What You Will Learn

• How tool size and curve match the part
• When to run a part on top vs. bottom
• Easy rules to pick a good polisher size

Simple Explanation

The tool should match the part’s shape, so the pitch and the liquid (slurry) spread out evenly.

• Convex part (domes out): use a concave tool (curves in) with a slightly longer radius.
• Concave part (curves in): use a convex tool (domes out) with a slightly shorter radius.

Quick example:
If your convex lens is 50 mm diameter and has a 100 mm radius of curvature, start with a concave tool about 45 mm in diameter and a surface radius around 102–104 mm.
If your concave lens is 50mm diameter, and has a 100mm radius of curvature start with a convex tool about 60 mm in diameter and a body radius around 96–98 mm.

Pitch is the conformal material that is adhered to the polishing tool to create the conformal soft ductile surface that polishes the surface of a lens.  When is warmed, it gets soft and slowly molds to the glass surface.

• For convex parts that bow out, usually are run on the bottom spindle of the polishing machine. A good starting point is to choose a tool that is about ninety percent of the part’s arc diameter and make the tool’s radius two to four millimeters longer than the part’s radius of curvature. That small extra flatness helps protect the center once the pitch warms and presses in.
• For concave parts that curve in, usually run them on the top. A good starting point is to choose a tool that is about one hundred twenty percent of the part’s arc diameter and make the tool’s radius two to four millimeters shorter than the part’s radius.

The decision of the optic being in the polishing spindle or the polisher being in the polishing spindle is usually decided by size of the optic, the radius of curvature, and the difficulty of the irregularity specification needed

Top vs. Bottom (What Changes)

• Bottom (convex parts): provides a more natural pivot to the polisher with the oscillation motion of the polishing machine.
• Top (concave parts): provides a more natural pivot to the polisher with the oscillation motion of the polishing machine.

Pitch, Grooves, and Thickness

Pitch hardness can be useful for maintaining control of radius of curvature, figure error and surface quality.

Grooves are used to control slurry flow between the polishing pitch and the surface of the optic being polished.  Grooves also control the surface area of the polishing pitch changing its dynamic properties and flow.

Thickness of the pitch layer can affect the flow characteristics of the pitch.  Uniform pitch layer is desired for polishers.

Math in Context

You don’t need big math, just tiny offsets:

• Make the tool radius +2 to +4 mm for convex parts (longer).
• Make the tool radius −2 to −4 mm for concave parts (shorter).

Process Steps

First, make sure you know the part size (clear aperture), the curve (radius), and how clean the edge must be. Pick top or bottom run based on the surface: convex usually bottom, concave usually top. Use the 90% / 120% diameter rules and set the tool radius with the small +2– +4 mm or −2– -4 mm change. Groove the pitch surface to carry the slurry efficiently across the polisher surface area, then warm and press until the contact looks even. Let slurry flow and reach working temperature before timing your polishing.

To check contact, draw a light wax crayon grid on the lap, do a short warm press, and look: the lines should fade evenly toward the edge. Re-press if room or slurry temperature changes, or after a long break. Keep a simple notebook with tool IDs, measured radii, chosen diameters, and the offsets you used so you can repeat a good setup next time.