Redefining the boundaries of high precision machining

Citation

(2002), "Redefining the boundaries of high precision machining", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 6. https://doi.org/10.1108/aeat.2002.12774faf.001

Redefining the boundaries of high precision machining

Redefining the boundaries of high precision machining

Keywords: Machining, Manufacturing processes, Engineering

Introduction

To many manufacturing engineers, air bearing spindles have traditionally been esoteric pieces of equipment – for the initiated only. Over the last few years this view has changed, largely due to force of circumstance, namely the growth in the number of applications for high and ultra precision machining as well as the unrelenting demand for ever higher levels of machine performance.

Air bearing spindles in their many forms are evolving continually to meet these demands, offering improvements that optimise upon the inherent strengths of the high precision aerostatic design – low motion errors, high static and dynamic stiffness and low thermal growth – through increased customisation. This process involves tailoring spindle layouts to suit individual requirements; achieving increased integration of auxiliary components and, hence, better optimisation of the overall system as well as improvements in bearing design and the manufacturing process itself.

Realising these improvements is both an incremental and an ongoing dynamic process, so new developments are occurring all the time. Recently, these have focussed on four key areas: bearings; spindle stiffness and damping, motion errors and thermal distortion, all of which are yielding gains that substantially aid spindle performance.

Bearing stiffness

Taking bearings first: although a number of different bearing geometries are used in high precision machining, in air bearing spindles mostly cylindrical journal and plain thrust bearings are used as these can be manufactured to the high standards of precision and finish essential for low motion errors. In addition, these types of bearings also exhibit low thermal distortion, meaning that they are suitable for both high speed and low speed operation. This facility for high speeds must be put into context, however, as the speeds in the vast majority of high precision machining applications are modest and the emphasis on bearing design is usually to achieve maximum static stiffness for the required speed range.

The stiffness of an air bearing is inversely proportional to its internal clearance, therefore maximum stiffness is achieved by minimising clearance. But there are limits to how far clearance can be reduced; one is thermal distortion the other is the geometrical accuracy to which the bearings can be manufactured.

In allowing for thermal distortion, bearing clearances cannot be reduced to the extent that opposing surfaces come into contact under operational conditions and in practice the bearing is designed to take maximum load at a safe working gap. The larger the surface geometry errors, the larger this gap and the bearing design clearance. Happily, over the last two decades a combination of improved methods of manufacture, better understanding of thermal distortion and improved internal design with regard to air flows, has succeeded in reducing the bearing design clearances by approximately 40 per cent, resulting in much improved levels of stiffness which benefit spindle performance overall.

Spindle stiffness and damping

The role of bearings in the overall radial stiffness of an air bearing spindle, whilst important is hot the only major factor. Stiffness also depends on the rigidity of other components such as the shaft and work holding arrangement. Recently, computer models for determining the elastic deformation of a bearing – shaft system have enabled better optimisation of the spindle layout, minimising the cantilevering effect of an overhung work position. The models have also been vital in improving the dynamic behaviour of spindles.

The data they provide enables critical speeds in the operating speed range of the spindle to be avoided and dynamic stiffness at the work position to be optimised by judicial choice of system parameters.

An example showing the radial dynamic response of a lens turning spindle measured at the work position is featured in Figure 1. The spindle operates at speeds of up to 10,000 rpm and the first critical speed occurs at 295 Hz. The magnitude of the response at 610 Hz defines the dynamic stiffness of the spindle, which in this case-is 5.3 N/μm and compares to a static stiffness of 17.2 N/μm. The significance of these figures is that the dynamic stiffness of an air spindle is predominantly a function of bearing damping and the typical response of Figure 1 shows this to be high compared to the level of damping found in most mechanical structures.

Figure 1 Example of air spindle’s radial dynamic response

Motion error

In addition to their role in providing spindle stiffness and damping, bearings are also a factor in the penultimate area of concentration for recent air bearing developments: motion error. In air bearing spindles this is often characterised by three features: eccentricity, higher order synchronous motion and asynchronous motion errors. In general, these are primarily a function of such factors as geometrical errors within the bearings, magnetic imbalances in the spindle drive motor and mechanical out of balance.

Many ultra precision machining applications now require peak-peak motion errors better than 0.05 μm, so addressing of all these sources is critically important. Basic eccentricities in motion error are normally addressed by attention to bearing alignments and out of balance, while reducing synchronous and asynchronous motion requires a combination of attention to air flows and bearing geometry.

One of the major factors in ensuring low asynchronous motion error is an air supply that is free from pressure pulses and contaminants. Minimising air turbulence within the bearing and random pressure fluctuations in the exhaust air also helps in this respect. In contrast, synchronous motion errors arise if the bearing geometry departs from perfect round, (or flat in the case of a thrust bearing). The air film in an air bearing averages the effect of bearing geometry errors to a low level and ratios of motion error/ geometry error as low as 0.05 have been observed in practice. However, sub micron precision of bearing surfaces is still required to meet most ultra precision motion specifications. Despite these problems, improvements in manufacturing methods have had an important effect on motion error and have enabled errors to be reduced from about 0.2 μm (typically) on commercial machining spindles 20 years ago to the present level of 0.05 μm peak-peak.

Whereas motion errors in air bearings contain both synchronous and asynchronous components, spindle drive motors are usually dominated by the former. In the air bearing spindle motor the magnetic forces necessary to produce required drive torques are high and although the motors have poles arranged opposite each other to balance out the forces there are several factors which can leave a residual magnetic imbalance. Therefore, on air spindles fitted with integral frameless motors the motion errors that can result are determined by the magnitude of the forces and the stiffness of the air bearings.

An alternative configuration in high precision air bearing spindles is the use of a self-contained precision motor mounted inline at the rear of the spindle. Such motors benefit from the additional stiffness of precision grade ball bearings, which are effective in limiting the motion errors due to magnetic forces. The ball bearings can, of course, still be a source of motion error, but the vibration forces are generally small compared to those, which would otherwise result from magnetic imbalance and are isolated from the air spindle by a flexible coupling.

Over the last 10 years much development work has been undertaken with the objective of reducing magnetic imbalance on both framed and frameless type motors. In addition, motor design and manufacturing methods have improved significantly over the period. The net result of this work is that both types of motors are now available with imbalanced magnetic forces of less than 1 N.

Thermal distortion

In contrast to the relative complexity of the development work to reduce motion errors, the initiatives to limit heat build-up and – resulting thermal growth are comparatively straightforward. In standard high precision machining spindles thermal growth is limited by locating the thrust bearing closer to the work position and placing the main heat source the drive motor, towards the spindle rear. However, in more critical applications, forced cooling is required to gain control of thermal growth. The type and degree of cooling is normally matched to application requirements. Chilled water-cooling affords the best results, allowing changes in spindle dimension s with time, to be kept as low as 1 μm.

Effectiveness of new developments exemplified in new spindle designs

The effectiveness of the recent developments in air bearing spindle technology is evident from the new designs that have emerged as a result of the work. These include high precision spindles for turning contact lenses, cup grinding and diamond turning. All are particularly noteworthy for the levels of precision they provide. However even among spindles that achieve surface finishes better than 0.l μm Ra, the cup grinding spindle stands out.

The spindle (see Plates 1 and 2) is fitted to Cranfield University’s Tetraform “C”, a grinding machine that is required to achieve optical surface finishes from grinding of both brittle materials and hard metallic surfaces at relatively high material removal rates. In terms of spindle performance these requirements demand high static and dynamic stiffness and low motion errors over a wide speed range up to 6,000 rpm. A loadpoint spindle provides these qualities, with the result that, in the grinding of materials such as glass, silicon and M50 tool steel, it is achieving surface finishes consistently better than 10 nm Ra and as low as 1 nm Ra.

Plate 1 Example of Loadpoint spindle fitted to Cranfield Universities Tetraform ‘C’ grinding machine for optical surface finishes

Plate 2 Loadpoint diamond turning spindle with integral slide way

The future

In terms of precision the question with air bearing spindles appears to be: “how low can you go”?

At present there is no definitive answer to this question, just in the same way that no one can be quite certain of how minute integrated circuits will become in their final evolution. What is certain, however, is that developments with air bearing spindles are not yet at their peak, and that ongoing improvements in some of the key areas mentioned in this article are consistently redefining the boundaries of what can be achieved.

Details available from: Loadpoint Ltd, Tel: +44 (0)1793 751160; Fax: +44 (0)1793 750155; E-mail: all@loadpoint.co.uk; Web site: www.loadpoint.co.uk