WO2008015455A1

WO2008015455A1 - Rotary tool holder assemblies

Info

Publication number: WO2008015455A1
Application number: PCT/GB2007/002960
Authority: WO
Inventors: John David Stratton
Original assignee: GSI Group Ltd
Current assignee: Novanta Technologies UK Ltd
Priority date: 2006-08-03
Filing date: 2007-08-02
Publication date: 2008-02-07
Anticipated expiration: 2009-02-03
Also published as: GB0615457D0; TW200906519A

Abstract

A rotary tool holder assembly (1) for holding a tool and comprising a collet (5) disposed within a shaft (4) and spring means (11) for acting on the collet, the collet being moveable relative to the shaft between a tool gripping position, for gripping a tool for rotation, and a tool release position, the spring means (11) being arranged to bias the collet towards the tool gripping position, the tool holder comprising a memory metal portion (9) which is controllably transformable in response to change in temperature between a first shape and a second shape, the memory metal portion (9) being arranged to act on the spring means to increase the biasing of the collet towards the tool gripping position when the memory metal portion transforms towards the second shape.

Description

ROTARY TOOL HOLDER ASSEMBLIES

This invention relates to rotary tool holder assemblies, in particular it relates to rotary tool holder assemblies which are to be used in high speed drilling or machining.

In such applications it is necessary to be able to hold tools securely whilst they are rotated at high speed.

One such way of holding a tool is to clamp the shank of the tool using a tapered collet. Axial spring forces are used to draw the collet back into a shaft taper imparting high gripping force on the tool shank. For high speed and high accuracy applications, for example PCB drilling, the performance of existing tool holder assemblies using tapered collet arrangements is not always adequate.

In applications such as PCB drilling, the tool may be rotated at speeds in the order of, or in excess of, 300,000 rpm. As the shaft speed increases so the shaft taper grows due to centrifugal force and moreover grows at a faster rate than the collet grows. The spring force will tend to draw the collet further up inside the taper but now less gripping force is exerted on the tool shank due to the reduction in spring pre-tension/pre-compression. The end result is that there will be a speed at which the growth of the taper reduces the gripping force on the shank to below that necessary causing excessive runout, tool slippage, and eventually complete release of the tool. Further, as the gripping force reduces, high vibration can occur as parts of the tool holder assembly begin to move.

It is desirable to alleviate the problems mentioned above.

According to one aspect of the present invention there is provided a rotary tool holder assembly for holding a tool and comprising a collet disposed within a shaft and spring means for acting on the collet, the collet being moveable relative to the shaft between a tool gripping position, for gripping a tool for rotation, and a tool release position, the spring means being arranged to bias the collet towards the tool gripping position, the tool holder comprising a memory metal portion which is controllably transformable in response to change in temperature between a first shape and a second shape, the memory metal portion being arranged to act on the spring means to increase the biasing of the collet towards the tool gripping position when the memory metal portion transforms towards the second shape.

It will be understood that the spring means may act on the collet directly, or the spring means may act on the collect indirectly through a mechanical link (for example, through a spacer). Similarly, the memory metal portion may act on the spring means directly or indirectly.

The spring means may be arranged to provide an axial force between the collet and the shaft. There may be a pair of facing shoulder portions against which the spring means may act, one of the shoulder portions being associated with the collet and one with the shaft. The shoulder portions may have surfaces that are transverse to the axis of the collet. Preferably, the surfaces are substantially perpendicular to the axis of the collet.

The tool holder assembly may comprise a bobbin mounted on the collet. Preferably, the bobbin comprises a shoulder portion against which the spring means can act.

Preferably, the shaft and the collet are arranged so that an accommodating cavity is provided within the shaft. The accommodating cavity may be formed between the shaft and the collet along part of the axial length of the shaft and the collet.

Preferably, the cavity is annular. The spring means may be accommodated within the shaft. The memory metal portion may be accommodated within the shaft.

The spring means may be accommodated in the cavity. The memory metal portion may be accommodated in the cavity.

The spring means may be circumferentially disposed around the collet.

The memory metal portion may be circumferentially disposed around the collet.

The collet may have a tail portion. The bobbin may be mounted to the tail portion. The spring pack and/or memory metal portion may be disposed around the tail portion of the collet. The collet may comprise a plurality of jaw portions for gripping an inserted tool. At least one of the collet and the shaft may be tapered so that axial movement of the collet relative to the shaft causes or allows the jaw portions of the collet to move in a direction transverse to the axis of the collet for gripping and releasing of an inserted tool.

The spring means may bias the collet relative to the shaft in an axial direction which is such that, as the collet tends to move in said axial direction, the jaw portions tend to move in a direction transverse to the axis of the collet for gripping an inserted tool. Said transverse direction may be inwards towards the axis of the holder. Said axial direction may be in a direction away from a tool receiving end of the holder.

The inside surfaces of the jaw portions can be considered to define a tool receiving recess which is shaped to receive a complementary shaped tool. Preferably, the tool receiving recess is substantially cylindrical.

The jaw portions may be disposed at a tool receiving end of the collet. Preferably, the jaw portions are circumferentially equispaced around the axis of the collet.

Preferably, the memory metal portion is arranged to transform towards the second shape if in the first shape and at a temperature on one side of a first temperature threshold and towards the first shape if in the second shape and at a temperature on the other side of a second temperature threshold. Preferably, the memory metal portion is arranged to transform towards the second shape when at a temperature above the first temperature threshold. Preferably the memory metal portion is arranged to transform towards the first shape when at a temperature below the second temperature threshold. This can allow the memory metal portion to increase the biasing of the collet towards the tool gripping position at higher temperatures.

This is useful because, generally speaking, as the rotational speed of a tool holder assembly is increased, frictional forces generate heat and so the assembly increases in temperature. Therefore, an increase in rotational speed will yield an increase in the temperature of the tool holder assembly (and memory metal portion) thereby increasing the biasing force. Also, heat generated within the motor rotor windings, particularly at high speed, will contribute to the heating effect.

The first and second temperature thresholds may be at the same temperature or spaced in temperature from one another.

The cavity and memory metal portion may be arranged so that when the memory metal portion is in the second shape it reduces the volume of the cavity occupiable by the spring means compared to the volume of the cavity occupiable by the spring means when the memory metal portion is in the first shape. The spring means may occupy part of an axial spacing between the facing shoulder portions and the memory metal portion may occupy another part of the axial spacing. The memory metal portion may be arranged to expand in an axial direction towards the second shape. The memory metal portion may be arranged to contract in an axial direction towards the first shape. The axial length of the second shape may be greater than the axial length of the first shape. The thickness of the memory metal portion in the radial direction relative to the axis of the tool holder may be smaller when in the second shape than when in the first shape. The volume of the memory metal portion in the first shape may be the same as in the second shape.

The tool holder may be arranged so that the length of the axial spacing occupiable by the spring means when the memory metal portion is in the first shape is larger than when the memory metal portion is in the second shape. The axial spacing may vary as the collet moves relative to the shaft.

The memory metal portion may be shaped to conform to the shape of the accommodating cavity. Preferably, the memory metal portion is substantially annular or part annular in cross section.

There may be a plurality of memory metal portions. Each of the plurality of memory metal portions may be located within the accommodating cavity. Where there is a plurality of memory metal portions, preferably each of the plurality of memory metal portions is part annular in cross section. The memory metal portions may be circumferentially arranged around the collet. The or each memory metal portion may be seated on one of the shoulders. A spacer may be disposed between the or each memory metal portion and the spring means.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 schematically shows a longitudinal section of a rotary tool holder assembly; and

Figure 2 schematically shows a longitudinal section of part of the shaft and collet of the rotary tool assembly of Figure 1.

Figure 1 shows a rotary tool holder assembly 1 which has a body 2 within which there is mounted a pair of bearings 3, typically air bearings. A hollow shaft 4 is journalled for rotation within the body 2. Within the shaft 4 there is a collet 5 for receiving and holding a tool (not shown). In the present embodiment the tool holder assembly is a drilling spindle and the collet 5 is provided for holding a drill bit (not shown). The collet 5 has an internal bore 6 running throughout its length. An adjustment screw 7 is disposed within the internal bore 6, approximately halfway between the two ends of the collet 5 and is threadingly mounted to the collet 5. A bobbin 8 is mounted to the collet 5 at an end of the collet 5 which is remote from the tool receiving end. Adjacent to the bobbin 8 is a memory metal portion 9, a spacer 10 and a spring pack 11. The bobbin 8, memory metal portion 9, the spacer 10 and the spring pack 11 are each disposed between the collet 5 and shaft 4.

The collet 5 is arranged for axial, but not rotational, movement relative to the shaft 4 under action of the spring pack 11 for gripping and releasing of the tool (not shown).

The tool holder assembly 1 is arranged to rotatingly drive the shaft 4 and hence the collet 5 and a carried tool about the axis of the tool holder assembly 1.

In the present embodiment, rotational drive is provided by a motor M comprising a stator Ml and rotor windings M2. The stator Ml surrounds the shaft 4 and is provided between the two air bearings 3. Rotor windings M2 are plated onto the external surface of the shaft 4. Such AC motor drive arrangements are well known for use in air bearing rotary tool assemblies and are not of particular pertinence in the case of the present invention. Therefore no further description of the structure or operation of the motor M will be given. It should also be noted that DC motor drive arrangements may be used where a permanent magnet is provided on/in the shaft 4.

Referring to Figure 2, the collet 5 comprises jaw portions 5a and a tail portion 5b. In alternatives the tail portion 5b of the collet 5 may be solid. The jaws portions 5a are arranged for gripping an inserted tool. In this embodiment, four jaw portions 5a are provided. The jaw portions 5a are located towards the tool receiving end of the rotary tool holder assembly 1 (to the right in the orientation shown in Figure 2). The jaw portions 5a are spaced from one another by way of slots 5d running parallel to the axis of the tool holder assembly. The slots 5d run from the tool receiving end of the rotary tool holder assembly 1 to part of the way into the collet 5 and terminate at stress relieving ends 5e. The slots 5d allow the jaw portions 5a to flex relative to the tail portion 5b of the collet 5. In other embodiments there may be differing numbers of jaw portions, for example there may be three, six or eight jaw portions.

The external surface of each jaw portion 5a has an external taper 5t which mates with an internal taper 4t in a front portion 4a of the shaft. When the collet 5 is moved axially relative to and into the shaft 4 these tapers 5t, 4t interact to cause the jaw portions 5a to move closer to one another, to grip an inserted tool. Similarly this allows the jaw portions 5a to move away from one another, to release an inserted tool, as the collet 5 is moved out of the shaft 4.

Rubber inserts 5c are provided in each of the jaw portions 5a to bear on the internal taper of the front portion 4a of the shaft 4. These inserts provide a dust seal to prevent or limit the ingress of dust into the assembly via the slots 5d.

As well as the front portion 4a mentioned above, the shaft 4 has a rear portion 4b. During assembly of the rotary tool holder assembly 1, the front portion of the shaft 4a is introduced into the rear portion of the shaft 4b and the two portions are welded to one another at weld locations 4c. The internal bore 6 of the collet 5 has a threaded portion 6b into which the adjustment screw 7 is mounted for rotation so as to allow axial adjustment of the adjustment screw 7. Axial adjustment of the adjustment screw 7 allows setting of the amount of a shank of a tool which can be inserted into the collet 5.

The tail portion 5b of the collet 5 has an outer threaded portion 5f on which the bobbin 8 is mounted via a complementary thread 8f. Therefore the collet 5, the adjustment screw 7 and the bobbin 8 are fixed together so that they move with one another when the collet 5 is moved axially relative to the shaft 4.

The bobbin 8 has an annular shoulder 8a which supports the memory metal portion 9 adjacent to the shoulder 8a. The memory metal portion 9 is separated by the spacer 10 from the spring pack 11 which is circumferentially disposed around part of the tail portion 5b of the collet 5. Thus, the spring pack 11 is when viewed in the axial direction, is annular in shape. Likewise, the spacer 10 is annular.

The spring pack 11 is arranged to provide a resilient axial force with one end of the spring pack 11 abutting against the spacer 10 and the other end of the spring pack 11 abutting against an annular shoulder 4d of the front portion 4a of the shaft. During assembly of the tool holder assembly 1, the springs of the spring pack 11 are compressed. Therefore, when assembled, the compressed springs of the spring pack 11 tend to drive the spacer 10, the memory metal portion 9 and the bobbin 8 further into the shaft 4 thereby retracting the collet 5 into the shaft 4. In the present embodiment, there is one annular ring-like piece of memory metal which is the memory metal portion 9. In alternatives there are a plurality (typically four or more) of memory metal portions equispaced circumferentially around the collet 5. In such alternatives, if the memory metal portions were viewed from the axial direction each would be seen to be part annular in cross section. Therefore, in such an alternative when the plurality of memory metal portions are in position in the rotary tool holder assembly 1 and viewed from the axial direction, the overall cross sectional shape they form is that of a broken ring. Other shapes and arrangements of memory metal portion or portions may be used, for example, the memory metal portion may be shaped as a disc spring.

To insert or release a tool, the collet 5 needs to be moved forwards relative to the shaft 4. A separate push-rod (not shown) is used to obtain this movement. In use, the push-rod is used to push on the bobbin 8 and/or tail portion 5b of the collet 5 moving the collet 5 forward such that the jaw portions 5a of the collet 5 spread to allow for the insertion of the tool. When the force of the push rod is removed, the collet 5 retracts under the resilient force of the springs of the spring pack 11 into the shaft 4 causing the jaw portions 5a to close onto and grip the inserted tool.

Once a tool has been loaded into the collet 5, the motor M can be activated to spin the shaft 4, the collet 5 and the held tool up to operational rotational speed.

It will be appreciated that as the shaft 4 is spun, heat is generated by friction (in particular, at the interface between the bearings 3 and the shaft 4). This tends to cause an increase in temperature throughout the entire rotary tool holder assembly 1 and so this will increase the temperature of the memory metal portions 9. Heat is also generated in the motor rotor windings M2 which heat will conduct through the shaft 4 to the memory metal portions 9. Generally speaking, higher rotational speeds generate more heat.

As mentioned above, the spring pack 11 is used to draw the collet 5 into the shaft taper 4t thereby imparting a gripping force onto the inserted tool shank. As mentioned in the introduction, with devices such as this, as the shaft speed increases, the shaft taper 4t grows at a faster rate than the collet taper 5t due to centrifugal force. As this occurs, the spring force will tend to draw the collet 5 further up inside the taper 4t. If nothing else is done, less gripping force will be > exerted on the tool shank due to the reduction in spring compression leading to the problems mentioned above.

In the present embodiment, reduced grip at high speeds is, at least in part, counteracted by the memory metal portion 9 being arranged to change shape in response to increased temperature in order to 'take up the slack' caused by flaring out of the shaft taper 4t.

It will be noted that it is a known characteristic of certain types of memory metal (for example, certain memory metals made from an alloy comprising Nickel and Titanium - so called 'NiTiNOL alloys') to change shape in response to temperature. This change in shape is due to a transformation, or phase change, between martensitic and austenitic structures. This is documented in, for example "Hodgson, DarelE., and Jeffrey W. Brown, Using Nitinol Alloys. San Jose, CA: Shape Memory Applications, Inc. 2000". It is a matter of standard practice in the art of memory metals to "train" portions of memory metal to perform required changes in shape at required temperatures. Appropriately trained pieces of memory metal are used in the present apparatus.

In this embodiment, the memory metal portion 9 is arranged to expand and contract in the axial direction in respond to a change in temperature. Specifically, an increase in temperature beyond a first threshold temperature will cause the memory metal portion 9 to expand significantly in the axial direction.

When the temperature of the memory metal portion 9 falls back below a second threshold temperature the portions will relax to their rest state and therefore contract significantly, either spontaneously or under the action of an external force.

The first and second threshold temperatures will generally be different from one another and are dependent on the memory metal portions used, in particular their composition and training.

As mentioned, at higher rotational speeds, the temperature of the memory metal portion 9 will tend to be greater than at low speeds. The memory metal portion 9 is chosen/trained so that the temperatures obtained during high speed rotation are above the first threshold temperature causing an "automatic" expansion in axial extent of the portion 9 during high speed rotation. Axially the portion 9 will expand much more than say the thermal growth of the surrounding material (steel in this embodiment). Generally, there will be a resulting contraction in radial dimension, but this is not of particular significance.

When the memory metal portion 9 is expanded, the springs of the spring pack 11 are compressed more than if the memory metal portion 9 is contracted, - it has less space to occupy. This increase of the compression of the springs of the spring pack 11 will increase the resilient force that they provide. This correspondingly increases the grip which the jaw portions 5a may exert on an inserted tool, compared with that which would be exerted if there had been no such expansion.

Because this transition occurs under high speed rotation (due to the resulting heat) the additional grip is provided just when it is required as the grip provided by an unassisted spring pack, might begin to fail.

After a drilling operation, the shaft 4 is 'spun down' over a period before coming to a stop. Once the shaft 4 has stopped rotating, a tool change operation may be required.

If the memory metal portion 9 is still in its expanded state, it may be impossible to remove the tool. At the very least the force required for the push-rod to release a gripped tool will be greater than if the memory metal portion 9 is in a contracted state. In some cases, the force required to release a gripped tool may be greater than that which can be applied by the push-rod under normal operating conditions or the arrangement may otherwise not allow the release of a gripped tool. In such cases it may be necessary to allow the memory metal portion 9 to cool below the second threshold temperature before tool release can occur. Once below the second threshold temperature force applied via the push rod may be used to encourage the memory metal portion 9 back towards its first shape, ie to "reset" the memory metal portion 9. Cooling may be assisted.

Similarly, heat which causes the memory metal portion 9 to expand does not necessarily have to originate only from the heat generated by the motor or the frictional forces between the moving parts of the rotary tool holder assembly, although this is particularly attractive because of its simplicity. Heating may be assisted by way of heating systems.

In either case (whether heating/cooling is passive or assisted) it is an important consideration to allow for heat to be easily propagated to and from the memory metal portion 9. Therefore, the memory metal portion 9 will typically be placed into good thermal contact with regions of the rotary tool holder assembly to/from which heat is transmitted. Heating/cooling channels may be provided within the rotary tool holder assembly allowing better control and responsiveness of the shape-changing action of the memory metal portion 9. It will be recognised that the use of a memory metal portion m such a tool holder assembly may provide advantages with regard to characteristics of the tool holder assembly such as its dimensioning, design and weight.

For example, it may be possible that the spring force required to grip a tool in the manner described above during high speed rotation can be offset by the action of the memory metal portion. This might allow lower pre-compression to be used. This might allow the use of smaller, lighter springs to be used, or ease assembly.

Further, the use of a memory metal portion to enhance grip in the present devices is attractive in general terms because of the absence of moving parts and complicated mechanisms, which is particularly important for systems which are to be rotated at high speed.

In some alternatives an electrical heating current can be passed through the memory metal portions to heat them, and a suitable source of electric current included in the tool holder assembly.

Claims

CLAIMS:

1. A rotary tool holder assembly for holding a tool and comprising a collet disposed within a shaft and spring means for acting on the collet, the collet being moveable relative to the shaft between a tool gripping position, for gripping a tool for rotation, and a tool release position, the spring means being arranged to bias the collet towards the tool gripping position, the tool holder comprising a memory metal portion which is controllably transformable in response to change in temperature between a first shape and a second shape, the memory metal portion being arranged to act on the spring means to increase the biasing of the collet towards the tool gripping position when the memory metal portion transforms towards the second shape.

2. A rotary tool holder assembly according to claim 1 in which the spring means is arranged to provide an axial force between the collet and the shaft.

3. A rotary tool holder assembly according to claim 1 or claim 2 in which there are a pair of facing shoulder portions against which the spring means acts, one of the shoulder portions being associated with the collet and one with the shaft.

4. A rotary tool holder assembly according to claim 3 in which the spring means occupies part of an axial spacing between the facing shoulder portions and the memory metal portion occupies another part of the axial spacing.

5. A rotary tool holder assembly according to any preceding claim in which the shaft and the collet are arranged so that an accommodating cavity is provided within the shaft and the spring means and memory metal portion are accommodated in the accommodating cavity.

6. A rotary tool holder assembly according to claim 5 in which the cavity and memory metal portion are arranged so that when the memory metal portion is in the second shape it reduces the volume of the cavity occupiable by the spring means compared to the volume of the cavity occupiable by the spring means when the memory metal portion is in the first shape.

7. A rotary tool holder assembly according to any preceding claim in which the memory metal portion is circumferentially disposed around the collet.

8. A rotary tool holder assembly according to any preceding claim in which the collet comprises a plurality of jaw portions for gripping an inserted tool, and at least one of the collet and the shaft are tapered so that axial movement of the collet relative to the shaft causes or allows the jaw portions of the collet to move in a direction transverse to the axis of the collet for gripping and releasing of an inserted tool.

9. A rotary tool holder assembly according to claim 8 in which the spring means bias the collet relative to the shaft in an axial direction which is such that, as the collet tends to move in said axial direction, the jaw portions tend to move in a direction transverse to the axis of the collet for gripping an inserted tool.

10. A rotary tool holder assembly in which the memory metal portion is arranged to transform towards the second shape if in the first shape and at a temperature above a first temperature threshold.

11. A rotary tool holder assembly according to any preceding claim in which the memory metal portion is arranged to expand in an axial direction towards the second shape.

12. A rotary tool holder assembly according to any preceding claim in which the tool holder is arranged so that the length of the axial spacing occupiable by the spring means when the memory metal portion is in the first shape is larger than when the memory metal portion is in the second shape.

13. A rotary tool holder assembly according to any preceding claim in which the memory metal portion is substantially annular or part annular in cross section.

14. A rotary tool holder assembly according to claim 3 or any one of claims 4 to 13 when dependent on claim 3 in which the memory metal portion is seated on one of the shoulders.

15. A rotary tool holder assembly according to any preceding claim in which a spacer is disposed between the memory metal portion and the spring means.

16. A rotary tool holder assembly according to any preceding claim in which there are a plurality of memory metal portions.

17. A rotary tool holder assembly according to any preceding claim which comprises a body in which the shaft is journalled for rotation and a motor for driving the shaft relative to the body, wherein heat generated by at least one of friction as the shaft rotates relative to the body and the operation of the motor is used to heat the memory metal portion to cause the memory metal portion to transform towards the second shape.

18. A rotary tool holder assembly according to claim 17 wherein the only source of heat provided for heating the memory metal portion is at least one of friction as the shaft rotates and operation of the motor.

19. A method of operating a rotary tool holder assembly according to any one of claims 1 to 16, wherein the rotary tool holder assembly comprises a body in which the shaft is journalled for rotation and a motor for driving the shaft relative to the body, and the method comprises the step of heating the memory metal portion to transform the memory metal portion towards the second shape using heat generated by at least one of friction as the shaft rotates relative to the body and operation of the motor.

20. A method according to claim 19 wherein the step of heating the memory metal portion consists of using only heat generated by at least one of friction as the shaft rotates relative to the body and operation of the motor.