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

FIGURE 14-1 A nylon cord frame holds the lenses in place with a nylon cord that slips into a groove on the edge of the lens.

FIGURE 14-2 Some frames have thin metal rims. The lens edges are grooved and the rim slips into the groove on the lens and is tightened.

cause small edge chips or microcracks. These may reduce the impact resistance of the lens by a significant amount. A previous OLA technical director, George Chase, suggested that if the laboratory decides to make a grooved glass lens at the insistence of the prescriber or dispenser, a warning label should accompany the eyewear. The suggested wording is as follows:

To Prescriber/Dispenser: The glass lenses in this rimless mounting were made according to your work order. Instruct your patient to take care to avoid damaging the edge. Should the exposed edge of the lens become damaged, have it replaced immediately. Edge damage

reduces the original impact resistance of the lenses. Plastic lenses2 are strongly recommended for such mountings.

Edge Thickness for Grooving

The edges of a rimless lens will be visible and should be as thin as possible for best cosmetic effect. However, lens edges need to be thick enough to allow them to be grooved but still have enough lens material left on either side of the groove so that it will not chip off.

Chapter 13, on rimless eyewear, includes a section titled “Edge Thickness for Rimless Lenses.” If the reader has not yet read or does not recall the section, that section should be reviewed now, including the subsections on plus and minus lens edge thickness. These sections include cosmetic, safety, and functional considerations for edge thickness and apply equally to grooved mountings.

Another factor is important to remember when grooving a lens. The groove for the nylon cord is 0.5 mm wide. After the lens is grooved, enough thickness must remain on either side of the groove to keep the edge from chipping. Apart from safety factors, ideas on just how much extra thickness is required vary. Most practitioners recommend a minimum edge thickness of 2.3 mm.

Many polycarbonate and high-index minus lenses have thin centers. When these are edged for small frames, the lens edge can be less than 2.3 mm in places. These lenses may not be thick enough to be grooved. To keep the same frame, either a different finished lens will have to be selected, or lenses will have to be surfaced to ensure enough edge thickness for grooving.

Grooving Methods

The groove can be cut into the edge of the lens by using either a specially equipped lens edger, or a separate piece of equipment called a lens groover. The groover is currently the most common method.

The first type of edger to incorporate a grooving option was a dry-cut router-blade-type edger. By replacing the normal V-bevel blade with a flat blade having a small cutting nib, the lens may be edged flat and grooved at the same time (Figure 14-3). Several types of edgers have

2Polycarbonate and Trivex lenses are included in this broad plastic lens material classification.

FIGURE 14-3 The cutting blade that can edge and groove a lens simultaneously. The arrow shows where the blade cuts the groove.

a grooving option as part of the edging cycle (Figure 14-4). This topic is discussed again later in the chapter.

Lens Grooving Method

Although several different types of manually operated groovers are available, the explanation that follows is primarily for one commonly occurring type of groover. Each groover operates somewhat differently. However, the basic principles are the same.

PREPARING THE LENS FOR GROOVING

A lens groover requires a lens that has been edged flat, as if it were to be drill-mounted. Lenses to be grooved should have a safety bevel—but only a moderate one. Excessive safety beveling of a thin-edged lens that is to be grooved causes the lens edge to be weak after grooving.3

Setting Up the Groover

The cutter wheel in a lens groover is a water-cooled diamond abrasive wheel. Because the wheel is thin and so narrow, only a wet sponge is needed to keep it cooled and cleaned. The sponge is placed against the wheel from underneath and should be wet thoroughly before each use if it is not already sitting in a reservoir of water (Figure 14-5).

The cutter wheel protrudes through a wheel dome. The edge of the lens rests on the wheel dome and the lens turns on the groover. The wheel cuts into the lens at the same depth that the wheel protrudes from the dome.

The groover has a groove depth adjustment. To calibrate groove depth, the depth dial is turned until the cutter wheel is flush with the wheel dome. The depth dial should register zero (Figure 14-6). If it does not, the zero-adjuster knob is used to reset the dial to zero.

3Rimlon mounting instructions, Hoya Corporation, 1983, p 4.

FIGURE 14-5 A wet sponge keeps the cutting wheel of a lens groover clean and cool.

Before the grooving process begins, the depth for the desired cut is set. The groove depth required for nylon cord frames is normally between 0.4 and 0.5 mm.

POSITIONING THE GROOVE ON THE LENS EDGE

The position on the lens edge on which the groove is placed depends on the power of the lens and sometimes on the type of lens. When a lens has a fairly thin edge that is uniform in thickness all the way around the circumference of the lens, the only logical position for a groove is halfway between the front and the back surfaces. When the lens edge thickens or varies in thickness around the lens, the possibilities for groove location increase. The groove can be placed in one of the following positions:

•In the middle of the edge

•At a constant distance from the front edge and usually closer to the front

•At a constant distance from the back edge and usually closer to the back

Grooving the Lens in the Middle of the Lens Edge

When the lens is thin, the groove is placed in the middle of the edge (Figure 14-7, A). This occurs with low minus and low–plus-powered lenses and keeps the groove from getting too close to either surface.

FIGURE 14-6 When the cutting wheel of the lens groover is flush with the wheel dome, the cutting depth dial should register zero. The top arrow shows the cutting wheel.

A

B

C

FIGURE 14-7 A, For the thin-edged, low-powered lens shown in A, the groove is placed exactly in the middle of the lens edge. B, For thick-edged high minus lenses, the groove position is the same distance from the front edge of the lens, all the way around the lens. This is especially the case for high minus lenses with high cylinder. C, High plus lenses often base the groove location on the back surface of the lens. These lenses are viewed from the top. Lenses, particularly lenses B and C, vary in edge thickness around the perimeter of the lens.

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To center-groove a lens, the operator begins by correctly setting the guide arms. The groover has two spring-controlled guide arms with rollers. The rollers follow along the front and back surfaces of the lens as the lens rotates to hold it on center. To keep the lens centered for center grooving, these arms must be locked together in such a way that they may spread apart evenly as the lens passes between them. The two guide arms are locked together from underneath when two coupling pins connected with a spring are set for centering (Figure 14-8, A). In addition, a centering pin (Figure 14-8, B) locks the unit into a centered position.

The lens is placed between the two lens groover chucks (Figure 14-9). The lens should be positioned in the groover so that the front surface is always facing in one direction. For the groover shown, the correct direction is toward the right. The lens must face this way because the cutting wheel is angled slightly so that the groove angle conforms to the normal meniscus curve of the lens (see Figure 14-5). Once the pins are set, the guide arms are spread and the lens is lowered onto the wheel between the arms (Figure 14-10).

The groover has two switches. One switch causes the lens to rotate; the other turns the small grinding wheel on and off. The switch to rotate the lens is turned on and the lens begins to rotate. After the lens rotates about one quarter turn, the cutting wheel is turned on and begins to groove the lens.

After the lens has been grooved all the way around, the sound of the cutter wheel changes. When this happens, the cutter wheel is turned off. Afterward the lens rotation switch is turned off.

The lens is taken out of the groover and rinsed to remove ground up lens material. To remove ground lens material from the groove, either a toothbrush or an ultrasonic cleaner, or both, are used (Figure 14-11). When using an ultrasonic cleaner and leaving the lens in the unit for cleaning, the operator should be sure the lens is placed face up to prevent surface scratching. The operator should not reach into the water in the ultrasonic unit before turning the unit off. Continued exposure of the joints of the fingers to ultrasonic vibration causes arthritis. The unit is turned off or a metal basket used to lower objects into and remove objects from the ultrasonic cleaner.

Grooving the Lens toward the Front of the Lens Edge

The groover may be reset so that the groove will be based on a specific distance from the front of the lens edge rather than positioned in the center of the lens edge. The groove position normally is based on the front edge of the lens for higher minus lenses, especially when the lens has more than a small amount

C H A P T E R 1 4 N Y L O N C O R D A N D O T H E R G R O O V E M O U N T I N G S

Coupling pins positioned for groove centering

A

Centering pin

B

FIGURE 14-8 A, To cut a lens groove in the middle of the lens edge, first the two spring-connected coupling pins need to be positioned for centering. B, A centering pin locks the groover so that the lens groove will center.

of cylinder power. (The cylinder normally is created on the back surface of the lens.) This is not to say that the groove needs to be close to the front of the lens. The distance the groove may be offset from the front

surface may be varied. The distance from the front of the lens is based on how the groove will be positioned when it reaches the thinnest edge of the lens. Therefore groove location should be set at the location of the thinnest portion of the edge. Once set, the groove maintains that same distance from the front surface (see Figure 14-7, B).

To change the settings of the groover so that the groove location is based on the front of the edge, one of the spring-coupled pins is moved to the “F” or front position (Figure 14-12, A). The centering pin is removed. (The centering pin was shown in Figure 14-8, B.)

The lens is chucked and lowered between the guide arms. Next the operator looks to see where the cutting

FIGURE 14-11 When the groove is cleaned with an ultrasonic cleaner, the fingers must not dip into the solution with the unit running.

wheel will meet the lens edge and decides how far the groove should be from the front surface of the lens. The groove position control lever is turned (Figure 14-12, B). This will move the front surface guide arm, shifting the lens left or right, until the groove location is at the desired position.

To help in deciding exactly where the groove should be located, the operator rotates the lens and observes where the groove will fall. Noting where the groove will be as it comes around to the thinnest part of the lens edge is especially important. Opinions vary regarding what distance from the front surface to put the groove. One suggestion is to position the groove just far enough back from the front surface so that when it reaches the thinnest part of the minus lens edge it bisects the edge.4

4Lisa J: Answers to challenges that arise when grooving and drilling lenses, (contributed by Chris Farley, Gerber/Coburn), Eyecare Business, October 1998, p 20.

AB

FIGURE 14-13 Grooved Executive lenses must have the groove based on the back surface of the lens. Otherwise the groove can go off the lens edge at the ledge. The lens shown in A was grooved based on the front surface. The groove drops off the ledge from the top and then must reposition itself on the lower half of the lens. This type of groove is unusable. The lens shown in B was based on the back surface of the lens and does not encounter the ledge.

B

FIGURE 14-14 A, The setting for basing the groove position on the rear surface of the lens. B, Turning the groove position control lever to the left moves the groove farther from the back surface of the lens. Here the lever is to the extreme left. This is farther than normally would be used.

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The groove position control lever is turned until the groove location is at the desired position (Figure 14-14, B).

For Franklin-style lenses, the lens is rotated to the places where the lens shelf separates distance and near portions and the position of the groove is noted. (For any style lens, the operator should look to see where the groove will be located when it tracks along the thinnest part of the lens edge.) When it is certain that the groove will be placed properly along the entire circumference of the lens edge, the operator starts the lens rotating and turns on the cutting wheel.

For a summary of groove placements, see Table 14-1. See Figure 14-15 for another type of groover.

TOUCHING UP THE GROOVE

If one part of the lens groove is not exactly as wanted, the groove may be touched up by hand. The plate supporting the guide arms is lifted, which exposes the cutting wheel. The operator holds the lens in the hand and smoothes out the groove where needed.

C H A P T E R 1 4 N Y L O N C O R D A N D O T H E R G R O O V E M O U N T I N G S

TABLE 14-1

Groove Position by Lens Type

SAFETY BEVELING THE GROOVE

Grooves on the edge of lenses chip if they are too sharp. This is true especially for lenses with wider grooves. Two ways exist to safety bevel the inner groove edges5:

5DeFranco LM: Rimless and faceted eyewear, (contributed by Matt Vulich, AIT Industries), Eyecare Business, December 2000, p 24.

C H A P T E R 1 4 N Y L O N C O R D A N D O T H E R G R O O V E M O U N T I N G S

FIGURE 14-16 One method of safety-beveling the inside edges of the lens groove is to hand-smooth it with the corner edge of a ceramic block.

1.A ceramic wheel with a 90-degree angle is used. The groove is touched lightly to the corner of the wheel. The lens is turned evenly without pressure.

2.A ceramic block is used to rub a corner of the block along the inner edge of the groove by hand (Figure 14-16).

GROOVING POLYCARBONATE

Polycarbonate material softens with heat. Therefore, making multiple passes around the lens when grooving is advisable. The first pass is done at one half the groove depth. The second pass is done at or close to the final groove depth.6

The following steps can help ensure a nice-looking polycarbonate lens:

1.Groove the lens dry.

2.Leave only a little depth ungrooved.

3.For the last pass, groove the lens wet. This eliminates the frosted look and gives a more polished appearance to the groove.

If the edge of the lens is to be polished, it should be polished before it is grooved, instead of afterward. Polishing over a groove will wallow out the groove,

6DeFranco LM: Eight tips for processing rimless (or semi-rimless) eyewear, Eyecare Business, August 1999, p 37.

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affecting appearance and possibly security of the lens in the frame.

GROOVING FOR A RECESSED METAL RIM

Some frames are made with a metal eyewire rim that slips into a groove in the lens edge (see Figure 14-2). The rims of these types of frames are thin but not as thin as a nylon cord. This means that the groove in the edge of the lens must be wider than the normal 0.5 mm width.

Because the groove must be wider, the minimum edge thickness for these lenses also will be greater than for regular nylon cord frames. Minimum edge thickness recommendations may vary. A recommendation of 2.4 edge is minimum.7 A 2.4 edge thickness works best with high-index or polycarbonate lenses. For CR-39 lenses, a 2.8 minimum edge thickness is less likely to chip in the course of normal handling.

To create a wider groove, either a wider cutting wheel must be used, or the lens must be grooved more than once. Using a wider wheel is certainly easier and is more likely to give a better-looking end result.

Grooving for Recessed Metal Rims with a Wide Cutting Wheel

The groove-cutting wheel for recessed-rim mountings is 0.3 mm thicker than the standard cutting wheel.8 The cutting wheel protrudes through a slot in a small dome (see Figure 14-6). The slot is wide enough for the standard-width wheel but might be too small for the new, wider-width wheel. If the slot is not wide enough, the dome will need to be replaced with one that has a wider slot.

Grooving for Recessed Metal Rims with a Standard-Width Cutting Wheel

To groove the lens more than once, the groover is set to trace off either the front surface or the back surface of the lens. (It should not be set for centering, otherwise the additional groove simply will overlap the original groove.) The groove is set at a certain distance from the front (or back) surface of the lens. The lens is grooved as usual. Next the lens is reset slightly farther away from the front (or back) surface than it was before. If the groove is 0.3 mm larger than the standard groove, then the groove is moved back an additional 0.3 mm and regrooved for this distance. This widens the groove by an amount equal to the difference between the first setting and the second setting.

7Air Titanium Rim, Optician’s video. Section C. Frichsparken, Denmark, Lindberg Optic Design (undated).

8Air Titanium Rim, Optician’s video. Section C. Frichsparken, Denmark, Lindberg Optic Design (undated).

Grooving a lens twice with a narrow wheel and achieving the desired result is more difficult than grooving the lens once with the correct wheel width.

The process for mounting a lens-recessed rim is similar to that for lenses for nylon cord mountings. A ribbon is used to pull the rim around the lens. This procedure is described later in the chapter.

Grooving in the Edger

Many newer patternless edgers offer the option of grooving the lens in the edger. The increased precision of these edgers allows control that includes variable groove depth, groove width, and groove location. Figure 14-17 shows how the screen appears for lens grooving on one such edger.

Mounting a Grooved Lens

The procedures for mounting a new grooved lens in a nylon cord frame or replacing a broken cord and remounting the lens are similar. Restringing requires a few extra steps but includes everything required for mounting a new lens in a new frame. The following is the procedure for restringing:

1.Remove the old cord. Sometimes it is difficult to remove the end of the cord from the frame groove because it is wedged in the groove. If this is the case, a dental pick can be useful for pulling the end of the cord out of the groove (Figure 14-18).

2.Cut the end of the new cord at an angle. The end of the cord should be cut at an angle to make threading easier. A razor blade works best (Figure 14-19). If the old cord is unbroken, a new length of cord can be cut to match the old. Many choose to purchase monofilament fishing line as replacement line for nylon cord frames. The line is 0.4 mm (0.016 inch) in diameter. The old cord is used as a guide and the new cord length is cut to match.

3.Thread the cord into one side of the frame. The cord must be attached to the mounting at two locations. Each point of attachment consists of two small holes. Some people prefer to start with the nasal point of attachment; others the temporal. For illustration purposes, this example begins temporally.

Starting with the temporal point of attachment, one end of the nylon line is threaded into the lower hole from the lens side. Then the same end is

FIGURE 14-17 This grooving screen shows the lens shape. The highlighted groove % icon shows that 50% of the edge of the lens is in front of the groove center (leaving 50% behind the groove center). In other words the groove is centered on the edge. The almost vertical line in the lens shape shows where the second box down on the right is measuring finished lens thickness. The distance measures 1.1 mm from the center of the groove to the front and 1.1 mm from the center of the groove to the back. The third box down on the right side shows that the groove depth is 0.60 mm. The third icon from the left shows a groove width of 0.8 mm.

FIGURE 14-18 When it is difficult to remove a nylon cord from the lens groove of a nylon cord frame, a dental pick makes the job easier. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, ButterworthHeinemann [Figure 7-18].)