Perfect vision with CAD
The Engineer’s Digest, June 1986
Perfect vision with CAD
Precision means different things to different people. But machining small hemispherical parts only 35 microns thick to a tolerance of 1 micron is ultra-precise by any engineering standards. And when you take in the complex geometrical forms involved and the demands for high productivity and consistent quality, the performance of the Polytech Contact Lens Turning System is quite remarkable.
Designed and built by Contact Lens Supplies (CLS) in High Wycombe, the Polytech is a CNC turning machine which took nearly four years to develop. A prototype was introduced back in 1981 and to date 20 lathes have been built. As well as using the machine itself for contact lens production, CLS sells the Polytech system world-wide with installations in Japan and the USA as well as in Western Europe.
There’s no doubt that Contact Lens Supplies has a lead in the technology of turning plastic (special polymers of perspex like HMA) contact lenses with the Polytech CNC machine. A key feature in the continuing development processes of the diamond tooling used on the machine, and contact lens design itself, is computer-aided design (CAD). The system selected to help CLS maintain its competitive edge in the field of both contact lens design and production is AutoCAD, a general-purpose 2D, micro-based package from Autodesk.
AutoCAD was bought in September ’84 during a visit to the Design Engineering show at the NEC in Birmingham. Ian Handricks, EDP Manager at CLS, initially went to the show to look at computerised inventory control systems but after seeing a demonstration on the AutoCAD stand (the first time, incidentally, he had ever seen CAD in action), he was convinced that micro-CAD could really optimise the . company’s development programme. He immediately introduced a series of benchmark tests on lens design (such as bringing curves together to see the junction and shifting the curves to different positions) results were very encouraging.
So AutoCAD was ordered directly from the Design Engineering Show and used on an IBM PC XT and other items of hardware for lens design, engineering drawings and preparing R&D projects for intra-ocular lens work (where the contact lens is actually stitched into the eye). Ian Handricks saw AutoCAD as; . . . ’Giving us the only opportunity to represent and simulate what the Polytech machine actually does’. To appreciate this, it’s worth examining the configuration of the Polytech turning machine to see just how.far it is technologically ahead of conventional contact lens turning machines.
The Polytech system offers flexibility in lens design, manufacture, quality and reproducibility of product which cannot be matched by any other existing technique. The operator is guided through the programming sequence by a series of simple and interactive menu-type screen displays.
In terms of element design, the lathe has a number of advanced features which enable it to maintain high accuracies consistently from lens to lens. The headstock incorporates a precision air-bearing spindle, a high frequency AC motor provides steplessly variable spindle speeds from 2000 to 14,000 rev/min and an electronic dynamic braking system brings the spindle to rest in only 6 seconds from the top speed. Headstock traverse is achieved by pre-loaded cross roller slides (so there’s absolutely no play or backlash) and is powered by a stepper motor driving through zero backlash lead screw and nut.
Spindle chucking is by way of an integral feed-through vacuum system, while diamond tool alignment involves the use of a demountable closed circuit TV screen and a high powered (X400) microscope. The tailstock is horizontally mounted on large diameter, pre-loaded ball bearings and the radius slide (which actually holds the tool) is a pre-loaded roller bearing slide. Adjustment of the latter also uses a stepper motor with zero backlash lead screw. Air, vacuum and electrical supplies to the machine are all monitored by the micro controller.
Lead over manual
Compared with 'hand crank' turning machines - where the radial slide is fed-in by hand - the performance of the Polytech lathe is quite outstanding. In terms of accuracy, the CNC model can machine to a repeatable accuracy of 1 micron over a 60mm travel. This compares with an accuracy of 0.01mm (100 times worse) on the traditional type of contact lens lathe. On the productivity front, the Polytech can produce 130 identical lenses in an 8 hour day. A ’hand crank' machine produces less than half this amount in the same time (but such machining is very much operator dependent).
In terms of setting up, it only takes around 5 minutes to change the diamond tool on the CNC machine, whereas it would take between 1 and 2 hours to set-up on a conventional lathe. Another plus is that one operator can look after four Polytech machines, while with 'hand crank' lathes it’s a strict 1:1 ratio.
However, such impressive performance characteristics don’t come cheap initially, a Polytech costs just over £,40,000 but increased productivity, labour and floor space savings give a 24-30 month payback.
As far as the use of AutoCAD at CLS goes, the CAD system was introduced after the first production series machines had already been built. But for the soon-to-be-released Polytech 2000, a second generation version of the original design, AutoCAD has made a real impact. As well as helping to make ergonomic changes to the machine, it has also produced all the engineering drawings and specifications for the Mk II version. Such draughting routines have saved at least 12 months on project lead times compared with manually produced drawings.
AutoCAD as also helped optimise tool design, too – with these natural diamond tools costing around £200 a time, CAD has proved to be a highly effective and economical design tool. By varying tool design in terms of angles, rakes and clearances on the screen and simulating the parameters of the resulting swarf (its thickness, radius and shape), the best tool design for the job! material can be calculated mathematically rather than by a series of trial and error tests actually carried out on the machine.
Such CAD routines have led to the design of diamond tools which produce better finishes (within 20 Nm peak to valley) and last longer (at least 5000 surfaces before a re-lap, instead of only 1000 as before).
Quality control is another area where AutoCAD has made its presence felt at CLS. The system can generate perfect overlays of tooling designs for use on an optical projector to check the form of the tool when it’s delivered. Plots of tool insert and shank superimposed on lens surfaces are also used in the QC department to highlight any potential problems.
Lens design is the other major area of CAD usage. As well as offering speed and flexibility, AutoCAD also acts as a tool for Ian Handricks and his team to access the full potential of the Polytech 2000 machine. Lens styles, magnifications, optical qualities, edge and corner design can now be generated, examined very closely and modified quickly. Design work like this used to take weeks, even months to complete. On AutoCAD, the same tasks can now be carried out in just a few hours.
Once a new style of lens has been created on the screen it’s a simple matter to generate the necessary co-ordinate moves of the headstock and radius slide for inclusion in the part program fed into the Polytech machine.
However, Ian Handricks is currently working on 3D CAD (with special additional software developed in-house) for a new range of intraocular contact lenses. Here, full CAD/CAM will be used so that the lens can be designed on AutoCAD and the part program automatically generated and verified on the screen. Manufacturing data will be transmitted down to a prototype NC milling machine in the works.
Such has been the use of AutoCAD in a variety of domains that CLS has already built up 30 MB of files (mainly drawings) in just over a year. And with plans already in the pipeline for an even higher performance Polytech machine, AutoCAD seems destined to take on an even greater workload in the future.