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

C H A P T E R 1 0 H A N D E D G I N G

BOX 10-1

Factors Affecting Cutting Speed in Hand

EdgingCutting Speed Will Increase…

•If pressure against the wheel increases

•If speed of rotation decreases

•If a rougher grit wheel is used

•At a corner of the lens shape

•As lens edge thickness decreases

Cutting Speed Will Decrease…

•If pressure against the wheel decreases

•If speed of rotation increases

•If a finer grit wheel is used

•Along a straight side of the lens shape

•As lens edge thickness increases

a wavy, irregular tone, all of which relate to faulty technique:

•A wavy sound. A wavy sound occurs if the angle of lens tilt is being altered. This results in a wavy bevel apex and an uneven, irregular appearance of the face of the bevel.

•Variations in sound volume. Changes in sound volume indicate that pressure on the lens is uneven. An uneven, wavy bevel results and the lens may not have the same shape as before. Applying too much pressure on corners causes corner gaps that will show up after the lens is inserted into the frame.

•Short, choppy sounds. Short, choppy sounds indicate that the lens is being lifted from the wheel too often. This can cause the surface of the bevel to lose its smoothness. Smooth, long motions result in smooth bevel surfaces.

•A smooth sound interrupted by periodic wavy sounds. A smooth sound interrupted by periodic wavy sounds indicates an attempt to regrip the lens without first lifting it from the wheel. Each time the grip is changed, the lens must first be lifted from the wheel. Maintaining a correct wheel/lens relationship is impossible while simultaneously shifting the grip on the lens.

SUMMARIZING A FEW BASIC

HAND-EDGING RULES

The following rules apply generally, not just for edge smoothing:

1.Maintain a constant angle between the wheel face and lens.

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2.Never reposition a grip on the lens while the lens is against the wheel.

3.Travel as far around the bevel as possible before lifting the lens from the wheel.

4.Listen to the sound of the lens on the wheel.

PRACTICING EDGE SMOOTHING

Practicing proper smoothing begins with choosing a lens large enough to permit an easy grip. A low-minus lens has an optimum edge configuration. Before advances were made in edger wheel quality, lenses would leave the edger with a “frosted” appearance. That frosted surface had to be removed by edge smoothing. A lens with an edge that is as “frosted looking” as possible should be used so that it is easier to see what has been accomplished.

After each major “pass” around the lens, the lens edge is wiped dry and the “frosted” area checked. If translucent-like “frosted” area is being removed near just the apex, for example, the lens is being held at the wrong angle and should be tilted so that the lens is closer to the wheel. The hands are too unsteady if the translucence that remains is scattered here and there on the bevel. Resting the hands firmly on the hand rest and maintaining a consistent angle of tilt helps solves this problem.

Pin Beveling

Lenses need to be pin beveled. It does not matter if the lens has been just edged or edged then edge smoothed by hand. The only reason not to pin bevel is if the edger includes the pin beveling process.

REASONS FOR PIN BEVELING

Three primary reasons exist for pin beveling, as follows:

1.Breakage prevention: If the intersection is allowed to remain as a sharp corner, the risk of chipping or flaking at the interface between the two surfaces is considerably higher. Hence the alternative term, safety bevels.

2.Cosmetic considerations: Pin beveling removes microchips or “stars” left as the abrasive wheel grinds away lens material. Microchips are seen by holding the lens up to a light source and turning it slightly. Starlike reflections on the edge indicate their presence.

3.Wearer safety: Pin bevels may reduce the risk of injury to the wearer’s face if the glasses are struck and then impact the wearer’s face. For this reason some

bevel

suggest that safety lenses should have a greater-than- normal pin bevel.

PIN BEVELING A LENS

The basic procedure for pin beveling uses much the same technique as is used for edge smoothing. For pin beveling the front and rear surfaces, the lens is held in basically the same manner as it was for edge smoothing. The difference is in the angle the lens makes with the cutting plane. This needed angle for pin beveling is only half of what it was for edge smoothing. Figure 10-12 compares the angles used for edge smoothing and pin beveling.

To pin bevel the apex, the lens is held vertically with the right hand. The lens may be guided with the left hand but should be held loosely enough to allow free rotation (Figure 10-13). Some people safety bevel the apex with only one hand.

The following are some key points in the pinbeveling process:

•Little pressure should be applied in pin beveling. The lens is permitted to rotate almost from the upward pull of the wheel alone.

•Speed of rotation is much faster than for edge smoothing.

FIGURE 10-13 Pin beveling the bevel apex is done with very little pressure.

•When completed, the pin bevel should be noticed only by the absence of edge sharpness and microchips. It does not need to be visible.

Table 10-2 summarizes the sequence of steps in the edge-smoothing and pin-beveling process.

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TABLE 10-2

Order of Steps in Hand Edging of a Lens

Pin Beveling the Rear Surface of a High-Minus Lens

Back-surface pin beveling may prove difficult if a lens has a high-minus back curve. It is made even more difficult when the frame being used has a narrow B dimension. This happens because the center of the back bevel spans the surface of the wheel and cannot be reached (Figure 10-14). The only way to reach the center is by use of either the edge of the wheel or a curved hub. Figure 10-15 demonstrates how to pin bevel the rear surface of a high-minus lens using the rounded edge of a traditionally shaped hand-edger wheel.

Face-type hand edgers do not have an exposed edge, but they usually have a curved central hub area. This curved central hub portion is used for pin beveling the back surface of the lens. Figure 10-16 shows how this is done.

Pin Beveling for Rimless Lenses

A pin bevel has an angle that is halfway between the two surfaces it separates. For beveled lenses the pin bevel is

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light. For glass, flat-beveled lenses the pin bevel may be a bit heavier (Figure 10-17). Rimless pin bevels are more obvious than regular pin bevels and for plastic lenses should be done very lightly (see Figure 10-17).

Reducing Lens Size by Hand

Reducing the size of a lens by hand is easier when it is to go into a plastic frame. Metal frames are harder to do. One of the most difficult tasks in hand edging is to reduce the size of an edged lens for a metal frame and have it fit right. Much skill is required to maintain the integrity of the lens shape during the process. The following list outlines the steps used to reduce a lens size by hand:

1.Apply pressure to the rear bevel surface and rotate the lens exactly as was done during the edgesmoothing process.

2.Continue edge smoothing with more pressure than usual until the apex of the bevel moves toward the front of the edge.

3.Next, edge smooth the front bevel until the apex of the lens bevel returns to its proper position.

If only a slight size reduction is required, just repeating the edge-smoothing process without attempting to move the bevel position may be sufficient.

CHECKING METAL FRAME LENS SIZE

Even a small lens size reduction can make a large difference in how well a lens fits. If the frame is available, the lens should be checked often; it is placed in the frame and the eyewire closed. The best way to check eyesize is to remove the screw and squeeze the eyewire barrels together with a pair of eyewire closure

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A

B

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pliers. This was shown in Chapter 8 (see Figure 8-26). Backing the eyewire screw out just far enough to allow the lens to be removed and reinserted may seem easier. Unfortunately, the edge of the lens often flakes when being shoved back into the eyewire. It is better to just remove the screw.

Simply being able to screw the eyewire barrels flush together does not indicate a good fit. The eyewire may exert undue pressure on the lens. In plastic lenses this causes the lens to warp, and in glass lenses a stress pattern is set up within the lens. If nothing is done to relieve the stress, edge flaking will occur when the rims receive even a slight blow. (Edge flaking is surface chipping at the lens edge.)

Stress also may result when the curve of the lens edge does not match the curve of the frame’s rim. In this case, either the rim must be shaped to match the

lens curve, or the lens bevel must conform to the same base curve for which the frame was designed.

The practitioner checks the lens for stress by placing it in a colmascope,4 as would be done to check for a heat-hardened glass lens. Viewing the lens in this state shows a zigzag color-fringed pattern around the lens edge wherever a stress buildup occurs. If this encompasses the lens completely, as was shown in Figure 8-27, the lens is still too large. It should be taken down evenly all the way around the lens. If, however, the stress pattern occurs only in one place, note that place. The edge should be made to better conform to the frame in that area.

4A colmascope consists of two polarizing films, illuminated from below. The lens is placed between the two for viewing.

FIGURE 10-16 The spherical section of the face hand edger is designed to pin bevel the hard-to-reach back edge of high-minus lenses.

REDUCING LENS SIZE FOR HIDDEN-BEVEL LENSES

To reduce lens size for a lens having a hidden bevel, a hand-edger wheel with a V-groove is helpful. The lens is held perpendicular to the wheel with its bevel in the V-groove (Figure 10-18).

HAND EDGING FOR FRANKLIN-STYLE LENSES

The Franklin-style (Executive) lens is ruined easily in inexperienced hands. Because of lens design, a large portion of the front bevel ends abruptly where it meets with the segment ledge on both the temporal and nasal sides of the lens. Ledge corners form small points. In smoothing the front bevel surface near these corners, little pressure should be used. Lens material is removed extremely rapidly. This same precaution is necessary during safety beveling. A poor job of safety beveling will look worse than if the lens had never been safety beveled.

HAND EDGING OF POLYCARBONATE

After edging, sometimes a polycarbonate lens may have a residue of plastic material clinging to the lens bevel. This buildup is known as flash or swarf and has been described as looking like white shredded wheat. (The amount of swarf remaining on the lens can be

FIGURE 10-17 Glass lenses have a pin bevel that is a bit heavier than the pin bevels on plastics lenses will be, since glass lenses are more likely to chip along any sharp edge. A and B show approximately what the size of glass pin bevels are for regular and flat-beveled lenses. Plastic lenses with a regular bevel (C) will have less of a safety bevel. The back surface interface with the bevel will have more of a safety bevel than the front surface interface or the apex. Before pin beveling this is generally the sharpest edge, especially for lenses with minus lens power. Plastic flat-beveled lenses (D) have sharper angles and have minimally more of a safety bevel.

reduced by increasing edger head pressure.) The polycarbonate lens should be pin beveled dry. Dry means the coolant to the hand edger is turned off. Pin beveling must be done lightly because the wheel cuts faster when it is dry.

Two exceptions exist. One is for rimless bevels and the other is for antireflection (AR)-coated lenses. These are pin beveled wet. Wet pin beveling of polycarbonate gives a smaller, less noticeable pin bevel and a polished appearance. After pin beveling, any remaining swarf is removed by scraping the lens edge with a razor or knife blade held perpendicular to the lens surface (Figure 10-19).

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FIGURE 10-18 It is helpful to brace one hand on the tray while reducing size or reshaping a lens with a hidden bevel edge configuration on a grooved wheel.

FIGURE 10-19 After a lens has been hand edged, it will have curled threads of polycarbonate material clinging to the edges. This is removed by scraping it with a single-edged razor blade held perpendicular to the edge.

HAND EDGING ANTIREFLECTION-COATED LENSES

Some AR coatings are more sensitive to heat, pressure, and flexing. Any of these may cause some coatings to craze.

In hand edging of lenses with an AR coating, the practitioner should not press too hard or too long against the wheel. Pressing the lens hard against the

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hand-edger wheel causes a plastic lens to flex. A wet hand-edger wheel should always be used.

Re-edging a Lens for a Different Frame

Most wearers just assume that putting their old lenses into a new frame is a simple matter. It is not. Several factors must be considered before deciding to try it. The following are a few:

•The distance between optical centers needs to remain the same. Even if old lenses do fit into the new frame without re-edging, the new PD may not be correct. If the distance between lenses (DBL) of the new frame differs from that of the old, the PD will no longer be correct. The lens major reference point (MRP) must be located close enough to the wearer’s PD to prevent prism that would exceed ANSI Z80 standards for prescription ophthalmic lenses.

•The lenses must be large enough. The old lenses have to be large enough for the new frame. If the new frame is metal, then the old lens must be big enough so that the entire lens opening is covered.

•Rotating the old lens so it fits the new frame may not work. Lenses that have a cylinder component to the prescription may not be turned from their prescribed axis, nor may most segmented multifocals and progressive addition lenses.

•Glass lenses need to be rehardened. If the lens has been heat-treated it must be dehardened5 before re-edging. Fortunately heat treating of glass lenses is now rare. Chemically hardened lenses may be re-edged as they are. However, both types need to be rehardened before dispensing.

If all these factors have been considered, the lens may be reshaped.

Lenses are not re-edged if the chosen frame is plastic and can be stretched to accept the old lenses. Stretching the frame’s eyewire usually eliminates unwanted air spaces between the lens and the eyewire.

HAND EDGING A LENS FOR A DIFFERENT FRAME

If possible, the lenses are blocked and re-edged with an edger. If this is not possible, the following is one method for hand edging the lenses for a new frame:

5Dehardening is done by placing the lens in the heat-treating unit for the same length of time as is required for hardening. As the lens is removed from the furnace area, instead of fast cooling with forced airto create internal stress, the air is turned off and the lens is allowed to cool slowly.

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1.Spot the lenses in the lensmeter according to the wearer’s prescription.

2.Measure on the frame from the center of the frame bridge to half the wearer’s prescribed PD.

3.Hold the lens over the frame with its dotted MRP at the wearer’s PD. The lens opening must still be completely covered by the lens. (If the lens does not cover the opening, and moving it left or right slightly6 will not cover the gap, do not go any further.)

4.Note those places where the lens is too big and must be edged away.

5.Reshape the lens on a hand edger.

6.Once the first lens has been satisfactorily shaped, use it as a model for the second to ensure left-right symmetry.

Changing a Frame’s Lens Shape

Sometimes practitioners request that a lens be made to a different shape than the shape of the chosen frame. The purpose for substitution of a different lens shape may be simply cosmetic. Some frame shapes may compliment a particular facial shape. Or a shape change may be requested to obtain either a better bridge fit or a slightly larger B dimension. Commonly requested shape alterations are for nasal cuts, nasal adds, and increases in the B dimension. These requested changes work primarily for plastic or rimless frames.

NASAL CUT

A nasal cut is requested when the wearer’s nose broadens more toward the nostril area than the frame does. When this happens, the frame rests on the lower nasal corners of the rims rather than on the bridge area of the frame. The amount of nasal cut desired is specified in millimeters or with a tracing or drawing, as Figure 10-20 shows.

If the frame specified is rimless, the matter is simple. Assuming an extra pattern is available, the pattern is marked for a nasal cut and the marked area filed away. Figure 10-21 is an example of this method. The lens is edged normally on the modified pattern.

If the lens is for a plastic frame, this procedure will not work. Filing the pattern with no other compensation only causes an inferior nasal gap between lens and frame groove. Following is a list of some alternative possibilities.

6The lens is moved only to the left or right from where it should be if certain that the prismatic effect created will not exceed ANSI standards.

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3-mm nasal cut

FIGURE 10-20 This lens will have lens material removed to the point where the dotted line appears. The person wearing the lens has a nose that flares out at the bottom. If the lens shape were not given this “nasal cut,” the lower nasal rim of the frame would rest on the wearer’s cheeks. If the lens were for a rimless frame, the lens itself might otherwise touch the cheek.

FIGURE 10-21 A simple method used to achieve a symmetrical nasal cut on both lenses is to reshape the pattern.

Method 1

If the lens is plastic, the frame is heated and reshaped. Then either the frame is traced to create an electronic pattern or a new pattern is made for the shape.

Method 2

The frame’s existing pattern is modified for a nasal cut by filing. If the nasal cut is extreme, the lenses are edged somewhat larger to take up slack in the rim. Any time a change in eyesize is made, decentration must be recalculated.

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Example 10-1

The frame to be used has an A dimension of 50 and a DBL of 18. The wearer has a 65-mm PD. If the lenses are edged to a 51-mm eyesize, what is the change in the amount of decentration required?

Solution

With a 50-mm eyesize, the decentration per eye is as follows:

50 + 18 – 65

2

= 1.5 mm

However, if the lenses are edged to a 51-mm eyesize to allow for a nasal cut, decentration per lens is as follows:

51 + 18 – 65

2

= 2 mm

or 2 mm per lens.

Method 3

The lenses are edged slightly large using the normal pattern and the nasal cut is done on the hand edger. The practitioner determines whether enough lens stock has been removed by comparison of the lens to the frame and the paired lens. When results are satisfactory, the other lens is hand edged to conform to the new shape.

NASAL ADD

The nasal cut technique just described was used to reshape the lens to better fit a wide nose. The reverse modification is to add more lens area to the inferior nasal portion of the lens shape. The purpose is to achieve a better fit for wearers who have less than the normal amount of nasal flare. Figure 10-22 illustrates how a lens is edged to create a nasal add. Results are somewhat less predictable than with the nasal cut from both a cosmetic and fitting viewpoint. If a lens were to be ordered with nasal add, the amount of nasal add also is specified in millimeters. The following are some different methods of producing a nasal add.

Method 1

Best results are obtained by finding a pattern that almost duplicates the desired shape. If an exact pattern cannot be found, a closely related shape can be used and the lenses modified on the hand edger to conform to the shape desired.

Method 2

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3-mm nasal add

FIGURE 10-22 If a suitable alternative pattern cannot be located, nasal add is more difficult to achieve by hand. The lens must be edged large, then reduced everywhere except in the nasal-add area.

the hand edger on all sides, skipping only the area of the requested nasal add.

INCREASING THE B DIMENSION OF THE FRAME

When frame styles are large, seldom does a need exist for requesting an increase in the B dimension. Narrow frame styles, though, may prevent effective use of trifocals and progressive addition lenses. By far the best solution is to select an appropriately shaped frame so that modifications are not needed. However, a frame may be fine in every aspect but one: it is too narrow vertically. In other words the B dimension of the frame is too small. If this is the case, the following method details how to increase vertical lens size.

Method 1

The easiest way to increase the B dimension is to find a pattern that is nearly the same as the chosen frame, except with a larger B dimension.

Method 2

To increase the B dimension of the shape, this method is done best using the frame’s original pattern7 and graph paper to perform the following steps:

When only a small amount of add is required, the lenses can be first edged large without compensating the decentration. The lens is then reduced to size on

7If no pattern is available, it is possible to use the coquille (dummy lens) that comes with the new frame or to trace the inside of the frame’s lens opening.

C H A P T E R 1 0 H A N D E D G I N G

1.Place the pattern on the graph paper and trace the shape. The pattern must be correctly oriented along the 180-degree line and must not be tilted.

2.Slide the pattern downward by the amount the B dimension is to be increased. Take care not to tilt the pattern away from the 180-degree line.

3.Retrace the lower half of the pattern. This vertically increased shape represents the new shape needed.

4.Take a pair of scissors and cut out the new shape.

5.Make a new pattern from this cut-out shape. (See Chapter 3 for instructions on how this is done.)

This method permits the upper half of the lens to fit into the frame exactly as it should, even if a nylon cord–style frame is used.

The method also may be used to decrease the B dimension of a frame. The only difference is that after the original pattern is traced, the pattern is moved upward —instead of downward—by the amount the B dimension is to be decreased. The lower half of the lens is retraced, and this trace becomes the lower edge of the new shape.

Note: Whenever the B dimension of the frame is altered, care must be taken to ensure that any ordered multifocal segment or progressive addition fitting-cross heights have been measured with the new vertical depth taken into account. The new vertical depth is used to determine segment or fitting-cross drop or raise.

Correctional Modifications

After learning how to edge smooth and pin bevel, it is easier to do other things on a hand edger. The following are some possible corrections and lens modifications. For most single vision lenses, it is more costly to modify the lens than to simply replace it. These modifications should be done only for lenses especially worth salvaging.

REMOVING CHIPS

A large chip should not be ground out at the lens surface/lens bevel border with a heavy pin bevel. This creates a double-bevel appearance clearly visible to both wearer and observer. If the lens can be salvaged, the procedure is to steepen the rear bevel angle in the area of the chip. In other words, without moving the bevel apex forward, the rear bevel surface is smoothed in the chip area until it disappears. This area must be approached gradually and the bevel angle changed gradually from either side of the chip. This ensures that the bevel still looks normal. (In extreme cases, duplication of this bevel shape change on the other lens may be necessary to maintain prescription symmetry.) Afterwards the lens can be pin beveled lightly. This

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will work only for V-beveled lenses, not for lenses with hidden bevels.

SALVAGING AN OFF-AXIS LENS

After a lens is mounted in a plastic frame it is determined that the axis of the cylinder is off. If the lens shape is rounded, it may be possible to twist the lens slightly with lens rotating pliers without disturbing the appearance of the frame.

Shapes having distinct corners do not allow a lens that has actually been edged off axis to be twisted much. Rotated corners cause unusual humping. The best policy is to replace the lenses. Unless the lens pair is very expensive, replacement may be cheaper. Spending the additional time may not yield results that are successful anyway. However, an attempted correction should be conducted as follows:

1.Trace the lens on a piece of paper as it would appear were the axis correct (Figure 10-23, A and B).

2.Turn the lens on the tracing to the correct axis (Figure 10-23, C).

3.Note the areas of the lens that are outside the tracing and mark them, taking into consideration the overall lens shape (Figure 10-23, D).

4.Remove the marked areas by hand edging them away.

If the integrity of the lens shape is restored and the lens still fits snugly in the frame, it may be used. With experience, it will no longer be necessary to trace the lens out on a piece of paper.

This practice is limited to plastic frames. Metal frames do not have the elasticity required.

CORRECTING FOR UNWANTED VERTICAL PRISM

When a high-powered pair of lenses is edged, a small amount of error in vertical positioning can induce vertical prism. Assuming that the prescription is for a plastic frame, it may be possible to hand edge the lenses as follows to reduce the unwanted vertical prism to zero:

1.Remove the lenses from the frame and respot on their MRPs.

2.Hold the lenses front to front8 so that their shapes exactly overlap each other. Take care to keep the lenses from touching each other so that they are not scratched. With lenses so aligned, as in Figure 10-24, A, the spotted MRPs will be seen one above the other. (If both lens meter spots and lens shapes overlap

8Lenses are held front to front to prevent parallax. When lenses are held back to back, it is more difficult to determine when the marks are truly aligned.

exactly, then the measured imbalance was due to a frame irregularity. The lenses are not the problem.)

3.Slide one lens up or down until the two MRPs overlap, as in Figure 10-24, B.

4.Remove extra lens material that extends above and below9 the areas of overlap, as marked in Figure 10-24, C.

9Simply removing extra lens material from the top or bottom of one lens eliminates the lensmeter-measured imbalance because the lower rim will shrink up around the lens and allow the frame to drop on that side. However, if the glasses were turned upside-down and measured for vertical imbalance, the distances from the often thicker upper rim will have remained unchanged. Part or all of the unwanted vertical prism may still remain. Because the upper rim is more of a reference for frame straightness on the face, this measure is more definitive.

The total B dimension reduction will equal the amount that the two MRPs were separated initially. If edging away the overhanging material reduces the overall B dimension too much, then the lenses must be remade.

Edge Polishing

The term edge polishing can have different definitions, depending upon the material being described.

EDGE POLISHING GLASS LENSES

In relation to glass lenses, edge polishing refers to the edge-smoothing process carried out on a fine or extra-