Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003
.pdf
C H A P T E R 8 E D G I N G
•To determine the way the edge will be finished, as when rimless lenses are needed
•To help determine if the lens is to be edged exactly on size, or will be edged slightly larger, as with certain plastic frames. Overriding the traced size and edging the lens slightly larger or smaller is possible. Size difference may be set at the discretion of the operator.
Newer edgers that receive information from previous data entry points in the laboratory will not need to prompt for lens and frame material. They will already have obtained that information.
PATTERNLESS EDGER VIEWING SCREENS
The viewing screen shows what the lens will look like when finished. A viewing screen can allow a number of options. The following are some of those options.
Viewing Screen that Shows the Lens Shape
The most basic screen type shows the shape of the lens as it appears when viewed from the front (Figure 8-39). This is important for several reasons, as follow:
•If the lens tracer has hit an irregularity in the frame eyewire, this will show up on the screen.
•If the frame has become deformed while being traced,
177
the operator may be able to pick it up by looking at the traced shape.
•The shape may not match the frame. Sometimes someone else scans the shape. This could have been either at order entry or off-site with a remote tracer. In either case the shape on the screen needs to be compared with the frame. If the frame and the traced shape on the screen do not match, the error will be caught and the lenses will not be ruined.
Screens that Show the Edge of the Lens
Edgers that show the edge thickness and shape will, of necessity, have the capacity to “feel” the front and back surface of the lens as described above. However, what is possible from this point on varies considerably.
If the edger can show the way the lens edge will look after edging, it usually allows some operator discretion on bevel placement, otherwise there would be no point in showing the edge.
Edgers should allow the operator to move the bevel forward and backward. In other words, the bevel may be moved closer to the front or closer to the back of the lens edge. But, for maximum control, the bevel should be able to move forward or backward different distances for different points around the rims of the frame. This may be done on the screen first to see what it will look like before being edged as shown in Figures 8-40 and 8-41.
FIGURE 8-39 Being able to see the shape of a lens on the viewing screen of a patternless edger allows a quick check to be certain that the correct lens shape is being edged.
178 |
C H A P T E R 8 E D G I N G |
FIGURE 8-40 The lower right quadrant of this screen shows the way in which the lens edge will look in cross-section on the temporal side. The upper half of the screen shows where the bevel apex is located relative to the front and back edges of the lens all the way around the lens. (It is as if this thick minus lens edge were removed from the lens and spread out flat on the screen.) The vertical line at II (lower-left area) shows where the cross-section in the lower right hand quadrant of the screen is located.
FIGURE 8-41 The bevel on the same thick lens pictured in Figure 8-40 has been moved back on the lens edge. It is possible to leave bevel placement to the preprogrammed settings, or to override these settings with what the operator feels is most appropriate for the prescription.
C H A P T E R 8 E D G I N G
Taking Advantage of the Third Dimension
Frame tracers will either trace in two dimensions or three dimensions. If the tracer traces in two dimensions, it only gives basic lens shape, exactly like a pattern would. If it traces in three dimensions, it tracks forward and backward and finds how the frame is curved (Figure 8-42).
Figure 8-43 shows a frame front with an eyewire curvature made for lenses with moderately curved front surfaces. If a lens has a very flat or a very steep front surface, either the frame will have to be reshaped or the
179
lens bevel modified. It can no longer simply follow the front edge of the lens.
Figure 8-44 shows a high-minus lens with a flat front curve. This flat-front lens will not fit into a curved-rim frame without modifying either the frame or the lens bevel position. If the lens is edged with bevel positioning based on the front edge of the lens (Figure 8-45), then the frame must be reshaped.
For example, what if this frame were traced with a frame tracer capable of tracing in three dimensions? Knowing this information, the edger is set to follow the
yFirst dimension
Third dimension
x
z
Second dimension
FIGURE 8-42 This drawing shows the three dimensions referred to for tracing and edging lenses. The first and second dimensions correspond to x and y coordinates in mathematics and correspond to horizontal and vertical measurements on a flat piece of paper. They resemble a flat screen television. The third dimension corresponds to the z coordinate in mathematics. This dimension is the out and in direction, in front and in back of the paper. If television were in three dimensions, the viewer would feel like the action was happening in the room.
FIGURE 8-43 This frame is made for lenses that have a moderately curved front surface, as do most lenses that are neither high-plus nor high-minus in power.
180 |
C H A P T E R 8 E D G I N G |
FIGURE 8-44 This lens is high minus in power. The front surface has little curve. It is almost flat.
frame (Figure 8-46). This moves the bevel to conform to the curvature of the frame as shown in Figure 8-47. (This is not a good frame choice for this particular lens prescription, but it is used in this discussion for demonstration purposes only.)
However, a frame tracer may trace in three dimensions but the patternless edger may not be able to use that information. The edger must be capable of interpreting the information given to it by the tracer and capable of edging in three dimensions.
LENS DRILLING AS AN OPTION
WECO makes an edger that has a drilling option (see Figure 13-53). The edger will drill holes, blind holes, and slots and blind slots. Blind holes and blind slots are holes and slots that go only halfway into the lens. A part of the rimless mounting presses into the blind hole or slot to give the finished spectacles more stability.
OPTIONS AVAILABLE ON PATTERNLESS AND PATTERNED EDGERS
Some options on patternless edgers are also options on patterned edgers. These include, but are not limited to, edge polishing, safety beveling, and edge grooving.
SELF-DIAGNOSTICS WITH EASY
CALIBRATION
When a piece of equipment becomes computer controlled, it should include some level of selfdiagnostic ability. At start-up it should be able to run through a program of self-check that will set up needed standards and check the various system components to ensure that they are functioning as they should. If they are not, the faulty system should be displayed.
FIGURE 8-45 A lens can be edged so that the bevel follows the front edge of the lens. In this image the edger is set to follow the front of the lens, maintaining a constant distance from the front edge.
C H A P T E R 8 E D G I N G |
181 |
FIGURE 8-46 A lens also can be edged so that the bevel follows the curvature of the frame, instead of the lens. This edger is set to follow the frame curve. If this option is chosen, the bevel will not always be an even distance from the front of the lens.
Patternless edgers should have some kind of system for easy calibration. This would include things such as wheel differential, eyesize, and axis. Some edgers dress the grinding wheels when they become worn and are not cutting as crisply as they should be. The extent of the calibration and diagnostics for patternless edgers varies considerably.
DECENTRATION CALCULATIONS
Patternless edgers may calculate decentration for the edger operator, but require the operator to decenter the lens in the layout/blocking process. Other edgers do not require the operator to physically decenter the lens. These edgers are made to work with the layout blocker. If a direct interface exists between blocker and edger, the lens does not have to be physically decentered nasally by the operator. The operator just positions the spotted lens so that the optical center (or major reference point) is in the middle of the blocker grid as if no decentration existed at all. Then one of the following two things happens:
1.The blocker moves the lens block over to where it would normally be positioned.
2.The lens is blocked right in the middle and the edger takes that factor into consideration when it is edging the lens.
FIGURE 8-47 The bevel’s appearance when the lens in Figure 8-44 is edged for the frame in Figure 8-43. Notice that the bevel is close to the front of the lens at the top but is farther back from the edge on both temporal and nasal sides of the lens. (Note: This lens/frame combination is not a good one but is chosen only for the purpose of clearly illustrating the way in which bevel location can be made to follow the curve of the frame instead of the curve of the lens.)
182 |
C H A P T E R 8 E D G I N G |
ABILITY TO INTERFACE WITH THE REST OF THE LABORATORY
A feature of growing importance for patternless edgers is the ability to interface with the rest of the laboratory. Patternless edgers are only one part of an optical laboratory. Increasingly optical laboratory equipment is being interconnected, tied together with laboratory management software programs.
A patternless edger may be completely self-contained, including frame tracing, blocking, edging, safety beveling, and edge polishing. It will prompt for frame dimensions and the wearer’s PD measurement. However, what if the order has been sent to the laboratory electronically with all of that information already entered? By having the capacity to accept frame tracings, frame dimensions, and PD from a laboratory software program it is no longer necessary to reenter all of the information. This saves time and eliminates one more source of human error that can occur by reentering existing information.
Using a bar code reader, the operator may scan a job tray (Figure 8-48). This tells the edger which job to call up. Any missing information may be entered and the job processed. It also tells the software program where the job is located. If any questions about the job arise, the tray may be located easily.
FIGURE 8-48 Bar coding each job allows information to be downloaded into the edger and at the same time keep track of the exact location of the job in the laboratory.
P r o f i c i e n c y Te s t Q u e s t i o n s
1.Definite relationships exist between pattern size and edger setting. Assuming a “standard pattern size” of 36.5 mm, fill in the missing information for each of the following lens size/pattern size combinations.
EYESIZE |
PATTERN SIZE |
SET NUMBER |
EDGER SETTING |
|
|
|
|
50 |
50 |
a |
b |
48 |
c |
–10 |
d |
45 |
e |
f |
37 |
g |
44.5 |
h |
44 |
50 |
36.5 |
i |
j |
k |
l |
–5 |
57 |
50 |
51.5 |
m |
n |
52 |
50 |
o |
p |
|
|
|
|
2.If a pattern is marked, “set –5,” which of the following is the pattern’s A dimension?
a.37.5
b.41
c.45
d.41.5
e.Cannot be determined from information given
3.A pattern measures 56 mm. If the frame to be used is a 58 20, which of the following edger settings will result in a correctly edged lens, assuming a correctly calibrated edger?
a.Set at 36.5 mm
b.Set at 38.5 mm
c.Set at 54 mm
d.Set at 56 mm
e.Set at 58 mm
4.A pattern has a B dimension of 47 and a pattern difference of 4. Which of the following is the pattern set number?
a.–14.5
b.–4
c.–6.5
d.–40.5
e.None of the above
C H A P T E R 8 E D G I N G |
183 |
5.Suppose a pattern has a size of 36.5 mm and has a “pattern difference” of 6. If the edger is correctly calibrated and set for 52 mm, which of the following is the measurement for the B dimension of the edged lens?
a.42.5 mm
b.30.5 mm
c.46 mm
d.58 mm
e.None of the above
6.An edger has a setting dial with a zero, plus-minus scale. A pattern has an A dimension of 53. The frame has an A dimension of 49. Which of the following indicates the edger setting?
a.–2 mm
b.–12.5 mm
c.–16.5 mm
d.–4 mm
e.None of the above
7.An edger has a setting dial with a zero, plus-minus scale. The pattern is marked “set –10.” The frame has an A dimension of 52. To which of the following is the edger dial set?
a.–10
b.+5.5
c.62
d.–15.6
e.None of the above
8.A “lenses only” order comes with the name of the frame, an A, a distance between lenses, and the lens circumference. The circumference measurement is 151.4 mm. You have the pattern. The pattern has a circumference of 146 mm. Which of the following would you use for an edger setting?
a.34.9
b.36.5
c.38.3
d.41.9
e.Not enough information given
9.An order comes with the patient’s old glasses, including the lenses. You are to put new lenses in the old frame. To be sure you get the right size, you measure the circumference of the old lenses. The circumference is 157 mm. You have the pattern and measure it. The pattern measures 161.8. Which of the following edger settings would you use?
a.35.1
b.36.5
c.38.1
d.41.3
e.Not enough information given
10.True or False? The groove for a nylon cord lens retention system can be produced on some edgers.
11.Which of the following statements about faceted lenses is true?
a.When the facet is used, high-minus lens edge thickness is reduced.
b.When the facet is used, high-minus lens center thickness increases.
c.When the facet is used, plus lens center thickness decreases.
d.When the facet is used, plus lens edge thickness decreases.
12.True or False? The term polished edges always means the edge has the same high luster as is found on the surface of the lens.
13.Which of the following pliers is helpful in the process of determining whether a lens is the correct size for a metal frame?
a.Fingerpiece plier
b.Endpiece plier
c.Half-padded plier
d.Eyewire closure plier
e.Lens rotation plier
14.“Mineral lens” is another name for which of the following?
a.High-index plastic
b.Polycarbonate
c.Glass
d.CR-39
184 |
C H A P T E R 8 E D G I N G |
15.Which of the following lenses are said to emit a sulfurous odor when edged?
a.Glass
b.High-index glass
c.CR-39 plastic
d.Polycarbonate
e.High-index plastic
16.Which of the following is/are some possible reason(s) for a bevel coming out incorrectly on a thin, high-index lens? (Note: More than one answer may be correct.)
a.A flexible plastic lens block was used instead of a stable, metal block.
b.A metal block was used instead of a flexible plastic block.
c.The wrong base block was chosen for the base curve of the lens being edged.
d.The lens was edged on a patternless edger equipped with measuring arms. As the thin, flexible lens was cut down, the curvature of the lens changed and caused the bevel to be misplaced.
17.Which of the following actions will not help prevent a high-index plastic lens from cracking during edging?
a.Using the edger’s reverse rotation option
b.Blocking the lens on a steeper base block
c.Reducing edger clamping pressure
d.Reducing the head pressure the edger exerts on the lens
18.Which of the following edging methods and/or a similar variation to this method yields a more stress-free polycarbonate lens?
a.Edging the lens wet
b.Edging the lens dry
c.Edging the lens wet until partway through the finishing cycle, after which the coolant is turned off and the cycle finished dry
d.Edging the lens dry until partway through the finishing cycle, after which the coolant is turned on and the cycle finished wet
19.Match the refractive indices to the correct lens material.
crown glass polycarbonate CR-39 high-lite glass
a. |
1.425 |
e. |
1.586 |
b. 1.498 |
f. |
1.621 |
|
c. |
1.523 |
g. |
1.70 |
d.1.530
20.Which of the following is not a way that may reduce spoilage of antireflection-coated lenses? (Note: Not every response may be necessary in every instance but should at least be considered helpful in some instances.)
a.Use surface-saver tape or discs on the lens.
b.Reduce chuck pressure on the edger.
c.Avoid heat on the lens when heating the frame and lenses together.
d.Leave the lenses blocked overnight before edging to allow initial stress to subside.
21.True or False? It is always best to increase the base curve to approximately +8.00 D when using prescription lenses in a wrap-around frame.
22.Which of the following is not a workable suggestion for keeping the bevel on the edge of an Executive lens?
a.Instead of a free-float bevel placement system, use a guided bevel system.
b.Use a patternless edger with measuring arms that “feel” lens thickness at the proposed location of the lens edge.
c.Edge the lens a bit large. Bring the final lens size down by hand to control bevel placement.
d.All of the above are workable suggestions.
23.True or False? Some patternless edgers are still capable of directly edging a lens from a pattern.
24.True or False? A patternless edger may ask for the frame material being used. Frame material will determine which roughing wheel should be used.
25.True or False? If a patternless edger viewing screen shows the proposed location of the bevel on the lens, that bevel position can be moved uniformly forward or backward on the lens edge.
26.True or False? If a patternless edger viewing screen shows the proposed location of the bevel on the lens, it may be possible to move the bevel selectively (e.g., moving the bevel back on the nasal and temporal sides while leaving the bevel toward the front of the lens at the top and bottom).
27.True or False? All frame tracers trace in three dimensions and send three-dimensional frame information to the patternless edger.
9Deblocking
The method used to deblock a lens depends on the type of blocking. Each is explained by
blocking method used.
Deblocking Lenses Blocked by
Suction
The method used for deblocking a suction-blocked lens is self-evident. When the suction cup seal is broken by lifting the edge of the suction cup, the lens deblocks easily. Grasping it with a plier and twisting it off also removes the lens.
Deblocking Metal Alloy–Blocked
Lenses
As mentioned in Chapter 7 (“Blocking of Lenses”), metal blocking of lenses is used rarely. It is not as environmentally friendly and has disadvantages when used with certain types of lenses.
Only two ways exist to deblock plastic lenses. The first is to twist or torque the lens to pop off the block (Figure 9-1). Unfortunately this is appropriate only for uncoated lenses. Flexing coated lenses should be avoided because this can damage the coating.
The second is to use a hot water system of deblocking. The blocked lens is placed in hot water and the block melts off and sinks to the bottom. If this method is used, the water must then be processed before it can be
185
186 |
C H A P T E R 9 D E B L O C K I N G |
FIGURE 9-1 If a plastic lens is twisted slightly, adhesion is broken and the alloy block drops off.
FIGURE 9-2 When pliers are used to deblock an alloyblocked lens, the long axis of the block is squeezed to deblock, not pulled.
poured down the drain because it contains heavy metals. This method works for glass lenses.
Metal blocks also may be removed from glass lenses using a large-jawed plier. The jaws are oriented so that they span the longest axis of the block, as in Figure 9-2. Deblocking is not accomplished by pulling the block off but rather by squeezing. As pressure is applied to the longer axis, slight flexing of the metal occurs, which breaks the seal between lens and alloy. The pliers never touch the lens. This method does not work well on plastic lenses because the plastic lens flexes with the block and does not release.
Deblocking ‘Wax’-Blocked Lenses
Deblocking the waxlike blocks used in the GerberCoburn Step Two blocking system is done with deblocking pliers. The block is grasped with the pliers and twisted from the lens.
Deblocking Adhesive Pad–Blocked
Lenses
Lenses blocked with adhesive pads use either metal or plastic blocks. Deblocking differences between metal blocks and plastic blocks are slight.
DEBLOCKING ADHESIVE-PADDED METAL BLOCKS
To deblock an adhesive pad–blocked lens, the lens is held firmly with a tool and twisted off. The “classic” type of deblocking tool is shown in Figure 9-3. It is shaped to accept the block in the same manner the edger chuck does.
The lens is held with a laboratory towel when twisting it off. If the lens is held without a towel the technician
FIGURE 9-3 An adhesive pad deblocker is simply a holding mechanism for the block. Deblocking occurs when the lens is twisted off the block. (This deblocker resembles the “classic-type” deblocker but is an “in-house” constructed model.)