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Date: |
Aug 2004 |
Making Stronger Three-Piece Eyewear
Lens preparation
Pay particular attention to the hole quality and the best
drilling method for the various lens materials.
To achieve the best performance, each material can have a very different
machining requirement. Keep a sample of
popular materials at your lab’s drilling station for practice and inspection,
before drilling finished work. A 5X
magnifier is adequate to judge the quality of the hole. The smoother the wall of the hole or slot,
the stronger the mounted lenses. A variable speed drilling motor, which has a
low cutting bit run out, is required. Holding a bit in a drill motor with a
setscrew will not produce high finish quality holes. If the speed or drilling
rate is too fast, the lens can sustain high levels of stress that can be
observed with a polarizing filter. A chamfer should be done with a tool that smoothly
grinds at approximately a 45 degree angle to the hole wall. In the photo on the
right, it is easy to see the need to chamfer both sides of the drilled holes.
The hole on the right has good wall and chamfer quality. The broken hole on the
left was enlarged with a file. Filing
cannot be substituted for drilling the proper sized hole or chamfering. Note
the roughness of the wall.
Chamfering
tools
The correct chamfering tools will produce a lens finish that will increase the strength of the drilled hole. The photo at the right shows a tool which has an angle too steep and which will leave a rough finish. The preferred tool will chamfer the hole at approximately a 45 degree angle with a very smooth finish. This tool, available from our Technical Service Department, is made from a Dremel Tool #83142 Silicon Carbon Grinding Stone that was shaped to the proper angle with a #415 Dremel Trueing Stone. (Note that the fine grit smoothing stone is green.) The chamfering tool is used by hand rather than in a motorized device. You may prefer to hold the tool in a pin vise for 1/8” shank drills. These items can be purchased wherever Dremel Tool accessories are sold.
Our Test Results
Based on inspecting the holes of broken lenses, the roughness of the hole wall, as well as the absence of soft plastic bushings and chamfers to smooth the hole edges, were factors contributing to the failures.
In this test, 1.498 index lenses, of 0.00 diopters, 2.0 mm thick were edged to a 65mm in diameter.
Holes were drilled with their centers at 4mm from the edge.
Testing variables
- Holes were made with a Dremel fluted bit and with a special drill set, which came with our drilling machine. We varied the speed and boring rate to get the smoothest hole possible.
- Holes were left sharp or chamfered to smooth the edges of the holes.
- A plain steel rod or one covered with soft plastic was used.
Test 1 results
Multiple holes were made in 0.00 diopter 1.498 index resin lenses.
The breaking force was recorded in grams per centimeter.
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Burr bit, no bushing, no chamfer |
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Drill bit, no bushing, no chamfer |
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2173 grams/cm |
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1749 grams/cm |
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Burr bit, no bushing, chamfer |
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Drill bit, no bushing, chamfer |
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2703 grams/cm |
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2332 grams/cm |
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Burr bit, bushing, no chamfer |
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Drill bit, bushing, no chamfer |
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3180 grams/cm |
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3445 grams/cm |
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Burr bit, bushing, chamfer |
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Drill bit, bushing, chamfer |
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3657 grams/cm |
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3445 grams/cm |
The strongest contributors to increasing the lens strength were the chamfer, combined with the simulated bushings on both sides of the lens. The difference between using either a high quality drill or fluted bit was insignificant, if care was taken to optimize the hole quality. It may, however, be that one type of bit is more versatile with various materials and drilling techniques.
Screw mounted lenses
The ideal condition is to use bushings on both sides of the lens. Some frames are not compatible with the use of a bushing on the front surface. The chamfer is even more critical in the case where only one bushing can be used.
Inspect your returned broken lenses
The photos below show carefully drilled holes with and without chamfering. It is obvious why chamfering reduces lens cracking. Inspecting any broken eyewear for the quality of the hole and the use of grommets can be very helpful to insure that the ideal mounting process is being used.
