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

FIGURE 13-16 A hex wrench is used to tighten a nut down. The head of the bolt is held in place with the finger of the other hand to keep it from turning.

a nut. A small metal washer is used between the nut and the bushing to keep the nut from crushing the bushing and possibly gouging the lens.10

The screw is clipped flush with the nut or strap (Figure 13-17). Next the screw is filed flush. A good file to use is a riffler file (Figure 13-18).

DETECTING MOUNTING FLAWS

One common mounting flaw is to tighten the mounting screws too hard. Screws should be tight. A visible slight amount of pressure distortion on the lens surface is even acceptable. But they should not be so tight that distortion is obvious.11 This means heavy pressure is in the area and an increased potential exists for the lens breaking at that point. Overtightening a screw also may cause an antireflection (AR) coating on a lens to craze. One suggested method for getting the correct tightness is to tighten the screw as far as it goes, then back it off a half turn.12

If two mounting holes are on one side of the lens for style or stability, those two holes must be spaced accurately. If they are too close together or too far apart, they will cause more stress on the lens. Using a high-end manual

10Yoho A: Keeping the rimless lens tight, Eyecare Business, August 2000, p 34.

11Yoho A: Keeping the rimless lens tight, Eyecare Business, August 2000, p 34.

12Herrick T: Three-piece mountings, when lenses are the frame, Lenses & Technology, April 2000, p 22.

FIGURE 13-17 Once the screw is in place, it needs to be clipped with a cutting pliers to remove excess length.

FIGURE 13-18 A clipped screw has a rough end. The end of the screw is smoothed after it has been clipped. The file being used is a spoon-shaped riffler file. Its form helps keep the rough file surface away from the surface of the lens.

lens drill or a computer-assisted drill system will reduce this type of flaw.

Notching Lenses

Lenses may be notched on either side and held in place with clips on the frame that fit into the notches. This type of mounting is called a Balgrip mounting or spring tension semirimless mounting. The clips are on either side

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of the top bar of the frame. The top bar is curved so that it must be pulled apart slightly so that the clips fit into the lens. The lenses are held in place with spring tension from the top bar. Use of notches alone to hold the lenses in place is rare. Instead notches are used in combination with drilled holes to increase the stability of the mounting.

When mountings with notched lenses were used more commonly, it was not unusual to see small machines called Balgrip Lens Groovers in the laboratory. These used a narrow grinding wheel to notch the lenses. When a request for notched lenses occurs, a piece of machinery like this is not necessary. Lenses even may be notched by hand with a rat-tail file. However, to use the drill with a milling bit to make the notch is much easier.

USING A MILLING BIT TO NOTCH A LENS

A milling bit may be used to notch the edge of a lens. It is much easier than trying to notch a lens by hand and does a better job. The milling bit may be lowered onto the edge of the lens as in drilling (Figure 13-19), or the lens may be pressed against the side of the milling bit. After the notch is created, the edges of the notch are smoothed (Figure 13-20).

FIGURE 13-19 A lens may be notched with a milling bit. In this photo there is a small piece of cardboard under the temporal half of the lens. This allows the lens to be angled some and also allows extra space under the lens for when the milling bit comes out the other side.

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Once drilled or notched lenses have been mounted, the drawn 180-degree lines should be along the 180 of the frame (Figure 13-21).

DRILLING POLYCARBONATE LENSES

For polycarbonate lenses the lens should not be drilled straight through in one pass. Instead about 0.5 mm at a time should be drilled. After each 0.5 mm, the drill is backed out and the drill operator waits for the material to cool. Then the next half-millimeter is done. Pressing through all at once “will push the material through and blow it out in a hump on the back side of the lens.”13 It also will cause the hole to be larger than intended and result in an unstable mounting.14 Drilling with a dull bit will also create unwanted heat, even when proper drilling techniques are applied. This, too, may result in a hole that is slightly larger than intended.

AN HISTORICAL NOTE ON HAND DRILLS

With plastic lenses, drilling a lens without an electric drill is possible but not necessarily efficient. The correct size drill bit can be mounted in a hand chuck with a swivel handle. To start the drill hole, the lens should be indented with a pointed instrument. The drill bit is placed in the indentation and turned but without a lot of pressure. Too much pressure with a hand or electric drill causes hairline cracks around the hole. When using a hand drill, the operator should not drill more than halfway through the lens before turning it over and drilling from the other side. Again, the operator begins by indenting the back surface of the lens with a sharp instrument at the intended point of drilling, then drills from the back. Applying too much pressure will cause plastic to break away next to the hole.

Notes Regarding Glass Lenses

Because plastic, polycarbonate, and Trivex lens materials are so much less likely to break in a rimless mounting than is glass, glass is no longer considered a good option for rimless mountings. If glass lenses are used in a rimless mounting, the lens must be drilled alternately first from one side, then the other, until the two holes meet halfway through the lens. This prevents the hole from chipping and ruining the lens as the drill comes out the other side. Glass lenses almost always chip out around the hole unless this is done. When drilling glass the operator should lubricate the place where the lens

13DeFranco LM: Eight tips for processing rimless (or semirimless) eyewear, Eyecare Business, August 1999, p 37.

14DeFranco LM: Drilling polycarbonate can be hot, Optical Dispensing News, No. 1, September 26, 2000.

and drill meet with oil to prevent heat buildup. Without oil, breakage increases and drill bits are ruined quickly.

Once again, glass lenses are not recommended for rimless or grooved mountings. The lens is weaker than a normally beveled and mounted lens and, though not illegal, is not advisable.

USING THE FRAME TO MARK FOR SEMIRIMLESS MOUNTINGS

With semirimless mountings, the shape of the top of the lens matches the curve of the top bar of the mounting. The following is the procedure:

1.With the frame flat on the table, hold the lens in front of the mounting as it should appear after it has been mounted.

a.The top of the shape should follow the curve of the top bar.

b.When correctly aligned, the top bar should have about one third of its thickness showing above the edge of the lens.15

2.Use a fine-point, felt-tipped marking pen to dot the location of the nasal screw hole.

15Levoy BM: How to work with rimless, Optometric Weekly, February 17, 1977, p 39.

FIGURE 13-22 This is a semirimless mounting. When no chart is available, the frame may be used as a guide for marking and drilling the lenses. First the nasal hole is marked and drilled. The lens is mounted in the nasal hole. With nasal hole drilled and lens mounted nasally, the lens is swung into position as it should appear when finished. Next the location of the temporal hole is marked as shown here.

FIGURE 13-23 On this semirimless mounting the nasal hole has been drilled and mounted. The temporal hole has been marked with the lens mounted nasally. Now the tem-

described in the section on lens drilling.

6.Mount the nasal side of the lens, leaving it loose enough to swing it away from its normal position.

7.Hold the half-mounted lens as it should look when correctly mounted. Mark the location of the temporal screw hole (Figure 13-22). (The biggest pitfall here is the possibility of misaligning the axis of the cylinder.)

8.Either remove the lens from the mounting and then drill it, or swing the lens upward or downward and drill the temporal hole with the lens still in the mounting (Figure 13-23).

9.Repeat the process to mark and drill the temporal hole for the other lens.

edging (as discussed earlier in the chapter). After edging, transparent tape is placed on the front of the lens to protect the surface. The nasal and temporal lens drilling positions are marked by using the notches on the edge of the pattern as guides (Figure 13-24). Next a strip of double-sided adhesive tape is placed on the inside of the drilling guide to hold it to the lens (Figure 13-25).

To drill the nasal holes, the drill guide is stuck on the lens so that the center line of the guide is parallel to the 180-degree line marked on the lens. For the temporal holes, the bottom line of the guide is parallel to the lens’ 180-degree line (Figure 13-26). The lens is drilled by pressing the drill bit into the holes in the drill guide (Figure 13-27).

DRILLING GUIDES

Some frame manufacturers make drilling guides available to simplify drilling. Silhouette uses a plastic guide for double-hole mountings. To use this system, the 180-degree line is marked on the uncut lens before

Improvements in Electric Drills

A variety of options are available with basic electric drills. These options become increasingly important because

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FIGURE 13-24 Here the pattern has been made to serve as a type of template in the drilling sequence. The edge of the lens is marked at the level of the notch on the pattern. This establishes the height where holes will be drilled.

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FIGURE 13-26 Here the lens guide is being applied to the lens.

of (1) the large variety of rimless designs available, (2) the necessity for drilling more than just two holes in one lens, and (3) the increased number of lenses that are slotted or notched. The following list outlines some improvements available with some electric stand drills (with options varying according to drill manufacturer):

•An edge gauge: An edge gauge allows the hole to be positioned a known distance inward from the edge of the lens (see Figure 13-8).

•A clamp-down holding system to hold the lens securely for drilling

•The ability to transfer measurements from one lens to the other: If the drill base guide has horizontal and vertical scales, one lens is marked first. It is then aligned and clamped in the drill base guide. The second lens then is clamped symmetrically in place. Hole location measurements are transferred to the second lens.

•A lens-holding system that leaves the adhesive-pad blocks on the lenses and uses the blocks to hold the lenses in the drill 16

(Figure 13-28): This system has several advantages. It assures that the lenses will be on axis. The 180degree line is exactly as it was for edging. It confirms that both lenses will be drilled the same. It makes replacement of a single lens easier if one lens was incorrect or if one lens has to be replaced later because of damage to the lens or a prescription change.

•The ability to tilt both lenses at once: When drilling a lens, the hole should be drilled approximately perpendicular to the first surface. If both lenses are mounted at the same time and one lens is angled for drilling, the second lens will be angled identically. This way both nasal holes will be tilted the same and

drilled. Then after readjusting the tilt, the operator may drill both temporal holes.

•The ability to transfer the frame manufacturer’s hole location specifications to the scale on the drill base guide: For example, vertical hole location could be specified as distance above or below the boxing center; horizontal location as distance from the edge of the lens. For example, if a lens has two temporal holes their locations could be specified as 5 mm above center and 3 and 6 mm in from the temporal edge of the lens, respectively.

•Twin drill bits: Twin drill bits approach the lens from both front and back sides at the same time. Drill bits meet in the middle and produce chip-free guideholes that will ream out smoothly.17

•Variable-hole diameters: Hole diameters can be varied without having to change drill bits.

•The ability to move the drill or table in a controlled manner to allow for slotting and notching of lenses

•Reflective screen: The reflective screen option allows a parallax-free view of the lens and drill.18

DRILLING BLOCKED LENSES

Some drills allow the lenses to remain blocked during drilling. Because the lenses are held in place by the

17The LAB-Tech DM-3 Drilling System (LAB-Tech, Inc., Miami) has this feature.

18The Drillrite Automatic Lens Drill (Nu-Tec Optical Equipment, Richardson, Texas) has this feature.

blocks, the cylinder axis cannot be off if the lens was correctly edged. The following is a representative drilling sequence for the type of drill that was shown in Figure 13-28.

Before edging, the base curve of the lens is measured (Figure 13-29). Once the lens is edged, it is mounted on the drill base. The lens is angled by tilting the lens to the corresponding base curve setting (Figures 13-30 and 13-31). This angles the lens so that the drill enters the front surface at the correct angle. (The importance of this was discussed earlier and illustrated in Figure 13-10.)

Drilling charts designed for ease of use with this particular drill show both horizontal and vertical measurements for all the holes and notches in the mounting. These charts also tell which drill bit sizes are needed for each hole (Figure 13-32). The vertical distance from the 180-line to the first hole is dialed. This moves the lens vertically (Figure 13-33).

With the horizontal scale set to zero, the edge of the lens is “zero-ed in” by sliding the base of the stand until the edge touches the drill (Figure 13-34). Now the stand is moved to the notch or hole location indicated on the chart using a screw-mechanism to set the horizontal distance on the scale (Figure 13-35). At this point the lens may be notched or drilled as needed (Figure 1336). In this example a notch and a hole are on the nasal side. After notching the lens, the measurements are reset. When necessary, a different size bit is used to drill the second hole (Figure 13-37).

FIGURE 13-28 This lens drilling system holds the lenses in place by the lens blocks. Because lenses are still blocked, it is not necessary to draw a 180-degree line on the lens. The clamping system ensures an accurate 180-degree alignment.

Slotted Lenses

Some lenses are held in place with a clip (Figure 13-38). The clip is slipped into a slot in the lens and stays in place by a tensile spring effect. This happens because the slot is slightly smaller than the distance between the two sections of the wire that makes up the clip. Clips may come in different lengths, depending upon lens thickness at the location of the slot. To provide enough lens substance for the clip, the lens must be no less than 2.7 mm thick at the position of the slot.19

19Air Titanium Optician’s Video (Lindberg Optic Design,

Frichsparken, Denmark).

FIGURE 13-29 With certain drills the lens may be tilted for drilling according to base curve. This lens has a +3.50 D base curve.

The lens is marked with a 180-degree line, R or L, and an N for the nasal side and placed on the drilling chart. In preparation for slotting, the center of the slot is dotted (Figure 13-39). Then the lens is marked with a line drawn perpendicular to the center of the slot (Figure 13-40). Unless the nasal and temporal slots are at exactly the same height, two lines will be on the lens, one for each slot.

If the slot is vertical, the line(s) will be parallel to the 180. The lens is secured in place on the table and the desired position of the slot centered (Figure 13-41). The table is placed on the base of the stand20 and locked into position (Figure 13-42).

A drill cuts into a plastic lens by being pressed into the lens from above. However, with use of a “milling drill bit,” the “drill” can now cut sideways. To cut a slot in a lens, the milling bit must have the ability to move laterally.21 The milling drill bit is now set to move left

20A space bar is used between the back edge of the table and the stand. The thickness of the bar causes the location of the slot to come out as needed.

21It would be possible to construct a table for the drill stand that moved the lens laterally.

FIGURE 13-30 The +3.50 base lens from the previous figure has been edged. It is still blocked and has been mounted on the drill. For this drill, lenses with a base curve of +4.00 D and below are considered flat enough not to require any tilt when drilled.

and right from the center point of the slot by equal amounts. The total length of the move corresponds to the length of the slot.

The milling bit is lowered onto the lens and drilled through the center of where the slot will be (Figure 1343). A locking mechanism is loosened (Figure 13-44), the milling bit pressed slightly into the lens surface, and then moved laterally. This removes plastic along the whole length of the slot (Figure 13-45). The bit is lowered into the lens a little at a time, each time moving the bit back and forth. This process is repeated about 15 to 20 times until the slot goes all the way through the lens. (The number of times varies, depending upon lens thickness.)

Once the slot is through the lens, the bit is raised. It is moved to one end of the slot and the end of the slot “drilled.” This process is repeated for the other end. This smoothes the inside ends of the slot.

Next the slot needs to be chamfered (safety beveled). This may be done with the drill, a cone-shaped tool, or by hand with the same type of tool. When done with the drill, the cone-shaped tool is mounted like a drill bit. The lens is held in the hands and moved back and forth

FIGURE 13-31 For comparison we see how much tilt is required to drill this +8.00 D base curve lens. The drill is basically perpendicular to the front surface of the lens.

across the spinning cone in the drill. Both the front and the back of the slot must be beveled. When safety beveled with the hand tool, the cone-shaped hand tool is rotated in the slot (Figure 13-46).

The clip is cleaned with alcohol (Figure 13-47) and the clip/slot fit is checked (Figure 13-48). The clips should slide into the lens easily. If force is used, stress will be introduced into the lens and the possibility of breakage increases. Clips from looped wire may be expanded or compressed (Figure 13-49). However, the top and bottom wire must remain parallel with each other so that they contact the ends of the slots evenly.

When all is prepared, glue is added to the inside slot of the lens or to the clip (Figure 13-50) and the clip secured in the lens (Figure 13-51). When glue is applied to the inside of the slot, excess glue should be wiped off the lens surface with alcohol before the clips are pressed into the slots.

22Freeburg S et al: L & T review, OptiDrill CNC Drilling System, Lenses & Technology, April 2001, p 20.

FIGURE 13-33 To set up the nasal side for drilling for the mounting in the previous figure, the hole is set for 4 mm above the midline.

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FIGURE 13-34 With the horizontal scale set for zero, the stand table is moved until the edge of the lens is pressed tight against the tip of the drill bit. This establishes the horizontal zero point for the nasal holes of the right lens.

FIGURE 13-35 In preparation for cutting the notch, the table is moved 0.5 mm using a crank handle.

cost of computer-assisted drills is justifiable only for laboratories that have a high volume of drill mountings.

Computer-assisted drills have, or should have, a capacity to store and recall a database of drill, slot, and notch positions for a large number of mountings in their full range of available sizes and shapes.

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FIGURE 13-36 The drill accurately “notches” the lens 0.5 mm.

Other features available for computer-assisted drills will be similar to those features listed for manual drills. In evaluating computer-assisted drills, the same features that are attractive for manual drills should be searched for in a computer-assisted drill.

Computer-assisted drills generally interface with a personal computer (Figure 13-52).

THE IN-EDGER DRILL

WECO has an edger that drills the lens while it is still in the edger (Figure 13-53). To use this option, the WECO blocker also must be used. An image of the drilled demo lens or the wearer’s old drilled lens is projected on a screen in the blocker during layout. Then a cursor is moved to the location of the hole and an X is placed over the future position of the holes. The edger edges the lenses then drills them as indicated in the blocking process.

Working with Older-Style,

Double-Strap Mountings

Older style rimless mountings have a double-strap assembly and were used extensively in years past.