Turning to higher accuracies

Tom Shelley reports on a visit to the UK's leading maker of very high precision machine tools


Bearings with the highest degrees of precision, destined for nano-precision machine tools, are in most cases hydrostatically supported, with aerodynamic support available for the highest speeds at the expense of load carrying. It is currently possible to reduce error motions and rotational accuracy to no more than 50nm, but new designs using water should halve this.
Economic forces demand that metals be machined to ever-greater degrees of precision. Gearbox, engine and jet engine components made to tighter tolerances remove select-on-assembly requirements and enhance functional efficiency. And the degree of precision achieved in the manufacture of computer hard disks and their bearings determines how much data they can store. Digital readouts may indicate resolutions of 100nm or even 10nm, but actually machining to such accuracies with conventional machine tools ranges from extremely difficult to completely impossible.
Cranfield Precision has for some years specialised in machine tools capable of the very highest degrees of precision for the aerospace, defence, optical and electronic industries. But R&D engineer Mike Pierse explains that even ABEC 7 and 9 precision ball bearings can only achieve rotational accuracies of 0.5 microns at best. Rolling element bearings are limited by constant changes in the relative positions of balls or rollers, shaft and raceway, and by elastic deformation of raceways as the balls are pressed into them.
Fluid film bearings, on the other hand, only depend on the roundness of the bearing bore, and much more importantly, the rotor. In addition, according to Pierse, "the squeeze film effect leads to enormous hydrostatic damping". Given the precision achieved at Cranfield, the engineers have had to consider even tiny effects such as those resulting from rotors having been held in a three jaw chuck when machined, and interaction between the resulting triple lobes and different numbers of fluid pockets. Values for calculated 'shape ratios' include 16% for three pockets, 41% for four pockets, and 5% for five pockets, the preferred choice at Cranfield. In theory, seven pockets should achieve 2.5% and 11 pockets, 1%. The pockets include plates to ensure the fluid flowing round them is laminar and not turbulent, for maximum thermal efficiency.
The team has studied a wide range of different working fluids. An ideal fluid needs to have a low viscosity to reduce heat generation, and a high heat capacity, to take heat away. It also needs to be non-volatile, not easily inflammable and non-toxic. It should come as no great surprise that the ideal fluid turns out to be water. The only snag is that it requires components in contact with it to be resistant to corrosion.
A favourite measure of bearing spindle performance is its DN ratio (diameter in mm x speed in rpm). Air bearings can be made with DN ratios of up to 3 million, but at the expense of stiffness. Highest tilt and axial stiffness is achieved with inside-out bearings in which a hub rotates around a fixed shaft. Such bearings have been made at Cranfield for a machine for the bearing industry that was actually belt driven. The wheel spindle DN ratio was 340,000 and the work spindle, 520,000. A hydrostatic cam grinding spindle has been made with a DN ratio of 1,200,000 and a target DN of two million has been set for a spindle being developed in conjunction with Cranfield's School of Industrial and Manufacturing Science. In this particular instance, the shaft is supported on water pumped in through the sides of a porous ceramic sleeve. The target for rotational accuracy is 25nm, which should be achievable because of the absence of pockets and the near infinite number of restrictors.
One of the challenges when making spindles of very great precision is measuring exactly how smoothly and accurately they rotate. Conventional probes are totally inadequate, so Cranfield uses a non-contact capacitance probe in its Spindle Error Analyser. This has a precision ball as its probe element. Stand off distance is 50 microns, and the machine can detect 16 undulations at 100,000rpm, or 160 at 10,000rpm with a 25nm resolution.
Edric Brown, Division Manager
Mike Pierse, Engineer - R&D Projects
Pointers
: Hydrostatic machine tool spindle bearings can be made which achieve rotation accuracies down to 50nm and DN ratios of up to 1,200,000
: New developments using water as a working fluid are expected to improve these figures to 25nm and two million respectively

Author
Tom Shelley

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