The present ideas are particularly suitable for use in drilling spindles and shafts, for example for use in high speed drilling applications such as printed circuit board (PCB) drilling.

With the trends in continuing miniaturisation of electronic equipment and the continually increasing sophistication of such equipment there is pressure to be able to drill smaller holes more quickly and more closely together in printed circuit boards in order to aid the overall manufacturing process. In general terms these desires lead to it becoming desirable to run drilling spindles at increasingly fast rates of rotation to achieve good drilling characteristics. There is also a requirement to keep drilling spindles as light as possible so as to help fast manoeuvring of the spindle relative to the printed circuit board or other workpiece.

Existing spindles for use in drilling printed circuit boards and other components typically make use of air bearing technology to achieve the necessary smooth running. The same is true of some other machining spindles.

In general terms there are two complementary drive technologies which may be used in such spindles:- dc motor drives and ac motor drives.

It is generally preferred to use ac motor drives rather than dc motor drives because ac motors generally have simpler construction, are easier to install and repair and will often be more reliable. It is also true that parts for and the whole of efficient ac drives are typically easier to obtain and cheaper.

Of course in ac motor drives, windings of some sort need to be provided on the driven part of the spindle, i.e. typically the shaft. One conventional way in which this is achieved is to mill slots into a steel shaft and plate these slots with copper to achieve the desired winding pattern on the shaft. Such drives are typically set up to operate as brushless induction motor drives.

Such motor drive technology is conventionally used in spindles which operate at say, 180,000 rpm. However, difficulties are encountered in attempting to run such spindles at higher rotational speeds.

This is partly because the operation of the motor drive relies on the magnetic properties of the (usually steel) shaft. The use of steel shafts can be become undesirable at high rotational speeds. Furthermore, at these high rotational speeds, there is a risk that the copper plating provided in the slots in the shaft will tend to fly out due to the very high centrifugal forces involved.

It is an object of at least some embodiments of the invention to address one or more of these problems.

According to a first aspect of the present invention there is provided a machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the shaft relative to the sleeve, wherein a portion of the shaft forms part of the motor drive such that in use magnetic flux flows through said portion and at least said portion of the shaft is of a metal-matrix-composite material.

According to a second aspect of the present invention there is provided a machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the sleeve relative to the shaft, wherein a portion of the sleeve forms part of the motor drive such that in use magnetic flux flows through said portion and at least said portion of the sleeve is of a metal-matrix-composite material. According to a third aspect of the present invention there is provided a shaft for a machining spindle, the shaft comprising at least one portion which is of a metal-matrix-composite material.

The metal-matrix-composite material, of course, needs to be selected so as to have suitable magnetic properties to enable the motor drive to function. The metal-matrix-composite may be Ferro-Titanit.

Preferably, the shaft comprises a main body of a first material carrying windings for forming part of the motor drive. The windings may comprise a second material deposited on the main body. At least one portion of the windings may be deposited in a multi-fluted groove provided in the main body. A preferred form of deposition is plating. In this specification plating is used to refer to processes of electro-chemical deposition. Other possible deposition techniques include hot metal, or flame, spraying. Preferably the windings comprise a second material plated onto the main body.

The windings may be provided on the portion of the shaft which is of a metal- matrix-composite material and through which magnetic flux flows in use.

According to a fourth aspect of the present invention there is provided a machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the shaft relative to the sleeve, wherein the shaft comprises a main body of a first material which carries windings forming part of the motor drive, the windings comprise a second material deposited onto the main body and at; least one portion of the windings are deposited in a multi-fluted groove in the main body.

According to a fifth aspect of the present invention there is provided a shaft for a machining spindle, the shaft comprising a main body of a first material carrying windings for forming part of a motor drive, the windings comprising a second material deposited onto the main body and at least one portion of the windings being deposited in a multi-fluted groove in the main body.

The technique of depositing windings on a steel shaft, by plating, to form the rotor of a motor drive is well established in the field of air bearing spindles but problems occur if the shaft is to be rotated at very high speed, say speeds tending towards 200,000 rpm. First there is a problem with shaft strength and stiffness, which makes it undesirable to use steel shafts. On the other hand the use of alternating current drive is desirable and this requires the shaft to have suitable magnetic properties. Another problem is that with conventional shafts carrying plated windings, there can be insufficient adhesion between the main body of the shaft and the windings so that plating will tend to fly out of the shaft causing failure of the spindle. This problem becomes worse as rotational speeds increase. The provision of a multi-fluted groove in the main body of the shaft helps solve this problem by providing a greater surface area for adhesion. It might be thought that the provision of a re-entrant shaped slot or groove or providing some other type of "bulge" shaped recess for plating could solve the problem. However, it has been found that such shapes will generally not plate properly, tending to leave voids in the windings, which impair performance.

The second material, which is deposited onto the main body of the shaft, may be copper.

The main body may comprise a plurality of multi-fluted grooves each of which is provided with a material that differs from that of the main body.

The or each multi-fluted groove may be open mouthed. Each flute in the or each multi-fluted groove may be open mouthed. Here the expression open mouthed is used to mean that the respective groove or flute does not have a re¬ entrant shape, or in other words, is shaped so that the portion of material deposited onto the shaft is not captured in the shaft due to its shape but is rather held in place by adhesion between the deposited material and the 7 material of the remainder of the shaft.

According to a sixth aspect of the present invention there is provided a machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the, sleeve relative to the shaft, wherein the sleeve comprises a main body of a first material which carries windings forming part of the motor drive, the windings comprise a second material deposited onto the main body and at least one portion of the windings are deposited in a multi-fluted groove in the main body.

Preferably, the spindle is arranged so that the driven portion of the spindle (the shaft or the sleeve) can be driven at in excess of 200,000 rpm.

According to a seventh aspect of the present invention there is provided a machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the shaft and sleeve relative to one another, wherein the spindle is arranged so that the driven portion of the spindle can be driven at in excess of 200,000 rpm.

Preferably, the spindle is an air bearing spindle including at least one air bearing allowing relative rotation between the shaft and the sleeve. In most implementations the spindle sleeve comprises the air bearing.

According to another aspect of the present invention there is provided an air bearing machining spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the shaft relative to the sleeve at in excess of 200,000 rpm, wherein the shaft comprises a main body of a metal-matrix-composite material which carries windings forming part of the motor drive, the windings comprise a second, different, material deposited onto the main body and at least one portion of the windings are deposited in a multi-fluted groove provided in the main body.

In each of the above cases the machining spindle may be a drilling spindle and moreover the shaft may be for a drilling spindle.

According to another aspect of the present invention there is provided an air bearing drilling spindle comprising a shaft, a spindle sleeve and an alternating current motor drive for rotatingly driving the shaft relative to the sleeve at in excess of 200,000 rpm, wherein the shaft comprises a main body of a metal- matrix-composite material which carries windings forming part of the motor drive, the windings comprise a second, different, material plated onto the main body and at least one portion of the windings are plated in a multi-fluted groove provided in the main body.

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

Figure 1 is a section through part of a PCB air bearing drilling spindle;

Figure 2 is an isometric view of a shaft of the drilling spindle shown in Figure 1;

Figure 3 schematically shows the profile of a slot milled into a prior art shaft of a similar kind to that shown in Figure 2; and

Figure 4 schematically shows the profile of a slot milled into the shaft shown in Figure 2 for plating with copper.

Figure 1 shows a section through part of a PCB drilling spindle which embodies the present invention. The PCB drilling spindle generally comprises a shaft 1 which is journalled within a spindle sleeve 2 and which comprises a collet (not shown) for holding a drill bit (not shown). In operation the shaft 1 is driven at high speed relative to the spindle sleeve 2 such that the drill bit may be used for drilling printed circuit boards (or other work pieces for that matter). In many respects the drilling spindle, of which part is shown in Figure 1, is conventional. Furthermore there are many ways in which those conventional aspects might be implemented and therefore drawings and description of those parts have not been included in this specification in the interest of brevity. In the present specification those parts of the drilling spindle which are non-conventional and therefore of interest reside in the materials used in the shaft and the nature of various aspects of the motor drive used to drive the shaft 1 relative to the sleeve 2. These aspects will now be described in more detail with reference to Figures 1 to 4.

The spindle comprises a motor drive 3 which generally comprises a set of drive coils 31 provided in the spindle sleeve 2, a portion 32 of the shaft 1 which is located adjacent to the drive coils 31 and windings 33 provided in the form of copper plating on the shaft 1.

The drive coils 31 are generally conventional and comprise magnetic material cores and appropriate electrical windings.

As best seen in Figure 2, the copper plated windings 33 comprise a plurality of generally axially orientated strip portions 33a and a pair of circumferential band portions 33b. One of the band portions is provided at a first end of the strip portions 33a joining these ends of the strips together and the other band portion 33b is provided at the other end of the strip portions 33a joining these other ends of the strips together. Thus, the copper plated windings 33 form a ladder like structure wrapped around the shaft 1. In Figure 1, the copper plated windings 33 are shown in solid black. It will be noted that in Figure 1 on one side of the shaft, a strip portion 33a and two circumferential band portions 33b can be seen in section, whereas on the other side of the shaft, only the two circumferential band portions 33b can be seen.

In use, suitable currents are supplied to the electrical windings of the drive coils 31 provided in the sleeve 2. As a result currents are induced in the copper plated windings 33 in a conventional and well understood way so as to cause the shaft 1 to be rotatingly driven relative to the sleeve 2. The shaft 1 acts as a copper plated rotor in the motor drive 3.

At this level the present drilling spindle is conventional. Such motor drive systems including copper plated rotor windings are known, as are the necessary drive current schemes for achieving rotation of the shaft 1 relative to the sleeve 2. It will be understood that it is a straightforward matter for a person skilled in the art to produce suitable drive coils 31, power supplies and controlling electronics and so on.

In producing a copper plated rotor shaft 1 of the type shown in Figure 2 appropriate slots or grooves are first milled into the main body of the shaft 1 and these are then plated to create the windings 33.

In a conventional shaft similar to that shown in Figure 2, the slots milled into the shaft for plating have the simple generally semi-circular profile of the type shown in Figure 3. However, in the present embodiment a more complex or convoluted slot profile is used. In the present embodiment, this more complex or convoluted slot profile is used for all the slots Ia milled into the shaft 1 which are to be plated by copper. That is to say a more complex or convoluted slot profile is used for the slots Ia milled to form each of the strip portions 33a of the copper plated windings and each of the circumferential band portions 33b. In alternatives however, only selected portions of the milled slots may be milled having a more complex or convoluted slot profile. In particular the slots Ia for the band portions 33b may have a simple profile.

Figure 4 shows a cross section through an arbitrary one of the slots Ia milled in the shaft 1 prior to plating to form one of the strip portions 33a. It will be seen that in this embodiment, the slot profile has what might be termed a baby's bottom profile. Thus the slot Ia is a double fluted groove. Whilst this particular profile is preferred because of its effectiveness and ease of machining, other profiles may be used. Such suitable profiles may more generally be referred to as being profiles which are such as to provide slots or grooves which are multi-fluted.

An effect of providing a more complex or convoluted slot profile providing multi-fluted grooves is to increase the surface area for adhesion between the material of the main body of the shaft 1 and the deposited material, in this case copper, used to form the plated windings 33. Increasing the surface area for adhesion between the main body of the shaft 1 and the deposited material improves the overall adhesion between the deposited portions and the body of the shaft 1 which enables the shaft as a whole to better tolerate very high speed rotation, of say 200,000 rpm or more.

It should also be noted that, as can be seen from Figure 4, each of the multi- fluted grooves Ia provided in the shaft 1 is open mouthed. That is to say the overall width of the groove at the surface of the shaft is equal to or greater than that at any point within the groove, or to put this another way the groove does not have a re-entrant shape. This is a preferred characteristic of the shape of the groove, since if a re-entrant shaped groove is used (which would better capture in place the plated material) satisfactory plating of the groove tends not to be achieved. As the plating of a re-entrant shaped groove takes place, the surface will tend to close over the exterior of the groove leaving voids within the body of the plating which will lead to degraded performance and/or increased risk of failure of the shaft during operation. Thus, whilst initially the idea of capturing the plated material within the shaft would seem attractive, in practice this is not the best way forward.

The slot width at the surface of the shaft 1 provided by using the multi-fluted groove profile as shown in Figure 4 is substantially the same as that achieved using the simple semi-circular profile shown in Figure 3 which is important for the electrical and magnetic properties of the shaft in operation.

In determining the best shape of slot Ia to use in order to improve adhesion between the main body of the shaft 1 and the plating material, consideration needs to be given to the machining processes which need to be undertaken to produce the slots. Thus, the baby's bottom shaped profile or another profile in which the or each groove comprises two flutes is preferred to other alternatives such as providing a larger number of flutes or providing a small scale roughening or knurling of the slot surfaces, for example. In the present embodiment, again to facilitate very high speed rotation of in excess of 200,000 rpm, the shaft 1 is made of a metal-matrix-composite and in particular in the present embodiment it is made of Ferro-Titanit metal-matrix- composite. This material is particularly strong and stiff which makes it suitable for use in very high speed shafts. Moreover however, the material has reasonable magnetic properties such that it may be used in a plated shaft type of spindle as described above.

It will be appreciated that for the motor drive described above in relation to Figures 1 and 2 to function, magnetic flux must be able to flow through the material of the shaft 1. This is not a problem where steel shafts are used, but as rotational speeds increase, and the use of steel shafts becomes undesirable or impractical, there is a considerable difficulty in finding an appropriate material from which the shafts can be made.

In other applications, for example, those using dc motor drive techniques where permanent magnets are provided on the shaft, ceramic materials have been used to achieve very high rotational speeds. However, such materials cannot be used where magnetic flux is needed to flow through the material of the shaft itself.

Therefore, a metal-matrix-composite material that can function as a magnetic material should be selected for at least the relevant portion of the shaft.

Drilling spindles of the type described above should be operable at speeds of up to say 220,000 rpm.

Whilst the above description has been written in terms of drilling spindles and the invention is particularly suited for use in drilling spindles and shafts for drilling spindles it should be appreciated that the invention applies more broadly and can be used in other machining spindles such as, for example, light engraving/routing spindles and bore grinding spindles.

Further, whilst the description refers to the plating of material onto the shaft other types of deposition technique may be used, for example hot metal, or flame, spraying, provided that sufficient adherence is achieved.