tool holder 3 together generate a resultant force as
illustrated by arrow f. The resultant force f can be divided into
components f.sub.f, f.sub.p and f.sub.v. As appears from FIG. 1,
the dominating component is f.sub.v which designates the force
required to remove material from the workpiece 2.
[0028] FIG. 2 exemplifies an embodiment of the inventive device and
how this embodiment is used in turning. FIG. 2 is a schematic
longitudinal cross-section of a tool in the form of a turning
insert 21, and a tool holder in the form of a turning insert holder
23, which correspond to the tool 1 and the tool holder 3,
respectively, in FIG. 1. A rotating workpiece is shown in
cross-section at 22. The inventive device is in this example
arranged to reduce/counteract the vibrations caused by the force
component f.sub.v and indicated by arrow C. The device comprises
converting means, which consist of plate-shaped sensors 24, 25 and
plate-shaped actuators 26, 27. The actuators 26, 27 comprise active
elements, here one element each, which in this embodiment consist
of piezoceramic elements which change dimension when an electric
voltage is applied across them. The dimensional change is related
to the voltage. A piezoceramic element can in turn be designed as a
unit or advantageously be made up as a so-called stack and/or of
several partial elements. Thus, the element can be a solid body or
a plurality of individual, but composed and interacting bodies. The
sensors 24, 25 are piezoelectric crystals which generate an
electric voltage when subjected to forces. The device further
comprises a control unit 28 which is electrically connected to the
sensors 24, 25 and the actuators 26, 27 via a conduit 29 containing
a plurality of conductors. For the sake of clarity, only those
conductors 30-33 are shown in the tool holder 23 which are
connected to the actuators 26, 27, but of course conductors are
also arranged for the sensors 24, 25.
[0029] The active elements, i.e. the piezoceramic elements, 26, 27
are embedded in the tool holder 23. In this case, and as a
preferred embodiment, the embedding is made by casting. The casting
is carried out by forming for each active element 26, 27 a recess
in the body of the tool holder 23, whereupon the active element 26,
27 is arranged therein and covered by casting. The active element
26, 27 is glued preferably to the bottom surface of the recess. The
sensors 24, 25 are fixed by casting in the same way as the active
elements. The conductors 30-33 are also cast into the tool holder
23.
[0030] The converting means 24-27 are oppositely arranged in pairs
and in parallel, in the form of one pair of sensors 24, 25 and one
pair of actuators 26, 27. An upper sensor 24 of the sensors 24, 25
is arranged close to the upper side 23a of the tool holder 23, and
a lower sensor 25 of the sensors 24, 25 is arranged close to the
underside 23b of the tool holder 23. The actuators 26, 27 are
arranged correspondingly, i.e. with an upper and a lower actuator
26, 27 arranged close to the upper side 23a and the underside 23b,
respectively, of the tool holder 23.
[0031] In FIG. 5 another advantageous embodiment is shown. The
purpose of FIG. 5, which is even more simplified than the other
figures, is to disclose a desirable positioning of the active
elements in relation to the engagement of the tool holder in the
machine, which is here a turning lathe. As shown most schematically
in a cut away view in FIG. 5, the tool holder 23 is held in a
mounting recess 53 of the machine 51, and, more specifically, for
example in a foundation or rigid part thereof. For comparing
purposes the tool holder 23 of FIG. 5 is corresponding with the
tool holder 23 of FIG. 2 and corresponding referentials are used.
For sake of simplicity merely two active elements 26, 27 are shown,
as indicated with dashed lines. The tool holder 23 is rigidly
mounted in the machine 51, which is symbolically shown with
fastening screws 55. What is specific for this embodiment is that
the active elements 26, 27 are positioned along the length of the
tool holder 23 in such a way that they extend into the recess 53
when the tool holder 23 is properly mounted therein. Preferably
about one half of each active element 26, 27 is placed inside of
the recess 53 and the other half thereof is placed outside of the
mouth 57 of the recess 53. This is due to location of the maximum
of the turning moment, and more specifically the bending, of the
tool holder 23. It can be shown that this maximum is located at the
very mouth 57 of the recess 53. Traditionally it has typically been
assumed that the tool holder acts like a protrusion of the rigid
foundation wherein it is fastened. However, in practise the forces
acting upon the tip of the tool holder causes a bending thereof
also within the recess 53, which bending has to be taken into
account. In order to obtain a maximum effect of the dimensional
changes of the active elements 26, 27, they should be positioned
like in FIG. 5.
[0032] The operation of the device will no be described in
conjunction with FIG. 2. However, the similar operation applies to
the device as shown in FIG. 5. When during turning the tool 21 and
the tool holder 23 vibrate up and down according to arrow C, the
sensors 24, 25 are subjected to alternating pulling and pressing
forces. Each sensor 24, 25 then generates a voltage which varies
concurrently with the variations in forces. The sensor voltages are
detected and analysed by the control unit 28. The control unit 28
generates two control voltages, in the form of A.C. voltages, which
are supplied to an actuator 26, 27 each and are applied across the
piezoceramic elements 26, 27. The piezoceramic elements 26, 27 are
elongate in the longitudinal direction of the tool holder 23, and
the conductors 30-33 are connected in pairs to a piezoceramic
element 26, 27 each in their respective front ends 26a, 27a and
rear ends 26b, 27b. When voltage is applied to the actuators 26, 27
by means of the control voltages, the piezoceramic elements 26, 27
are thus extended to a greater or smaller degree depending on the
magnitude of the voltages. In other words, each piezoceramic
element 26, 27 obtains a dimensional change in its longitudinal
direction, which in the present example is also the longitudinal
direction of the tool holder 23. The piezoceramic elements 26, 27
preferably have power-transmitting surfaces, in this case their end
surfaces at the ends 26a, 26b, 27a, 27b which abut directly against
surfaces in the body of the tool holder 23. Moreover, the
piezoceramic elements 26, 27 are spaced from the centre axis I-I of
the tool holder 23. The expression "spaced from the centre axis"
means in general terms that the centre axes of the piezoceramic
elements 26, 27 do not coincide with the centre axis of the tool
holder 23. If the centre axes should coincide, no turning moment
would be obtained, but merely a pure longitudinal change of the
tool holder 23. In the preferred embodiment, the piezoceramic
elements 26, 27 are arranged close to the surface for the moment
arms to be as long as possible. In the present example, the
dominating vibration is vertical, which means that the forces
induced by means of the piezoceramic elements 26, 27 in the first
place strive to bend the end of the tool holder 23 upwards and
downwards.
[0033] The turning moments thus act round an axis which is
perpendicular to the centre axis I-I and are controlled so as to
operate in opposition to the turning moments induced by the
workpiece 22 during working owing to its rotation. This reduces the
vibrations. Thus the control unit 28 generates such control
voltages that the forces induced by the actuators 26, 27 are in
opposition to the forces detected by the sensors 24, 25.
[0034] The control unit 28 is selectable among many different
types, such as analog, fed-back control unit, conventional PID
regulator, adaptive regulator or some other control unit suitable
in a current application. Preferably the control unit strives to
control the vibrations towards an optimal state. The control can
imply, for instance, minimising of the vibrations in one or all
directions, in which case the optimal state can be completely
extinguished vibrations. A large number of known control algorithms
are available. It is desirable to find the most efficient one for a
certain application. Regarding the above-described embodiment in
connection with turning, the analysis of the sensor signals, i.e.
the voltages generated by the sensors, and the generation of the
control signals, i.e. the control voltages, to the piezoceramic
elements 26, 27 occur as follows.
[0035] A preferred embodiment of the control system which the
control unit 28, the sensors 24, 25 and the piezoceramic elements
26, 27 constitute, is fed back and based on a so-called "Filtered-X
LMS-algorithm". It is true that this algorithm is per se known to
those skilled in the art. FIG. 3 illustrates an equivalent block
diagram of the fed-back control system in a digital
description.
[0036] Block 301, which is also designated C, represents the
dynamic system controlled, which contains the actuators 26, 27 and
the sensors 24, 25. The other blocks represent an implementation of
said algorithm. Block 305 represents an FIR filter with adjustable
coefficients, block 307 represents an adaptive coefficient
adjusting means, and block 309 represents a model (C*) of the
dynamic system 301.
[0037] Seen from a functional, mathematic perspective, the dynamic
system constitutes a forward filter, whose output signal, i.e. the
response of the dynamic system, is y.sub.c(n). The coefficient
adjusting means 307 strives to optimise the coefficients of the FIR
filter so that an error signal e(n) is minimised. The error signal
e(n)=d(n)-y.sub.c(n) where d(n) is a desirable output signal. The
determination of the error signal is carried out by means of a
summer 311. To ensure that the coefficient adjusting means
converges each time independently of its initial state, it is
supplied with a reference signal r(n) from the model 309 of the
forward filter.
[0038] In mathematical terms it is possible to describe the effect
of the invention by saying that it changes the transmission of the
tool holder and, more specifically, changes the properties of one
or more forward channels, each forward channel being associated
with an excitation direction. This way of looking at the matter is
equivalent to the effect of the invention being that control
vibrations are generated, which influence the vibrations of the
tool holder. It should thus be pointed out that in many cases the
forward channel cannot be considered time-invariant, i.e. a
traditional linear systems theory is in many cases not applicable.
The system is usually non-linear.
[0039] The invention is applicable not only to turning but
functions also for other types of cutting, such as milling or
drilling, in which also the above described control algorithm is
applicable.
[0040] In milling, the workpiece does not rotate, but instead the
tool itself and its tool holder. FIG. 4 shows a milling tool holder
41, whose direction of rotation is indicated by an arrow. The
milling tool holder 41 has embedded sensors and active elements, of
which two active elements 45, 47 are schematically shown. The most
important vibrations that arise in milling are caused by torsion of
the milling tool holder 41 owing to the engagement of the cutting
edges 43 in the material of the workpiece. The milling tool holder
41 is also subjected to a certain degree of bending. The resultant
forces are mainly helically directed round the axis of rotation of
the milling tool holder 41. A preferred arrangement of the active
elements 45, 47b therefore is in a band round the milling tool
holder 41 so that the active elements have an essential extent and
simultaneously a direction of action helically round the axis of
rotation of the tool holder 41. Thus, the resulting turning moments
act essentially in the same directions as said torsion. A
conceivable variant of or combination with the helical arrangement,
however, is also to arrange the active elements parallel with the
axis of rotation.
[0041] In drilling, like in milling, the tool and the tool holder
rotate. Drills have a tool in the form of drilling teeth supported
by a tool holder. The teeth are usually welded to the holder.
However, also so-called high-speed-steel drills are available, in
which the tool holder and the tool are integrally made. Also in
that case, however, the drill comprises in terms of definition a
tool in the form of the actual teeth at the end of the drill and a
tool holder in the form of the remaining part of the drill. In
drilling, the circumstances resemble those prevailing in milling. A
clear distinction, however, is to be found in the direction of
feed, which in drilling is parallel with the axis of rotation of
the tool holder whereas it is perpendicular to the axis of rotation
of the tool in milling. A further distinction is that the entire
tool abuts against the workpiece in drilling whereas in milling the
abutment is only partial. Therefore, in drilling the vibrations are
almost exclusively related to torsion. Active elements and sensors
are arranged in about the same way as in milling, but at a greater
angle to the axis of rotation.
[0042] Also vibrations in planing tools and other cutting tools can
be controlled according to the invention.
[0043] An alternative arrangement of sensors is, in connection with
turning, between the actual insert and the tool holder, i.e. below
the insert. In that case, a pressure-sensitive sensor is used.
[0044] Besides, the sensors can be of different types. In addition
to those mentioned above, use can be made of e.g. accelerometers
and strain gauges. The latter, however, are less suitable than the
piezoelectric sensors from the environmental point of view.
[0045] Also the active elements can be of different types within
the scope of the invention. In the future, even thinner elements
than those used today will probably be conceivable, for instance in
the form of piezofilm (PZT). The currently preferred type, however,
is piezoceramic elements.
[0046] The above-described arrangements of the sensors and
actuators are examples of arrangements and many variants are
possible, such as a combination of those shown or other numbers of
actuators. For instance, in turning, it is possible to arrange two
pairs of actuators in each direction or a plurality of actuators
adjacent to those shown. In its simplest embodiment, the inventive
device comprises only one actuator which comprises one active
element. This, however, results in a more non-linear control
system, which causes unnecessary technical difficulties in
controlling. Therefore it is an advantage to balance the system by
arranging, like in the embodiment shown in FIG. 2, the active
elements in pairs opposite each other, i.e. opposite each other on
each side of the centre axis of the tool holder. A still greater
linearity is achieved if each actuator is besides formed of two
active elements which are joined, for example by gluing, with each
other, large face to large face, into a double element. The double
element will certainly be twice as thick as a single element, but
gives a more dynamic effect, which sometimes is preferable.
[0047] The active elements are in respect of form not bound to be
rectangularly parallelepipedal and plate-shaped as the elements
shown, but the form may vary according to the application. The
plate shape, however, is advantageous since it contributes to
minimising the volume of the element. Moreover, an elongate form is
a good property which also contributes to imparting to the element
a small volume. It is preferred for the dimensional changes to
occur in the longitudinal direction of the element.
[0048] The arrangement of the active elements in the tool holder
may vary and certainly also affects the form. In addition to the
above-described, preferred mounting where the elements certainly
are glued to the base of the recess but two opposite
power-transmitting surfaces essentially generate the turning
moments, other alternatives are possible. One alternative implies
that the dimensional change is fully transferred via the glue
joint, which in principle is possible with today's strongest glues.
Also other variants are contained within the scope of the
invention.
[0049] The active element is covered by casting, using a suitable
material. As an example, plastic materials can be mentioned.
Preferably, however, a cover of metal is arranged on top and on the
same level as the remaining tool holder surface.
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