GB2467948A

GB2467948A - Thermal shrinking chuck with inner gripping member thermally decoupled from chuck body

Info

Publication number: GB2467948A
Application number: GB0902930A
Authority: GB
Inventors: Ralf Dupont
Original assignee: Air Bearings Ltd
Current assignee: RALF DUPONT; Air Bearings Ltd
Priority date: 2009-02-20
Filing date: 2009-02-20
Publication date: 2010-08-25
Also published as: WO2010094918A1; TW201032921A; GB0902930D0

Abstract

A chuck 40 for gripping a tool comprises a body 42 made of thermally sensitive material that is excited by a thermal excitation element 50 to change the perimeter of a bore 44 in the body 42, and a gripping member 52 that is disposed inside the bore 44, the thermal expansion of the gripping member 52 being decoupled from the thermal expansion of the body 42. The gripping member 52 may be integral with the body 42 or may be formed from a separate part to the body 42. The separate part may comprise a layer inside the bore 44. A plurality of contact points 54 may be formed by the gripping member 52. The gripping member 53 may be formed from ceramic material. A coil 50 for conveying AC current may circumscribe the outer surface of the body 42 in order to thermally excite it.

Description

CHUCK

The invention relates to chuck, i.e. a device for clamping a tool to another piece of apparatus, e.g. a shaft.

The chuck may be used in any apparatus that needs to hold an operative tool, e.g. in devices for drilling, milling, grinding, punching or the like.

Figs. 1A and lB are schematic views of a known collet chuck 10 used to clamp tools to a shaft. Fig. 1A is a cross-sectional view and Fig. lB is a plan view. The chuck 10 includes a sleeve 12 fixed to or integral with a pinion (not shown) . The sleeve 12 has a bore 14 for receiving a collet 16. A spring 18 provides a force to pull the collet 16 into the bore 14. Tapering surfaces on the inside of the sleeve 12 and the outside of the collet 16 cooperate to force inwards the sides of the collet 16 adjacent a split channel 20 formed therein to clamp a tool (not shown) disposed in the channel 20.

To release a clamped tool, the collet 16 is pulled out of the sleeve 12 against the biasing force of the spring 18.

Fig. 2 shows a schematic perspective view and a cross-sectional side view of another known chuck 22, often called a Thermal Shrinking Chuck (TSC). This chuck 22 comprises a monolithic body 24 having a bore 26 therein for receiving the tool (inner body) 28. The body 24 is made of heat conductive material e.g. steel and is surrounded by a heating coil 30 that is arranged to heat its outer surface by induction.

Other sources of heat may be used. Heating the outer surface of the body 24 causes it to expand, which opens the bore 26 to enable a tool to be inserted (or removed) . Upon cooling, the body 24 shrinks and the bore 26 closes, thereby gripping the tool 28.

At its most general, the present invention proposes a thermal shrinking chuck in which an inner volume of the heated body is omitted to facilitate improvement of the magnitude and speed of bore opening.

According to the invention, there may be provided a chuck for gripping a tool (which may have any shape), the chuck comprising: a body made of thermally sensitive material, the body having a bore therein for receiving the tool; a thermal excitation element arranged to cause thermal excitation of the body to change the perimeter of the bore; and a gripping member disposed inside the bore to contact the tool and secure it in the chuck upon a decrease in the perimeter of the bore; wherein thermal expansion of the gripping member is decoupled from thermal expansion of the body and the gripping member includes a tool contact region that protrudes inwardly from the perimeter of the bore. Decoupling of the thermal expansion (and contraction) of the gripping member and body means that the thermal expansion (and contraction) of the body is not constrained by the gripping member. The combination of this property with an inwardly protruding region permits removal of an inner part of the body, i.e. effectively to increase the perimeter of the bore, when compared with a conventional TSC. This allows the chuck to open wider whilst avoiding an increase in the thermal stresses experienced by the body. Moreover, it may allow the outer perimeter of the body to be increased without necessarily causing higher thermal stresses, which can also enable wider bore openings to be achieved.

The tool contact region may extend radially further towards the centre of the bore than the innermost point of the body by a distance that is 2% or more of the outer perimeter of the body, e.g. to enable the body to achieve a sufficient improvement of opening. The outer perimeter of the body may be calculated as iz, where d is the average outer diameter of the body.

The gripping member may be formed with the body as a monolithic member, e.g. as shown in the embodiment depicted in Fig. 5. In this arrangement, the inner perimeter of the bore may be formed by machining a slot (e.g. an air space) between the body and the gripping member) . The slot may be formed by an electrical discharge machining (EDM) wire cut. The gripping member may protrude from the body on a stem of material. The stem may have narrow circumferential extent to limit the heat conducted to the gripping member and hence provide the decoupling function.

Alternatively, the gripping member may be a separate part, attached to the body. The gripping member may be made of material that is more thermally inert than the thermally sensitive material. For example, the material of the gripping member may have a thermal expansion coefficient of lower magnitude than the thermal expansion coefficient of the thermally sensitive material. In other words, the relative expansion rates of the gripping member and body may be different. The decoupling of the thermal expansions may be arranged to permit relative movement between contiguous surfaces of the gripping member and body (i.e. between portions of the gripping member and body which touch one another) during a change in the perimeter of the bore.

Herein, the perimeter of the bore may mean the cross-sectional outline of its inner surface. A change in the perimeter of the bore may correspond to a change in the cross-sectional area of the bore.

The thermally sensitive material may have a positive thermal expansion coefficient, whereby thermal excitation of the body causes an increase in the perimeter of the bore, i.e. the bore opens when the body is heated. Thermal excitation may mean heating the outer surface of the body. The thermal excitation element may be arranged to thermally excite the body by induction heating. For example, the thermal excitation element may comprise a coil for conveying AC current having one or more turns which circumscribe the outer surface of the body. The changing magnetic field generated by the AC current in the coil causes induction currents and therefore heating in the body. The frequency of the AC current may be selected to control the depth from the outer surface into the body where heating occurs.

Induction heating may enable high power densities to be achieved very quickly (i.e. in the order of seconds) and efficiently (i.e. up to 95% heating efficiency) . Other heat sources may be used to heat the body, e.g. a burner or a UV light heater.

An advantage of heating the outer surface only is that the energy demand may be less than when the entire volume is heated. Moreover, heating only the surface can prevent the inner parts (e.g. the tool) for being heated. However, as mentioned above, a possible disadvantage of heating the outer surface to a temperature necessary to achieve a sufficient change in bore perimeter is that a hot volume on the outside and a colder volume on the inside can result in high thermal stresses within the body, which can also restrict the range of opening. This disadvantage is illustrated with respect to a standard thermal shrinking chuck in Fig. 2. The desirability to prevent heat from being transmitted through the body 24 means that a hot outer volume 32 and a cold inner volume 34 are created. In this case, the thermal stress and yield strength of the material of the body 24 limit the allowed thermal gradient and therefore limit the opening and the heating time. The invention overcomes this problem by permitting removal of the cold volume, i.e. to effectively increase the inside diameter of the body.

The tool contact region may define a plurality of contact points for contacting the tool. The chuck may be arranged so that only the gripping member contacts the tool in use. For example, the gripping member may comprise a plurality of (preferably three) jaw elements attached around the inside surface of the bore to form a multi-point pincer for gripping the tool. The jaw elements may be separate from each other.

They may be attached, e.g. bonded, to the inside surface of the bore at regular intervals around the bore. The means of attachment may provide flexibility to permit the body to freely change shape during thermal expansion or contraction.

Recesses may be formed in the body to receive the jaw elements, which may be form-fitted and/or a force-fitted (e.g. exact fit or interference fit and/or punch marking) therein.

The jaw elements may thus be clamped to the body without constraining its thermal expansion.

The tool contact regions may be shaped to form a point contact or a line contact with the tool. For example, each jaw element may be an elongate member extending in an axial direction, whereby the contact regions grip the tool along a portion of its length received in the bore.

The gripping member may be made of a material having a lower thermal conductivity than the thermally excitable material. Heat may therefore be prevented from passing to the tool without necessarily increasing the thermal stress experienced by the body.

S

The gripping member may be made of a material having a higher abrasion resistance than the thermally excitable material. Since the gripping member may also experience a change in temperature in operation (e.g. due to thermal excitation of the body), it is preferably made of a material having a thermal expansion coefficient of low magnitude, e.g. less than 5 x 10-6 K1, to prevent or minimise thermal expansion of the gripping member into the bore from decreasing the opening for the tool. The presence of the gripping member may thus provide an additional advantage over a conventional thermal shrinking chuck, in which aluminium alloys were often necessary to form the body because only their thermal expansion coefficients were high enough to achieve an acceptable change in bore perimeter. However, these alloys are not abrasion-resistant, which limited the life cycle of a conventional thermal shrinking chuck.

The gripping member may be made of an insulating material, such as ceramic. For example, ceramic materials such as Si3N4 have a relatively low thermal expansion coefficient of less than 5 x 106 K' and a relatively high hardness of more than 1500 HV.

The thermally excitable material may be an aluminium alloy, or other metallic material (e.g. steel) with a relatively high thermal expansion coefficient of e.g. more than 10 < 106 K', preferably more than 20 < 106 K'.

The components of the chuck may be arranged in a single housing or at separate locations in an apparatus. In one embodiment the invention may provide apparatus comprising: an operating shaft; a chuck according to any preceding claim for gripping a tool; and a housing for supporting the shaft during operation; wherein the body of the chuck is mounted on the shaft and the thermal excitation element is mounted in the housing at a region adjacent to the body. The apparatus may be any type of working apparatus, e.g. for drilling, stamping or the like. The shaft may be any member arranged to hold the body of the chuck. It need not move in operation (e.g. the apparatus may be arranged to move the work piece relative to the shaft in operation) . Alternatively, the shaft may be arranged to perform rotational and/or axial movement.

This arrangement may provide a compact and simple chuck without any moving parts, which may be desirable in rotating high-speed apparatus. The housing may contain additional components. For example, the housing may include bearings, e.g. gas bearings, for supporting a rotating shaft.

Examples of the invention are described in detail below with reference to the accompanying drawings, in which: Figs. 1A and lB are schematic views of a known collet chuck and are described above; Fig. 2 is a schematic perspective and cross-sectional side view of a known thermal shrinking chuck and is described above; Fig. 3 is a schematic perspective and cross-sectional side view of a thermal shrinking chuck that is an embodiment of the invention; Fig. 4 is a schematic cross-sectional side view of a drilling apparatus that is an embodiment of the invention; and Fig. 5 is a schematic front and perspective view of a monolithic body for use in a thermal shrinking chuck that is another embodiment of the invention.

Fig. 3 is a schematic perspective and cross-sectional side view of a thermal shrinking chuck 40 that is an embodiment of the invention. The chuck 40 comprises a cylindrical body 42 of heat conductive material which has an axially extending bore 44 and is adapted to be mechanically connected to a rotating shaft (not shown) at a proximal end 46 thereof. The outer circumferential surface 48 of the body 42 has an induction heating coil 50 formed therearound at a distal end thereof. In this embodiment the coil 50 is a substantially C-shaped winding which is located around the outer surface 48 and substantially circumscribes the body 42.

The heating coil 50 is arranged to receive an AC current from a power supply (not shown) at a frequency selected to cause induction heating in an outer layer of the body 42. The body 42 is made of a conductive material (e.g. metal such as aluminium or a steel alloy) with a. positive expansion coefficient (i.e. about 24 > 106 K1 for aluminium alloys and about 10 x 106 K' for steel alloys) such that induction heating of the outer layer causes the perimeter of the bore 44 to increase (thereby causing it to open, i.e. its cross-sectional area to increase) . When heating is stopped, the body 42 cools and the perimeter of the bore decreases accordingly.

Three ceramic rod elements 52 are attached inside the body 42. Each rod element 52 is aligned with the axis of the body 42 and includes a protruding contact region 54 that extends radially away from the inner surface of the body 42 into the bore 44. In this embodiment, the rod elements 52 are arranged symmetrically around the inside surface of the body 42 so that the axially extending contact regions 54 define a three-point pincer suitable for contacting and securing a tool shank or cylinder that is inserted therein. For example, in Fig. 3 a cylindrical tool 56 is inserted into the pincer. The contact regions 54 are shaped to conform with the outer surface of the tool 56. In this embodiment, the contact surfaces are arranged to form a line contact with the tool 56.

The ceramic rod elements 52 may be fixed to (i.e. prevented from moving in a radial sense with respect to) the body 42 by a form-fitted and/or a force-fitted fixing (they may be additionally glued when a form-fitted fixing only is used) . In this embodiment, correspondingly shaped recesses are formed in the body 42 to receive respective rod elements 52. The rod elements 52 are fixed to the body 42 by an interference fit in the recesses. Alternatively or additionally, the ceramic rod elements 52 may be attached by bonding or the like to the body 42.

In use, when the cross-section of the bore 44 decreases or increases in accordance with heating or cooling of the outer layer of the body 42, the ceramic rod elements are moved together or apart to close or open the pincer for securing or removal/insertion of the tools. The attachment of the rod elements 52 to the body 42 is in a manner whereby their thermal expansions are decoupled, i.e. the thermal expansion of the body 42 is not constrained by the attachment to the rod elements (which do not expand as quickly) . The arrangement thus effectively operates as a gear, with the movement of the body 42 under thermal excitation being transferred to the rod elements 52.

Because the ceramic rod elements 52 protrude radially inwards from the body 42, the inner perimeter 58 of the body 42 may be greater than the outer perimeter of the tool that is being secured. As explained above, this can reduce the thermal stress experienced by the body and facilitate efficient opening of the bore.

Fig. 4 shows a schematic cross-section through a drilling apparatus 60 that is another embodiment of the invention. The drilling apparatus comprises a rotating shaft 62 supported by a housing 64 above a surface 66 to be drilled e.g. a PCB or the like. The rotating shaft 62 may be supported by gas bearings (not shown) formed in the housing 64.

At the distal end of the rotating shaft 62 a chuck 68 similar to that described in Fig. 3 is attached so that it rotates with the shaft 62. As shown in Fig. 4, a drill-bit 70 is secured in the chuck 68. The chuck 68 differs from Fig. 3 in that the heating device is not part of the chuck body. In this embodiment, the heating device 72 (e.g. coil) is provided in regions of the housing 64 adjacent to the distal end of the chuck 68. The chuck 68 may thus be compact because its outer perimeter corresponds to the outer perimeter of the body. In other embodiments, the coil may also being integrated into the body of the chuck itself.

Fig. 5 is a schematic front and perspective view of a monolithic body 80 that can be used in a thermal shrinking chuck that is another embodiment of the invention. In this embodiment, the monolithic body 80 is a cylindrical member having a bore 81 formed along its axis. Using a EDM wire cut technique, a plurality of curved gripping members 84 are formed at the inner surface 85 of the bore 81 by cutting one or more radial slots 83 into the inner surface 85 of the bore 81 and then cutting one or more circumferential slots 86 inside the body 80 from the bottom of the radial slots 83.

The radial slots 83 and circumferential slots 86 may extend axially through the whole body 80. The adjacent ends of neighbouring circumferential slots 83 are separated from each other by a narrow stem 87 of body material that connects the integrally formed gripping members 84 to an outer layer 82 of the body 80.

In operation, the outer surface 88 of the outer layer 82 is heated to cause outward expansion thereof, which acts to open the bore 81. The stems 87 are narrow enough to restrict the amount of heat that is conducted into the gripping members 84 to decouple thermal expansion of those members from thermal expansion of the outer layer. The circumferential slots 86 also aid the decoupling function. By decoupling the thermal expansion of the gripping members 84 from the outer layer 82, the gripping members can be prevented from thermally expanding into the bore 81 when the outer layer is heated, which improves the range of opening of the bore that can be achieved.

Claims

CLAIMS1. A chuck for gripping a tool, the chuck comprising: a body made of thermally sensitive material, the body having a bore therein for receiving the tool; a thermal excitation element arranged to cause thermal excitation of the body to change the perimeter of the bore; and a gripping member disposed inside the bore to contact the tool and secure it in the chuck upon a decrease in the perimeter of the bore; wherein thermal expansion of the gripping member is decoupled from thermal expansion of the body and the gripping member includes a tool contact region that protrudes inwardly from the perimeter of the bore.
2. A chuck according to claim 1, wherein the gripping member is a separate part from the body and is attached thereto in a manner to permit relative movement between contiguous surfaces during a change in the perimeter of the body.
3. A chuck according to claim 2, wherein the gripping member is a coating with a thickness of at least 2% of the outer perimeter of the body, the outer perimeter being calculated as,zd, where d is the average outer diameter of the body.
4. A chuck according to claim 2 or 3, wherein the gripping member is made of material that is more thermally inert than the thermally sensitive material.
5. A chuck according to any preceding claim, wherein heating the body causes an increase in the perimeter of the bore.
6. A chuck according to any preceding claim, wherein the tool contact region defines a plurality of contact points for contacting the tool.
7. A chuck according to claim 6, wherein the gripping member comprises multiple jaw elements attached around the inside surface of the bore to form a multi-point pincer for gripping the tool.
8. A chuck according to any preceding claim, wherein the gripping member is made of a material having a lower thermal conductivity and/or a higher hardness than the thermally excitable material.
9. A chuck according to any preceding claim, wherein the gripping member is made of ceramic.
10. A chuck according to any preceding claim, wherein the thermal excitation element is arranged to thermally excite the body by heating.
11. A chuck according to claim 10, wherein the thermal excitation element comprises a coil for conveying AC current having turns which circumscribe the outer surface of the body to cause thermally excitation by induction heating.
12. Apparatus comprising: a operating shaft; a chuck according to any preceding claim for gripping a tool; and a housing for supporting the shaft during operation; wherein the body of the chuck is mounted on the shaft and the thermal excitation element is mounted in the housing at a region adjacent to the body.