ROTARY TOOL HOLDERS

This invention relates to rotary tool holders. It relates, in particular to rotary tool holder assemblies which may form part of spindles, for example air bearing spindles in manufacturing apparatus, which are used for example, in PCB drilling and other high speed drilling operations.

Such a drilling apparatus may be arranged to rotate a drill bit at in excess of 250,000 rpm or even in excess of 300,000 or 350,000 rpm to perform drilling operations.

As drilling machines are required to run at higher and higher rotational speeds and drill bits become smaller and smaller, it becomes more and more difficult to produce tool holder assemblies which will adequately hold a tool and prevent slippage during drilling.

A conventional way to hold tools such as drill bits in these type of machining apparatus is to make use of a tool holder including a tapered collet disposed within a tapered bore in a surrounding shaft. The collet and shaft are arranged such that if the collet is drawn into the shaft, the tapers interact with one another to cause the collet to grip an inserted tool. A spring pack is usually provided to act on the collet and tend to draw it into the tapered bore.

However a stage has been reached where the rotational speeds which need to be obtained and/or the desired miniaturisation of a tool holder can make it difficult or

impossible to produce a conventional collet based tool holder which will adequately hold the tool during machining.

Other known rotary tool holders make use of centrifugal effects to increase the gripping on a tool at high rotational speeds. However, where such a gripping mechanism is used there is an issue of ensuring that there is sufficient grip of the tool at lower rotational speeds, and ensuring that the tool can be released.

It is the aim of the present invention to provide an alternative form of tool holder which aims to alleviate at least some of the problems associated with the prior art.

According to one aspect of the present invention there is provided a rotary tool holder assembly comprising a tool receiving portion and tool gripping means for gripping a tool received in the tool receiving portion, the tool gripping means comprising at least one tool gripping portion which is moveable between a tool gripping position and a tool release position, and a moveable mass portion which is arranged to move, during rotation of the tool holder, away from a main axis of the tool holder under action of centrifugal effects and is arranged in so doing to move the tool gripping portion towards the tool gripping position, the tool gripping means further comprising a memory metal portion which is controllably transformable in response to change in temperature between a first shape and a second shape and is arranged, at least whilst centrifugal effects have not taken hold, to move the tool gripping portion towards the tool gripping position, as the memory metal portion transforms from the first shape towards the second shape.

The memory metal portion may be arranged to act upon the moveable mass portion to move the moveable mass portion away from the main axis of the tool holder as the memory metal portion transforms from the first shape towards the second shape, to move the tool gripping portion towards the tool gripping position.

According to another aspect of the present invention there is provided a rotary tool holder assembly comprising a tool receiving portion and tool gripping means for gripping a tool received in the tool receiving portion, the tool gripping means comprising at least one tool gripping portion which is moveable between a tool gripping position and a tool release position, and a moveable mass portion which is arranged to move, during rotation of the tool holder, away from a main axis of the tool holder under action of centrifugal effects and is arranged in so doing to move the tool gripping portion towards the tool gripping position, the tool gripping means further comprising a memory metal portion which is controllably transformable in response to change in temperature between a first shape and a second shape and is arranged, at least whilst centrifugal effects have not taken hold, to act upon the moveable mass portion to move the moveable mass portion away from the main axis of the tool holder as the memory metal portion transforms from the first shape towards the second shape, to move the tool gripping portion towards the tool gripping position.

The gripping means may comprise at least one arm that comprises the gripping portion. The arm may comprise the moveable mass portion. The tool holder may

comprise a tool holder body portion. The at least one arm may be connected to the tool holder body portion and arranged so that as the gripping portion moves between the tool gripping position and the tool release position, the gripping portion moves relative to the tool holder body portion.

The arm may be arranged so that deflection of the arm relative to the tool holder body portion causes the gripping portion to move between the tool gripping position and the tool release position.

The arm may be arranged so that as the arm moves such that the moveable mass portion moves away from the axis, the gripping portion moves towards the axis.

The arm may be integrally joined to the tool holder body portion.

The arm may be attached to the tool holder body portion via a deformable portion which allows movement of the gripping portion between the gripping and release positions. The arm may comprise the deformable portion. The deformable portion may be provided between the arm and the tool holder body portion.

The arm may be joined to the tool holder body portion by a respective web. The web may comprise the deformable portion. The web may project radially inwards from the tool holder body portion.

In some embodiments the arm may be cantilevered from the tool holder body portion. The arm may extend inwards away from the tool holder body portion in a direction transverse to the axis of the tool holder.

In other embodiments the arm may be joined, at a position part way along its length, to the tool holder body portion. In such a case, the gripping portion may be on one side of said position and the moveable mass portion may be on the other side of said position. The arm may extend in two directions away from said position, which directions are generally parallel to the axis of the tool holder.

The arm may comprise at least one projection for contacting with the memory metal portion. The projection may be arranged for locating the memory metal portion.

The memory metal portion may comprise an elongate member. The elongate member may extend in a direction which is generally parallel to the axis of the tool holder. In some embodiments, the elongate member may be spaced from the axis of the tool holder. In other embodiments, the elongate member may be coincident with the axis of the tool holder. In some embodiments where the memory metal portion comprises an elongate member, the memory metal portion is arranged to expand in axial extent when transforming from the first shape to the second shape to act on the moveable mass portion. In other embodiments where the memory metal portion comprises an elongate member, the memory metal portion is arranged to expand in a direction transverse to the axis of the tool

holder when transforming from the first shape to the second shape to act on the moveable mass portion.

The memory metal portion may comprise a ring. The ring may be arranged around and centred on the axis of the tool holder. The ring may be arranged to expand in overall radial dimension when transforming from the first shape to the second shape to act on the moveable mass portion.

The memory metal portion may be arranged to transform from the first shape towards the second shape when in the first shape and exposed to a temperature above a first threshold. This can allow the memory metal portion to move the gripping portion towards the gripping position at higher temperatures.

The memory metal portion may be arranged to transform from the second shape towards the first shape when in the second shape and exposed to a temperature below a second threshold.

hi the present embodiments the memory metal portion is used to provide grip on a carried tool when the tool holder assembly is being rotated as speeds below which centrifugal effects give sufficient grip. Ensuring that the temperature of the memory metal portion is above the first temperature threshold can ensure that this gripping effect is provided. Ensuring that the temperature of the memory metal portion is below the second temperature threshold can allow this gripping effect to be removed to aid release of the tool.

Heating and/or cooling means can be provided to control the temperature of the memory metal portion. These means can be completely external to the tool holder assembly or integrated with it. At a most simplistic level, the tool holder assembly may be exposed to a hot environment to heat it and a cold environment to cool it.

The rotary tool holder assembly may comprise at least one fluid channel for supplying heat exchange fluid to heat and/or cool the memory metal portion, or at least one of the memory metal portions. Typically the fluid will be air. Preferably the fluid channel comprises at least one channel portion which allows direct contact between carried fluid and a respective memory metal portion. The memory metal portion may be hollow. The channel portion may be provided within the respective memory metal portion and an internal wall or walls of the memory metal portion may define the channel portion.

The channel portion may be partially defined by an external wall of the respective memory metal portion. The respective memory metal portion may have a groove that partially defines the channel portion. Structure surrounding the respective memory portion, for example, the tool holder body portion, may comprise a groove that partially defines the channel portion. The or each memory metal portion and a respective void in which it is disposed may be shaped and dimensioned to define the channel portion.

In another aspect of the invention there is provided a spindle comprising a rotary tool holder assembly as defined above. The spindle may comprise cooling and/or

heating means. Cooling and/or heating means may be provided externally to the spindle. The cooling and/or heating means maybe arranged to supply heat exchange fluid to the tool holder assembly.

According to another aspect of the invention there is provided a machining apparatus comprising a rotary tool holder assembly. The machining apparatus may comprise cooling and/or heating means.

The tool holder maybe arranged to apply a restoring force to the memory metal portion to encourage the memory metal portion to transform from the second shape to the first shape. This can be useful because whilst the transformation from the first shape to the second shape can typically be made spontaneous, the transformation back to the first shape can require assistance.

The tool holder may be arranged to apply a restoring force to the memory metal portion to encourage the memory metal portion to transform from the second shape to the first shape under predetermined conditions. The predetermined conditions may comprise the tool holder being stationary, or not rotating at a rate above a threshold rate. That is the tool holder may be arranged to apply a restoring force when centrifugal effects are below a given level or have been removed.

The moveable mass portion may be arranged to act on the memory metal portion to apply the restoring force.

Resilience in the material of the tool holder may provide the restoring force.

The tool holder may comprise restoring force energy storage means for storing energy as the memory metal portion transforms from the first shape to the second shape and for outputting that energy as a restoring force is applied to the memory metal portion to encourage it to transform from the second shape to the first shape.

The tool holder may comprise a resilient restoring force storage portion which is deformed as the memory metal portion transforms from the first shape to the second shape and when so deformed, the resilience of which, provides a restoring force for acting on the memory metal portion to encourage it to transform from the second shape to the first shape.

The tool holder may comprise a resilient restoring force storage portion which is deformed as the moveable mass portion moves away from the axis of the tool holder and when so deformed, the resilience of which, acts on the moveable mass portion to tend to drive the moveable mass portion back towards the axis of the tool holder. In such a case, at least when centrifugal effects fall below a predetermined level, the moveable mass portion can apply a restoring force to the memory metal portion.

Preferably the tool holder comprises a plurality of tool gripping portions. There may be a plurality of arms each comprising a respective gripping portion. There

may be a plurality of moveable mass portions. There may be a plurality of memory metal portions.

Where there are a plurality of moveable mass portions at least two moveable mass portions may be connected to one another by a resilient restoring force storage portion.

The tool holder may comprise a plurality of arms and a plurality of memory metal portions where each arm comprises at least one projection and each memory metal portion is located between one projection from a first arm and another projection from a second arm. In such a case, each arm may comprise two projections, one of a first type and one of a second type. Each memory metal portion may be disposed between a projection of a first type from the first arm and a projection of the second type from the second arm. The first type of projection may be for contacting a carried tool and may comprise the gripping portion. The second type of projection may comprise the moveable mass portion. The projections, in particular the second type of projection may be arranged to apply a restoring force for acting on the memory metal portion.

The memory metal portion may be arranged to tend to drive the projections away from one another when transforming from the first shape to the second shape. In arrangements such as this the memory metal portion may act on two gripping portions, driving them towards the gripping position.

In one set of embodiments the tool holder assembly comprises a plurality of gripping arms each having a respective gripping portion, the gripping portions being arranged around the tool receiving portion and running the length of the tool holder body portion. In one set of embodiments there is a plurality of gripping arms each having a respective gripping portion, the gripping portions being arranged around the tool receiving portion and each arm being cantilevered from the tool holder body portion and extending inwards away from the tool holder body portion in a direction transverse to the axis of the tool holder.

hi another set of embodiments the tool holder assembly comprises two sets of gripping portions which are axially spaced from one another. This can provide gripping on a carried tool at two spaced regions. There may be two sets of arms, a first set of which comprises arms comprising respective gripping portions of a first of the sets of gripping portions and a second set of which comprises arms comprising respective gripping portions of a second of the sets of gripping portions. The two sets of arms may be axially spaced from one another.

Each arm in each set may comprise a respective mass portion. The mass portion of at least one arm in one set may be connected to the mass portion of at least one corresponding arm in the other set. Such a connection can help to provide a restoring force.

One or more common memory metal portions may be provide in these sets of embodiments for acting on the moveable mass portion of at least one arm in one

set and on the moveable mass portion of at least one arm in the other set. There may be one common memory metal portion that acts on all of the mass portions in both sets of arms.

fn another set of embodiments the tool holder comprises a collet and a main body in which the collet is disposed, the collet comprises a plurality of jaw portions having an external taper which fits with a complementary taper in the main body, each of the jaw portion comprises a respective gripping portion, the jaw portions are arranged for axial movement relative to the main body and the tapers are arranged so that axial movement of the jaw portions relative to the main body moves the jaw portions transversely to the axis of the tool holder so that the gripping portions move between the release position and the gripping position, the collet further comprises a tail portion comprising a plurality of moveable mass portions which are arranged to move away from the axis of the tool holder under action of centrifugal effects so shortening the overall length of the collet and drawing the jaw portions into the taper in the main body and moving the gripping portions towards the gripping position.

In this set of the embodiments, the collet and the main body together comprise the gripping means.

The tail portion of the collet may be arranged as hollow tube with generally axial slots which allow portions of the wall of the tube to act as moveable mass

portions. The slotted portion of the tube can then flair out under centrifugal effects.

The memory metal portion may be provided within the tail portion of the collet. The memory metal portion may be rod-like. The memory metal may be arranged to expand in radial dimension when transforming from the first shape to the second shape to act on the moveable mass portions.

The collet may be of resilient material so that deformation of the tail portion of the collet gives rise to a restoring force which may act on the memory metal portion when in the second shape.

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 drilling machine including a rotary tool holder assembly for holding a tool;

Figure 2 shows a schematic section of part of a first rotary tool holder assembly which may be used in an apparatus of the type shown in Figure 1 ;

Figure 3 schematically shows an end perspective view of the rotary tool holder assembly shown in Figure 2;

Figure 4 schematically shows a perspective view of a second rotary tool holder assembly of a type which may be used in the apparatus shown in Figure 1;

Figure 5 shows a section on a plane generally indicated by line V-V in Figure 4 of the second rotary tool holder assembly shown in Figure 4 when received in a shaft of an apparatus of the type shown in Figure I ₅ this shaft also being shown in section;

Figure 6 shows a schematic perspective view of a third rotary tool holder assembly of a type which may be used in an apparatus of the type shown in Figure i;

Figure 7 shows the third rotary tool holder assembly of Figure 6, but without a tool or memory metal portions in position;

Figure 8 shows a section on a plane generally indicated by line VIII-VIII in Figure 6 of the third rotary tool holder assembly shown in Figure 6 when received in a shaft of an apparatus of the type shown in Figure 1, this shaft also being shown in section;

Figure 9 shows a schematic perspective view of part of a fourth rotary tool holder assembly of a type which may be used in an apparatus of the type shown in Figure 1; and

Figure 10 shows a schematic sectional view taken on a plane generally indicated by line X-X in Figure 9 of the fourth rotary tool holder assembly shown in Figure 9.

Figure 1 shows a machining apparatus which in this case is a high speed drill apparatus for PCB drilling. The apparatus comprises an air bearing spindle 1 comprising a main body Ia and a shaft Ib which is journalled for rotation within the main body Ia. Mounted in the shaft Ib is a rotary tool holder assembly 2 which is arranged for holding a tool 3 during machining, in this case, drilling.

This specification is primarily concerned with the structure and operation of rotary tool holder assemblies 2 of such apparatus. Below description is given of a number of rotary tool holder assemblies, each of which may be used in apparatus of the type shown in Figure 1.

Figures 2 and 3 show a first rotary tool holder assembly 102 mounted in a hollow shaft Ib of an apparatus of the type shown in Figure 1. The rotary tool holder assembly 102 is shown holding a tool 3. The rotary tool holder assembly 102 comprises a tool receiving portion 103 which is arranged axially along the axis of the rotary tool holder assembly 102 and the axis of the shaft Ib.

The rotary tool holder assembly 102 comprises a generally annular tool holder main body 104 on which are carried a plurality of, in this case eight, arms 105. Each of the arms 105 in this embodiment is formed integrally with the tool holder

body portion 104 such that the arms 105 and tool holder body portion 104 are of a single piece of material. This single piece of material is machined to give the desired shape.

Each of the arms 105 is cantilevered from the tool holder body portion 104 and projects inwards generally towards the axis of the rotary tool holder assembly 102. Each of the arms 105 terminates in a tool gripping portion 151. The tool gripping portions 151 of the arms 105 are arranged around the axis of the rotary tool holder 102 and define a boundary of the tool receiving portion 103. Each of the tool gripping portions 151 has a tool contact surface 151a which faces an inserted tool and a tool engaging edge 151b which is arranged to engage with and bite into the shank of inserted tool 3. As best seen in Figure 2, the arms 105 and gripping portions 151 run axially along the length of the tool holder body portion 104. This means that the tool engaging edges 151b each provide a line of tool engaging contact with a carried tool.

As well as the tool gripping portion 151 which projects from a main stem of the arm 105, each arm also comprises another projection 152. Each of these other projections 152 act as movable mass portions which will tend to fly radially outwards from the axis of the rotary tool holder assembly 102 during high speed rotation. Moreover as these moveable mass portions 152 fly outwards under centrifugal effects this will cause deformation in the respective arms 105 and consequently cause the tool engaging edge 151b of each tool gripping portion 151 to engage with a carried tool. That is to say as the moveable mass portion 152

tends to move outwards under centrifugal effects this will cause deformation of the arm 105 in the region where it joins the tool holder body portion 104 in such a way that the tool engaging edge 151b of the tool gripping portion 151 will tend to move inwardly towards the axis of the tool holder. In the present embodiment the tool holder body portion 104 and arms 105 are formed of steel and the dimensions of the arms are chosen so that resilient deformation of the arms 105 can occur to allow the effects described above to occur.

It should also be noted that the tool holder body portion 104 and arms 105 of the present embodiment are arranged to exert off-centre sprag effect forces on a carried tool, when the tool holder assembly 102 is rotated in the desired direction for operation.

By way of explanation, a sprag effect force is a force due to an offset pressure in a direction opposed to that of rotation. Sprag effect forces were originally used on railway wagons in the nineteenth century where a sprag, or piece of timber, was placed against a wagon wheel on a railway line at an orientation directed slightly above the centre line of the wheel to act as a brake. Li such a case as the wheel tries to move towards the sprag the pressure increases due to the offset force of the sprag with respect to the axle of the wheel. Clutches have been devised based on the sprag effect where a wedging effect increases against the direction of rotation to provide for a clutching action.

Here, as a carried tool tries to rotate relative to the holder 102 (ie. resist being rotated in the desired direction), the force exerted by the tool engaging edge 151b of each gripping portion 151 will tend to increase. This is because a line from the region (or hinge), where the arm 105 joins the tool holder body portion 104, to the tool engaging edge 15 Ib does not pass through the axis of rotation of the tool holder 102.

The present rotary tool holder assembly 102 also comprises eight memory metal portions 106. Each of these memory metal portions 106 has an elongate shape and is arranged with its main axis parallel to the axis of the tool holder 102. Each memory metal portion 106 is disposed between one projection 151, 152 from a first arm 105 and another projection 152, 151 from an adjacent arm 105. That is to say each memory metal portion 106 is disposed between the projection forming the tool gripping portion 151 of one arm 105 and the projection forming the moveable mass portion 152 of an adjacent arm 105.

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, ie undergo a transformation, 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 for the memory metal portions 106 in this embodiment as well as in the embodiments described below with reference to Figures 4 to 10.

In the present rotary tool holder assembly 102, the eight elongate memory metal portions 106 are designed and trained to fit between the respective projections 151, 152 as described above. In a first state the portions 106 are arranged to exert no particular force on these projections 151, 152. However, they are further trained so as to expand in a direction transverse to their longitudinal axis, that is to say in a generally radial direction when disposed in position in the tool holder assembly 102, when transforming to a second state. In particular, the memory metal portions 106 are trained to expand in such a way when a critical temperature is exceeded. When this expansion occurs it serves to tend to drive the projections 151, 152 on either side of each memory metal portion 106 away from one another. This on the one hand means that it will tend to directly drive the tool gripping portion 151 and in particular, the tool engaging edge 151b, of one of the arms 105 into a carried tool, and on the other hand it will also tend to drive the moveable mass portion 152 with which it contacts in the opposite direction, that is generally radially outwards. It will be appreciated that this generally radial outward movement of the moveable mass portion 152 will be of the same general kind which occurs under centrifugal loading (CF loading).

Thus, as the arms 105 are arranged to encourage the tool gripping portion 151 to grip a carried tool 3 under CF loading, due to movement of the moveable mass portion 152, the same effect is being produced as the memory metal portion 106 transforms from its first state into its second state.

This means that provided the memory metal portions 106 are raised above their critical temperature, they will tend to drive the gripping portions 151 towards a gripping position when there is no rotation of the rotary tool holder assembly 102 or rotation at a insufficiently high speed to provide sufficient grip via centrifugal effects. At higher rotational speeds the centrifugal effects can take over.

When it is desired to release a tool 3 from the rotary tool holder 102 assembly, the memory metal portions 106 should be taken to a temperature below the appropriate threshold temperature which will allow them to tend to transform back towards their first relaxed state.

Whilst the transformation of the memory metal portions 106 from the first state to the second state should be spontaneous in response to the correct temperature, the transition from the second state back to the first state sometimes requires assistance. Here the arrangement in the present tool holder assembly 102 can provide such assistance in the form of a restoring force which is applied to the memory metal portions 106 due to the resilience in the arms 105 provided that the tool holder 102 is not being rotated at high speed. Of course when it is desired to remove the tool 3 the tool holder 102 will not be being rotated at high speed.

When the tool holder 102 is not being rotated at high speed each memory metal portion 106 is effectively squeezed between the projections 151 and 152 surrounding it.

As can be seen from Figure 3, the shape of the arms 105 means that channels C exist through the rotary tool holder assembly 102 through which heating and/or cooling air may be passed to heat the memory metal portions up above the threshold temperature to cause them to transform from the first to the second state and/or to cool the memory metal portions down below the threshold temperature to allow them to return from the second state to the first state. Moreover these channels C are partly bounded by the memory metal portions 106 themselves so that there can be direct contact between heat exchange fluid flowing through the channels C and the memory metal portions 106.

Apparatus of the type shown in Figure 1 can include appropriate means for supplying cooling and/or heating air to the memory metal portions, hi the present embodiments as the apparatus shown in Figure 1 comprises an air bearing spindle 1 there is a ready supply of air which can be used for heating and/or cooling purposes. Depending on the circumstances it may or may not be necessary to provide means for heating or cooling the air before delivery through the channels C. That is to say, heat generated in the device itself may be sufficient to raise the temperature of the memory metal portions 106 above the critical temperature to cause them to move from the first state into the second state and the delivery of ambient air through the channels C may be sufficient to cool the memory metal

portions 106 below the critical temperature necessary to allow the memory metal portions 106 to return to the first state from the second state.

Thus it will be seen that in the present embodiment a tool holder assembly 102 is provided which uses centrifugal effects to give good tool 3 gripping at high rotational speeds and makes use of memory metal portions 106 to increase grip at least at lower rotational speeds.

The embodiments described below achieve the same basic function, but in differing ways.

Figures 4 and 5 show a second rotary tool holder assembly 202 which in Figure 5 is shown mounted within a shaft Ib of an apparatus of the type shown in Figure 1. Again this tool holder assembly 202 comprises a tool receiving portion 203 and is shown carrying a tool 3. In this rotary tool holder assembly there are two, generally annular, tool holder body portions 204 each of which carries a respective six arms 205. Each of the arms 205 comprises a tool gripping portion 251 in this case in the form of a jaw and a respective moveable mass portion 252.

Each arm 205 is connected to its respective tool holder body portion 204 via a respective web 253. Each web 253 extends in a generally radially inward direction from the respective tool holder body portion 204 and is joined to the respective arm 205 part way along its length.

Each tool holder body portion 204 and its respective set of arms 205 is formed of a single piece of material. This material is typically steel and the material and dimensions of the various parts are chosen such that the arm 205 may move due to resilient deformation of the respective web 253. In particular when the rotary tool holder assembly 202 is rotated at high speed the moveable mass portions 252 will tend to fly out under centrifugal effects which in effect causes a pivot at the web 253 and causes the tool gripping portions 251 to deflect inwards and grip a carried tool 3.

The two holder body portions 204 and their accompanying arms 205 are arranged in a back to back configuration where the jaws/gripping portions 251 face outwards and the moveable mass portions 252 face one another. Between, the two sets of moveable mass portions 252 and around the tool receiving portion 203 there is provided a ring like memory metal portion 206.

This memory metal portion 206 is located in an accommodating recess 252a at the distal end of each of the moveable mass portions 252. Moreover, in a similar way as described above in relation to the first rotary tool holder assembly shown in Figures 2 and 3, this memory metal portion 206 is designed and trained to undergo a desired change in shape between a first state and a second state. In particular in its first state, as shown in Figure 5, the memory metal portion 206 resides around a received tool and within the accommodating recesses 252a without exerting any particular force on the moveable mass portions 252. However the portion 206 is designed and trained so as to increase in overall radial

dimension when reaching a temperature above its respective threshold temperature so transforming into a second shape.

When in the second shape the memory metal portion 206 has a greater radial dimension and the memory metal portion 206 acts on the moveable mass portions 252 to tend to drive them outwards in the same way as would occur under CF loading. Thus the effect of the action of the memory metal portion 206 on transformation from the first state to the second state is to force the gripping portions 251 into gripping contact with a carried tool 3.

Again, heating and/or cooling of the memory metal portion may be provided, for example, by delivering heating and/or cooling air to the memory metal portion through channels or spaces available within the rotary tool holder assembly 202.

Figures 6, 7 and 8 show a third rotary tool holder assembly 302 which has some similarities with the second rotary tool holder assembly 202 and is shown in Figure 8 mounted within a shaft Ib of an apparatus of the type shown in Figure 1. Here again, there are two generally annular collar like tool holder body portions 304 each of which supports a respective set of six arms 305 having tool gripping portions 351 and moveable mass portions 352. The arms 305 and body portions 304 are structured and function in much the same way as those in the second rotary tool holder assembly 202 described above in relation to Figures 4 and 5. However there are differences which will be described below.

In this embodiment there are six memory metal portions 306. Each of these memory metal portions 306 has an elongate shape and has a main axis arranged to be parallel to the axis of the rotary tool holder assembly 302, but spaced therefrom. Each memory metal portion 306 is received in accommodating recesses 352a defined in four adjacent moveable mass portions 352. In each case, two of these moveable mass portions 352 are associated with one of the tool holder body portions 304 and the other two moveable mass portions 352 are associated with the other tool holder body portion 304.

Again, the memory metal portions 306 are arranged and trained to have a first shape such that they can be disposed in these accommodating recesses 352a without exerting any particular force on the moveable mass portions 352 and a shape in the second transformed state which is such that the moveable mass portions 352 are forced outwards in the same or a similar way as they would be under CF loading to cause the tool gripping portions 351 to grip a carried tool. That is, the memory metal portions 306 exert an axial force, parallel to the tool holder axis, on the moveable mass portions 352, creating a turning moment on the moveable mass portions 352, hence forcing them outwards.

Further, in this embodiment, each moveable mass portion 352 associated with one of the tool holder body portions 304 is connected via a tab 352b to respective one of the moveable mass portions 352 associated with the other tool holder body portions 304. Thus when the moveable mass portions 352 move away from the axis of the tool holder assembly under centrifugal effects or under action of the

memory metal portions 306, they must do this against a resilient restoring force which is provided by the tab 352a. That is to say as each moveable mass portion 352 tends to move outwards this will deform the tab 352a and thus there will be resistance to this movement. This is useful because it means that the energy generated under centrifugal effects and/or as the memory metal portions 306 move from the first state to the second state can be stored to some extent and used to act on the memory metal portions 306 to help return them from their second state to the first state, once below the appropriate threshold temperature.

Again cooling and/or heating air may be supplied to and through the tool holder assembly 302 to control the temperature of the memory metal portions 306.

Figures 9 and 10 show part of a fourth rotary tool holder assembly 402 which may be used in an apparatus of the type shown in Figure 1 to grip a tool 3. Here the rotary tool holder assembly 402 comprises a one-piece tapered collet which is arranged to be disposed in a receiving shaft S with a correspondingly tapered bore schematically shown in Figure 10. This assembly may then be received as a whole in an apparatus of the type shown in Figure 1 - either in place of the shaft Ib, or within the shaft Ib. Here the tool gripping portions 451 take the form of jaws of the collet having an external taper which is to interact with the matching taper in the shaft S such that if the jaws 451 are drawn further into that taper the tapers interact with one another to drive the jaws 451 into gripping contact with a carried tool 3.

The collet has a tail portion 407. An end of the tail portion 407, which is remote from the jaws 451, is fixedly mounted in the shaft S. The tail portion 407 of the collet comprises a hollow slotted tube having four axial slots 471 (only one of which can be properly seen in Figure 9) terminating in stress relieving apertures 472. The parts of the hollow tube between the slots 471 act as moveable mass portions 452 which can move outwards away from the axis of the collet and hence the tool holder 402 under high speed rotation. When this occurs, there is an effective shortening in the overall length of the tail 407 of the collet. This serves to draw the jaw portions 451 further into the corresponding taper giving rise to a gripping effect on a carried tool 3.

As best seen in Figure 10, a rod-like memory metal portion 406 is disposed within the tail portion 407 of the collet. Again this is arranged and trained so that in a first state it may be accommodated within the hollow tube of the tail 407 of the collet 407 without exerting any particular force on the moveable mass portions 452. However, in a second state the memory metal portion 406 has a shape with a greater radial dimension such that it forces the moveable mass portions 452 outwards away from the axis of the collet so shortening the effective length of the tail of the collet and tending to increase the grip on a carried tool.

Again, cooling of the memory metal portion 406 may be achieved by supplying air to the region of the memory metal portion. In some alternatives the memory metal portion 406 may include a through bore through which cooling/heating air may pass.

It can be noted that in each of the above arrangements the tool holder assembly is mounted within a shaft, which in turn is (or can be) journalled with the bearings of a spindle/main body. Thus, there are provided integrated shaft solutions where the tool holder assembly is in effect part of the shaft as opposed to being separate. The tool holder assembly may be permanently mounted in the respective shaft. Adhesives and/or welds may be used in such permanent fixings, for example. In alternatives the tool holder assembly may be permanently captured in the shaft.