Precision machining workshop

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7 - 1 0 June 1982, St Paul, USA

Precision machining workshop If its reputation is any measure, Williamsburg in 1978 was always going to be a tough act to follow. Follow it they did; Jim Bryan and SME both deserve credit for organising an event with user participation, controversy, and challenging concepts all bound together with a smattering of practical ideas which could be applied to producing precision parts. The technical presentations can be split into three main categories; diamond turning machine design; alternatives for non-diamond turnable materials; and metrology. The miscellany that completed the programme also provided some of the highlights.

allow the developers of innovative machines time to talk about their own project. If each believed that he had a message, McKeown had two advantages in getting his across: he was first to the podium and he was talking about a machine which has removed metal. The vertical axis machine developed at Cranfield 1-3 has been designed specifically to produce X-ray telescope mirrors. These internal conicoids up to 1.4 m in diameter with a 0.6 m axial length are required to have: •

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Machine design Rather than presenting one man's view of the "latest and best" ideas, the sessions on diamond turning machine design were deliberately structured to

waviness of the axial profile with an amplitude less than 6 nm over 30 mm overall conicoidal form to within 1 /~m departures from circularity less than 1/lm

Without meaning to denigrate in any way the technological aspects of the machine, by far the most impressive aspect is the speed with which it has

been designed and constructed. It comes as little surprise, then, that in meeting a 22 month schedule, the CUPE team adopted many of the techniques with which they are most familiar; their own 0.1 jum resolution optical grating displacement transducers, synthetic granite, and the CUPROC 16 bit microprocessor. The machine is due to be installed in a purpose built facility at Cranfield shortly and it is hoped that a detailed appraisal of its performance will be published in due course. Diamond turning has long been a key process in Fresnel lens die production at OSG4. Bill Bryan described a 70 inch (1.75 m) horizontal axis machine s designed to handle parts weighing up to 4000 Ibs (1800 kg) with a z-axis travel of 30 inches (0.75 m). Built by Moore Special Tool 6, the stacked slide machine incorporates double vee rollerways, direct dc servo motors with lead screws,

Fig 1 The vertical axis diamond turning machine at CUPE

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J A N 1 9 8 3 V O L 5 NO 1

and laser interferometer displacement feedback. The hydrostatic bearing spindle was essential to support part loads while allowing speeds up to 2000 r/min. The stacked slide arrangement leads to a requirement that the ends of the X-slide bridge by supported. This is accomplished using hydrostatic flat bearings externally compensated to provide infinite stiffness oil films. It was this presentation which introduced some of the first practical hints; for example, the high pressure oil supply system can excite machine vibrations if flow in the pipe is turbulent. The presentation also contained the understatement of the week: a 70 inch, 4000 Ib part rotating at 2000 r/min was described as "getting exciting". Pardue lead the invasion of the acronyms with his progress report 7 on POMA, the 'point one micron accuracy" project at Y-12. Briefly, the program aims to provide the US Air Force Weapons Laboratory (AFWL) with a source of diamond turned optics up to 0.8 m in diameter. The design philosophy adopted in POMA (and, one presumes, in every other precision machine) was to: "Reduce the sources of error to as few as possible and reduce the remaining sources of error to acceptable levels". The implementation of this philosophy is that POMA relies entirely on two ULE reference bars located in the cutting plane. Metrology of these reference bars provides a significant challenge. At the time of the conference, repeatability studies on POMA were due to start "within a short period of time". Let us hope that, once again, a detailed description of the design and an assessment of performance will be published in due course. Hauschildt reported progess on LODTM (Large Optics Diamond Turning Machine), the 60 inch (1.5 m) diameter vertical spindle machine currently under construction at Lawrence Livermore National Laboratory (LLNL). Designed to handle components weighing up to 3000 Ibs (1350 kg) and up to 20 inch (0.5 m) axial height which are radically aspheric, the machine has error budgets quoted as 1.1/zin rms radial figure error, 0.5/~in peak-to-valley spindle error and 0.5 gin Ra surface roughness.

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Fig 2 70 inch diamond turning machine It will be fascinating to hear further progress reports. Carter reported progress on DTM 3 the design of which was described in detail in an earlier issues . It had seemed that obtaining smooth motion at very low speeds with the traction drives might prove difficult; Carter reported, however, that the drive will operate at 1 revolution per year! Another horizontal axis machine, this time with a capacity of 60 inches (1.5 m) was described by Rhorer. Details have appeared earlier 9 of the design which includes facilities for two interferometers on the Z-axis. Unfortunately, the software for this option has not been completed and no results are, therefore, available. The final part of the section on machine design was turned over to commercially available turn-key machine descriptions ~'1°'1~ . Kobayashi 12 surveyed diamond turning machine developments in Japan although, unfortunately, he did not describe specific machines in any great detail. Rather, he reported on research programmes, such as cutting tests on a range of plastics and investigations of the surface roughness obtained in a variety of conditions, and on components, such as the development of spindle bearings, the use of a hydrostatic screw etc.

Alternatives Casstevens opened the session titled 'Alternatives for non-diamond turn-

able materials' by talking about diamond turning; in particular he described diamond turning of steel in a carbon saturated atmosphere (a paper on the subject appears on pages 9 - 1 5 of this issue). Two papers focussed on precision grinding. Hahn 13 noted that grinding bodies of revolution to 5 micron tolerances is common industrial practice, but that 0.5 micron tolerances are not readily achievable despite the capability of obtaining positional repeatabilities in grinding machines to 0.25/~m or better. He suggested that incorporation of force sensors in work or wheel spindle would ultimately allow deflections to be calculated and compensated. Miyashita et a114 described the design of a grinding machine which has given considerable improvements in the surface finish obtained on brittle materials such as quartz. Attention then turned to alternative tool materials, in particular cubic boron nitride (cbn). Hauschildt reported that beryllium parts have been turned to contour tolerances of 0.5/Jm and finish better than 0.1/Jm Ra using cbn tools at LLNL. The dimensions of parts on which this has been achieved is not clear, although some indication may be drawn from the discussion of BODTM Is in the presentation. Commercial suppliers of cbn tooling were given the chance to extoll the virtues of their products - and did so without degenerating into a slanging match. None the less,

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some damage was done to the English language; one answer to crossquestioning about the feasibility of producing single crystal cbn tools was: "It is probably p o s s i b l e . . . " Plating has long provided a resolution of the conflicting requirements of surface and substrate. From Dini's presentation, and the discussion that followed, it became clear that plating is more an art than a science. The most important piece of practical advice seemed to be "Get to know your local platerf".

Metrology In the first metrology presentation, Teague discussed light scattering from manufactured surfaces. In a talk that drew heavily on the recent CI RP paper 16, he emphasised the discrepancy between the range of surface typographies of practical interest and the types of surface for which adequate theories are available. Although most light scattering techniques cannot be considered metrological tools, their value, or potential value, as on-line comparators should not be overlooked l~ . Layer addressed the question "lodine-stabilised lasers - are they really for use on machine tools?" His talk made it amply clear that the answer is yes (which must have come as a relief to the designers of LODTM who, two days before, had stated confidently that they would be using such lasers). In practice, the system showing great promise for such applications uses both iodine stabilisers and the standard HP laser to obtain the better features of the performance of each, ie the uncertainty and short-term stability of the iodine stabilised laser and the long term stability of the commercially available system. Bryan described the magnetic ball bar 18 , in its various configurations, and its applications to measuring machine and machine tool testing. Benjamin ~9 urged caution in first appraisal of diamond turned optics from judgement of a reflected image; he explained why concave surfaces tend to appear worse than convex surfaces of equivalent finish in such tests. Goodwin 2° described the use of castable rubber replicas for extracting "hard to get at" part features for

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measurement. Measurement has been attention is currently being paid to improving the quality of the replicas themselves.

automated and

Oil showered measuring machine Bryan eta/ 2~ described the up-grading of a Moore 3 Measuring Machine to improve its accuracy capability by an order of magnitude. They believe that they have now the world's first (and only?) oil showered general purpose measuring machine. The machine, built in 1960, is a plane way machine using the doublevee concept and X- and Y-slide travels of 18 and 11 inches (450 and 275 mm) respectively and a Z slide travel of 16 inches (400 mm). The Z slide has been converted to hydrostatic bearings. Displacement of the X and Y slides in the original design was measured by reading the graduated drum attached to the end of the precision lead screw while the Z axis had no displacement measuring device. Laser interferometry has been added to the machine for displacement measurement of all three axes. A rotary table mounted on the X-slide allows use not only as a 3-axis coordinate measuring machine but also as a 'Y-Z' or 'X-Z' measuring machine. In this mode, axisymmetric parts can be rotated on the C-axis to produce circumferential sweeps. The electronic gauge stylus is typically a ball or spherical radius tipped, single axis, linear, variable-displacement transducer (Ivdt) carried and positionid by the vertical Z-slide. Fig 3 shows the liquid showered machine. Starting at the machine sump, the oil flows through a filter located on the suction side of the pump, through the pump to a heat exchanger, onto the machine and back to the sump. The sump is a part of the collection base. To avoid transmission of vibration from the pump and motors to the isolated machine, the suction pipe is cantilevered from the floor and does not physically contact the sump or machine. The same concept is applied to the plumbing which distributes oil over the machine. The oil used for the liquid shower is a light weight mineral-base oil with a viscosity high enough to limit splash and airborne droplets

while flowing readily over the surface of the machine. Various pieces of sheet metal and wire screen are used to direct and smooth the flow and minimize splatter. The oil is pumped by a commercial 2 HP integral pump-motor unit. The pump is a constant displacement, rotary screw-type pump which is quiet and capable of delivering 40 gallons per minute at 20 psi supply pressure. This pressure is adequate to overcome line losses and provide good distribution pressure at the shower nozzles. The circulating oil is heated by the pump. Therefore, it is possible to control the oil temperature by cooling only in a shell and tube heat exchanger. The oil temperature at the exchanger outlet is sensed by a bare-bead thermister. An on-off controller actuates a solenoid valve to admit chilled water when the oil temperature exceeds 68 o F and to bypass it when the temperature is below 68 o F. It was found that the LLNL chilled water supply temperature varies as much as 5 ° F. This causes a variation in oil temperature of 0.05 °F. A second shell and tube heat exchanger and an on-off control is used to control the temperature of the chilled water. In this case, heat is added intermitently to the chilled water. Plant cooling tower water supplies the heat. The result is that the average temperature of the circulating oil is held constant to -+0.01 °F. The temperature oscillations of the oil have an amplitude of 0 . 0 8 ° F and a frequency of about 10 cycles per minute (6 seconds per cycle). At this frequency, the thermal inertia of the machine limits machine distortion.

Measuring machine performance For a drift test to be meaningful, it should include a typical test part in the structural loop, and people in the vicinity of the machine during the test. It should also be set up in a direction most sensitive to temperature changes. For the drift test on the Moore Measuring Machine, an 8 inch long aluminium cylinder was chosen. Aluminium, with a thermal coefficient twice that of steel provides a compromise between steel and plastics. A person was working around the machine during the test to provide

J A N 1983 V O L 5 N O 1

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-*4 Fig 3 Oil showered measuring machine. Placement of laser pathways satisfies the Abbe principle in the Z~xis and minimises off-set in X and Y.

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the additional amount of heat load to the machine that an operator would provide in normal use. The Z-axis of this machine is most sensitive to temperature changes due to geometry. The drift test was therefore set up in the Zdirection. The results (Figs 4 and 5) speak for themselves.

O t h e r features The accuracy of measurement was improved by adding laser interferometry to all 3 axes of the machine. A Hewlett-Packard 5501 machine tool laser was mounted in a casting mounted on the rear of the column. The beam is split 3 ways to feed the 3 axes. The plane mirror measuring system was used to obtain a 0.5 microinch resolution. The paths of the measuring beam were chosen to minimize Abbe errors. The laser paths are enclosed in either a casting, a pipe or a covered enclosure depending upon the location of the machine. The pathways contain room air and are slowly purged with filtered air to discourage the intrusion of oil. The intensity of the beam is severely lowered when oil is deposited on the optics. Enough oil will cause the system to fail. When the proper techniques of sealing and oil shower management are applied, reliability is very high. The successful application of laser interferometry to this machine, in terms of accuracy, is due to total thermal control. Adding a laser system would otherwise be counter productive. Most of the laser mirror and interferometer brackets are external to the machine's massive castings which protect the leadscrews. Exposed brackets quickly distort when exposed to temperature changes. These distortions are likely to cause measurement errors exceeding those of the original leadscrew. The temperature controlled oil shower eliminated thermal distortions. The Ivdt mentioned earlier is an air bearing type. The air vearing eliminates coulomb friction, a requirement when the direction of Ivdt travel is not normal to the part surface. This Idvt also uses air pressure to maintain stylus force. The typical stylus force is 0.2 grams. Even when the stylus tip

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radius is as large as 0.1 inch at a force of 0.2 grams, the oil does not keep the ball from following the part contour. Bryan and his team have found that the presence of flowing oil on the part is not a problem in contact gauging. Positioning and holding a part on the rotary table can be difficult in the presence of oil, especially when the parts are small or of relatively low density. The oil tends to get under the part and lift it. In this case a vacuum chuck is used. Centering is accomplished with an X-Y micrometer stage that has built-in piezo-electric crystals. The part can first be roughly centered with the micrometer dial and then finely centered 'hands-off' by adjusting the voltage supply to the piezo crystals. Adjustment of tilt is built into the rotary table top. As stated earlier the Z slide's plane ways were replaced with hydrostatic bearings. The weight of the Z slide is held by a roller chain which moves over 2 sprockets to a counterweight inside the column. One of the sprockets is driven by a handwheel on the side of the machine. The oil hydrostatic bearings have the advantage of eliminating static friction, are stiff, and are relatively easy to retrofit to the geometry. The leaking oil from the bearings is of no consequence since the machine is oil showered. The bearings are fed with a small portion of the filtered shower oil which is directed to these bearings through a high pressure pump. Bryan et a! conclude that accuracy and repeatability errors in a measuring machine should be significantly less than the errors in the part being measured. At LLNL, for the past 12 years, the temperature controlled diamond turning lathes have produced parts which had errors less than the measuring machines used for inspection. There has been, and will be, psychological resistance to oil showered inspection machines. Bryan et al believe that this initial resistance will be overcome by the need to progress and the lack of "practical alternatives".

Brute strength debate As regular readers of this journal will know 22 Bryan believes in the 'brute strenght' approach; that is to "directly address the heart of a problem with the intent to maintain the ultimate in

system simplicity". The most Often quoted examples are liquid shower temperature control and corrective lapping of lead-screws. Hocken argues that computer based error correction techniques meet all of Bryan's criteria. Predictably, the formal debate and ensuing discussion of this subject was entertaining as well as stimulating. One protagonist suggested that one does well to think of an error map much like a mechanical component; for example it distorts with temperature changes and wears. Perhaps the best summary of the debate was one delegate's remark that: "The better it is before you do error corrections, the better it will be afterwards", while adding the confessional caveat "provided you get the signs right in all the correction terms."

Horror stories Loewen introduced an almost spiritual aspect to the meeting with his "Horrorabilia" session which had all the characteristics of a very public confessional. As speakers left the dais it was clear that, for many, a great weight had been lifted from their hearts. The session was built on the concept that "Misery loves company"; the 'ideal' horrorabilia involved considerable manpower being devoted for a long period in search of a solution, which when found, was blindingty obvious. One example described was a test rig for which the vibration characteristics were critical. Significant effort was expended, at the design stage, on modal analysis etc. The rig was built and the vibration performance was awful; design calculations were checked and re-checked, instruments recalibrated, and 'new' ideas and suggestions sought from everyone. In desperation the rig was eventually redesigned; to make the major modifications, a partial teardown was necessary and it was then that it was discovered that the nuts holding the top cross-member were only "finger-tight". Another example was the team, relatively new to diamond turning, who were unable to get a decent surface finish. They discussed the problem, standing next to the machine, for hours without finding the solution!

J A N 1 9 8 3 V.OL 5 N O 1

turning machine. CI RP Annals, Volume 31 (1982)

When they w e n t for lunch, finish improved dramatically. Many other examples were presented, Sadly, however, it is not possible to extract from them a c o m m o n thread, be it a c o m m o n cause or a c o m m o n reason w h y it takes so long to find the answers.

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Moore Special Tool Co, 800 Union Ave, Bridgeport, CT 06607, USA

Comments

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Pardue R.M. POMA - A progress

It is not possible to summarise the meeting in a concise phrase. Practical ideas rubbed shoulders w i t h philosohpical debates (are random results always the consequence of random procedures or are there naturally rand o m processes and events?). I was entertained, stimulated, and i n f o r m e d ; what more can one ask of a conference? C. Evans

References 1.

2.

It is hoped that a paper on the machine will appear in this journal once it is installed and fully functional in its purpose built laboratory at Cranfield. See Ref 2 and 3 for published information McKeown P.A., Wills-Moren W.J., Read R.F.J., and Modjarrad H. The

design and development of a large ultra precision cnc diamond turning machine. SME Technical Paper MR82-931 3.

Wills-Moren W.J., Modjarrad H., Read R.F.J. and McKeown P.A. Some aspects

of the design and development of a large, high precision cnc diamond

4,

Optical Sciences Group, Petaluma, CA, USA

5.

Bryan W. Design and construction of a 70-inch diamond turning machine. SME Technical Paper MR82-935

report. Union Carbide Y/DX-424, June 1, 1982 8.

Bryan J.B. Design and Construction of an Ultraprecision 84 inch Diamond Turning Machine. Precision Engineering (Jan 1979) 1, I, 13--17 Anon. More diamond turning machines. Precision Engineering (October 1980) 2, 4, 225-227

10. Pneumo Precision, Precision Park, Keene, NH 03431 11. Kollmorgen Corporation, Wallingford, CT 06492 12. Kobayashi A. Diamond turning machine developments in Japan. SME Technical Paper MR82-929 13. Hahn R.S. Precision grinding bodies of revolution: an alternative to diamond turning. SME Technical Paper MR82928 14.

15.

Thompson D.C., Chrislock J.L. and Newton L.E. Development of an

inexpensive, high accuracy diamond turning machine. Precision Engineering (April 1982) 4, 2, 73--77 16. TeagueE.C., Vorburger T.V. and Maystre D. Light scattering from manufactured surfaces. CI RP Annals, Volume 30 (1981) 17. TeagueE.C. and Vorburger T.V. Optical techniques for on-line measurement of surface topography. Precision Engineering (April 1981) 3, 2, 6 1 - 8 3 18. Bryan J.B. A simple method for testing machines and machine tools: Part 1 Principles and applications. Precision Engineering (April 1982) 4, 2, 6 1 - 6 9 Part 2: Construction details. Precision Engineering (July 1982) 4, 3, 125-- 138 19. Benjamin R.J. Judging surface roughness of diamond turned parts by age can be misleading. SME Technical Paper FC82-934 20. Goodwin W.L. Use of castable rubber replicas for measurement of small dimensions. Union Carbide Report Y/DP-1827, May 18, 1982 21. Bryan J.B., Carter D.L., Clouser R.W. and Hamilton J.H. An order of magnitude improvement in thermal stability with use of liquid shower on a general purpose measuring machine. SME Technical Paper MR82-936

Yoshioka J., Kozumi K., Shimizu M., 22. Bryan J.B. The benefits ()f brute strength Yoshikawa H., Miyashita M., and Kanai A. (Comment). Precision Engineering

Surface grinding with a newly developed ultra-precision grinding machine. SME Technical Paper MR82-

(October 1980) 2, 4, 173. See also ' Letters' Precision Engineering (January 1981) 3, 1, 4 - 6

Precision balls Precision tungsten carbide balls are a critical part of the flight and landing control systems in Europe's latest multi-role c o m b a t aircraft, the Tornado. More than 1700 such balls are used in the 12 screw jacks used in the aircrafts variable nozzle and thrust reversal activation systems. Developed by Lucas Aerospace Ltd, these actuators are subjected tp extremely severe operating conditions at elevated temperature which preclude the use of conventional forms of lubrication. In the screw jack mechanisms, 3 m m diameter balls made by Spheric Engineering Ltd f r o m Wimet grade WN40 cemented carbide are used for the recirculating ball screw assembly which operates the reverse thrust deflectors. Similar units actuate the variable nozzle mechanism. Each engine employs t w o reverse thrust jacks and four variable nozzle jacks,

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each jack having a total of 140 cemented carbide balls. Power to these jacks is provided by a pneumatically driven m o t o r supplied f r o m the high pressure compressor of the engine and each p u m p is f i t t e d w i t h 12 Wimet grade WN20 cemented carbide balls. This use of carbide balls provided the solution to w h a t was originally a d i f f i c u l t design problem. Steel balls were employed for the p r o t o t y p e jacks, but the high temperature environment and heavy Ioadings applied resulted in rapid failure. Conventional forms of lubrication were totally unsuitable because of the heat problem, and until carbide balls were considered, the only alternative appeared t o be a major design modification to re-position the jack actuating mechanism. This w o u l d have created a more complex and heavier assembly. Sandvik supply cemented carbide

ball blanks for a wide variety of uses to Spheric Engineering. The sintered blanks are produced by pressing of the basic tungsten carbide and cobalt powders, f o l l o w e d by pre-sintering, sintering under stringently controlled conditions and finally, hot isostatic pressing. The final process is claimed to eliminate any possibility of m i c r o p o r o s i t y and provide a product of virtually 100% density. The range of tungsten carbide balls offered by Spheric Engineering covers sizes f r o m 0.5 m m diameter to 80 m m diameter and the application potential is enormous. Typical tolerances are said t o be 0.13/~m. Sandvik Wimet Wear Parts Ltd, PO Box 1 10, T o r r i n g t o n Avenue, Coventry, UK; Spheric Engineering Ltd, Fleming Way, Manor Royal, Crawley, Sussex, UK, RH10 2SQ

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