EP1755814A1

EP1755814A1 - Hydraulic chuck

Info

Publication number: EP1755814A1
Application number: EP05744388A
Authority: EP
Inventors: Jeff Pratt; Dave Goodwin; John Bradley
Original assignee: Westwind Air Bearings Ltd
Current assignee: Novanta Technologies UK Ltd
Priority date: 2004-05-25
Filing date: 2005-05-23
Publication date: 2007-02-28
Also published as: CN1980763A; GB0411686D0; TW200603924A; WO2005115665A1; JP2008500191A

Abstract

A chuck comprising a main body (1) defining a hydraulic fluid receiving enclosure (6) and comprising a tool receiving portion (4) with at least one deformable wall portion (41), the hydraulic fluid receiving enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool, the chuck further comprising a moveable piston (8) which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the piston (8) is in the first position is less than the volume displaced within the enclosure by the moveable piston when the piston is in the second position; and a centrifugal pressure intensifier (9) for increasing the pressure in carried hydraulic fluid in dependence on the speed of rotation of the chuck, said centrifugal pressure intensifier being arranged to act on the moveable piston, the moveable piston comprising a taper portion (85) and the centrifugal pressure intensifier comprising jaws (92) having a complementary taper. The jaws (92) are arranged to be driven outwards by centrifugal effects and further arranged, when so driven outwards, to interact with the moveable piston (8) taper portion (85) to drive the moveable piston in a direction which serves to increase the volume displaced within the enclosure (6) by the moveable piston (8).

Description

Hydraulic Chucks

This invention relates to hydraulic chucks, in particular it relates to hydraulic chucks which are to be used in high speed machining operations.

The traditional way to hold a precision 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. At low rotational speeds the system has been adequate. However, for very high speed and very high accuracy applications existing tapered collet chuck performance can reduce to a point where an alternative solution is necessary.

As 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. The end result is that there will be a speed where the growth of the taper reduces the gripping force on the tool shank to zero causing excessive runout, tool slippage, and eventually complete release of the tool. Further, as the gripping force reduces, high vibration can occur as the parts of the collet chuck and tool begin to rattle. There can also be a problem in trying to achieve very high degrees of accuracy with such collet chucks in that the collet taper and bore, as well as the shaft taper have to be machined separately and it is very difficult to machine these parts accurately enough to achieve the runouts which are now being required.

Further such collet chucks rely on sliding movement between the collet and the shaft taper as tools are inserted and removed from the chuck. Generally the contacting surfaces are greased. However grease dries out and also centrifuges away from the surfaces meaning that routine greasing maintenance is required. This is disadvantageous because it gives rise to machine down time and is a skilled operation.

Yet a further problem with existing collet chuck designs is that after a very large number of actuations, the springs used to draw the collet into the shaft taper can lose some of their strength and worse, can eventually crack due to fatigue.

It is therefore desirable to find alternative forms of chuck.

According to one aspect of the present invention there is provided a chuck comprising a hydraulic fluid receiving enclosure, a tool receiving portion with at least one deformable wall portion, and means for applying pressure to fluid when carried in the enclosure, the enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool.

The means for applying pressure may comprise a moveable member which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position.

It will be seen that in such a case, the pressure in fluid provided in the enclosure will tend to be higher when the moveable member is in the second position. Furthermore, increasing pressure in the enclosure will tend to deform the deformable wall increasing gripping force on an inserted tool.

Here the expression "partly within" is used to refer to an element with a portion which is within the enclosure in the sense that that portion displaces volume within the enclosure rather than merely forming part of the outer boundary of the enclosure. Thus, for example, a plug or a screw which closes a port into an enclosure is not considered to be within the enclosure and nor is such a port closing plug or screw normally able to displace volume within the enclosure.

The moveable member may be arranged for sliding movement between the first and second positions. The moveable member may be a piston. The moveable member may have a first portion having a first cross-sectional area and a second portion having a second cross-sectional area. The moveable member may have varying diameter along its length. The first portion may have a first diameter and the second portion may have a second diameter.

The moveable member and hydraulic fluid enclosure may be arranged so that when the moveable member is moved in one direction between the first position and the second position, the amount of the first portion of the moveable member which is disposed within the enclosure decreases and the amount of the second portion of the moveable member which is disposed within the enclosure increases, whereas when the moveable member is moved in the other direction between the first position and the second position, the amount of the first portion of the moveable member which is disposed within the enclosure increases and the amount of the second portion of the moveable member which is disposed within the enclosure decreases. The means for applying pressure may comprise pressure control means for applying selectable degrees of pressure to carried fluid to control the gripping force on an inserted tool. The pressure control means may comprise said moveable member.

The means for applying pressure may comprise pressure intensifier means for increasing the pressure in dependence on the rotational speed of the chuck. The pressure intensifier means may be centrifugally operated. The pressure intensifier means may be arranged to act on the pressure control means.

The pressure intensifier may be arranged to act on the moveable member. The moveable member may comprise a taper portion and the pressure intensifier may comprise at least one complementary taper portion. The complimentary taper portion may be arranged to be driven outwards by centrifugal effects and further arranged, when so driven outwards, to interact with the moveable member taper portion to drive the moveable member in a direction which serves to increase the volume displaced within the enclosure by the moveable member. It will be seen that this action of the complimentary taper portion tends to increase the pressure in carried hydraulic fluid.

A part of the enclosure may be defined by the deformable wall portion. The tool receiving portion may comprise three deformable wall portions. The or each wall portion may comprise a respective land for contacting with an inserted tool. The or each land may have a contact surface which is profiled to closely match the shape of the shank of a tool which is to be inserted. The or each land may have a concavely profiled contact surface for contacting with the shank of an inserted tool.

The contact surfaces may be machined to final shape in a common boring operation carried out in the tool receiving portion.

According to another aspect of the present invention there is provided a chuck comprising a main body defining a hydraulic fluid receiving enclosure and comprising a tool receiving portion with at least one deformable wall portion, the hydraulic fluid receiving enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool, the chuck further comprising a moveable member which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position.

According to another aspect of the present invention there is provided a chuck comprising a main body defining a hydraulic fluid receiving enclosure and comprising a tool receiving portion with at least one deformable wall portion, the hydraulic fluid receiving enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool, the chuck further comprising a moveable member which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position; and a centrifugal pressure intensifier for increasing the pressure in carried hydraulic fluid in dependence on the speed of rotation of the chuck, said centrifugal pressure intensifier being arranged to act on the moveable member, the moveable member comprising a taper portion and the centrifugal pressure intensifier comprising at least one complementary taper portion, the complimentary taper portion being arranged to be driven outwards by centrifugal effects and further arranged, when so driven outwards, to interact with the moveable member taper portion to drive the moveable member in a direction which serves to increase the volume displaced within the enclosure by the moveable member.

According to another aspect of the present invention there is provided a method of holding a tool comprising the steps of: providing hydraulic fluid into a chuck as defined above, operating the chuck to release pressure in the fluid carried in the enclosure, inserting the tool into the tool receiving portion and pressurising the fluid in the chuck to deform the deformable wall portion into contact with the inserted tool.

According to another aspect of the present invention there is provided a method of manufacturing a chuck as defined above and comprising a tool receiving portion having a plurality of deformable wall portions each having a respective tool contacting land, the method comprising the step of using a common boring operation to bore tool contact surfaces into the lands.

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 through a chuck; and Figure 2 is a section on line II-II of the chuck shown in Figure 1. Figure 1 schematically shows a chuck which comprises a main body or core 1

to which is welded a nose portion 2 and a rear shaft portion 3. Within the main

body or core 1 is provided a tool receiving portion 4 in which a tool 5 can be

located. The tool receiving portion 4 comprises 3 deformable wall portions 41 as best seen in Figure 2. The deformable wall portions 41 are connected together so as to form a triangular prism. Each of the deformable wall portions 41 comprises a respective tool contacting land 42 which has a concave tool contacting surface. In Figure 2 the lands 42 can be seen in contact with the carried tool 5. It will be noted that in Figure 1 the tool 5 can be seen in contact with one of the lands 42 and in the orientation shown in Figure 1 this is above the tool. On the other hand, in Figure 1 no land is shown in contact below the tool. This is because the other two lands 42 make contact with the tool at 120 degrees separation from the upper land and thus are not visible in the section shown in

Figure 1.

The tool contacting surfaces are formed by a common boring operation as one of the final stages in manufacture of the chuck. In this boring operation a

boring tool is introduced into the tool receiving portion and the lands 42 are

bored to the required size and shape by the boring tool. This operation ensures

that the lands are accurately machined so as to closely fit the shank of the tool which the chuck is being manufactured to hold.

On the side of each of the deformable wall portions 41 opposite to that which carries the respective land 42 is a hydraulic fluid chamber 61 which forms part of a hydraulic fluid enclosure 6 which will be described in more detail below.

As well as the tool receiving portion 4, the main body or core 1 also comprises a piston receiving bore 7 at its opposite end. A piston 8 is captured within the piston receiving bore 7 by a rear cover portion 71. The piston receiving bore 7 and end cover 71 both define outer boundaries of further parts of the hydraulic fluid receiving enclosure 6 such that the piston 8 is disposed partly within the hydraulic fluid enclosure 6.

There are also passageways or galleries 62 which serve to connect the hydraulic fluid chambers 61 adjacent to the deformable walls 41 to the piston receiving bore 7 so that there is a hydraulic fluid communication path between the piston receiving bore 7 and the hydraulic fluid chambers 61 adjacent to the deformable walls 41. Furthermore there is a hydraulic enclosure filling port 63 provided in the nose portion 2 and this port is provided with a screw plug closure 64. A spring 81 is carried by the piston 8 and whilst this is schematically shown as a coil spring in Figure 1, in practice, this spring is more likely to be a wave spring. Wave springs are more tolerant to high speed rotation than conventional coil springs. The spring 81 is positioned between a spring end stop sleeve 82 which is mounted in the main body of core 1 and a spring abutment portion 83 which is provided on the piston 8. The spring 81 serves to bias the piston 8 towards the right (in the orientation as shown in Figure 1). As will be explained in more detail below, this corresponds to a position in which pressure is exerted on the hydraulic fluid in the hydraulic fluid enclosure 6 such as to tend to deform the deformable wall portions 41 thereby gripping the inserted tool 5.

As will be described in more detail below, at the inner end of the piston 8 there is provided a centrifugal pressure intensifier arrangement 9.

Sealing means 84 are carried by the piston 8 in the region where the piston passes through the end cover 71 and through the spring stop sleeve 82. The sealing means 84, which may for example comprise O-ring seals, serve to prevent hydraulic fluid in the hydraulic fluid enclosure 6 from leaking out of the rear part of the chuck or leaking forwards into the centrifugal pressure intensifier arrangement 9. The piston 8 has two main portions 8 a, 8b which are disposed within the hydraulic fluid enclosure 6. These portions are provided on either side of the spring abutment portion 83. A first of these portions 8a is that part which carries the spring 81 and has a diameter A. The second main portion 8b is on the other side of the spring abutment portion 83 and has a diameter B which is smaller than the diameter A of the first main portion 8a.

It should be noted that the first main portion 8a is partly housed within the spring stop sleeve 82 and also that part of the second portion 8b is housed within the end cover 71. These portions housed within the spring stop portion 81 and end cover 71 are not considered to be within the hydraulic fluid enclosure 6 since they do not contribute to any displacement of volume in the hydraulic fluid enclosure 6.

Because these portions have differing diameters (A,B), if the piston 8 is moved axially relative to the main body or core 1 , the total volume occupied by the piston 8 within the hydraulic fluid enclosure 6 changes.

In particular, if the piston 8 is moved to the right from the position shown in Figure 1 then more of the second main portion 8b of the piston will move into the bore of the end cover 71 and more of the first portion 8a of the piston 8 will move out of the bore of the spring stop sleeve 82. Furthermore, since the diameter A of the first portion 8a is greater than the diameter B of the second portion 8b this movement will serve to increase the volume which the piston 8 occupies within the hydraulic fluid enclosure 6. This in turn will reduce the volume available for the hydraulic fluid and hence increase the pressure within the hydraulic fluid. If the piston 8 is subsequently moved back towards the left

(in the orientation shown in Figure 1) the situation is reversed and pressure in the hydraulic fluid will drop.

Thus it will be seen that movement of the piston 8 can be used to alter the pressure in the hydraulic fluid and consequently movement of the piston 8 may be used to alter the pressure felt in the pressure chambers 61 adjacent to the deformable walls 41. These pressure changes will allow the deformable walls 41 to relax as the pressure is lowered and cause the walls to be further deformed into gripping contact with an inserted tool as the pressure increases.

Therefore in basic setup and operation of the device, after the components have been assembled, the piston 8 is pushed, using a pushrod (not shown), towards the position shown in Figure 1 against the force of the spring 81 and hydraulic fluid is introduced into the hydraulic fluid enclosure 6. The screw plug 64 is then inserted into the fluid port 63 and sealed. At this point if the action of the pushrod (not shown) is removed, from the piston 8, the piston will tend to move to the right (in the orientation shown in Figure 1) under the action of the spring 81 so reducing the available volume in the hydraulic fluid enclosure 6.

This in turn will tend to deform the deformable walls 42 of the tool receiving portion 4 such that in this state a tool 5 can not be inserted. The chuck is then primed ready for use.

In operation when a tool is to be inserted a pushrod (not shown) again is used to push the piston 8 towards the position shown in Figure 1 so reducing the pressure in the hydraulic fluid enclosure 6 and so allowing the deformable walls 42 to relax such that a tool may be inserted. Once the tool 5 has been inserted the action of the pushrod may be removed so that the piston 8 again moves to the right (in the orientation shown in Figure 1) under the action of the spring so that the hydraulic fluid pressures serves to deform the deformable walls 41 into gripping contact with the inserted tool via the respective lands 42.

When it is desired to remove the tool 5, then the pushrod may again be used to push the piston 8 towards the position shown in Figure 1 to reduce the pressure in the hydraulic fluid and allow the deformable walls to relax. Whilst not shown in detail in the drawings, the pushrod actuation mechanism for moving the piston to allow the insertion and removal of tools can be part of the overall chuck design and disposed in the rear part 3 of the shaft. More normally, however, the pushrod mechanism will be mounted in a machine on which the chuck is also mounted. The pushrod does not rotate with the body of the chuck and need only contact the end of the piston 8 when the chuck is stationary.

Now that the structure and operation of the basic parts of the device have been described, a more detailed description of the structure and operation of the centrifugal pressure intensifier device 9 is given. The inner-most end of the piston 8 which is adjacent to the centrifugal intensifier is provided with tapered bore 85. The centrifugal pressure intensifier mechanism 9 comprises an inner tapered body 91 and a plurality of taper wedge jaws 92. The jaw portions 92 are carried by the inner tapered body 91 in a way which allows the jaws 92 to move radially outwards under the influence of centrifugal force. The jaw portions 92 have taper surfaces which are complementary to the tapered bore 85 in the end of the piston 8 such that as the jaw portions 92 move outwards under the effect of centrifugal force, the interaction between the jaw portions 92 and the internal taper 85 of the piston 8 drives the piston 8 to the right (in the orientation shown in Figure 1). This means that the intensifier 9 drives the piston 8 so that a larger part of the larger diameter portion 8a enters into the hydraulic fluid enclosure 6 in place of a respective part of the smaller diameter portion 8b such that the volume of the hydraulic fluid enclosure 6 is reduced.

This increases the hydraulic fluid pressure and increases the deformation of the deformable wall portions 41.

It will be appreciated that as the rotational speed of the device increases, and there might otherwise be a tendency for the tool to slip due to the higher forces involved or the tendency for the chuck to grow away from the tool, the increase in hydraulic pressure created by the intensifier 9 will tend to increase the grip on the tool 5. This can counter any tendency for the tool 5 to become loose. Of course, the higher the rotational speed of the chuck, the higher the centrifugal effects which the jaw portions 92 will experience, and the larger degree of movement of the piston 8 that will be achieved. Furthermore as the rotational speed drops the reverse process occurs and the gripping force is reduced.

It will of course be appreciated, that due to the incompressible nature of the oil typically used as hydraulic fluid, a relatively small change in volume of the hydraulic enclosure 6 will give rise to a significant increase in pressure which can give rise to a significant increase in gripping force. It will be noted that the spring stop sleeve 82 is provided as a separate component which is mounted into the core or main body 1. This gives an advantage that deferring lengths of spring sleeve 82 may be used to determine the rest position of the piston 8 within the assembly. This can determine the degree of pressure which is exerted on the hydraulic fluid when the piston is in this rest position.

As mentioned above, the lands 42 are finish bored to final size after the chuck has been assembled. Furthermore, this operation may take place with the complete assembly supported in its own bearings to minimise runout.

In producing the main body or core 1 , the tool receiving portion 4 is machined by wire Electro Discharge Machining (EDM) from a solid blank and this blank is then turned and bored to give the outer shape and so as it may receive the piston 8 assembly.

It might also be mentioned that in contrast to a conventional tapered collet the tool receiving portion (or "collet") in the present device is of constant cross section along its length. This means that the outer surfaces of the deformable walls form a generally uniform triangular prism and the profiled surfaces of the bored lands 42 bound a cylindrical cavity for receiving the tool 5.

Claims

CLAIMS:

1. A chuck comprising a hydraulic fluid receiving enclosure, a tool receiving portion with at least one deformable wall portion, and means for applying pressure to fluid when carried in the enclosure, the enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool.

2. A chuck according to claim 1 in which the means for applying pressure comprises a centrifugally operated pressure intensifier means for increasing the pressure in dependence on the rotational speed of the chuck.

3. A chuck according to claim 2 in which the means for applying pressure comprises a moveable member on which the centrifugally operated pressure intensifier means is arranged to act.

4. A chuck according to claim 3 in which the moveable member comprises a taper portion and the pressure intensifier comprises at least one complementary taper portion.

5. A chuck according to claim 4 in which the complimentary taper portion is arranged to be driven outwards by centrifugal effects and further arranged, when so driven outwards, to interact with the moveable member taper portion to drive the moveable member in a direction which serves, in use, to increase deformation of the deformable wall portion.

6. A chuck according to any one of claims 3 to 5 in which the moveable member is moveable between a first position and a second position wherein the volume of the enclosure when the moveable member is in the first position is greater than when in the second position.

7. A chuck according to any one of claims 3 to 6 in which the moveable member is disposed partly within the hydraulic fluid receiving enclosure.

8. A chuck according to claim 7 when dependent on claim 6 wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position.

9. A chuck according to any one of claims 3 to 8 in which the moveable member is a piston.

10. A chuck according to any one of claims 3 to 9 in which the moveable member has a first portion having a first cross-sectional area and a second portion having a second cross-sectional area.

11. A chuck according to claim 10 when dependent on claim 8 in which the moveable member and hydraulic fluid enclosure are arranged so that when the moveable member is moved in one direction between the first position and the second position, the amount of the first portion of the moveable member which is disposed within the enclosure decreases and the amount of the second portion of the moveable member which is disposed within the enclosure increases, whereas when the moveable member is moved in the other direction between the first position and the second position, the amount of the first portion of the moveable member which is disposed within the enclosure increases and the amount of the second portion of the moveable member which is disposed within the enclosure decreases.

12. A chuck according to any preceding claim in which the means for applying pressure comprises pressure control means for applying selectable degrees of pressure to carried fluid to control the gripping force on an inserted tool.

13. A chuck according to any preceding claim when dependent on claim 3 in which the moveable member is slidably moveable.

14. A chuck according to any preceding claim when dependent on claim 3 in which the movable member is arranged to move axially relative to a body of the chuck.

15. A chuck according to any preceding claim in which a part of the enclosure is defined by the deformable wall portion.

16. A chuck according to any preceding claim in which the tool receiving portion comprises three deformable wall portions.

17. A chuck according to any preceding claim in which the or each wall portion comprises a respective land for contacting with an inserted tool.

18. A chuck according to claim 17 in which the or each land has a contact surface which is profiled to closely match the shape of the shank of a tool which is to be inserted.

19. A chuck according to claim 18 in which the contact surfaces are machined to final shape in a common boring operation carried out in the tool receiving portion.

20. A chuck comprising a main body defining a hydraulic fluid receiving enclosure and comprising a tool receiving portion with at least one deformable wall portion, the hydraulic fluid receiving enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool, the chuck further comprising a moveable member which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position.

21. A chuck comprising a main body defining a hydraulic fluid receiving enclosure and comprising a tool receiving portion with at least one deformable wall portion, the hydraulic fluid receiving enclosure and tool receiving portion being arranged so that pressure applied to fluid in the enclosure can cause deformation of the deformable wall portion for gripping an inserted tool, the chuck further comprising a moveable member which is disposed partly within the hydraulic fluid receiving enclosure and which is moveable between a first and a second position wherein the volume displaced within the enclosure by the moveable member when the member is in the first position is less than the volume displaced within the enclosure by the moveable member when the member is in the second position; and a centrifugal pressure intensifier for increasing the pressure in carried hydraulic fluid in dependence on the speed of rotation of the chuck, said centrifugal pressure intensifier being arranged to act on the moveable member, the moveable member comprising a taper portion and the centrifugal pressure intensifier comprising at least one complementary taper portion, the complimentary taper portion being arranged to be driven outwards by centrifugal effects and further arranged, when so driven outwards, to interact with the moveable member taper portion to drive the moveable member in a direction which serves to increase the volume displaced within the enclosure by the moveable member.

22. A method of holding a tool comprising the steps of: providing hydraulic fluid into a chuck according to any preceding claim, operating the chuck to release pressure in the fluid carried in the enclosure, inserting the tool into the tool receiving portion and pressurising the fluid in the chuck to deform the deformable wall portion into contact with the inserted tool.

23. A method of manufacturing a chuck according to any one of claims 1 to

21 and comprising a tool receiving portion having a plurality of deformable wall portions each having a respective tool contacting land, the method comprising the step of using a common boring operation to bore tool contact surfaces into the lands.