Safety Beveling, Edge Polishing & Rimless Mounting

Why Safety Beveling Matters

Every lens that leaves an edger has a peripheral edge that is sharp enough to cut skin, scratch a frame, or—more seriously—shatter under stress if the sharp apex concentrates impact energy. A safety bevel removes that apex by grinding or stoning a small, flat chamfer around the front and back edge of the lens. It is not optional. Professional finishing standards require that no sharp edge remain on any ophthalmic lens before it is dispensed. Patient safety and frame integrity both depend on this step.

The Two Primary Methods

Bench-Stone Method

A fine-grit aluminum oxide bench stone is used for manual beveling. Hold the lens at roughly 45 degrees to the stone surface. Using light, consistent pressure, draw the lens edge across the stone in a smooth two-stroke motion—one stroke along the front edge chamfer, one stroke along the back edge chamfer. Rotate the lens a few degrees and repeat until you have worked all the way around the circumference. The goal is a uniform, narrow flat facet (approximately 0.3–0.5 mm wide) that eliminates the sharp apex without removing enough material to affect the bevel fit in the frame groove.

Key technique points:

• Keep the angle consistent throughout the rotation; rocking changes the chamfer width unevenly.
• Use only enough pressure to produce a visible flat—excess pressure removes too much material.
• Keep the stone clean and flat; a glazed or grooved stone produces uneven results.

Edger Safety-Bevel Wheel

Most modern automatic edgers include a dedicated safety-bevel wheel or a programmed safety-bevel cycle. After the main bevel is cut, the lens is repositioned and the machine applies a controlled chamfer. The operator selects the safety-bevel size in the edger software (commonly a small value, e.g., 0.3 mm). The automated method is faster and more consistent for production volume, but manual bench-stone technique remains essential when a machine is unavailable or when touching up irregular edges.

Verifying the Result: The Cotton-Swab Drag Test

The standard shop-floor test is simple and reliable. Draw a cotton swab slowly around the entire edge of the finished lens. If the swab fibers catch or snag at any point, a sharp edge or burr remains and the lens must be re-beveled at that spot. A properly beveled edge slides the swab cleanly around the full circumference. Also inspect the edge under a loupe (7× to 10× magnification) looking for chips, uneven chamfer width, or unbeveled segments.

Material Considerations

Different lens materials respond differently to beveling:

• CR-39 (allyl diglycol carbonate): Machines and stones easily; chamfer is visible and uniform. Minor chipping at the edge is possible if pressure is excessive.
• Polycarbonate: Tough and impact-resistant but prone to crazing or micro-fracture if bench-stone pressure is uneven or the stone is used dry. Use light, steady strokes. Many labs rely on the edger's safety-bevel cycle for polycarbonate.
• High-index glass and plastic: High-index materials can be brittle. Use minimal pressure and ensure the stone is clean. Inspect carefully under the loupe for micro-chips.

Rimless Lens Mounting

Rimless (drill-mount) frames attach directly to the lens through holes drilled near the nasal and temporal edges. Because there is no frame rim to distribute stress, the lens material around each drill hole bears the full mechanical load of the mounting hardware. Precision and gentleness are essential to prevent cracking.

Marking Drill Position

The drill position is taken from the frame's pattern or template. Place the pattern against the finished lens and mark the hole center with a non-permanent marker or a prick punch designed for this purpose. The hole is typically positioned 2–3 mm from the lens edge (follow the frame manufacturer's specification exactly). Too close to the edge invites cracking; too far inward may interfere with the optical zone.

Drilling

Use a drill press or dedicated lens drill with a sharp, lens-appropriate bit. Drill at low speed with light, steady pressure. Let the drill do the work—forcing it generates heat and cracks polycarbonate. Drill from the front surface, stopping just before breakthrough, then reverse the lens and finish from the back. This two-direction technique minimizes chipping and delamination at the exit hole.

After drilling, lightly hand-chamfer the inside of each hole with a countersink bit or a tapered stone to remove the sharp rim of the hole. This distributes stress from the mounting screw or peg over a broader area.

Installing Hardware and Torque

Thread the mounting screw or insert the mounting peg per the frame design. Use a torque screwdriver set to the frame manufacturer's recommended value—hand-tightening is not precise enough and over-torquing is a leading cause of drill-hole cracking. Tighten to snug; if the frame uses a nut, hold the nut steady while turning the screw to avoid rotating stress on the lens.

After mounting, inspect each drill hole under a loupe for any crack propagation (visible as a fine white or dark line radiating from the hole). Confirm that both lenses are aligned at the correct pantoscopic tilt and face form matching the original frame specification.

Safety Reminders

• Wear safety glasses whenever operating the edger or drill press.
• Handle lens blanks and finished lenses carefully—sharp edges can cause cuts before beveling is complete.
• Keep hands clear of the rotating drill bit; use a lens-holding clamp, never your fingers alone.
• Dispose of lens chips and swarf appropriately; fine polycarbonate or glass dust is a respiratory hazard—use ventilation or a dust mask.
• Never skip the cotton-swab test. A lens dispensed with a sharp edge can injure a patient.