GB2461011A

GB2461011A - Ceramic spindle with balancing provision for high speed drill

Info

Publication number: GB2461011A
Application number: GB0721755A
Authority: GB
Inventors: John David Stratton
Original assignee: GSI Group Ltd
Current assignee: Novanta Technologies UK Ltd
Priority date: 2007-11-06
Filing date: 2007-11-06
Publication date: 2009-12-23
Anticipated expiration: 2027-11-06
Also published as: GB2461011B; GB0721755D0

Abstract

A motor spindle and shaft assembly for high speed precision drilling of PCBs comprises a non-ferrous e.g. ceramic or Beryllium based shaft 22. The shaft carries a tool holder assembly 23 and a permanent magnet 21 which forms part of the motor. The shaft assembly also carries a balancing portion 214, which facilitates balancing by addition / removal of material. The balancing portion 214 is preferably mounted to the magnet 21. This arrangement avoids the need to remove material from the shaft itself, which would cause undesirable stress concentration.

Description

I

MOTOR SPINDLES AND SHAFT ASSEMBLIES FOR MOTOR

SPINDLES

This invention relates to motor spindles and shaft assemblies for motor spindles.

In particular this invention relates to motor spindles and shaft assemblies for such motor spindles which are used in high speed machining operations for example, high speed drilling of printed circuit boards (PCB's).

As there is a continual desire to be able to drill smaller and smaller holes more and more accurately in printed circuit boards, there comes a corresponding need to be able to drive drill bits at increasingly high speeds with corresponding accuracy.

The high speeds are required to ensure that there is sufficient tangential speed of the tool's cutting edge to satisfactorily perform drilling.

It is the aim of this invention to provide an improved motor spindle and/or shaft assembly therefor.

According to one aspect of the present invention there is provided a shaft assembly for a motor spindle comprising a non-ferrous shaft, a tool holder assembly for holding a tool and a permanent magnet to act as part of the motor, the tool holder assembly and magnet being carried within the shaft. I 12

I

It will be appreciated that, where, in this specification, reference is made to an item being carried within the shaft this does not preclude parts of that item projecting out of, or beyond the shaft.

The shaft assembly may comprise a thrust runner insert which is carried within the shaft. The thrust runner insert and tool holder assembly may be disposed at opposite ends of the shaft.

The present shaft assemblies are intended for high speed use. The use of a non-ferrous shaft, for example, a non-metallic shaft such as a ceramic shaft, or a Beryllium based shaft can aid in this -these material can be stiff and resistant to diametrical growth, but other issues can arise.

In general there is a need to balance a shaft assembly. To achieve a high level of balancing a typical technique involves an iteratitive process of testing and removal of small amounts of material from desired locations. With a ferrous (typically steel) shaft, it is usually possible to remove material from the shaft itself to achieve this. However, removing material from a non-ferrous, for example ceramic, shaft will often be impractical because of the likelihood of this leading to failure at high speed due to stress build up around removal sites.

If Beryllium is used there can be safety issues due to its poisonous nature -one possibility is Nickel plating a Beryllium shaft -Again removal of material from such a shaft can be impractical.

I

The shaft assembly may comprise a balancing portion which is, preferably, mounted to the permanent magnet. The balancing portion may be arranged to facilitate the addition and/or removal of material to achieve balance. The balancing portion may be a sacrificial balancing portion. The balancing portion may be of a relatively soft metallic material. The balancing portion may be of non-magnetic material. The balancing portion may comprise material receiving locations for receiving additional material to achieve balance. The balancing portion may have a plurality of pre-drilled holes for receiving additional material.

The additional material may be adhesive. The provision of a balancing portion eases the task of balancing the shaft assembly.

The material of the permanent magnet will typically be brittle and also when subjected to high centrifugal forces, at risk of break up/explosive bursting -any removal of material from the magnet is therefore not desirable. The provision of a balancing portion on the magnet provides a convenient way to facilitate material removal at that location to improve balancing.

According to another aspect of the present invention there is provided a method of balancing a shaft assembly according to any one of the above aspects of the invention comprising the step of removing material from at least one of the tool holder assembly, the thrust runner insert and a sacrificial balancing portion mounted to the shaft assembly.

I

According to another aspect of the present invention there is provided a method of balancing a shaft assembly according to any one of the above aspects of the invention comprising the step of adding material to andlor removing material from at least one of the tool holder assembly, the thrust runner insert and a balancing portion mounted to the shaft assembly.

The permanent magnet may be provided with a pair of supporting end pieces, a respective one of which is mounted on each end of the magnet. The end pieces may be of non-magnetic material. The end pieces may be provided to provide axial containment. This can be useful to guard against break up of the magnet at high speed. As such the end pieces may be subjected to high stresses, may be of hard material, and it can be undesirable to weaken them.

The balancing portion may be mounted on one of the end pieces and hence mounted on the permanent magnet.

Each of the end pieces may be an annular end ring. The balancing portion may be annular.

In some embodiments one or more of the end pieces may comprise the balancing portion.

One of, or any combination of, the thrust runner insert, permanent magnet, and tool holder may be bonded to an internal surface of the shaft.

I

The thrust runner insert may have an insert surface shaped and dimensioned to confront an internal surface of the shaft.

The tool holder assembly may have an insert surface shaped and dimensioned to confront an internal surface of the shaft.

The insert surface of the tool holder assembly and/or the thrust runner insert may comprise a plurality of land portions with at least one intervening recess. The land portions together may serve to locate the insert surface accurately within the shaft.

The or each recess may act as an adhesive reservoir.

The or each recess may be a circumferential groove. The insert surface of the tool holder assembly andlor the thrust runner insert may comprise a circumferential groove and preferably comprises a plurality of axially spaced circumferential grooves.

The permanent magnet may comprise a through bore. The permanent magnet may be annular. The shaft assembly may comprise a magnetic material member portion which passes through the bore of the permanent magnet and is moveable relative to the permanent magnet.

The shaft assembly may comprise an actuation rod for actuating the tool holder assembly. The actuation rod may comprise the magnetic material member portion.

Where magnet end rings are provided these may serve to guide the actuation rod.

Preferably the permanent magnet has no backing metal mounted directly to the bore of the magnet. In such a case reliance can be placed on the magnetic material member portion.

An air passage path may be provided through the shaft assembly. This can allow the delivery of a blast of air through the tool holder assembly for cleaning. The air passage path may pass through the bore in the permanent magnet. The magnetic material member portion may comprise a through bore -the air passage path may comprise this bore.

The shaft assembly may comprise a magnet containment sleeve disposed around the magnet. Where a containment sleeve is provided this is to hold the magnet together against centrifugal effects. The containment sleeve may surround an outer curved surface of the permanent magnet.

The containment sleeve may comprise a carbon fibre sleeve. The carbon fibre sleeve may be a shrink fit onto the magnet. The carbon fibre sleeve may be first wound on a mandrel, the mandrel expanded, and then the carbon fibre sleeve pressed onto the magnet and allowed to shrink fit around the magnet.

According to a further aspect of the present invention there is provided a method of manufacturing a shaft assembly according to any one of the above aspects of the invention, the method comprising the steps of winding a carbon fibre sleeve on a mandrel, expanding the mandrel to stretch the sleeve, pushing the sleeve onto the magnet and allowing the sleeve to shrink fit around the magnet.

The permanent magnet as contained within the containment sleeve may be bonded into position within the shaft.

The tool holder assembly may comprise a tool gripping portion and a spring pack for acting on the tool gripping portion to grip a carried tool. In some embodiments the spring pack is provided between the tool gripping portion and the permanent magnet. In other embodiments the permanent magnet is provided between the tool gripping portion and the spring pack. This can facilitate an increase in the area over which the load of actuating the spring pack is distributed. The tool holder assembly may comprise a plurality ofjaws each of which acts as a tool gripping portion.

The tool holder assembly may comprise a taper collet tool holder comprising a tapered collet having a plurality ofjaws for gripping a carried tool and a sleeve having a correspondingly tapered bore in which the collet is received. The collet may be of steel. The sleeve may have a contact portion including the tapered bore for contact with the tapered surface of the collet. The sleeve may have a secondary portion. The contact portion may be of different material from the secondary portion. The contact portion may be of steel. The secondary portion may be of Titanium. The secondary portion may have a plurality of voids. The secondary portion may have a plurality of axial through holes.

The permanent magnet may be disposed wholly within the shaft. A portion of the tool holder assembly may project from a respective end of the shaft. A portion of the thrust runner insert may project from a respective end of the shaft.

According to another aspect of the present invention there is provided a motor spindle comprising a spindle body in which a shaft assembly according to any one of the above aspects of the invention is journalled for rotation relative to the body.

According to another aspect of the present invention there is provided a motor spindle comprising a spindle body, in which a shaft assembly comprising a tool holder assembly is journalled for rotation relative to the body, and a motor for rotatingly driving the shaft assembly relative to the body, the body comprising a stator of the motor and the shaft assembly comprising a permanent magnet to act as part of the motor, wherein the shaft assembly comprises a non-ferrous shaft and the magnet and tool holder assembly are carried within the shaft.

An embodiment 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 DC motor spindle in section; Figure 2 schematically shows a shaft assembly of the motor spindle of Figure 1 again in section, but this time in isolation; Figure 3 schematically shows, in section, a first alternative shaft assembly of a type similar to that shown in Figure 2; Figure 4 schematically shows, in section, a second alternative shaft assembly of a type similar to that shown in Figure 2; and Figure 5 schematically shows a collet receiving sleeve portion of the shaft assembly shown in Figure 4.

Figure 1 schematically shows a DC motor spindle for use in high speed drilling operations, for example in the high speed drilling of printed circuit boards. The motor spindle comprises a main body 1 and a shaft assembly 2 which is journalled in the main body 1 for rotation relative thereto. A pair of radial air bearings 11 are provided in the spindle body 1 and the shaft assembly 2 is journalled in these bearings. The air bearings are jetted aerostatic air bearings and are shown only in highly schematic form in Figure 1 as the construction and functioning of these air bearings is conventional in the field of air bearing PCB drilling spindles and is not of particular pertinence to the present invention. Of course, the provision of such air bearings is useful in achieving the high drilling speeds which are to be achieved with the present spindle. The Applicants have developed a spindle of the type shown in Figure 1 which is able to operate at in the order of 370,000 rpm.

Currently a typical range of operating speeds for spindles of the present type is 250,000 rpm to 370,000 rpm. A motor M is provided for rotatingly driving the shaft assembly 2 relative to the spindle body 1. This motor M is a DC motor and comprises windings 12 disposed in the spindle body and a permanent magnet 21 mounted within the shaft assembly 2. Again, in general terms the structure and functioning of this DC motor is conventional in the field of high speed PCB drilling using air bearing spindles and thus no detailed description of those aspects is given herein.

The shaft assembly 2 comprises a hollow tube like shaft 22 which is of non-metallic material. In the present embodiment this shaft is of ceramic material.

The ceramic material is chosen to give strength and high resistance to diametrical growth of the shaft 22 when rotated at high speed. A tool holder assembly 23 for holding a drilling tool is carried at a first end of the shaft 22 and a thrust runner insert 24 is carried at the opposite end of the shaft 22. A thrust runner 241 of the thrust runner insert 24 is received in an axial air bearing (not shown) provided in the spindle body 1 to axially locate the shaft assembly 2 relative to the spindle body 1.

Referring now to Figure 2 as well as Figure 1 various aspects of the shaft assembly 22 will be described in more detail.

Both the tool holder assembly 23 and thrust runner insert 24 are carried within the shaft 22 and both have a respective insert surface 232, 242 which confronts the internal curved surface 221 of the shaft 22. Both of these insert surfaces comprise a plurality of axially spaced circumferential grooves 232a, 242a. *11

That is to say the outer curved surface represented by the insert surface 232, 242 has grooves around it which are spaced from one another axially (ie axially relative to the axis of the tool holder assembly 23, thrust runner insert 24 and the shaft 22 as a whole). The remainder of the insert surface 232, 242 can be considered to consist of a plurality of land portions 232b, 242b. These lands 232b, 242b closely and accurately fit within the bore of the shaft 22 to ensure accurate radial location of the tool holder assembly 23 and thrust runner insert 24 in the shaft 22. The tool holder assembly 23 and thrust runner insert 24 are bonded with adhesive into position in the shaft 22. It will be noted that both the insert surfaces 232, 242 have significant axial extent to provide a relatively large surface which may be bonded by the adhesive to the internal surface 221 of the shaft 22. This is to help increase the strength of the bond. Furthermore, the recesses provided by the grooves 232a, 242a provide reservoir locations for adhesive to improve bonding. Whilst there can be a significant amount of adhesive in these grooves to provide bonding to the internal surface 221 of the shaft 22, there will be little adhesive between the lands 232b, 242b and the internal surface 221 of the shaft 22.

In the present embodiment the thrust runner insert 24 and the part of the tool holder assembly 23 having the insert surface 232 are of titanium. In alternatives, particularly where only lower speeds of operation are required, these parts might, for example, be of steel.

The tool holder assembly 23 is a generally conventional taper collet tool holder and as such has a tapered collet 233 which includes a plurality of jaws 233a and which is received in a sleeve 234 having a correspondingly tapered internal bore.

A spring pack 235 is provided which serves to draw the collet 233 into the tapered bore 234 so squeezing the jaws 233a of the collet together around a carried tool.

It is the sleeve portion 234 which has its outer surface acting as the insert surface 232 discussed above.

A push-rod 236 is provided within the shaft assembly 2 for use in releasing a carried tool from the collet 233 during a tool change operation. This push-rod 236 may be used to push the collet 233 out of the tapered bore in the sleeve 234 against the action of the springs 235 so as to allow the jaws 233a to move outwards and reduce grip on a carried tool.

During such a tool change operation, force is applied to the push-rod 236 and there can be a significant axial shearing force between the shaft 22 on the one hand and the tool holder assembly 23 and the thrust runner insert 24 on the other.

This is one of the reasons for there being a desire to maximise bonding between the shaft 22 and the tool holder assembly 23 and the shaft 22 and the thrust runner insert 24.

The permanent magnet 21 is mounted within the shaft 22 and has an annular cross-section. The magnet 21 as such is in the form of a hollow cylinder, but with relatively thick side walls. A portion of the push-rod 236 is disposed within the bore 211 through the permanent magnet 21. However, the permanent magnet 21 is bonded to the internal surface 221 of the shaft 22. The push-rod 236 is thus movable within the bore of the permanent magnet 21 and is so moved during tool change operations. Having said this, at least the portion of the push-rod 236 which passes through the bore in the permanent magnet 21 is of magnetic material such that it can act as backing metal to the magnet 21 to improve the magnet's magnetic performance. It will be noted here that the magnet 21 does not have a tube of magnetic material, for example, steel, directly bonded to the internal bore as might more typically be the case, but rather reliance is placed on the presence of the magnetic material push-rod 236 in the magnet's bore to give a similar enhancing effect.

The permanent magnet 21 will typically be of a fairly structurally weak material such as samarium cobalt. Therefore, the permanent magnet 21 may not be well able to cope with the high centrifugal forces generated as the shaft assembly 2 is rotated at speeds up to the order of say 370,000 rpm. Thus, there is a risk of the magnet 21 breaking up and causing failure of the spindle or other damage/problems. As the magnet 21 is contained within the shaft 22, it might be thought that this would provide sufficient containment against the magnet disintegrating under centrifugal forces. However, even the walls of the shaft 22 may be insufficient to provide such containment without being undesirably thickened. To put this another way the magnetic rotor needs a compressive preload on the outside diameter to survive ultra-high rotational speeds, as the ceramic tube of the shaft has insufficient strength to retain the rotor without cracking. Therefore, in the present embodiment the permanent magnet 21 is provided with a carbon fibre containment sleeve 212 which surrounds the outer curved surface of the magnet 21. Further to guard against disintegration of the magnet 21, non-magnetic end-rings 213 are provided at each end of the magnet 21. These end rings have a central aperture/bore through which the push-rod 236 passes and by which the push rod 236 is guided. The circumferential surface of each of these end rings 213 are also covered by the carbon fibre sleeve 212 to provide a completely contained magnet package. However, again these end rings need to be able to resist being subjected to significantly high stresses and act as guides for the push-rod 236. Therefore the end rings 213 are of a relatively hard non-magnetic material metal (typically an alloy). Note that in alternatives the end rings 213 may be of magnetic material.

Mounted to the outside of this magnet 21 containment package and particularly mounted to the end ring 213 remote from the tool receiving end of the shaft assembly 2, there is a sacrificial balancing ring 214. This is of a softer material than the end rings 213 and is provided specifically to aid in balancing of the shaft assembly 2 as will be explained in more detail below.

When constructing the magnet package the carbon fibre sleeve 212 is first wound onto a mandrel. This mandrel is then expanded to stretch the carbon fibre sleeve 212 before it is pushed over the magnet 21 with its end rings 213 already bonded in place at each end of the magnet 21. The carbon fibre sleeve 212 can then be shrink fitted onto the magnet 21 and end rings 213 to provide the final package.

After this operation is complete the balancing ring 214 may be bonded to the respective end ring 213 and the package bonded into position within the shaft 22.

As mentioned in the introduction, when operating a spindle such as this at high speed, it is important that the spindle is well balanced. To achieve a very high degree of balance, one technique commonly used is to put the shaft assembly in a balancing machine to determine if there is any lack of balance and then remove minute portions of the shaft assembly to improve balance. Where in a traditional shaft assembly, steel is used for the shaft itself, it is usual to remove material from the shaft itself to achieve balance. However, in the present assembly, where the shaft is of non-metallic material (and in this embodiment of ceramic), removing material from the shaft is not practical as the removal sites would become points for concentration of stress and this would risk failure of the shaft. Typically it is desirable to be able to remove material from a region at each end of the shaft assembly and a region near the motor M as there is significant mass at this location and it is the centre of the shaft. Again, however, removing material from the magnet 21 itself is undesirable due to the nature of the magnetic material.

Furthermore, the end rings 23 are of a relatively hard material and have been provided to withstand possibly high stresses caused as the magnetic might be about to break up. Thus, removing material from these components is also undesirable.

Thus, in the present embodiment the sacrificial balancing ring 214 is provided and is chosen to be of a softer non-magnetic metallic material. This means that during the balancing operation it is an easier task to remove portions of the sacrificial balancing ring 214 to achieve the desired balance. These portions of the sacrificial balancing ring will be removed by introducing a long, small diameter drill/drill bit through the end of the shaft to cut away the material. This process may be repeated until the required balancing is achieved. Similarly, material may be removed from the tool holder assembly 23 and thrust runner insert 24 to give the ability to remove material at a position close to the nose and the rear of the shaft assembly 2.

It will be noted that this sacrificial balancing ring 214 does not need to and does not act as a guide to the push-rod 236.

A through bore 236a is provided through the push-rod 236 and this means that there is an air supply path from the rear of the spindle to the collet 233. This may be used to administer a blast of air for cleaning the collet 233 and tool holder assembly 23 in general. Of course, it is provision of a magnet 21 with a through bore 211 which facilitates the provision of this air flow path from the rear to the nose of the shaft assembly 2. In other embodiments there may be no push rod or no through bore through the push rod, but still an air path provided through the magnet 21.

In an alternative rather than achieving balance by removing material from a sacrificial balancing portion as described above, balance can be achieved by the provision of a balancing portion with pre-drilled holes (or other sites) arranged for the receipt of adhesive and the iteratitive process of checking balance and adding adhesive (or other material) to these sites. In such a case the balancing portion might, for example, be a balancing ring 214 or an end ring 213. Addition of material in this way can be combined with removal of material from the tool holder assembly 23 and thrust runner insert 24 and/or the addition of material to those parts in an analogous way.

As a further alternative rather than the shaft 22 being of ceramic material it may be of a Beryllium based material, for example Nickel plated Beryllium.

Figure 3 shows a first alternative shaft assembly 2' which is similar to the shaft assembly 2 shown in Figures 1 and 2 and may be used in a DC motor spindle of the type shown in Figure 1. Many aspects of the first alternative shaft assembly 2' are the same as the shaft assembly 2 shown in Figures 1 and 2. The same reference numerals are used to reference the corresponding parts and much description of the parts common between the two shaft assemblies is omitted below in the interest of brevity.

In the first alternative shaft assembly 2' the tool holder assembly 23 again is a taper collet tool holder assembly comprising a tapered collet 233 disposed in a sleeve 234 having a correspondingly tapered bore, and a spring pack 235.

However, in this case, the tool holder 23 assembly has a split design. The spring pack 235 is not disposed adjacent the sleeve 234 as is the case in the tool holder assembly 22 of Figures 1 and 2, rather the spring pack 235 is axially spaced from the sleeve 234. The spring pack 235 is on "the other side of' the permanent magnet 21. Thus the permanent magnet 21 is disposed between spring pack 235 and the sleeve 234 and collet 233 -or to put this another way between the gripping portion of the tool holder assembly 23 and the spring pack 235.

Furthermore a spacer tube 222 is disposed within the shaft 22 between the collet 233/sleeve 234 and the magnet 21.

The spring pack 235 has a casing 235a which is bonded to the internal surface of the shaft 22 and against which one end of the springs react. The spacer tube 222 is also bonded to the internal surface of the shaft 22 as are the magnet 21 package and tool holder insert surface 232 as described above. The spring pack casing 23 5a, magnet 21 package and spacer tube 222 are in contact with one another at their respective axial ends. This means that there is a whole chain of components bonded to the internal surface of the shaft 22.

Because of the re-positioning of the spring pack 235, what was a push-rod 236 in the shaft assembly 2 of Figures 1 and 2 becomes a draw bar 236' in the present shaft assembly 2'. The drawbar 236' transmits the force provided by the spring pack 235 to the collet 233 to draw the collet 233 into the taper of the sleeve 234.

The draw bar 236' is still hollow with a central bore 236a' and is of magnetic material. It can thus still assist in the performance of the magnet 21 and provide an air passageway from the rear of the spindle to the collet 233. The draw bar 236' can slide relative to the magnet 21 but rotates with the shaft assembly 2'.

The draw bar 236' terminates in a flange 236b' which contacts the end of the springs of the spring pack 235. As the end of the magnet 21 is inaccessible in this shaft assembly, there is no balancing ring at the end of the magnet package in contrast with the assembly of Figures 1 and 2. However, the outwardly facing surface of the flange 236b' can be used in balancing the shaft assembly 2' -ie as a balancing portion via the removal of material from and/or addition of material to the flange 236b' or in alternatives, as an attachment point for a separate balancing portion.

When a tool is to be inserted into and/or released from the collet, a force is applied to the flange 236b' to compress the springs in the spring pack 235. As discussed above in relation to Figures 1 and 2 such an operation can generate a large shearing force between the shaft 22 and the components bonded to the shaft 22.

In the present shaft assembly 2', the force applied to the flange 236b' is applied to the coUet 233 via the draw bar 236' and is reacted (via the springs) by the spring pack casing 235a, the magnet 21 package arid the spacer tube 222 each of which is bonded to the internal surface of the shaft 22 as mentioned above. This spreads the shearing load over a larger area than in shaft assembly 2 of Figure 1, where the tool holder assembly insert 23 must bear this load. In fact in the present assembly the front portion of the tool holder assembly, ie the sleeve 234, is protected from this load by an axial spacing between the sleeve 234 and the spacer tube 222. These features reduce the chances of bond failure.

Figure 4 shows a second alternative shaft assembly 2" which is similar to the shaft assembly 2 shown in Figures 1 and 2 and may be used in a DC motor spindle of the type shown in Figure 1. Many aspects of the second alternative shaft assembly 2" are the same as the shaft assembly 2 shown in Figures 1 and 2.

The same reference numerals are used to reference the corresponding parts and much description of the parts common between the two shaft assemblies is omitted below in the interest of brevity.

In the second alternative shaft assembly 2" the tool holder assembly 23 again is a taper collet tool holder assembly comprising a tapered collet 233 disposed in a sleeve 234 having a correspondingly tapered bore, and a spring pack 235.

However, in this case, the sleeve 234 has a different design. In the tool holder assembly 2 of Figure 1, Titanium is used for the sleeve -this is useful due to its low density and strength but there can tend to be a risk of galling, ie a risk of the collet (typically of steel) locking up with the sleeve 234.

in the present shaft assembly, the sleeve 234 comprises a first, contact, portion 234a which includes the correspondingly tapered bore which is to contact with and interact with the tapered surface of the collet 233. The collet 233 and first contact portion 234a are of steel. This helps ensure good performance as the steel surfaces run against one another. On the other hand the mass of the parts when using steel will be higher. To help counter this, in the present shaft assembly 2", the sleeve comprises a secondary part 234b, which is shown in isolation in Figure 5. This secondary part does not include the tapered bore, and is of Titanium.

Moreover the secondary part 234b has a plurality of mass reducing through bores 234b'. This arrangement can allow higher operational speeds (due to the reduced mass) than use of a wholly steel sleeve 234 whilst giving a reduced risk of galling.

The secondary part 234b of the sleeve is bonded to the internal surface of the shaft 22 and its axial ends contact the contact portion 234a of the sleeve 234 and the spring pack 235. Thus the transmission of axial (reaction) force from the spring pack 235 to the contact portion 234a can occur and this shaft assembly can operate in the same way as that of Figures 1 and 2.

In general terms, of course, the ideas of the different shaft assemblies above can be used together where the context allows. In one particular alternative, the sleeve 234 of the second alternative shaft assembly 2" shown in Figure 4 can be used in place of the more conventional sleeve 234 in the first alternative shaft assembly 2' shown in Figure 3 to provide a shaft assembly combining the ideas of the first and second alternative shaft assemblies 2', 2".

The use of a stiff, lightweight shaft as is used in the embodiments described above, can have the advantage of dramatically increasing the natural bending frequency of the shaft. It is extremely inadvisable to run a shaft through, or even close to, its natural frequency in an air bearing system, due to the relatively low stiffness and damping characteristics (compared with more conventional mechanical type bearings). Further, where the shaft has a very high natural frequency, this also reduces the chance of sub-harmonics from, for example, the drilling process, or the machine, resonating together with the shaft causing poor surface finish to, for example, holes drilled using the spindle.

Claims

CLAIMS: 1. A shaft assembly for a motor spindle comprising a non-ferrous shaft, a tool holder assembly for holding a tool and a permanent magnet to act as part of the motor, the tool holder assembly and magnet being carried within the shaft.
2. A shaft assembly according to claim 1 in which the shaft is one of, of non-metallic material, for example, ceramic material and Beryllium based.
3. A shaft assembly according to any proceeding claim which comprises a balancing portion which is, preferably, mounted to the permanent magnet.
4. A shaft assembly according to claim 3 in which the balancing portion is arranged to facilitate the addition and/or removal of material to achieve balance.
5. A shaft assembly according to claim 4 in which the balancing portion is of a relatively soft metallic material.
6. A shaft assembly according to any one of claims 3 to 5 in which the balancing portion comprises material receiving locations for receiving additional material to achieve balance,
7. A shaft assembly according to any one of claims 3 to 6 in which the permanent magnet is provided with a pair of supporting end pieces, a respective one of which is mounted on each end of the magnet, the end pieces being provided to provide axial containment and the balancing portion being mounted on one of the end pieces and hence mounted on the permanent magnet.
8. A shaft assembly according to any preceding claim which comprises a magnet containment sleeve disposed around the magnet for holding the magnet together against centrifugal effects.
9. A shaft assembly according to claim 8 in which the containment sleeve surrounds an outer curved surface of the permanent magnet.
10. A shaft assembly according to claim 8 or claim 9 in which the containment sleeve comprises a carbon fibre sleeve, which may be a shrink fit onto the magnet.
11. A method of manufacturing a shaft assembly according to claim 10, the method comprising the steps of winding a carbon fibre sleeve on a mandrel, expanding the mandrel to stretch the sleeve, pushing the sleeve onto the magnet and allowing the sleeve to shrink fit around the magnet.
12. A shaft assembly according to any preceding claim which comprises a thrust runner insert which is carried within the shaft.
13. A shaft assembly according to any preceding claim in which one of, or any combination of, the thrust runner insert, permanent magnet, and tool holder is bonded to an internal surface of the shaft.
14. A shaft assembly according to claim 12 or claim 13 in which the thrust runner insert has an insert surface shaped and dimensioned to confront an internal surface of the shaft.
15. A shaft assembly according to any preceding claim in which the tool holder assembly has an insert surface shaped and dimensioned to confront an internal surface of the shaft.
16. A shaft assembly according to claim 14 or claim 15 in which the insert surface of the thrust runner insert or the tool holder assembly respectively comprises a plurality of land portions with at least one intervening recess, the land portions together serving to locate the insert surface accurately within the shaft and the at least one recess acting as an adhesive reservoir.
17. A shaft assembly according to any one of claims 14 to 16 in which the insert surface of the tool holder assembly andlor the thrust runner insert comprises at least one circumferential groove, and preferably comprises a plurality of axially spaced circumferential grooves.
18. A shaft assembly according to any preceding claim in which the permanent magnet comprises a through bore.
19. A shaft assembly according to claim 18 which comprises a magnetic material member portion which passes through the bore of the permanent magnet and is moveable relative to the permanent magnet.
20. A shaft assembly according to claim 19 which comprises an actuation rod for actuating the tool holder assembly, the actuation rod comprising the magnetic material member portion.
21. A shaft assembly according to claim 19 or claim 20 in which the permanent magnet has no backing metal mounted directly to the bore of the magnet, but reliance is placed on the magnetic material member portion.
22. A shaft assembly according to any preceding claim in which an air passage path is provided through the shaft assembly.
23. A shaft assembly according to claim 22 when dependent on claim 18 in which the air passage passes through the bore in the permanent magnet.
24. A shaft assembly according to any preceding claim in which the tool holder assembly comprises a tool gripping portion and a spring pack for acting on the tool gripping portion to grip a carried tool.
25. A shaft assembly according to claim 24 in which the spring pack is provided between the tool gripping portion and the permanent magnet.
26. A shaft assembly according to claim 24 in which the permanent magnet is provide between the tool gripping portion and the spring pack.
27. A shaft assembly according to any preceding claim in which the tool holder assembly comprises a taper collet tool holder comprising a tapered collet having a plurality of jaws for gripping a carried tool and a sleeve having a correspondingly tapered bore in which the collet is received, wherein the sleeve has a contact portion including the tapered bore for contact with the tapered surface of the collet and a secondary portion which is of different material from the contact portion.
28. A shaft assembly according to claim 27 in which the secondary portion has a plurality of voids.
29. A motor spindle comprising a spindle body in which a shaft assembly according to any one of claims 1 to 10 and 12 to 28 is journalled for rotation relative to the body.
30. A motor spindle comprising a spindle body, in which a shaft assembly comprising a tool holder assembly is journalled for rotation relative to the body, and a motor for rotatingly driving the shaft assembly relative to the body, the body comprising a stator of the motor and the shaft assembly comprising a permanent magnet to act as part of the motor, wherein the shaft assembly comprises a non-ferrous shaft and the magnet and tool holder assembly are carried within the shaft.
31. A method of balancing a shaft assembly according to any one of claims I to 10 and 12 to 28, the method comprising the step of adding material to and/or removing material from at least one of the tool holder assembly, the thrust runner insert and a balancing portion mounted to the remainder of the shaft assembly.