Aug 9, 1975
“Computer aided design is helping contact
lens manufacturer CLS, produce lenses to micron accuracy”
Engineer’s
Digest
June 1986
COMPUTER AIDED
DESIGN
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
Turning technology
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.
Exhibition idea
AutoCAD was bought in September ’84 during a visit to the Design Engineering
show at the NEC in
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.
Lathe design
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.
Tool design
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.
<< In the Media index < Auckland company signs deal with US investors - The Photographers Mail, Vol. 11, 2002 | Hirstlens again in private ownership - NZ Optics - June 1988 >
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