The first half of The Nuts and Bolts of Rimless (L&T May 2006) covered edge thickness, lens materials, tintability, drilling techniques and frame adjustment. Part Two of the article covers techniques for fabricating the four basic types of rimless eyewear: screw mounts, hole and notch, tension mounts and slot mounts.
Screw mounts are the classic three-piece rimless style. It is also the rimless style that has the most diversity—and a style offered by a wider range of manufacturers. One distinguishing feature is the number of screws per lens, which can be as high as three or four per eye piece. (That’s six or eight per lens!
Specific fabrication tips for scew mounts are:
• Distance between the holes needed for each piece of hardware must exactly follow manufacturer specifications or the mounting will be unstable, the lens can be off axis and spoilage risk increases during eyewear assembly.
• The screw head should be flush to the surface of the lens. An unevenly protruding screw head results in the frame pieces being crooked on the lens and almost always, an unstable mounting. In general, a screw head not being flush to the lens surface is a result of the hole quality being below par or the hole being drilled at an angle that does not correspond to the lens base curve.
• With screw mounts, frame pieces are affixed to the lens with a screw and nut on the back. But there are also metal and plastic washers. Even when using a metal washer for under the screw head and under the nut, plastic washers must also be used, positioned beneath the metal washer. The purpose is not to let metal screw, nut, metal washer or frame hardware—come in contact with the surface of the lens. Plastic washers act as a cushion between metal and lens, preventing lens fractures when tightening the screw or adjusting the frame.
• A plastic washer that is inserted into the hole of the lens is mandatory. This washer acts as a gasket around the screw so the screw threads do not come in contact with the hole interior lens material.
• Up to five washers—two metal, two plastic and the plastic “gasket” washer—can be used. Sometimes only the plastic washer that lines the hole interior, with ends that protrude around the entry and exit points of the hole, is necessary since it also acts as an extra layer between the lens and frame metal, screw head or nut. The number of washers is dependent both on the frame design and optician’s preference.
• Applying “Lok Tite” or a similar glue around the threads of the screw can enhance mounting stability. Do not apply the substance too liberally and avoid any contact with the lens surface. Wipe any excess glue off the lens immediately. Glue can cause coating crazing and even lens fracture.
• For most jobs, the screw will have to be clipped and the pointy tip has to be filed down. While filing the screw sounds easy—and essentially it is a simple step—one slip can scratch the lens and even the frame. In addition, one of the relatively new tools on the market is an all-material clipper. Screws used to be made only of stainless steel, but now they come in a range of materials, so a versatile clipper is needed.
• There are final touches to prevent the nut from coming off the screw. One method is using a “nut cap,” an additional nut that covers the tip of the screw. A nut cap will modify how much protruding screw is clipped and filed. Another, more “old school” technique, is mushrooming the screw tip. With this method, a cloth is placed over the frames and an optical hammer is used to tap the ends of the screw until it is peened or blunted. The visual effect resembles a mushroom, the practical effect is blunting the tip takes the sharp point away and holds the nut on tighter. Some optical tool suppliers now offer manual and electric “screw burnishers,” which can mushroom the screw tip without the obvious risks of the old school hammer technique.
• Screw mounts require the opticinary bench carry extra inventory of screws, nuts and washers. One size does not fit all the different frame designs. Just as you wouldn’t use a 1.2mm drill bit for a 1.0mm screw, you can’t use a 1.2mm washer or nut for a 1.1mm screw. In addition, a variety of lengths of plastic washers that slip into the hole must be on-hand in order to accommodate a variety of edge thickness.
Affixed Screw Mounts
Some screw mount designs use affixed screws, which require a slightly different mounting procedure. Affixed screw designs feature a threaded screw tip mounted to the ends of the frame pieces. This style eliminates the screw, but the washers, nuts and glue are still necessary. The screw tips of these frames pieces must be clipped and filed.
Hole and Notch
Drilling hole and notch styles can be the trickiest of all three-piece mounts. The holes, in which screws are inserted follow protocols similar to screw mounts. The “notch”—think incomplete circle—is basically carved into the edge of the lens using a helical router drill bit. Frame temple pieces are held in by a screw and a post or “prong” on the tip of the temple, which is lodged into the notch. Thus, the temple piece requires two drilling steps—one for the notch to hold the post, one for the hole to insert a screw. (The bridge piece is attached to each lens by a single screw.)
Specific fabrication tips for hole and notch are:
• Placement of the hole and the notch must be precisely positioned, per the frame manufacturer’s guidelines or else the assembly won’t work. It the positions are misplaced by even 1/10th of a millimeter in either direction on the lens surface results in an unsuccessful mounting.
• A helical router drill bit can drill a hole as well as a notch, but only a few manufacturers feature a temple prong the same size as the screw. Check with the frame manufacturer to ensure you are using the proper size bits for each type of opening on the lens.
• To carve a notch, the movement, unlike drilling a hole, is not up and down, but sideways. Newer drills feature a moveable drill table. While the drill is kept stationary, the table is moved laterally and the router carves the notch into the edge of the lens. The movement should correspond to the size notch required—for example, a 1.1mm notch requires the table to be laterally moved 1.1mm. Techs must ease the edge of the lens into the spinning helical router to evenly cut the notch. Instead of a drill bit, sometimes a notch file is used to remove the necessary lens material to form the optimum notch size. The bottom line is, because a notch is different than a hole, do not use a typical drill bit.
• Notch quality is as crucial as hole quality. If the notch is too large, the temple prong will wiggle in the notch. Any rotation is bad and will mean an unstable mounting. On the other hand, a small notch is easily noticeable—the prong won’t fit—and is easily enlarged by a few swipes of a file or in some cases, re-drilling. Once the notch is created, smooth out the notch with a file then chamfer, as you would a drill hole. The notch should be made straight or parallel to the edge of the lens. Otherwise, alignment can be off, causing the eyewear to rest lopsided when worn and even cause the lens to off axis.
Tension and Mounts
Tension mounts are held together by the tension created between the frame pieces and lens. Some styles require only one hole per temple piece, one for the bridge piece, others styles require two holes per piece because instead of a single tip, the dual tips are “male” and “female” prongs. A clear, flexible plastic sleeve or bushing, encloses the prongs. The tension is created by the compression of the plastic between the prongs and the lens.
Specific fabrication tips for tension mounts are:
• Tension mounts have two important variations—front or back insertion. Some eye pieces go into the back of the lens, other eye styles “loop” around the edge of the lens and are inserted through the front lens surface. The bushing however, which is inserted first, is always pushed through the drill hole from the back to the front of the lens. (Check with the frame manufacturer for the appropriate insertions.)
• Drilling methods follow screw mount procedures. (Follow manufacturer instructions for selecting drill bit size.) Then insert the bushings. The bushings tend to clamp closed upon insertion and must be eased open by a small object, such as a safety pin. At this point, techs use clippers (some frame manufacturers supply clippers) or a straight edged razor. The bushing should be flush to the front lens surface.
Depending if it’s a front or back mounting insertion, the frame hardware is inserted inside the plastic bushings. Assembly for single hole mounts should be completed upon insertion. For the tension mounts requiring two holes for each piece, the tips of the hardware, referred to as prongs, are simply squeezed together.
Slot mounts success depends on a hole that is oval in shape, and approximately 5mm in length and 1.0mm in width. Slot mounts usually require a router drill bit and drill table that can be moved back and forth. Some slot frame manufacturers offer a special drill designed solely for this procedure and some of the newer, more automated drills also feature slotting capabilities.
Specific fabrication tips for slots are:
The quality of the oval is crucial. The shape must be perfect, the interior of the opening smooth, so filing and chamfering is almost always necessary. A special, manufacturer-supplied glue is applied, either to the lining of the ovals generally depending on optician preference. Only a small amount is necessary and any excess glue must be wiped off with an alcohol solution (acetone can corrupt coatings).
The glue, which takes about an hour to cure, has adhesive properties, but compared to other glues, dries to a rubbery consistency. It acts as a gasket or bushing for the hardware as well as an affixing agent, but since it is thinner than a plastic washer or sleeve, exact size and smoothness of the slot is a more crucial factor in the success of this mounting.