Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

Emergency Stops

As with any type of equipment, occasionally stopping the operation in midcycle becomes necessary. Edgers have some system to stop the cycle and lift the lens off the wheel, resetting the machine to start.

Checking Size Accuracy

Once a lens has been edged, the block should not be removed until it is certain that the lens fits into the frame exactly. This is especially true for metal frames, in which small size increments determine the difference between illand well-fitting lenses.

To check lens size accuracy for a metal frame, the lens is placed in the eyewire with the block still on. Then, using eyewire closure pliers, the two sections of the eyewire are squeezed together, as in Figure 8-26. This draws the eyewire around the lens. If the upper and lower halves of the barrel fail to come together, the lens is still too large. The eyewire should close fully and leave no gaps between the lens and the eyewire, yet without putting undue stress on a lens.

Undue stress from a metal eyewire causes a plastic lens to warp and a glass lens to exhibit stress when viewed with the aid of a colmascope.3 Figure 8-27 shows how the strain appears with a colmascope.

While the block remains on the lens, an operator may make slight steps in size reduction by running the lens through the finishing cycle again. However, once

3A colmascope consists of a light source behind two crossed polaroids. When a lens is placed between two polaroids, internal lens stress becomes visually apparent.

the block is removed, it is extremely difficult to reblock the lens accurately enough to allow an even removal of only a few tenths of a millimeter.

Because plastic frames stretch upon lens insertion, some variation in lens size is possible without serious consequences. Such lens size variation is not possible when using a metal frame because the frame will not expand or contract. It is best to have the frame in the laboratory when the lenses are edged, but if this is not the case, a more accurate indication of size than a ruler or a Box-o-Graph (Kosh Manufacturing Co., Ft. Lauderdale, Fla.) can be obtained using a circumference gauge (Figures 8-28 and 8-29). By knowing the circumference necessary for each eyesize of a given metal frame, precision in duplication is possible. Therefore for metal frames that are uniformly manufactured to precisely repeatable sizes, an exact fit can be obtained without the frame being in the laboratory. (Note: Patternless edging combined with a remote tracer placed on location at the dispensary ensure an accurate fit even without having the frame in the laboratory, especially if used in conjunction with circumference measurements.)

Variations in Edging Lenses of

Different Materials

Although the edging process remains approximately the same in spite of differences in lens materials, some aspects require special attention. These are discussed separately on the basis of the lens material used.

FIGURE 8-28 Knowing the desired circumference of the lens and then edging the new lens size to match this desired circumference will provide more accurate results than edging using only eyesize measurements.

an index of 1.523. Glass lenses are also referred to as mineral lenses.

PHOTOCHROMIC GLASS

Photochromic glass, which darkens as light intensity increases, is treated almost the same as crown glass during the edging process. Because of its composition the photochromic lens generates more heat when being edged and is harder on edger wheels. As a result, some wheels have been designed for use in situations in which a high percentage of photochromic lenses are used.

When a large proportion of photochromic lenses are edged, the coolant must be of good quality and of sufficient concentration to produce the desired effect. Failure to maintain a clean, efficient coolant flow causes unnecessarily rough lens edges and a reduced abrasive wheel life.

CROWN GLASS

Crown glass lenses are considered to be the standard for glass ophthalmic lenses. Lens fabrication equipment ordered for glass assumes crown glass lenses as the standard glass lens. Crown glass for ophthalmic use has

HIGH-INDEX GLASS

Some clear glass lenses are made from materials that result in a higher refractive index than the standard crown glass lens. Common refractive indices for these so-called high-index lenses are 1.60, 1.70, and 1.80.

Because of basic property differences in the material, high-index lenses should be edged with less pressure than standard crown lenses.

It is advisable to avoid large temperature shocks with high-index glass, as when a hot lens is rinsed with cold water. A standard crown lens can generally withstand a 70° C difference in temperature, whereas a 50° C maximum difference is average for high index.4

High-index lenses should not be notched or drilled because these processes reduce impact resistance.

LENSES MADE FROM PLASTIC

Edging of plastic CR-395 lenses can be performed on the same equipment and with the same edger wheels as are used for glass if relatively few of the lenses are plastic. Because most lenses are plastic and not glass, this is impractical. When the percentage of plastic lenses increases, the plastic begins to glaze the wheel. Glaze means that the spaces between abrasive diamond cutting edges become clogged with plastic, preventing good, clean cutting.

A glazed wheel results in increased cutting time for lenses being edged. An extremely glazed wheel can even cause the edges of a glass lens to chip to a depth of 4 to 5 mm into the lens surface, ruining the lens. This does not occur when only a few plastic lenses are

4High-Lite, S-1005 High Index, Low Density, Schott Optical Glass, Inc., Duryea, Pa., 1979.

5Columbia Resin 39, a trademark of Pittsburgh Plate Glass Co.

interspersed with glass because the wheel is cleaned by the glass before plastic can build up to a glaze.

The solution to the problem of glazing can be handled in one of two ways. The first is to use two edgers—one for glass, the other for plastic. The second solution is an edger with three or more cutting wheels. Most threewheel edgers made to handle two different lens materials are designed with one finishing and two roughing wheels. One roughing wheel may then be used for glass, and the second for plastic. All lenses may use the same finishing wheel because the same kind of finishing wheel is often used for both plastic and glass. Glass lenses must not inadvertently be allowed to rough on the wheel designed for plastic because wheel damage will occur.

HIGH-INDEX PLASTIC

With the exception of polycarbonate, lenses made from high-index plastic material are edged in essentially the same manner as are CR-39 plastic lenses. Some highindex lenses emit a sulfurous odor when edged. Although not considered hazardous, ventilation should be considered, especially if the laboratory is within proximity to a retail dispensary.

POLYCARBONATE LENSES

Polycarbonate lenses are highly impact resistant and are excellent for regular wear and as protective lenses in industrial situations. Although of a higher refractive index (1.586) than CR-39 plastic, polycarbonate is a

softer material and easily scratched unless coated. A protective coating is standard.

Care should be taken to ensure that all surfaces or points that come into contact with the lens are free from rough edges or burrs. Lensmeter marking points should not be overlooked.

Lens surfaces should be free of dirt and oils in order to prevent slippage during edging. For this reason, it is best to hold the lenses by their edges.

Wet or Dry?

Even though the edging process for polycarbonate lenses can be performed either wet or dry, a combination of both processes yields a cosmetically better, more stress-free lens. In this combination process, the lens is rough edged with no coolant. Halfway through the finishing cycle the coolant pump is activated and the edging process completed with coolant.

Polycarbonate lenses are also edged on dry-cut, rotating blade edgers. Although this system gives nicely configured edges, National Optronics (Charlottesville, Va.) makes an edger that first dry-cut edges the lens, then shifts the lens to a diamond polishing hub. During polishing this hub is misted with water to produce a clear edge finish.

Stress in Polycarbonate Lenses Resulting from Edging

If the speed of the diamond wheel or rotating blade is too slow, stress develops in a lens. Although stressed, lenses may still edge rapidly and look acceptable from a cosmetic viewpoint. Stress also can develop when cutting with a dull cutter. Over a period of time, this lens stress is released in the form of cracking or surface crazing.

Completing the Polycarbonate Process

Like other lenses, polycarbonate lenses should be safety beveled. Variations in this process are discussed in Chapter 10 on hand edging.

The lens must be edged and mounted stress-free to prevent future cracking or surface crazing. Although improper edging techniques can be the greatest source of stress in a polycarbonate lens, mounting the lenses too tightly also yields significant stress.

Cleaning of polycarbonate lenses may not employ harsh chemicals. Acetone damages the exposed edge of a polycarbonate lens. A mild detergent should prove sufficient. Alcohol is used to remove progressive lens manufacturer’s markings.

ANTIREFLECTION-COATED LENSES

develop microcracks over the surface called crazing. Heat, pressure, and lens flexing can cause the coating to craze. With these things in mind, reducing the possibility of subjecting the lens to these factors also reduces the possibility of lens spoilage. The following list provides some ways to reduce AR spoilage.6 Although not all are directly involved with edging, they are included together for convenience, as follows:

•Hold the lenses by the edges, not the surfaces.

•Edge AR lenses on an edger not used for glass. If glass lenses have been edged, or if the edger has not been cleaned recently, clean the edger so that debris will not recirculate over the AR-coated lens.

•Use surface-saver tape or surface-saver discs on the lens. This will protect it from scratching and will cushion the lens when it is chucked in the edger. If more cushioning is needed, use an adhesive blocking pad on the back surface of the lens.

•Reduce the chuck pressure on the edger, especially if the lens is also a high-minus, high-index plastic lens.

•Use the largest block and chuck possible to spread the pressure over a greater area.

•Do not use an edger with a bevel guide that touches the front of the lens.7 (If this type of edger is used, then surface-saver tape also should be used to protect the surface.)

•Many of today’s patternless edgers have self-adjusting features that keep chucking pressure and other stress forces to a minimum. They also are often able to get the lens exact the first time and pin-bevel it simultaneously so that the lens is handled less. For best results, use one of these.

•Do not leave the lenses blocked for long periods of time before edging them.

•Deblock the lens carefully.

•Rinse the lens after edging to remove any debris.

LAMINATED LENSES

When lenses are composed of two or more materials laminated together, the lens must be edged on a wheel that is compatible with both materials.

Special Edging Situations

EDGING LENSES FOR WRAP-AROUND FRAMES

Some sunglass frames are made primarily for plano sunglasses that are made with a high base curve to give

Antireflection (AR) coated lenses present some challenges in edging. A number of factors cause the coating to

6Herrick T: The A-R upgrade: you’re ready to sell S-R coated lenses, but can you finish them? 20/20 January:54-56, 2000.

7Edging instructions for Reflection Free Plus stock lenses, undated.

a wrap-around effect, which adds to protection from the sun and fashion appeal. One of the problems encountered is that prescription lenses are made on a variety of base curves, depending upon lens power. When a lens with a low base curve (flatter front curve) is edged for one of these frames, they do not always want to fit in and stay in the frame. There are two ways that may be tried to make the lens fit into the frame.

A Steeper Base Curve

The first method for making the lenses fit better in a wrap-around frame is to use a lens with a steeper base curve. This means ordering a lens with a base curve of about 8.00 D. The average prescription has a base curve of about 5 or 6 D. For lenses of low power this may not interfere significantly with the overall optical performance of the lens. However, each lens power has a specific base curve that is optically correct. Changing the base curve will decrease the optical performance when viewing objects in the periphery of the lens.8 Therefore caution should be taken when changing base curves. The higher the lens power, the more significant will be the effect optically.

An Edger with a Guided Bevel System

The best way to edge a lens for a wrap-around frame is to use an edger with a guided bevel system. It is possible to use a lens with the correct base curve for the power of the prescription. To get the lens to fit properly, use an edger that allows any base curve configuration to be placed on the lens. In other words, a lens with a +6.00 D base curve can be edged with an 8.00 D bevel, which will better fit the curvature of the wrap-around frame (see Figure 8-21). The main drawback is that the edge of the lens must be thick enough to prevent the bevel from going off the lens edge completely.

WHEN THE BEVEL LEAVES THE LENS

Some types of lenses are notorious for causing the bevel to leave the edge of the lens, which makes it difficult to get the lens to stay in the frame or requires that the lens be re-edged completely. This often occurs on Franklinstyle (Executive) lenses (Figure 8-30) and on lenses with high base curves. The following discussion presents some solutions.

A Guided Bevel System

When edging “Executive” lenses or high base curve lenses, the bevel may run off the lens edge. This is

Bevel leaves the lens

FIGURE 8-30 A bevel tracking around the lens a certain distance from the front surface often leaves the lens when it reaches the ledge of an Executive bifocal. This ruins the lens.

especially the case for long corners of the lens farthest from the center.

The best answer is to go to an edger that allows for manually guided bevel placement. Good results may be obtained on some lenses using a patternless edger that is auto-guided. These systems “feel” the thickness of the lens where the bevel will go and often show on-screen exactly where the bevel should appear on the lens after it is edged. Best results are obtained when this information may be actively used by the operator by allowing for manual adjustment for optimum bevel guidance all the way around the lens.

MAKING A PATTERN FROM AN EXISTING LENS

Some edgers have a kit that will allow the edger to cut a pattern from an existing lens. The wearer’s lens or a coquille9 is blocked with an adhesive pad so that the special block is exactly in the middle of the lens. The blocked lens is mounted where the pattern would normally be placed and a pattern blank is blocked and chucked in the edger as if it were a lens. The edger is set as if it were edging a lens for a rimless frame with no bevel.

8Changing the base curve of a lens successfully without affecting peripheral lens optics is possible if the lens is redesigned in aspheric form for that particular base curve.

9A coquille is the dummy lens that comes in the frame for frame display purposes.

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PATTERNLESS EDGING

Patternless edgers and patterned edgers work in basically the same way. There are two main differences.

1.The patternless edger does not use a physical pattern. Instead it uses a digitized shape.

2.The patternless edger is already electronically sophisticated, so it builds in many more computercontrolled options. This makes it considerably easier to get a more refined end result.

Electronic Shape

A patternless edger is like a regular edger, except that it accepts frame shape information electronically instead of mechanically. This means that a patternless edger must work together with a frame tracer. The frame tracer creates a pattern shape electronically. (Frame tracers are explained in Chapter 3.)

Frame tracers can be a separate piece of equipment, hooked by cable to the patternless edger (Figure 8-31). Or the frame tracer can be a part of the edger, built right in to save space (Figure 8-32). Some patternless edgers are even more self-contained and have the layout blocker built into the housing of the edger (Figures 8-33 through 8-36).

C H A P T E R 8 E D G I N G

Because each frame is traced individually, it is as if the electronic pattern is the same size as the edged lens will be.

Patternless Possibilities

What are some of the extras that can go along with a patternless edger? Not all of the options listed in this chapter’s section on patternless edging are on every patternless edger. Each edger is unique in what it offers and will vary in how effectively and skillfully it is able to carry out that option.

USING PATTERNS WITH SOME

PATTERNLESS EDGERS

Most patternless edgers cannot use patterns. Yet from time to time it is easier to use a pattern. For this reason, some patternless edgers are made to also run with a pattern. They are more the exception than the rule.

‘FEELING’ THE LENS FOR EDGE THICKNESS

One of the better options on a patternless edger is the ability to “feel” the lens where the edge will be located after edging. Many edgers use two “measuring arms,”

one on the front of the lens and the other on the back. Before the lens is actually edged, the edger turns the uncut lens in the edger. These two measuring arms touch the front and back surfaces of the lens (Figure 8-37). As the lens turns, the arms trace the future shape of the edged lens at the exact lens size dictated. This

tells the edger how thick the lens is at each point on the edge. The edger uses this information to decide an optimum bevel position. How well the edger makes this decision depends on the sophistication of the computer software. (Other ways exist for measurement of lens thickness besides use of measuring arms.)

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REQUIRED LENS AND FRAME MATERIALS

Patternless edgers often have a prompt that asks for the lens and frame material being used (Figure 8-38). The edger needs to know which lens material is used for the following reasons:

•The edger will rough the lens on the correct roughing wheel. The correct roughing wheel for

A

C H A P T E R 8 E D G I N G

glass is not the correct roughing wheel for plastic or polycarbonate.

•If the lens is polycarbonate, the coolant flow will cycle on and off at different times. Other materials have a continuous coolant flow.

•The edger presses the lens against the wheel with the appropriate pressure.

Knowing the frame material/bevel style is important for

the following reasons: