What polish-free lathing demands from the tool edge

Why lathe without polishing

• Shape: polishing removes a little material unevenly. On complex designs, such as ortho-k reverse zones, toric surfaces or the large back surface of a scleral lens, it can change the geometry the lathe cut so precisely.
• Time and handling: one step less per lens, and fewer chances to scratch or contaminate it.
• Consistency: the result depends on the tool and the lathe, not on a polishing operator.

What the tool edge must deliver

On a polish-free lens, every feature of the edge appears in the optical surface. These are the points that matter:

• A truly sharp edge. The edge has to cut the material cleanly rather than push or smear it. Single crystal diamond, natural or MCD, is the only tool material that can be polished this sharp.
• No chips, not even tiny ones. A chip invisible to the eye draws a ring on every lens. Edges for polish-free work are inspected under high magnification.
• The right nose radius for the feed. Each turn leaves a tiny scallop whose height is roughly feed² ÷ (8 × nose radius); the finishing radius and feed are chosen so these marks are far too small to matter optically.
• Controlled waviness. The round edge must be very close to a perfect circle, because any waviness is printed into the lens surface.
• Rake and clearance for the design. Usually a neutral rake, and enough clearance for the steepest zone of the lens, such as reverse zones or a deep scleral back surface, without the flank rubbing.

For the geometry in detail, see diamond tool geometry: nose radius, rake, clearance and waviness.

What the lathe and the setup must deliver

• A stable spindle without runout, typically on air bearings, and a rigid machine; vibration shows directly on the lens.
• The tool exactly on center. A tool above or below center leaves a small pip or ring at the lens center.
• A light, steady finishing cut after roughing, with an even allowance.
• Clean chip removal, so chips do not drag across the fresh surface.

What the material must deliver

• Consistent buttons: stress-free and from a consistent batch; the same settings can behave differently on another material.
• The right condition: hydrogels must be fully dry when cut; some soft materials need cooling to cut cleanly.
• Stable temperature, so the material and the machine do not move during the cut.

Keeping it polish-free: tool care

A diamond edge wears slowly, and on polish-free work the first sign is on the lenses: a slight haze, faint rings, or lenses that suddenly need polishing. Many labs track how many lenses each finishing tool has cut and change it on a schedule, before the quality drops. A worn edge is not the end of the tool: it can be relapped to its original radius and waviness. See resharpening diamond tools.

What Bulunmaz does

Bulunmaz makes single crystal diamond finishing tools for contact lens and IOL lathes in natural diamond and MCD, with the nose radius, rake, clearance and waviness your lenses need, inspects every edge, and resharpens tools when they wear. See our diamond tools for lathe-cut contact lenses and IOLs.