More diamond turning machines

More diamond turning machines Traditionally, high precision diamond turning machines have been individually designed to the end user's particular specification, although in many cases these special purpose designs have been based on 'standard' machines. Moore Special Tool Company* recognised that many of the growing spectrum of applications for these special purpose machines could be accommodated on a relatively standard diamond machining centre and that elimination of individual design and specification could give considerable cost advantages. The first batch of four of these machining centres has been built and delivered and a second batch of six will be produced during 1981.

horizontally or vertically over their full travels and the best f i t straight lines through the straightness profiles are square to within 1 arc-sec. Total yaw of the slides is less than 1 arc-sec over their full travels. Total radial run out of the C axis is specified as not more than

0.25/~m (10/~in) through 360 ° of rotation at the spindle centre line when moving at a speed of 1 rev/min or less. The maximum positioning error of this axis is specified as 10 arc-sec between'any two points. The 250 mm (10 in) air bearing

D i a m o n d m a c h i n i n g centre The M-18 Ultra Precision, Continuous Path, Numerically Controlled, Horizontal Turning, Single Point Diamond Turning and Aspheric Generating Machine (Fig 1) can accommodate workpieces up to 400 mm (16 in) in diameter with a spindle load of up to 90 kg (200 Ib). The machine, available with encoders or laser interferometers, has two linear slide motions (X,Y) in a stacked slide configuration and a rotary (C) motion to provide continuous path contouring in a variety of modes. Maxi mum X axis travel (perpendicular to the spindle axis) is 450 mm (18 in) and 275 mm (11 in) in the Y axis. These figures are reduced to 400 mm (16 in) and 225 mm (9 in) respectively if laser interferometers are fitted. Specifications for the mechanical accuracy (at 20°C) include a maximum positioning error of 3 / l m (120/~in) from any point to any other point in the full travel of the X and Y slides moving under tape control in either direction without rezeroing and a maximum of 0.8/~m in any 25 mm (30/~in in any inch). The maximum separation between two parallel straight lines which just include the straightness profile of either slide does not exceed 0.5/lm (20/~in) *Moore Special Tool Company Inc, 800 Union Avenue, Bridgeport, Connecticut 06607, USA

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Fig I Moore diamond turning and aspheric generating machine

Fig 2 High speed jig grinding head for glass and metal grinding

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PIODUCTION POXINT$ spindle has a speed range of 50-2000 rev/min. Maximum spindle errors are specified as 0.08/~m (3/1in) tir radial, 0.05/~m (2/~in) axial, and 0.2/~rad angular change of spindle axis. As well as tooling for single point diamond turning, Moore supply one of their standard high speed jig

grinding heads (Fig 2) and a suitable coolant supply system for glass and metal grinding. 1.0 m c a p a c i t y Introduction of the diamond machining centre concept does not mean that Moore have renounced their interest in

Fig 3 Laser interferometers on either side of the spindle will provide Z axis yaw compensation for the LASL machine built on a Moore Number 5 measuring machine base

Fig 4 Layout of the laser interferometer system

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producing one-off special purpose machines. Far from it; they have recently delivered a diamond turning machine (Fig 3) catering for components up to 1.0 m (40 in) in diameter. This machine, built on a Moore Number 5 measuring machine base, is now at the Los Alamos Scientific Laboratories + (LASL) in New Mexico. A 150 mm (6 in) radius spherical air bearing spindle manufactured by Pneumo Precision ~ is mounted on the large table of the measuring machine base to provide a 750 mm (30 in) Z axis travel and a Moore rotary table mounted on the bridge table provides a 750 mm X axis motion. A Hewlett Packard 5501 laser interferometer system (Fig 4) provides 0.025/~m (1/1in) pulses for feed back to an Allen Bradley 7320 controller. Brackets on each side of the spindle hold the retroreflectors for measuring Z axis motions, while still providing the necessary clearance for the workpiece. The interferometer paths are in tubes with a split in the bottom and soft flexible wiper seals. Both axes are driven by Inland ten pole permanent magnet dc servo motors on the end of the lead screws. The basic machine is installed at LASL and producing components. The acceptance test results given below are on the basic machine with only one Z axis interferometer, although LASL plan a number of modifications to improve machine performance. In a ring test the machine was programmed to contour around a 300 mm (12 in) master disc x with an electronic indicator mounted on the rotary table, which is programmed to rotate the probe so that it is always normal to the disc. The results (Fig 5) indicate a maximum error of about 2/lm (80/~in). The element of nonroundness in the ring test data could be explained if there is a positioning error in either axis. Fig 6 shows the results of X axis positioning tests using a 600 mm Moore step gauge +Los Alamos Scientific Laboratories, PO Box 1663, Los Alamos, New Mexico 87545, USA ~Pneumo Precision Inc, Precision Park, Keene, New Hampshir~03431, USA XOriginally made by Moore Special Tool Company for Bendix, KansasCity, and borrowed from them to check the machine

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P' ',ODUCT ON POINTS centred on the X axis travel. The slope of the error in the first 300 mm (12 in) of travel can be explained and compensated for by laser compensation. The non-linear effect in the second 300 mm is a sizeable positioning error, probably related to movement of the laser brackets due to bridge or base deflections as the bridge nears the end of its travel. A software solution to this repeatable effect is being developed. Other parameters measured during acceptance testing are axis straightness to about 0.75 #m in 300 mm, squareness of X to Z of 0.5 arc-sec using the centre 300 mm of both axes, and spindle errors without the" belt drive of 0.1/~m axial and 0.15/~m radial.

180° m= + X Direction

+ Z Direction

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Machine improvements Developments planned by LASL to improve machine performance include the addition of axis calibration to the Allen Bradley controller and a vibration isolation system. LASL will also develop the software needed for the dual Z axis laser interferometers to provide yaw compensation. For any diamond turning machine, thermal stability is crucial. From the outset LASL recognised that temperature control would be necessary to improve on the original accuracy specifications, which Moore satisfied, and will be adding an oil shower system. In addition, this will eliminate the effect of higher than normal sensitivity to temperature fluctuations inherent in mounting the retroreflectors for 2-axis compensation on long arms. Finally, LASL plan to provide a metrology frame system for straight edges and to support the laser components. They believe that this is probably the only way of obtaining the ultimate accuracy of the machine.

PRECISION ENGINEERING

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Fig 5 Ring test results (taken from pre-print LA-UR 80-929)

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Fig 6 X axis positioning errors (taken from pre-print LA-UR 80-929)

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