Mineral Lenses

Surfacing Ophthalmic lenses

Production systems for the mass manufacture of spherical glass lenses using therough grind, smooth and polish processusually start with a tool that has fine diamond particles embedded into a metalbond. A system using this method into a UK Optical factory over 25 years ago. It enabled 30-second grinding, 30-second smoothing and two-minute polishing to be achieved on spherical glass lenses where one operator produced a finished lens every 30 seconds from a bank of 12 machines (a grinder, smoother and four polishers for the convex surface and the same for the concave surface).

This system of using lapping tools for all stages is only suitable for mass production because of the large investment required in tooling. Most surfacing production uses a
generation stage (see next), in which one machine can be adjusted to generate different curves as described in the next section.

Generate, smooth and polish
The key component of a generate, smooth and polish system is the generating machine. This usually consists of a tubular shaped 'cup-wheel' rotating cutter with a
curved end. The cutter is mounted on a swinging arm and can be tilted so that the combination ofswing and tilt can produce the two different curves of a toroidal surface.
Cutting with a tilted circular cutter does not actually produce a circular cut, but an elliptical curve. The elliptical error is not large enough to be significant to the optical
power of a lens, but can be significant to the manufacturing process. It should be remembered that the smoothing and polishing tools may be accurate circular shapes,
Make the major portion of the lens using a traditional process Cut a curved section to accept the bifocal segment.

• Make the low index portion of the lens using a traditional process.
• Make the high index portion of the lens using a traditional process
• Combine all parts by a heat fusing process
• Remove excess material by grinding
• The manufacture of glass multifocal lenses by fusing

Rough grind, smooth and polish

This is the oldest system in existence. Apart from the use of an electric motor to rotate the lenses against the grinding tool, Leonardo da Vinci would have fully comprehended any modern process of grinding, smoothing and polishing a lens surface. A poker arm machine is the most basic version, and is illustrated in At each stage, finer and finer materials are used working on a ratio of about 10:1 between stages. For example, the first process is to obtain a rough shape by the removal of about 2mm of glass using a 0.2mm diameter grit. This leaves imperfections in the surface of about 0.2mm. The second stage removes about 0.2mm using 0.02mm abrasive, and leaves imperfections of about 0.02mm. The final polishing stage usually removes about 0.02mm, using 0.002mm abrasive and leaves a surface with 0.002mm imperfections (0.002mm = 2 microns). Since the eye cannot see individual particles less than about 20 microns, the resulting surface appears highly polished.

There are many clever techniques used in production to ensure that the correct curvature of the surface is achieved. For example, the lapping tool always oscillates so that a constant curvature without surface waves is obtained, the radius of curvature can be gradually decreased by preferentially oscillating towards the edge of the tool and vice versa. More accurate curves can be although with many variants for special applications particularly in relation to the production of progressive power lenses.

They are:

• Rough grind, smooth and polish,
where all processes involve a lapping tool. Modern systems usually do the initial roughing stage using a lapping tool with a diamond abrasive embedded into a metal matrix. This system is only used in large volume glass production processes because it requires separate grinding, smoothing and polishing laps for each
curvature

• Generate, smooth and polish, where a cup-shaped cutting wheel is used to generate a spherical or toroidal curve, followed by lapping tool smoothing and polishing. This is the method used in most prescription laboratories. Again, different smoothing and polishing laps are required but a common machine is used for the grinding stage

• Cut, smooth and polish, where a single point cutter is used in a process usually referred to as 'freeform'. This is particularly applicable to progressive lens surfaces. After single point cutting, the surface can be smoothed and polished in the same way as if a cup-wheel generator had been used. However, current developments are aimed at omitting the smoothing stage or may have been produced using a process that also involves elliptical errors. It is the discrepancies between the elliptical curvatures of the generation, smoothing and polishing tools that may cause some difficulties in the lens making process (see later).
The key part of a generate, smooth and polish process is the generator itself. Very old machines had to have the radii of the two curvatures set manually before the cutting commenced, although this is now done automatically through electronic controls.

When a generator is combined with some smoothing and polishing machines, it creates a prescription lens manufacturing system suitable for a small Rx laboratory. Such a laboratory requires not only the generator, smoothing and polishing machines, but also a rack of lapping tools, a blocking and de-blocking unit and other ancillary equipment.

A considerable inventory of lapping tools is required to produce the combination of spherical and cylindrical curves commonly used in a prescription laboratory. At the very least, tools for every quarter dioptre, from perhaps plano to 12.00D curves with zero to 6.00 cylinders - not an inconsiderable number is required.

Because of the wide variety of different refractive index materials now in use, many more tools, normally in eighth dioptre increments are required, as well as a few tools of each sphere and cylinder combination. To prevent damage and wear to the accurate tool curves, a system of applying replaceable abrasive and polishing pads onto the same tool has been evolved. The pads have a special shape to allow them to adapt to the curvature of the tool

Other techniques, where for example the polishing stage is replaced by lacquer coating, have been created and these are described later in the section on recent developments.

Mention should also be made here of the number of smoothing stages. With modern equipment, it is usually possible to go from generating to polishing with only one smoothing stage, but historically two stages of smoothing were often used. Obviously there is a trade-off between the amount of material removed and the smoothness of the surface. Remember that sufficient material must be removed to clear surface cracks
and curvature inaccuracy (including elliptical errors) left by the generating stage. Advice on the choice of smoothing and polishing pads is freely available from the suppliers of prescription laboratory consumables, who can provide complete systems as a turnkey operation, including operator training. In addition to the generating, smoothing and polishing machines, many other pieces of equipment are needed to complete the
surfacing process, including lens tool calculation, lens surface protection, blocking

A lens generator
The first essential is to calculate the curvature of the lens surface to be produced. This requires knowledge of the semi-finished curvature, the material refractive index and the desired lens power. For plus power lenses, a knowledge of the frame shape is also required so that the thinnest possible uncuts may be produced allowing just sufficient thickness at the critical point around the circumference. For aspheric and progressive

A small prescription laboratory showing (clockwise): lens design computer, surface saver tape dispenser, lens blocker, lens deblocker, lapping tool rack, lens generator, smoothing machine and polishing machines

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