L&T: L&T 101

Jul
2008

On The Edge

Finishing Wrap Lenses


Top to bottom: Normal hide-a-bevel; edge cut-out fluting; sharp V bevel; shelf bevel; polished bevel

Photo by NEDJELJKO MATURA. Lenses courtesy of Tom Buell, Tom’s Sportique Eyewear, Boulder, Colo.


This article is the second in a two-part series about ordering and fabricating wrap sunwear. The first part, “Wrap It Up” appeared in the August issue of 20/20.


Finishing wrap lenses, whether for sunwear or streetwear, depends on skilled handwork. Prescription wrap lenses usually have thicker edges, especially in the nasal and/or temporal areas—due to the prescription, lens size and decentration—and can be a particular problem in minus powers given the use of steeper base curves. Wrap frames, available in a variety of plastic and metal full-frames or three-piece drilled rimless styles, all have a wrap or vertical tilt (a tilt around the vertical axis) of 10 to 20 degrees, which complicates lens finishing. Balancing these components mandates accuracy in lens sizing, beveling and mounting and in-office lab personnel must be prepared for a more labor intensive and meticulous fabrication process when it comes to excelling at wrap styles.

“Wrap sunlenses take more handwork,” says David Finley, lab manager of Dixie Eye Center in St. George, Utah. “You can’t do them in the normal production flow.”

Prescription wrap sunlenses defy standard lab protocols. The critical finishing aspect for all eyewear is probably lens sizing, a step made easier and more accurate by tracing technology. But even the latest generation of tracers can have trouble accurately reading the shape of wrap eyewires because of the frame wrap. “Many frame tracers require that the frame is ‘unwrapped’, or flattened to be traced, and the horizontal dimension and vertical dimension ratio can be wrong,” says Mark Mattison-Shupnick, CE coordinator for 20/20 magazine. “A lens that fits the frame’s A size will be too small in the B size. As a result, without the availability of a true 3D tracing, the entire size must be increased and some handwork is usually required.”

Tracing wrap frames requires experience: knowing which frames trace easier than others and when to bump up the size. “If I am not sure of the trace, I will cut it a little big and bring it down by hand,” says Kathleen Staffieri, lab manager for Site For Sore Eyes, San Jose, Calif. “It depends on the frame style, how familiar I am with the style, the curvature of the frame and the curve of the lens. Wraps just take more handwork. It’s something you have to accept if you are an optician and want to sell them.”

With full-frame wraps, not only is attaining the exact lens size trickier compared to other eyewear, but there’s also less leeway with sizing. Even a slightly over-sized wrap lens may “fit” into the frame, but it will be susceptible to popping out. Lab techs recommend verifying frame and lens measurements during and after edging. David Holland, general manager of West Coast Lens Lab in Huntington Beach, Calif., uses digitized calipers. “This takes away some of the guess work, especially as you take the lens down on the hand-edger. What you don’t want is a lens too tight in the frame. Not only is it more likely to pop out, but there can be compression marks—especially in polarized lenses—and the patient will see flecks of light-to-dark gray or brown.”

Beveling Edges
Beveling is another edging step technicians approach differently with wraps. Bevels must be positioned close to the front surface of the lens and must be “V” shaped. “A ‘U’ shape or a hide-a-bevel often won’t work on wrap frames because those bevels won’t hold the lens in the frame,” says Mattison-Shupnick. “Because of the way the eyewire turns, it is necessary to have more of an angle on the edge of the lens to stay in the eyewire bezel. A conventional bevel won’t hold in the eyewire.”

Some patternless edgers feature a “V” bevel setting, which can work fine in some wraps, though bevel touch-up by handstone is often needed. For many styles and prescriptions, the lens must be hand-beveled. “You need to steepen the back bevel, create a V groove,” says Staffieri. “The bevel has to conform to the curvature of the frame and sometimes you can only get that bevel by handwork.”

Ledges and Edges
Due to forward positioning of the bevel as well as its V shape—and the fact that in minus prescriptions on base curves of six or higher, lens edges tend to be thick—there is an excess of several millimeters of lens material behind the bevel. The rim of the eyewire in many frame styles can hide this excess, although techs recommend polishing the back edge to make the overall eyewear more aesthetically pleasing.

However, this excess of material can be an additional nuisance when mounting many wrap styles, especially severe wraps or wraps intended for plano sunlenses. These wrap styles often feature what techs refer to as a ledge (or lip) located at the temporal and nasal areas on the rim of the frame. The structural purpose of this design feature is an added brace to the plano lens. It also covers the temple hinges in some frames.

Prescription lenses are thicker than plano, particularly in the nasal and temporal edges, depending on prescription. Lab technicians require skill, patience and more than a little creativity to perform the handwork needed so Rx lens edges better mimic plano lens edges and can be inserted into these frames. “You have to relieve the lens material behind the bevel, especially when it comes to the lip on the rim of some frames,” says Thomis Buell, owner/optician of Tom’s Sportique Eyewear in Boulder, Colo. “Due to the frame design and the thickness of the edge, the handwork can be intricate. You basically are fluting the edge behind the bevel, similar to what is done with faceted lenses.”

Some techs will attempt to file down the lip on the frame in order to mount the lens. Not only does this take a high amount of skill, as you are essentially modifying the cosmetics of the frame, but also a great deal of caution—you risk spoiling the frame as well as compromising its construction, especially if you damage the temporal hinge.

If the edge behind the bevel is too thick, it will impede the removable side shields featured on several performance-oriented wrap styles, points out Buell. “Side shields, especially the leather side shields, need the clearance,” he says. “If the edge is protruding, the shields will wobble or not set correctly on the stems and you won’t be able to take them off and put them on the frame.” Buell creates what he calls a “shelf bevel” on the handstone, essentially squaring off the lens edge behind the bevel. “It’s almost like scalloping off this portion of the edge so there’s no protrusion,” he adds.

This edge thickness problem is worse of course, in higher prescriptions. Lab techs should be aware of the frame style when ordering a prescription in order to ensure that the edge can fit into the frame design. For opticians who have more control over their stock, the problem is easily solved. “I look for frames without the ledge,” says Maria Robbins, owner/optician of Optik Eyewear in Langhorn, Pa. “Most of the new ophthalmic wraps have done away with the ledge.”

Polarized Pitfalls

Bevel type and placement can be an acute spoilage pitfall for polarized lenses, perhaps the most popular lens for wrap sunwear. The polarizing film on many plastic lenses is located beneath a front laminate “cover,” about 1mm. thick. The back lens, which is always thicker than the front, is clear. If the bevel is placed on the film separating the two lenses, some dispensers suggest that delamination may result.

“You can create a stress point if the deepest part of the bevel is on the film,” says Mattison-Shupnick. However, he notes “this is no longer an issue with some of the newer plastic polarized lenses because they are not laminated. Instead, the film is cast into the lens blank.”

Another polarized wrap issue is lens color uniformity if the edge of the back lens is exposed. “If the bevel is not close enough to the front, you can create a clear ring that is visible along the edge of the lens, not behind the rim of the eyewire,” says Mattison-Shupnick. “In the case of hard resin polarized lenses, dip dye the lens edge to minimize the clear or whiter appearance.”

With polarized polycarbonate, the film is practically at the front surface of the lens, and as in plastic polarized, the back lens is clear. Extreme edge thickness in wrap sunlenses manifests itself mainly in a visible back, clear lens. Although polycarbonate, because of its high-index properties, makes a thinner lens, that advantage may be mitigated by the combination of minus prescriptions on steeper base curves. Bevel placement must be as close to the front surface as feasible—and if the prescription power is too high, and/or the wrap is too severe (i.e., a steeper tilt)—the clear back edge of the lens may be exposed. “If you get the bevel placement wrong on the poly polarized, the lens can be outlined by a clear edge,” says Finley. “With poly you tint the back coating, not the material, so the edge won’t tint. We do a lot of poly polarized, but some prescriptions won’t work in some wraps.”

Easy Rimless

Three-piece wraps can be an easier finishing task than full wraps because no beveling is needed. In addition, the new, adjustable drill mount systems allow the drill bit to enter the lens at an angle perpendicular to the front curve of lens, making the drilled hole even and its positioning exact. “The curves make the surface steep, so you need the angling ability of the drill so the head of the screw is flush to the surface of the lens,” says Robbins.

Some rimless wrap styles can be compromised by lens thickness. In general, screw mounts can be less of a problem—technicians can use a longer screw than in the original frame in order to affix the lens to the mounting. Compression mounts are more problematic because the prescription lens can be too thick for the plastic sleeves to go through the entire thickness of the lens. For these prescriptions, reducing the eyesize may solve the problem. Otherwise, compression or tension mounts should be avoided in these Rxs.
While drilling and assembly may be comparable to regular streetwear frames, wrap sunwear frame alignment can pose additional risks “With wraps, you will have to handle the eyewear more. There is more alignment required, because you are dealing with different angles and curves,” warns Holland. He adds the one essential handtool for rimless wraps are alignment pliers with a nylon covered jaw. “In many of the new wrap designs, the frame pieces seem to be easily dented or marred, and using padded pliers avoids that frame damage.”

Tech Tips for Wrap Artists
  1. To compensate for less than perfect wrap tracing, cut lenses large and bring them down to size on the handstone. Size reduction is usually required in the “A” dimension.
  2. Bevel placement should be close to the front surface of the lens.
  3. Use V bevels for most wraps.
  4. Check lens size before mounting. Even lenses a fraction too large can pop out of wrap frames.
  5. Adjust the angle of the drill for rimless to correspond to the steeper lens curve. The drill bit must be perpendicular to the lens surface.

 

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