GB2427375A

GB2427375A - Rotary tool holder assembly including friction reducing coating

Info

Publication number: GB2427375A
Application number: GB0617376A
Authority: GB
Inventors: Christopher John Pavey; Michael Clinton Tempest
Original assignee: Westwind Air Bearings Ltd
Current assignee: Westwind Air Bearings Ltd
Priority date: 2003-05-22
Filing date: 2004-05-24
Publication date: 2006-12-27
Anticipated expiration: 2024-05-24
Also published as: GB0617376D0; GB2427375B

Abstract

A rotary tool holder assembly comprises a collet 2 carried by a shaft 1. The collet 2 is moveable relative to the shaft 1 between a tool gripping position, in which an inserted tool can be gripped for rotation, and a tool releasing position. A spring means 7 is disposed in a spring receiving portion 1b of a main bore of the shaft 1, for biasing the collet towards the gripping position. A friction reducing coating, such as tungsten disulphide, is provided between at least a portion of the spring means 7 and the spring receiving portion 1b of the main bore of the shaft 1. The collet may be carried on a bobbin 4 arranged for axial movement within a further portion 1c of the main bore of the shaft 1.

Description

2427375

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Rotary Tool Holder Assemblies This invention relates to rotary too] holder assemblies, particularly rotary tool holder assemblies used in the printed circuit board manufacturing industry.

i Typically, rotary tool holder assemblies are used in the printed circuit board manufacturing industry for high speed drilling or high speed machining using router bits.

Such rotary tool holders need a simple and effective means for gripping die 10 tool to operation because of die high speeds of rotation used.

Because a relatively large number of tools will be used over time, rotary tool holders, and especially their mechanism for clamping and releasing tools are susceptible to wear.

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Conventionally greases are used on surfaces within rotary tool holders which are liable to wear.

Furthermore, in at least some types of rotary tool holders, very low friction 20 sliding between moving components within the tool holder can be important for providing a sufficiently strong grip on an inserted tool to allow proper operation. The provision of grease on the appropriate surfaces can at least initially provide the desired low friction between the sliding surfaces. However, if grease is lost or dries, the frictional forces will increase, impairing the 25 performance of the rotary tool holder.

If the gripping force of a rotary tool holder reduces to the point where tool slippage occurs during use, damage can be caused to the tool holder assembly

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and the tool. Typically a rotary tool holder assembly will comprise a collet in which the drill shank is held and this collet is particularly susceptible to damage. Other components in the rotary tool holder assembly can suffer fretting damage to their surfaces.

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Greasing is almost universally used at present but has disadvantages. Firstly, whilst grease may be applied to almost any surface during the original manufacture of the tool bolder, the further application of grease as part of a servicing operation can be difficult or impossible. Furthermore, in the high 10 speed drilling and machining operations with which, this application is particularly concerned, grease can be driven out from its desired location due to centrifugal effects.

Therefore, over die life of a rotaiy tool holder its performance can be IS significantly degraded due to wear and lack of lubrication.

It is an object of this invention to provide rotaiy tool holder assemblies which alleviate at least some of the problems associated with the prior art

20 According to a first aspect of the present invention there is provided a rotary tool holder assembly for high speed rotation comprising a collet and a shaft, the collet being moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for rotation, and a tool release position, die shaft comprising a bore for receiving die collet, an inner surface 25 of the shaft being shaped for contact with an outer surface of the collet, the shaft and collet shaped such that when the rotary tool holder assembly is rotated at a predetermined high speed the shape of the inner surface of die shaft substantially matches the shape of the outer surface of the collet and

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wherein a friction reducing coating is provided between at least a portion of the inner surface of the shaft and the outer surface of the collet.

According to a second aspect of the present invention there is provided a rotary. 5 tool bolder assembly comprising:

a collet carried by a shaft, the collet being moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for rotation, and a tool release position; and spring means disposed in a spring receiving bore for biasing the collet towards 10 the gripping position, a friction reducing coating being provided between at least a portion of the spring means and the spring receiving bore.

According to a third aspect of the present invention there is provided a rotary tool holder assembly comprising a collet carried by a shaft, wherein the collet 15 is moveable relative to the shaft between a tool gripping position in which an inserted tool can be gripped for rotation and a tod release position, die collet is canied by a bobbin arranged for axial movement within a bore of the shaft and a friction reducing coating is provided between at least a portion of the bobbin and the bore of the shaft.

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In this specification, a friction reducing coating is a coating which gives rise to a lower frictional force between relatively moving parts than would occur if the coating were not present

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The rotary tool holder will typically be arranged for high speed rotation which would typically be in excess of 80,000 ipm. The tool to be gripped will typically be a drill bit or a router bit

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Preferably, at least one portion of the collet is coated with a friction reducing coating.

In one set of embodiments, the shaft comprises a bore for receiving the collet 5 At least part of an outer surface of the collet which faces the internal surface of the shaft bore may be coated with a friction reducing coating. Part or all of the internal surface of die shaft bore may be coated with a friction reducing coating.

10 The collet may comprise a plurality of jaw portions for gripping an inserted tool. Typical numbers of jaw portions are three, four, six or eight

The'collet and/or shaft may be tapered so that axial movement of die collet relative to the shaft causes or allows the jaw portions of die collet to move in a 15 direction transverse to the axis of the collet for gripping and releasing of an inserted tool. Preferably the tapering is such as to force the jaw portions into gripping contact with an inserted tool under movement of die collet relative to the shaft in one direction and the collet is arranged so that the jaw portions are biased away from gripping contact, so that an inserted tool will tend to be 20 released as the resnlt of movement of the collet relative to the shaft in the opposite direction.

The taper surfaces of the collet and/or the shaft may be coated with a friction reducing coating. It is particularly prefered for some or all of the taper surface 25 of die collet to be coated with a friction reducing coating. In cases where both die collet and die shaft have tapers, the taper angle of the collet may be greater than the taper angle of the shaft. The taper angle is the acute angle of the taper relative to the axis about which the tool holder assembly is arranged for

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rotation. The difference in taper angle between the taper of the shaft and the taper of the collet may be between 1 and 10 arc minutes.

The collet will typically be generally cylindrical and substantially the whole of 5 the outer curved surface of the cylinder may be coated with the friction reducing coating.

The shaft will typically be arranged to be joumalled in a tooling machine. The surfaces of the shaft which are arranged to be received in the bearing(s) of the 10 tooling machine may be coated with a friction reducing coating.

In one set of embodiments the collet may be carried by a bobbin arranged for axial movement within the bore of the shaft The collet may be carried cm the bobbin by virtue of being mounted on a stud retained within die bobbin. A -15 guidebush insert may be provided within the bore of the shaft and the bobbin arranged for axial movement within die guide bush. Spring means may be provided for biasing die collet towards die gripping position. The spring means may be arranged for acting on the bobbin to bias the collet towards the gripping position. The spring means may be disposed in a spring recdving bore 20 which may be provided in the shaft

At least a portion of one of, or any combination of, the following components may be coated with a friction reducing coating: the bobbin, the guide bush, the spring means, the spring recdving bore.

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In . one embodiment each surface of each component of the assembly that moves in contact with die surface of another assembly component during the insertion and/or release of a tool is coated with a friction redudng coating.

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Preferably the internal surface of the collet, and especially the inner tool gripping surfaces of the jaw portions of the collet are kept free of friction reducing coating.

5 The coating preferably has a very low coefficient of friction, say in the region of 0.1 or lower.

Preferably the coating is applied to parts using a low temperature process to avoid changing the properties of the materials of the coated components. The 10 process may be conducted at room temperature.

Preferably the coating is thin. Mart preferably still, the coating is sufficiently thin that the coating may be applied after the finishing processes have been carried out on the components and, after coating, the components remain within IS the selected manufacturing tolerances.

Preferably the coating is applied evenly over die coated surfaces. This can ensure that the geometry of the components is maintained and there is no edge build up of coating material. The coating is preferably useable on heat treated 20 materials without damage.

The coating is preferably compatible with at least one of, or any combination of: solvents, lubricating oils and greases.

25 The coating preferably has sufficient hardness to resist handling damage. The coating may have a hardness in the region of 30 Rc (Rockwell Hardness scale

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The coating may be a tungsten di-sulphide coating.

According to a fourth aspect of the present invention there is provided a method of manufacturing a rotary tool holder assembly comprising a collet 5 canied by a shaft, wherein the collet is moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for rotation, and a tool release position, and a spring means disposed in a spring recdving bore for biasing die collet towards the gripping position the method comprising die steps of machining and finishing a plurality of component parts of the 10 assembly within selected manufacturing tolerances and after die machining and finishing steps, applying a friction reducing coating between at least one portion of the spring means and die spring receiving bore without causing the dimensions of the coated component to fall outside of die selected tolerances.

15 According to a fifth aspect of the present invention there is provided a method of manufacturing a rotary tool holder assembly comjprising a collet carried by a shaft, wherein the collet is moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for rotation, and a too) release position, and the collet is carried by a bobbin arranged for axial 20 movement within die bore of die shaft, die method comprising die steps of machining and finishing a plurality of component parts of the assembly within selected manufacturing tolerances and after the machining and finishing steps, applying a friction reducing coating between at least one portion of the bobbin and the bore of the shaft without causing the dimensions of the coated 25 component to fall outside of the selected tolerances.

According to a sixth aspect of the present invention there is provided a rotary tool holder comprising:

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a collet carried by a shaft, the collet being moveable relative to the shaft between a tool gripping position, in which an insetted tool can be gripped for rotation, and a tool release position; and spring means disposed in a spring receiving bore for biasing die collet towards 5 the gripping position, a friction reducing coating being provided between at least a portion of the spring means and the spring receiving bore,

wherein the collet is canied by a bobbin arranged for axial movement within a bore of the shaft, a friction reducing coating being provided between at least a portion of the bobbin and the bore of the shaft

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According to a seventh aspect of the present invention there is provided a rotary tool holder comprising:

a collet canied by a shaft, the collet being moveable relative to die shaft between a too] gripping position, in which an inserted tool can be gripped for 15 rotation, and a tool release position; and a spring disposed in a spring receiving bore few biasing the collet towards the gripping position, a friction reducing coating being provided between at least a portion of the spring and the spring receiving bore,

wherein the collet is canied by a bobbin arranged for axial movement within a 20 bore of die shaft, a friction reducing coating being provided between at least a portion of die bobbin and the bore of the shaft

According to an eighth aspect of the present invention there is provided a A rotary tool holder comprising:

25 a collet carried by a shaft, the collet being moveable relative to the shaft between a tool gripping position, in which an insetted tool can be gripped for rotation, and a tool release position; and means for biasing the collet towards the gripping position disposed in a

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receiving bore, a friction reducing coating being provided between at least a portion of the means for biasing the collet and the receiving bore,

wherein the collet is canied by a bobbin arranged for axial movement within a bore of the shaft, a friction reducing coating being provided between at least a 5 portion of the bobbin and the bore of die shaft

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

10 Figure 1 is a sectional view of part of a rotaiy tool holder assembly;

Figure 1A is a schematic representation of part of the rotaiy tool holder shown is Figure 1;

15 Figure 2 is a sectional view of a collet of the rotaiy tool holder assembly shown in Figure 1; and

Figure 3 is an isometric view of the collet shown in Figure 2.

20 Figure 1 shows a rotaiy tool holder assembly which comprises a shaft 1 which carries a collet 2 for holding a too] (not shown). The collet 2 is arranged for movement within the shaft 1 between a gripping position in which an inserted tool is firmly gripped for rotation and a release position in which the tool may be relatively easily inserted into and removed from the collet 2.

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The release position corresponds to the collet 2 projecting further out of the shaft 1 and the gripping position corresponds to the collet 2 being drawn back into the shaft 1. That is to say movement of the collet 2 towards die left in the

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orientation shown in Figure 1 will release the gripping force on an inserted tool, whereas movement to the right will increase the gripping force on an inserted tool.

5 As is perhaps most clearly seen in Figure 3, the collet 2 is of generally cylindrical shape and although not shown in the drawings die shaft 1 similarly has a generally cylindrical shape.

As best seen in Figure 3, the collet 2 in this embodiment comprises four jaw 10 portions 2a which are arranged for gripping an inserted tool. Slots 2b terminating in stress relieving apertures 2c are provided in die body of the collet 2 to allow the jaw portions 2a to flex relative to the remainder of the collet 2. In other embodiments there may be differing numbers of jaws, for example there may be three, six or eight jaw portions.

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A central portion of each jaw portion 2a carries an external taper which mates with an internal taper in a front portion la of die shaft 1. When the collet 2 is moved axially relative to the shaft 2 these tapers interact to cause the jaws 2a to move closer to one another, to grip an inserted tool, as the collet 2 is moved 20 into the shaft 1 and similarly allow die jaw portions 2a to move away from one another, to release an inserted tool, as die collet 2 is moved further out of the shaft 1.

Rubber inserts 2d (not shown in Figure 3) are provided in each of the jaw 25 portions 2a to bear on the internal taper of the front portion la of die shaft 1. These inserts provide a dust seal to prevent or limit the ingress of dust into the assembly via the slots 2b.

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The body of the shaft 1 comprises a main bore which has two portions of different diameter. The first portion of the bore is adjacent to the front portion la of the shaft and has a diameter which is larger than die diameter of the second portion of die bore lc which is disposed further into the body of the 5 shaft 1.

The collet 2 is threadingly mounted on a threaded stud 3, which itself is . retained in a bobbin 4 arranged for movement within the shaft 1. The collet 2 is threaded onto the stud 3 during assembly and is locked in position using a 10 locking grab screw 5. Thus in normal operation the collet 2, stud 3 and bobbin 4 move together relative to die shaft 1.

The collet 2, stud 3 and bobbin 4 are housed within die second portion of die main bore lc of the shaft 1. A guide bush insert 6 is provided within the bore 15 of the shaft 1 and is arranged to guide die movement of the bobbin 4 and hence the collet 2.

The first part of the main bore lb bouses a set of springs 7 which act on a shoulder 4a of the bobbin. The springs 7 .are put under compression during 20 assembly so that the springs 7 tend to drive the bobbin 4 further into the shaft 1 so retracting the collet 2. As described above, due to the mating tapers of the collet 2 and front portion la of the shaft, retraction of the collet 2 causes the collet jaw portions 2a to grip an inserted tool.

25 Therefore, when a tool is to be inserted into the rotaiy tool holder, a push rod is used to push the bobbin 4, and hence collet 2, forward such that the jaw portions 2a of the collet may spread to allow the insertion of a tool. When the force of the push rod is removed, the collet 2 and bobbin 4 retract under force

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of the springs 7 into the shaft 1 causing the jaws 2a to close onto and grip the inserted tool.

It will be noticed that during the movement of the bobbin 4, stud 3 and collet 2 5 relative to die shaft 1, there' are various components which are in sliding contact with one another.

This has two implications. Fust of all these components are susceptible to wear and, secondly die factional force between these sliding components must be 10 kept to a minimum if a good gripping force is to be. exerted on an inserted tool.

In the present embodiment, to give good wear characteristics and maximise die gripping force which is exerted on an inserted tool, components within the tool IS holder assembly are coated with a friction reducing coating.

In particular, the outer curved surfaces of the collet 2 are provided with a friction reducing coating.

20 Other surfaces in the tool holder assembly can also be coated with a friction reducing coating. These surfaces include: the internal curved surface of the guide bush insert 6; the taper surfaces of the front portion la of the shaft; the outer cylindrical surface of the bobbin 4; the internal cylindrical surface of the first pait of die main bore lb which houses the springs 7; and the whole of die 25 surfaces of the springs 7, or at least those parts which come into contact with the bobbin 4 and/or the internal cylindrical surface of the first part of the main bore lb.

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On the other hand however, the interna] cylindrical surface of the collet 2 and especially the interna] surface of die jaw portions 2a is kept clean of friction reducing coating to ensure that maximum grip is exerted on an inserted tool.

S The provision of friction reducing coating on these components, and first foremost on the externa] curved surfaces of the collet 2, in particular the taper surfaces of the collet 2, can give rise to a very low maintenance product which will retain a high gripping force on an inserted tool over a long period of time without the need for any regressing.

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The friction reducing coating should have a low coefficient of friction, preferably a very low coefficient of friction of in the region of 0.1 or lower. It is preferred that the friction reducing coating can be applied to parts using a low temperature application process, for example a room temperature IS application process. This is to avoid changing the material properties or conditions of the components during coating.

The coating is preferably thin, and in a preferred manufacturing process, the components are machined and finished to die desired size within chosen 20 manufacturing tolerances and die friction reducing coating is then applied to the relevant surfaces of the components whilst staying within the manufacturing tolerances. Compared with die typical assembly tolerances (of say 2.S microns on diameter) in devices of interest, the coating should be thin and say in die order of 0.5 microns or less.

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The coating should follow the geometry of die existing machined surfaces and exhibit no edge biiild up;

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The coating should be usable on heat treated materials without damage and, furthermore, should be compatible with solvents, lubricating oils and greases which may be used deliberately or may come into contact with the coated surfaces during assembly or use.

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The coating needs to have a reasonable degree of hardness to resist handling damage. A hardness of 30 Rc (Rockwell hardness scale Q or greater is considered to be appropriate.

10 Is preferred embodiments of die present invention the coating is a tungsten disulphide coating.

Although not shown in die drawings, the rotary tool holder assembly will be arranged to be received in a tooling machine for rotaiy drive. The shaft 1 15 therefore has journal surfaces to be received in appropriate rotaiy bearings and these journal surfaces can be coated with a friction reducing coating to give low friction starting and anti-scuffing where air bearings are used, and to generally assist with bearing lubrication where mechanical bearings are used.

20 In an alternative embodiment, the rotary tool holder described above with reference to Figures 1 to 3 includes a tapering mis-match between the external tapers of die jaw portions 2a and the internal taper of the front portion la of the shaft 1. This mis-match even if it were shown in Figure 1 would not be readily discernable because the amount of mis-match is small compared to the 25 dimensions of the components shown in Figure 1. This mis-match may,

however, be seen in Figure 1A, which is a schematic representation of part of the rotary tool holder shown in Figure 1, but modified to include the tapering mis-match of this alternative embodiment

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Figure 1A shows part of the rotaiy tool holder in a stationary configuration. In this configuration the taper angle of the internal taper of the shaft 1 (which is the acute angle formed between die internal taper and the axis of the shaft) is less than the taper angle of die jaw portions 2a. The difference between the 5 taper angle of the shaft and the taper angle of die jaw portions 2a is typically several arc minutes. Thus when the rotaiy tool holder is stationary die internal taper of die shaft 1 has only line contact with the taper of the jaw portions 2a at die very front of the shaft

10 When the rotaiy tool holder is in use, it is typically rotated in excess of 80,000 rpm. At these speeds die shaft 1 tends to expand radially. This effect tends to be more pronounced towards the front end la of die shaft 1 where die shaft 1 is thinner. As the shaft 1 expands, it changes shape such that the mis-match between the interna] taper of the shaft 1 and die external taper of the jaw 15 portions 2a decreases. In preferred embodiments, the arrangement is such that the mis-match disappears and the taper of die shaft and the external taper of the jaw portions 2a become the same as one another. During this process, the jaw portions 2a are urged axiafly into the narrow end of the internal taper of the shaft by the springs 7 causing the collet 2 to slide relative to die shaft 1. 20 The shape of the jaw portions 2a also changes as a result of rotation and so the tapers are ideally chosen with this in mind.

The presence of the friction reducing coating on the external taper of the jaw portions and the internal taper of the shaft is advantageous as the contact forces 25 between the front portion la of die shaft 1 and the jaw portions 2a are initially very high due to the small area of contact The friction reducing coating tends to allow die collet 2 to move relative to the shaft 1 without jamming or fietting of the surfaces.

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Claims

CLAIMS:

1. A rotaiy tool holder assembly comprising:

a collet carried by a shaft, the collet being moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for 5 rotation, and a tool release position; and spring means disposed in a spring receiving bore in the shaft for biasing the collet towards the gripping position, a friction reducing coating being provided between at least a portion of the spring means and the spring receiving bore.

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2. A rotary tool holder assembly according to claim 1 which is arranged for high speed rotation.

3. A rotary tool holder assembly according to claim 1 or claim 2 in which at least one portion of the collet is coated with a friction reducing coating.

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4. A rotaiy tool holder assembly according to any preceding claims in which the collet comprises a plurality of jaw portions for gripping an inserted tool, 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

20 to move in a direction transverse to the axis of the collet for gripping and releasing of an inserted tool and at least some of the taper surfaces of the collet and/or the shaft are coated with a friction reducing coating.

5. A rotaiy tool holder assembly according to any preceding claim in

25 which the collet is carried by a bobbin arranged for axial movement within a bore of the shaft.

6.

A rotary tool holder assembly according to claim 5 in which the spring

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means is arranged for acting on the bobbin to bias the collet towards the gripping position.

7. A rotaiy tool holder according to any preceding claim wherein at least a 5 portion of the spring means is coated with a friction reducing coating.

8. A rotary tool holder assembly according to any preceding claim in which the coating is applied to parts using a low temperature process to avoid changing die properties of the materials of the coated components.

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9. A method of manufacturing a rotaiy tool holder assembly comprising a collet carried by a shaft, wherein the collet is moveable relative to the shaft between a tool gripping position, in which an inserted tool can be gripped for rotation, and a tool release position, and a spring means disposed in a spring

IS receiving bore for biasing the collet towards the gripping position die method comprising the steps of machining and finishing a plurality of component parts of die assembly within selected manufacturing tolerances and after die machining and finishing steps, applying a friction reducing coating between at least one portion of the spring means and the spring receiving bore without 20 causing the dimensions of the coated component to fell outside of die selected tolerances.