Jun
2003

Lens Choice

Rimless eyewear combines craftsmanship with construction—expert drilling and meticulous mounting transforms the lens into a frame without sacrificing optics. Drilling and mounting rimless eyewear requires attention to detail and this category’s expanding selection means more details to pay attention to.
In general, three-piece frames fall into in four distinct categories: Screw Mounts, Hole & Notch, Tension Mounts and Slot Mounts. Each style, and sometimes each brand, requires specific techniques, but there are universal production concepts finishing labs must adopt if they want to succeed at three-piece mounts.

The process begins with the lens edge. Laboratory technicians recommend a minimum edge thickness of 2mm and a maximum edge thickness of 5mm. As with all lenses, thinner is better as long as the edge is not thinner than 2mm.
To guarantee this mandatory edge width, in addition to precise geometric frame and pupillary distance measurements, opticians should notify the surfacing lab that the lenses are for a drill job.  Today’s lab software, with features such as cribbing and prism thinning, can create a lens with an optimum rimless edge thickness. “Discuss each job with the wholesale lab,” says Bryan Shaw, president of Rimless Connection, a Miami finishing lab specializing in rimless eyewear. “In some higher-plus powers, a smaller eye size will result in a better job. The thin edges will be cut away, so the area drilled will be 2mm. Sometimes a base-curve change will be necessary if the prescription can withstand it, especially if it is a wrap-around style.”

Drill Hole Quality
The dual objectives of three-piece fabrication are a well-fitted frame and stable mounting. Both objectives are dependent on how successful the frame hardware—temple and bridge pieces—are attached to the lens. The quality of that attachment rests solely on the quality of the drill hole. Drilling the hole requires the same meticulousness and precision as any skill-intensive opticianry procedure.

Placement, angle and caliber determine drill hole quality. Recently, several equipment manufacturers have introduced multi-functional, automated and semi-automated, drilling systems designed specifically for lenses. Unlike the guesswork that may be necessary when using 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.

Drill holes must be straight through the lens. To do so, the drill bit should enter the lens at a 90-degree angle, perpendicular to the curvature of the lens, so the hole is flush to the surface. “The drill angle is different for different base curves,” says Robert Wallner, president of The Cutting Edge Optical Specialties, a finishing lab in Wallkill, N.Y. that specializes in processing rimless jobs. “If the angle of the hole is off, the frame pieces will not mount correctly, the pieces will wobble and eventually fall out. The lenses can also be off-axis for the patient.”
Similar problems will result if the holes are not drilled in the precise spots.

Placement becomes even more difficult with mounting designs requiring 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. “If the holes are off, some people will try bending the frame to make it fit, but you are doing a patient a disservice and more often than not the frame will rest cock-eyed,” says Kent Miller, optician/owner of Spectacle Shoppe in Wichita, Kan.

To ensure proper hole coordinates, use either the frame manufacturer-supplied lens and/or pattern as a template. “I practice drilling with each new style,” says Alex Hegert, optician/ owner of City Optics in San Francisco. “Going through demo lenses is cheaper than spoiling a high-index progressive.”

The last issue with hole quality is the caliber of the hole. The exterior opening and the interior surface of the hole must be smooth. Pros touch-up each hole with a rat-tail or similar file. In addition, they use a chamfer tool, which de-burrs the hole of any excess debris, such as micro-chips of material. Chamfering enhances smoothness as well as preventing any spider web cracks forming around the hole’s perimeter.

How the holes are drilled can also affect their caliber. Techs prefer a pecking drill motion, meaning lowering the bit in and out of the lens, about three times. “It’s better not to make the hole in one fell swoop. The hole is smoother and there’s less chance of over-heating or cracking when you do the hole in two or three passes,” says Wallner.

Polycarbonate is the lens material most recommended by techs for drilling. PPG’s new Trivex (available in Younger Optics’ Trilogy lenses and Hoya’s Phoenix lenses) is also gaining favor—for many labs, it has become the lens material of choice for rimless. These two materials are less prone to over-heating and cracking than most plastic materials. Luckily, the newer drills offer variable speed settings. Polycarbonate, Trivex and some high-index plastics must be drilled at a lower speed than regular plastic in order to avoid overheating the material and warping the lens.

Mounting procedures differ for each frame style. But there is one protocol that holds true for all assemblies. The frame pieces must fit firmly in the lens. Any movement will get worse and the frame will eventually disassemble. However, too tight shouldn’t be mistaken for firm. A frame assembled too tightly can cause cracks in the lens. “You want things snug, but if you are forcing something in, that’s a sign of a tightness problem,” says Miller. “But, if things go in without effort, more than likely they will wobble. Knowing when the pieces feel right is a knack you pick up only through experience.”

Screw Mounts
Screw mounts are the archetypal three-piece frame style as well as the most popular. But prevalence has bred diversity—some styles feature one screw per frame piece (two screws per lens), while others feature two screws per eye piece (four per lens). With the latter, of course, the distance between each piece’s screw holes must be exact.

A clue that the angle of drill hole has been misaligned will be evident upon assembly—the screw head will not 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.
 
With screw mounts, there are more pieces to consider, thus more steps to follow. 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 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.

Inserting a plastic washer into the hole of the lens is mandatory. The washer acts as a gasket around the screw so the screw threads do not come in contact with the hole’s interior.

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 is necessary, because it also acts as an extra layer between the lens and frame metal, screw-head or nut. The ends of the washer protrude around the entry and exit points of the hole. The number of washers depends on both the frame design and the optician’s preference.

Most techs will use “Lock-Tight” or similar glue around the threads of the screw to enhance mounting stability. Some opticians even use nail polish. The rule of thumb is not to apply the substance too liberally in order to avoid it making any contact with the lens material (wipe any excess glue off the lens immediately). Glue can cause coating crazing and even lens fractures.

In most cases, after the washer and bolts are backed up on the screw toward the back surface of the lens, the screw will have to be clipped and the pointy tip has to be filed down. While filing the screw sounds easy, one slip can scratch the lens or frame.

Some opticians prefer a final touch that further prevents the nut from coming off. One method is using a “nut-cap,” an additional nut that covers the tip of the screw. A nut-cap will modify how much the 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. Using an optical hammer the ends of the screw are tapped until they are peened or blunted. Blunting the tip takes the sharp point away and holds the nut on tighter.

Screw mounts require an extra inventory of screws, nuts and washers. One size does not fit all the different frame designs. 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. Also, the drill-bit size must be the size of the screw. Forcing a screw into too small a hole will crack the lens, while compensating for too big a hole with glue or extra washers will mean a wobbling frame prone to disassembly.

One other variation of screw mounts requires a slightly different mounting procedure: “affixed screws.” They feature a threaded screw tip mounted to the ends of the frame pieces. This style eliminates the screw, though the washers, nuts and glue are still necessary and the tips must also be clipped and filed.
No matter what screw mount style is used, it’s important not to overtighten screws or nut. Over-tightening will damage a lens, sometimes cracking it outright, sometimes causing fractures that will spread and deepen over time. Over-tightening is often done to compensate for a hole that has been drilled off-angle or too large or small. However, it will never turn an unstable mounting into a stable one.

Hole & Notch
Drilling hole and notch styles can be the trickiest of all three-piece mounts. Both “hole” and “notch” require distinct drilling methods. 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.) 
Placement of the hole and notch must be precisely positioned per the frame manufacturer’s guidelines. If the positions are misplaced by even 1/10th of a millimeter in either direction on the lens surface, it can result 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 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. The edge of the lens should be eased into the spinning helical router to evenly cut the notch.

Some opticians use a notch file instead of a drill, removing the lens material until the notch size is optimum. Since a notch differs from a hole, using a typical drill bit won’t work.

If the notch is too large, the temple prong will wiggle in the notch. Any rotation is bad, and will mean an unstable mounting. Some opticians may try to compensate by bending the frame, but this will only put off the inevitable. 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, it should be smoothed out with a file then chamfered, as you would a drill hole.

Tension Mounts
An innovative frame assembly, tension mounts are held together by the tension created between the frame pieces and lenses. Some styles require only one hole per temple piece and one for the bridge piece for each lens; other styles require two holes per temple piece because instead of a single tip, the dual tips are “male” and “female” prongs.

A clear, flexible plastic sleeve, or bushing, encloses the temple tip; prong tips require a bushing on each tip. Tension is created by the compression of the plastic between the frame hardware and the lens.
Tension mounts have two important variations: front or back insertion. Some eye pieces go into the back of the lens while other styles “loop” around the edge of the lens and are inserted through the front lens surface. However, the bushing, 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 the manufacturers’ 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—which some frame manufacturers supply—or a straight-edged razor. The bushing should be flush to the front lens surface.

Regardless of whether 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 tension mounts requiring two holes for each piece, the tips of the hardware, referred to as prongs, are simply squeezed together.

Slot Mounts
A successful slot mount depends on a hole that is oval in shape and approximately 5mm long and 1mm wide. Slot mounts usually require a router drill bit and drill table that can be moved back and forth. Some manufacturers of slot mounts offer a special drill designed solely for this procedure and some of the newer more automated drills also feature slotting capabilities.
The quality of the oval is crucial. The shape must be perfect, the interior of the opening smooth, so filing and chamfering are almost always necessary. The optician may apply a special, manufacturer-supplied glue to the lining of the ovals. Only a small amount is necessary and 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 and dries to a rubbery consistency. It acts as a gasket or bushing for the hardware as well as an affixing agent. Because 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.

Technical consultant: Pierre Mermet-Marechal

 

|