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

Mountings that have the central bridge area attached to the endpieces with a metal bar that runs along the top, back side of the lens are called semirimless mountings (Figure 13-2).

Rimless mountings may be further classified by how the lenses are attached. A style of mounting that holds notched lenses in place by spring tension, with clips, is called Balgrip mountings. Notching is now seldom used alone. More often it is used in combination with drilling to lend stability to the mounting.

Frames whose lenses are grooved around the edge then held in place with nylon cord are called nylon cord frames, string mounts, or nylon supras. Nylon cord frames are a category by themselves. However, they still sometimes are referred to as rimless. Nylon cord frames are considered in the next chapter and are shown in Figure 14-1.

PARTS OF A RIMLESS MOUNTING

Parts of a rimless frame are unique and have specific terminology. If a rimless mounting attaches the lenses to the frame with screws, the screw passes through a part of a traditionally designed mounting called a strap. Technically the strap is an area of the mounting. In the original type of rimless mounting, this area consisted of the following parts (Figure 13-3):

•The shoe (or shoulder) that braces against the edge of the lens

•Between the shoe and the lens edge may or may not be a thin metal spring

•An ear (or tongue) extends behind the lens. Sometimes the lens has two of these—one on the front surface of the lens and one on the back. Together

C H A P T E R 1 3 D R I L L E D , S L O T T E D , A N D N O T C H E D M O U N T I N G S

Spring

Shoe (shoulder)

Strap area

Ear (tongue)

FIGURE 13-3 A close-up of the strap area of an oldfashioned rimless mounting. Newer mountings will not have as many parts. In most cases, only one “ear” or “tongue” is on either the front or back of the lens, but not on both sides. Or, this part of the strap area may not even exist on many new mountings. (From Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, ButterworthHeinemann [Figure 1-22].)

with the screw they clamp the lens in place. Most people call the ear the strap, even though technically the strap is the entire area. However, because of common usage, this chapter does the same.

Now, however, mountings are widely different in the specifics of how the frame is constructed at the point of attachment to the lens.

Types of Lens Drills

A number of options are available for drilling lenses. These include the following:

•Nonelectric hand-held drills

•Basic electric stand drills

•In-edger drills

•Computer-assisted drills

The information contained in this chapter begins with principles and builds with explanations of drilling options in the following sections. Procedures explained in sections on the most basics methods are applicable to other methods also and help in an overall understanding of how lenses are mounted.

Wearer Safety Issues

249

withstanding an impact of a 5/8 inch steel ball dropped from a height of 50 inches. All glass lenses had to be hardened to increase their impact resistance. The only available method for hardening glass lenses was heat tempering. Heat tempering creates internal stress in the lens. Drilling a lens and screw mounting it to the frame also sets up stress on the lens. The combination of heat tempering stress and lens mounting stress almost guarantees that the lens will break if struck on the edge. Because glass lenses were still a large segment of the market, this drastically reduced the use of rimless mountings.

Chemical tempering is now the method of choice for hardening glass. Chemical tempering makes it possible to drill-mount a glass lens. This type of lens passes the drop ball test but is still not a good option for rimless eyewear compared with other lens choices available in plastic materials. An Optical Laboratories Association (OLA) technical paper states that drilling (or grooving) a glass lens “produces microscopic surface cracking, which will inevitably propagate and spread. The OLA strongly advises its members against it.”1 For liability reasons most laboratories are not willing to put glass lenses in a rimless mounting, although no specific prohibitions exist against it.

COMPARING LENS MATERIALS FOR RIMLESS USE

Of the available lens materials, glass is the least desirable for rimless eyewear. For a number of years the standard lens material used in rimless eyewear was the standard CR-39 plastic. However, CR-39 is not as good as some other lens materials because it has a tendency to flake and crack at the mounting points.2

Polycarbonate material is used regularly for rimless eyewear, but its use is associated with a few downsides. Because polycarbonate lenses are softer than regular plastic lenses, if the lens is not drilled properly and too much heat is produced, the hole may enlarge and cause the mounting assembly to loosen. Because of internal stress when mounted, polycarbonate can still crack at the mounting points, although it is much less likely to do so than conventional plastic materials.

Trivex is emerging as a highly desirable material for rimless eyewear. Like polycarbonate, Trivex is also highly impact resistant, yet it does not crack or distort around the holes of a rimless mounting, even when surfaced thin.3

In 1971 the Food and Drug Administration passed a ruling that mandates that lenses must be capable of

1Bruneni JL: Ask the labs, Eyecare Business, February 1996.

2Morgan E: Working with Trivex, Eyecare Business, February 2003, p 30. 3Bruneni JL: Ask the labs, Eyecare Business, January 2003, p 30.

Edge Thickness for Rimless Lenses

No one wants a thick edge on a rimless lens. Everyone sees the edge. This should be a consideration for dispensers. On the other hand, rimless lenses have to have enough thickness for the drill mountings. This raises the question of what types of lens shapes and materials are most appropriate for rimless. Although the question addresses plus and minus lens edges, one type of lens is not well suited for rimless: the Franklinstyle (Executive) lens. This lens is unwieldy for drilled lenses and for the type of grooved mountings that are discussed in Chapter 14.

MINIMIZING MINUS EDGES

In making a minus-powered lens for a rimless mounting, thickness can become an issue, especially for higherpowered minus lenses. Edge thickness can be a problem from a cosmetic standpoint because, without frame rims, the lens edges are highly visible. Edge thickness also can make mounting the lenses more difficult. Dispensers should be advised of the possibilities of producing a thinner lens edge if the minus lenses they order are producing thick edges. The following are some ways to make the edges look better:

•Using a higher-index material

•Using a lens material that can be made thinner in the center, which produces thinner edges (e.g., polycarbonate, Trivex, and a number of high-index plastics)

•Using an aspheric design to reduce minus edge thickness

•Polishing the edges to reduce visibility

•Using as small an eyesize as feasible

•Using a lens shape that does not have long corners— in other words, a frame shape in which the effective diameter is not much bigger than the eyesize

•Using an antireflection coating to reduce lens visibility

PLUS LENS EDGE THICKNESS ERRORS

Plus lenses may have edges that are either too thick or too thin. Each situation is discussed individually, beginning with the plus lens with a thick edge.

Overly Thick Plus Edges

Usually two possibilities exist when a plus lens edge thickness is too thick for a rimless mounting. The first is that a plus lens has been taken from stock as an uncut lens and is edged for a small frame. To bring the edges down to a reasonable size, the whole thickness of the lens must be reduced. To do this, a semifinished lens is

surfaced to power for that particular frame. Using a large, higher–plus-powered finished lens blank for a small frame is inappropriate. (This was explained in Chapter 2 and shown in Figure 2-3.)

The other occurs through frame selection error. The following are some ways to equalize and minimize lens thickness around the whole circumference of a plus lens:

1.Choose a frame that does not require much decentration. In other words, the A and distance between lenses (DBL) of the frame should not be much larger than the wearer’s PD to prevent a thick nasal edge.

2.Choose a shape that is closer to a round shape than it is to a narrow oval shape. A frame that is narrow vertically causes a plus lens to have thick upper and lower edges.

3.Choose a shape in which the effective diameter of the frame is not much larger than the frame eyesize.

4.Use an aspheric lens design.

Of course high-index materials reduce overall lens thickness. However, a stock high-index lens will not necessarily have a thinner edge compared with a lowerindex lens that is custom surfaced for the wearer’s frame.4

Thin Edges

A plus lens edge can be too thin for a rimless mounting. A low minus lens with a thin center also can have an edge that is too thin to use for rimless mountings. Because no rim exists to protect the edge of the lens, an edge that is too thin may chip. It also must be thick enough to endure the stress of being mounted at only two points on the lens. A lens that is too thin can break or crack at the mounting point.

How thin is too thin? Opinions vary regarding how thin a lens may be. Some say that the minimum edge thickness for a rimless lens should not be less than 1 mm.5 If the particular lens material being used at this thickness would not be able to withstand the required impact test, 1 mm will be too thin. Others state the “absolute minimum” to be 1.5 mm, whereas still others say to plan for a minimum of 2.0 to 2.2 mm.6

Polycarbonate lenses and lenses made from Trivex are strong enough to hold up extremely well in rimless, even with somewhat thinner lens edges. Most other midto high-index plastic materials are suitable for

4For more on correct frame/lens selection, see Brooks CW, Borish IM: System for ophthalmic dispensing, ed 2, Boston, 1996, ButterworthHeinemann [Chapter 4].)

5Herrick T: Three-piece mountings, when lenses are the frame, Lenses & Technology, April 2000, p 20.

6DeFranco LM: Eight tips for processing rimless (or semirimless) eyewear, Eyecare Business, August 1999, p 36.

C H A P T E R 1 3 D R I L L E D , S L O T T E D , A N D N O T C H E D M O U N T I N G S

rimless, although they do not have as much resistance to fractures at the drilled areas as do polycarbonate and Trivex. The primary weak point with plastic lenses made from another material than polycarbonate and Trivex is that they may not have as much edge strength as desired if edges are thin.

Flatter Base Curves

It is generally easier to mount a rimless lens with a flatter base curve than one with a steeper base curve. If the plus-powered lens prescription is one that calls for a steep base curve, then an aspheric lens, instead of a regular, nonaspheric lens, should be used. Aspheric lenses may be designed with flatter base curves without degrading the optical quality of the lens. Going to an aspheric design is much more preferable than simply flattening the basic curve of a nonaspheric lens.

Marking and Drilling the Lens

Before drilling a lens, the operator must know exactly where the holes are to be located. If the hole is misplaced, the lens will be too high, too low, or tilted in the frame. Or it may be impossible to mount if the holes do not line up with the mounting points on the frame. Therefore the lenses must be marked or the drilling unit programmed for the correct hole locations ahead of time.

If the lens hole locations are to be marked, they may be marked with use of a drilling template or chart if it is available. If a chart is not available, then the actual frame may be used.

LENS-DRILLING CHARTS, GUIDES, AND TEMPLATES

A lens-drilling chart is a series of pictures of pairs of drilled lenses. These pictures are actual size drawings of the lens shape in every available eyesize. A number of different lens shapes may exist for the same mounting, because rimless lenses are not constrained by rims that limit their shape. The same mounting may look totally different when the shape of the lens changes.

Rimless frames should come with a drill-guide, a template, or an actual-size pattern with drill markings on the pattern. If none is with the frame, it should be available upon request. A drill guide is a picture of the lenses and where the holes are supposed to be. A template looks just like the lens and is placed on the lens. Holes are drilled through the template and on through the lens.

Before using a mounting chart, the operator must first think about the finished product. Even a mounting

251

chart must be considered in perspective. For example, older types of rimless eyewear have straps that end up on the front and back surfaces of the lens. If the lens is a high-minus lens, the holes in the straps will be closer to the edge of the lens than for a plus lens. This is because they have to be bent open farther to accept the thickness of the lens and therefore are closer to the edge of the lens. The operator should bear in mind how lens power and frame construction may affect lateral hole position before marking the lens for drilling. The vertical position of the holes always will be as indicated on the chart.

Steps to Mark a Lens Using a Drilling Chart

1.Find the size and shape that matches the edged lens.

2.Place the edged lens on the corresponding picture.

3.Line up the lens exactly over the drawn shape. To line the lens up exactly, one eye is closed and the open eye positioned exactly above the lens.

4.When ready to mark the lens, shift position so that the sighting eye is exactly over the location of the hole to be marked.

5.Use a fine-point felt-tipped marking pen to dot the location of the center of the hole with a small dot (Figure 13-4).

6.Reposition yourself so that your open eye is exactly above the next hole. Repeat the procedure for each additional hole to be drilled. Both lenses must be marked.

7.When both lenses have been marked, check to ensure that both lenses are identical by holding right and left lenses back-to-back. Their shapes must overlap exactly. Corresponding dots must be lined up over one another.

FIGURE 13-4 When using a drilling chart, the operator should be positioned with the sighting eye directly over the place on the lens being marked.

USING COQUILLES TO MARK THREEPIECE MOUNTINGS

Three-piece mountings have certain advantages over semirimless mountings regarding lens drilling. The distance between the screws on a semirimless mounting does not have much flexibility. The distance between the nasal and temporal holes in the lens cannot be incorrect.

For a three-piece mounting, the distance between nasal and temporal lens holes is not limited by the top bar. Nothing is connecting the central nosepiece area of the frame to the temporal endpieces.

Before the positions of the drill holes are marked, many recommend drawing a 180-degree reference line across the three lensmeter dots with a permanent felttipped overhead transparency pen. The line remains throughout the process. When the lenses are mounted, the lines on both right and left lenses should go straight across the frame. This line may be drawn before or after edging (Figure 13-5). After drawing 180-degree lines on the lenses, edged lenses are held back-to-back to be sure both have identically straight 180-degree lines (Figure 13-6). If they do not, the axis of one lens may be off. If one line is higher than the other, unwanted vertical prism may be present. Finding an error at this point will save wasted time later on.

Many frames come with thin plastic demonstration lenses to make the frame look better on display and to lend stability to the frame. These lenses are called coquilles, or dummy lenses. If the frame came with attractively

mounted coquilles, the coquilles may be used as guides rather than a chart. To use the coquilles, they are removed from the mounting. Some recommend holding the right edged lens in a back-to-back position with the left dummy lens. (When using this technique the lenses

FIGURE 13-6 Once the lenses are edged and the 180degree lines drawn on both, hold the lenses back to back. When the shapes are superimposed, the drawn lines should overlap exactly.

FIGURE 13-7 When using the frame’s sample lens or the wearer’s old lens as a guide, the eye should be positioned directly over the hole and its location marked.

must be right-with-left or left-with-right. This is harder than just holding the edged right lens directly over the dummy right lens, back-to-front. Those who advocate this right-with-left technique, as backward as this seems, maintain that marking right-with-right is more likely to cause the lenses to come out incorrectly.)7 With experience, each will choose the method that brings them the best results. When the two lenses are aligned, the eye is positioned directly over the place where the hole is to be drilled and the desired location of the hole is marked (Figure 13-7).

DRILLING THE LENS WITH BASIC ELECTRIC STAND DRILLS

The electric stand drill is made like a miniature drill press. The difference between the various types of stand drills is in the way the lens is steadied and held in place so that the drill will create the hole in the intended position on the lens.

A basic stand drill has a base that the back surface of the lens is rested against. The lens is hand held. The marked location for the hole is centered on the base and in some models may be braced against a small millimeter gauge that is rotated to increase or decrease the distance from the edge of the lens to align the location of the hole (Figure 13-8). Once positioned, the gauge is locked in position (Figure 13-9). The gauge is especially useful when the vertical position of the hole is marked on the lens. The horizontal position is set for a certain number of millimeters from the edge of the lens by using the gauge.

FIGURE 13-8 Some drills have a small-millimeter gauge. The gauge is rotated until the marked drill point is exactly over the proposed location of the hole.

The lens should be held so that the angle of the hole will be perpendicular to the front surface of the lens8 (Figure 13-10). Without turning the drill on, the drill bit is lowered to a position just above the mark on the lens. If the lens is accurately aligned, the drill is turned on and used to slowly drill through the lens (Figure 13-11). A foot switch is almost a must when drilling because both hands are usually busy with the lens. Pressing too hard causes the lens hole to chip as the drill passes out the back surface of the lens.

The drill bit should go through the lens smoothly. If it does not go through smoothly or goes through more slowly than it should, it is dull and needs to be replaced.

Unless the drill has a lens mounting system that allows the lens to be exactly flipped over, or unless the drill has two drill bits that drill the lens from both sides at once, it is not advisable to turn the lens over and drill from the other side. (Glass lenses are the exception.) Drilling from both sides without the appropriate mechanical assistance makes it too easy to line the hole up wrong and mess up the lens. This is especially true for high minus lenses with thick edges.

7Levoy BM: How to work with rimless, Optometric Weekly, February 17, 1977, p 34.

8Field C: Processing drill-mounted eyewear, LabTalk, 28(42):32, 2000.

254

FIGURE 13-9 Once the gauge is correctly positioned, it is locked.

FIGURE 13-10 The left-hand side of the figure shows the angle of a hole incorrectly drilled. The hole is perpendicular to the lens plane, but not perpendicular to the lens surface at the location of the hole. The right-hand side shows the correct angle. The lens must be angled so that the hole is approximately perpendicular to the front surface of the lens at the location of the hole.

C H A P T E R 1 3 D R I L L E D , S L O T T E D , A N D N O T C H E D M O U N T I N G S

FIGURE 13-11 When pressing the drill bit through the lens, do not press too hard. Drilling through the lens too quickly will cause the back side of the hole to chip.

If the angle of the hole is slanted instead of being perpendicular to the front surface the lens may not mount properly. If this is the case, the hole is straightened by putting the drill back in the hole and, with drill running, the lens is turned to the correct angle. This elongates the hole somewhat and may make it possible to mount the lens.

Chamfering and Smoothing Holes

A drill hole that has not had the sharp edges taken off is just like an edged lens that has not been safety beveled. It is likely to chip. Smoothing off the sharp edges of the hole is called chamfering. Chamfering is the equivalent term for safety or pin beveling. To chamfer the edges of the hole either a round diamond burr that is at least twice the diameter of the hole9, or a small cone tool (Figure 13-12, A) is used. The front and back entrances of the hole should be smoothed lightly (Figure 13-12, B).

The inside of the hole can be smoothed with a rattail file. A rat-tail file is a small-diameter, round file that starts narrow at the tip and increases slightly in diameter

9Yoho A: Keeping the rimless lens tight, Eyecare Business, August 2000, p 34.

C H A P T E R 1 3 D R I L L E D , S L O T T E D , A N D N O T C H E D M O U N T I N G S

255

A

B

toward the center of the file (Figure 13-13). The same file may be used to chamfer the edges in the absence of a diamond burr or cone tool (Figure 13-14). However, a rat-tail file cannot be expected to provide the same uniform smoothness.

ATTACHING THE DRILLED LENS TO THE MOUNTING

When assembling the drilled lens and mounting, the operator should use a screwdriver that has a plastic sleeve around the blade to reduce the chance of slipping and gouging the lens surface (Figure 13-15). Lens screws are no different from other screws in that they may

loosen with time. For this reason some people like to use a screw-locking compound on the screw to keep it from loosening. These locking compounds are available from optical suppliers.

Often hex nuts are used to hold the lens in place and are screwed on with a hex wrench (Figure 13-16). To cushion the lens and reduce lens fractures, use of plastic bushings in the drilled holes may be advisable. A plastic bushing looks like a miniature top hat. The plastic bushings are put in from both sides of the hole before the lens is mounted.

Many rimless mountings have only one strap. That strap may be on the front or the back of the lens. If the strap is on the front, the back side of the lens will have

256

FIGURE 13-13 Rat-tail files are tapered and come in a variety of sizes.

C H A P T E R 1 3 D R I L L E D , S L O T T E D , A N D N O T C H E D M O U N T I N G S

FIGURE 13-14 A rat-tail file may be used to smooth the inside of a drilled hole or to enlarge its diameter.