How Edging Precision Affects Drill Mounted Eyewear

By Chuck Fields
Processing Drill Mounted Eyewear (DME) requires greater axis accuracy than processing regular plastic or metal frames. A slight axis twist that could be overlooked or adjusted in other frame types will result in a re-do for DME. Here’s why. After I sold Multi-Facets Lab and was training labs to process DME for Silhouette Optical, I referred to the drilling section of the labs I trained as the “Bellyache” section, because if there were any problems in finishing, drilling would find it first. An axis twist can come from any of the main processing steps including blocking, edging or drilling. There are various ways to determine where the problem occurs, but over the six years of Smart Lab customer support calls, we have found that almost always the problem can be traced to the edger. Lens Shape Mirroring is the Reason There are amazingly efficient edgers on the market, but the reason why an axis twist coming off the edger is more noticeable than from either the blocker/tracer or drilling system is the way that edgers work using mirroring of the lens shape for right/left lens processing. In effect, any axis twist that occurs on the edger is doubled by the mirroring and so is easier to notice. The way this works is that the lenses are held in the edger by the finishing block inserted into the “chuck” or “cup holder” of the edger and if either the actual “chuck” is slightly off a true 180 line reference or if there is finishing pad slippage, then the right lens could be “nasal up” and the left lens would then be “nasal down”. Again the mirroring of the lens shape is the problem, because if either type of twist occurs, the blocked lens always moves in the same direction in the edger and against the block, but with lens mirroring, the right lens shape is moved in an opposite direction to the left lens shape. Why it matters more to DME The reason why this matters more in DME is that all the drilling systems, both computer and manual, use the finishing block to hold the lenses in the drill. And all the drilling systems are designed with the assumption that the lenses will be “on axis” from the edger. So when an axis twist has occurred in the edging cycle, and the lens shapes are twisted as seen in drawing one or two, then no matter how exactly on axis the blocks are locked into a drilling system, the end result will be different vertical heights of the holes in the right lens when compared to the left lens. This will be seen (Drawing three) when the lenses are already drilled and mounted, and unfortunately after any possibility of correcting the problem. Edger Misalignment or Finishing Pad Slippage You can identify which aspect of edging is causing the lenses to be misaligned by determining if you have a consistence or irregularly occurring axis twist. A consistent twist, such as the right nasal holes are always at a higher vertical height than the left nasal holes would be a mechanical problem. This means the block is held slightly off a true 180 alignment by the edger “chuck” so the same error occurs every time. (And please note that this means that ALL lenses are slightly twisted. It’s just that in DME frames the twist is more apparent.) All edgers have procedures for axis alignment of the “chuck” or “cup” holder and most patternless edgers have stored shapes with very sharp corners that help in determining in which direction the twist occurs. Check your service manual or call your edger’s customer service department. They should give you directions for adjusting this alignment. But remember you have to test the “doubling” of the error that happens with lens mirroring. This type of lens slippage can be caused over time by worn drive belts, an old back surface pressing/contact pad or other worn assemblies. (See Drawing one for an example of a Mechanical Twist) Also we have PDF’s of testing procedures on our Web site: smartlab.cc, that are helpful. The dotted line shows the true 180 line. The “chuck” of the edger is slightly off axis, but since lens shapes are mirrored, the nasal and temple sides are reversed. The other way your lenses could be off axis is an occasional twist that is usually caused by slippage of the lens against the finishing pad that occurs during edging and is more often seen with AR coated lenses, thicker minus powers or “softer” lens materials that tend to grip the wheel more. Another reason for pad slippage is when the blocking/clamping pressure is too low. There is a fine line where the clamping pressure is enough to securely hold the lens, but not too great to craze the lens coating. Also using a rigid finishing block that is not matched to the base curve of the lens will greatly decrease the surface contact area and allow slippage. Some of the newer finishing blocks are made using more flexible materials that will conform to any base curve and will greatly reduce operator error from incorrect base curve matching. (See Drawing two for an example of Pad Slippage) The dotted line shows the true 180 Line. The “chuck” of the edger is exactly on axis, but the finishing pad has allowed the lens to twist in the edging cycle. The twist again moves in the same direction, but because the lens shapes are mirrored, the nasals and temples sides move in opposite directions for both lenses. Regardless of what caused the lens twisting, the end result will be different vertical heights of the holes in both lenses. Look at Drawing #3 to see how the axis twist caused the hole placement to be different even when the drilling system used the same drilling procedure on both lenses The drilling system held the lenses by the finishing block and used the same vertical movements to place the holes in each lens. But, with the lenses coming from the edger with the lens shapes already twisted in the opposite directions in relation to the finishing block, the holes are then positioned at different vertical heights on the lens shape. Finishing Pad, Older Design or Just Old One important point to remember about finishing pads is that temperature variations and humidity can affect the adhesive. If you found a great deal on pads and bought a large amount, but had to store them in a room or warehouse that is not climate controlled, you might not have made that great a deal. One way of checking your pads is to stick two together and then try to separate them. It should be almost impossible to separate them, if that is not the case, then you have a greater chance that they will not hold that AR coated progressive that probably costs more then a couple of new boxes of finishing pads. If you have started having a problem with twisting at the same time that you introduced a new AR coating, your current finishing pad might not be the only one you need. Finishing pad suppliers are always working to improve the quality of their pads. They are ready to send samples of their latest designs for you to test. Call and get some and try them out. And remember there are pad designs for different lens treatments. Another solution is to use the larger round tape products that are applied on the lens and then the finishing pad attaches to the tape. This increases the surface area of contact for a better hold. Processing DME is easier now than anytime in the past, and with the new lens materials and mounting systems these frame and lens combos are more robust and secure than in the 80’s and 90’s. Making sure of the quality and look of the finished job seems to be the last bridge to cross so that drill mounted eyewear remains an active option for your patients. Chuck Field has been in the industry since 1970 and was the founder of Multi-Facets Labs and the designer of the Smart Drilling System. Currently is he president of Smart Lab and Awesome Eyes, a specialty lab that processes facets, rhinestone and sculpture designs. PDF’s of testing procedures and other help aids for processing DME’s can be found at smartlab.cc

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Lab Talk-February/March 2018