May
2006

RIMLESS


Rimless eyewear accentuates the natural beauty of eyes and facial features. The fashion concept is usually described as less is more. But to achieve this minimal look, more, not less, expertise is required in the finishing lab and that expertise is involved mainly with the lens. Rimless is all about the lens.

The crucial stage of three-piece eyewear is drilling holes in the lens. If issues such as hole location and quality are not exact, eyepieces will not remain properly attached to the lens and the eyewear dismantles.
Three-piece mounted eyewear requires attention to detail, understanding design and fabrication concepts, and following finishing and mounting protocols.

Expanding Diversity
In general, three-piece frames fall into in four distinct categories: screw mounts, hole and notch, tension and slots. What distinguishes each category is how the frame hardware attaches to the lens. Each style requires specific drilling and mounting techniques, but there are universal production concepts finishing labs must adopt.

Edge Thickness & Ordering
When fabricating rimless eyewear, most laboratory technicians recommend a minimum edge thickness of 2.0mm and a maximum edge thickness of approximately 5.00mm. As with all lenses, thinner is better as long as that edge thickness is not thinner than 2.0mm. To guarantee this mandatory edge width, in addition to precise geometric frame and pupilary distance measurements, notify the surfacing lab the lenses are for a drill job. Today’s lab software, with features such as cribbing and prism thinning, optimizes lens edges for rimless. With the proliferation of three-piece wrap styles in sunwear, sometimes a base curve change will be necessary, which can further impact drillability of the lens.

Material Matters
Lens manufacturers have worked with their material scientists to optimize drillability of substrates, addressing interrelated issues such tensile strength, flexural strength and notch sensitivity. Basically, rimless jobs require a lens material that when drilled, the hole made will retain its size and shape over time and the microscopic cracks caused by drilling will not spread and deepen. Pressure is created around the eye pieces during the life of the eyewear and the lens material must be able withstand this pressure and maintain the integrity of the frame. In general, polycarbonate, Trivex and some high-index materials (particularly 1.67 and 1.70) are recommended for rimless. Different materials usually require different drilling speeds. Newer drill systems offer variable speed settings for this purpose.

Tintability
For all tintable materials, dyes are absorbed throughout the lens so edging and drilling will not affect color. Some polarized lenses are actually colorized by the polarizing film within the lens, resulting in edges and drill holes that appear white and clear. Depending on the manufacturer, these lenses can be tinted post-edging and drilling—generally only short “dip” in the dye—to create a uniform color throughout the lens. For other polarized lenses (as well as some polycarbonate), lens tints can’t be absorbed and colored lens marking pens must be used for hole openings.

Lens Edge
In general, drilling a lens does not affect edging. The only aspect opticians must pay extra attention to is the lens edge itself. Because there is no rim, a bevel is not required but a safety bevel is. The safety bevel is also exposed, so not only must any edge sharpness be removed by safety beveling, the safety bevel itself must be cosmetically appealing. Edge polish is more noticeable due to its increased exposure. Whether it is a high-sheen or something less ostentatious, patients are more aware of how the edge looks and lab technicians will be more exacting to achieve the desired look. The latest generation of edgers boast optimized safety beveling and polishing cycles—and these may be adequate for some rimless jobs—but for other jobs, opticians have to be ready to touch-up the lens edge with the hand-stone and polisher to perfect the rimless look.

Stable Mounting
Three-piece eyewear depends on a stable mounting, i.e., how firmly temple and bridge pieces are attached, and remain attached to the lens. The frame pieces must be firmly in the lens—they can’t wobble—any movement will get worse and the frame will eventually disassemble. But don’t mistake too tight for firm. Too tight can cause cracks in the lens. Judging when something is tight, but not too tight is a knack techs attain through experience. One basic rule of thumb is that snugness is optimal, but forcing something in is often a sign of too much tightness and can impact assembly and lead to breakage.
The quality of that attachment solely rests on the quality of the drill hole. Drilling the hole is a skill-intensive opticianry procedure that requires meticulousness and exactitude.

Hole Quality
Drill hole quality is based on placement, angle and caliber. In recent years, several optical equipment manufacturers have introduced multi-functional, automated and semi-automated, drilling systems designed specifically for lenses. Unlike the guess-work days of a hand-held Dremel drill, the new drills feature an adjustable drill-tilt, enabling better angling of the drill and sophisticated hole-alignment functions, so the precise coordinates of drill hole placement can be followed.

Hole Angles
Drill holes must be straight through the lens. The drill bit must enter the lens at a 90 degree angle, perpendicular to the curvature of the lens so the hole is flush to the surface. This angle is different for different base curves. Newer drilling systems enable more exact synchronicity in matching drill bit angle with the lens base curve. Holes that are not straight through, i.e., have an off-angle, means the frame hardware won’t mount correctly, can wobble and fall out of the lens and also make the lenses off-axis for the patient.

Hole Placement
Where the drill holes are positioned in the lens is essential to the design of the frame. Placement becomes even more difficult with mounting designs that require more than one drill-hole per frame piece. Hole position must be in-synch and there is little-to-no-leeway—a hole misplaced by even 1/10th of a millimeter can corrupt the eyewear.

To ensure proper hole coordinates, use either the frame manufacturer-supplied lens and/or pattern as a template. Opticians can also utilize reference guides, such as the FRAMES Data Drilling Points Quarterly, which features updated hole coordinates of rimless frames. Often—although, not all the time—changing the eye-size of the lens to be drilled will change the hole coordinates of the job. If these new coordinates are in doubt, opticians recommend a run-through of a job by drilling demo lenses, especially if the lenses ordered for the final eyewear are a high-priced, premium lens and/or the frame style is new to the finishing lab.

Hole Integrity
How firmly frame hardware attaches to the lens is determined by the quality of the drill hole. The exterior opening and the interior surface of the hole must be smooth. To ensure this uniformity, touch-up each hole with a rat-tail or similar file. A chamfer tool, which de-burs the hole of any excess debris, such as micro-chips of material, also will help maintain hole quality. Chamfering enhances smoothness as well as prevents any spider-web (or star) cracks that can form around the hole’s perimeter. In general, chamfering is the final procedure before affixing frame hardware.

Clean Lenses
Wash the lenses completely before attaching the mounting pieces. Remove all excess debris and practices (including the openings and interiors of the drill holes) to reduce potential risks of lens scratches.

Adjustment
A well-assembled rimless frame should be nearly as sturdy as full-frame eyewear. But the eye pieces themselves can be more fragile and because the frame is completely exposed, adjustments can risk lens scratching. Tool manufacturers have introduced specialized three-piece adjusting pliers featuring nylon jaws that minimize scratching or otherwise fracturing the lens when adjusting the frame. In general, when adjusting rimless eyewear, the pliers should brace the area where the frame hardware passes through the lens (i.e., as near the drill hole as possible). Rimless frame adjustment requires more care and attention than other frame designs.

For more tips on fabricating three-piece mounts, look for Part 2 of ‘The Nuts and Bolts of Rimless,” in the June issue.”
LT


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