Optical glass polishing is limited by scratch depth, not by removal rate. That means the grade selection starts with the finest step you can tolerate, works backwards through a size ladder, and treats particle-size distribution as more important than the nominal grade.
Why Glass Punishes Bad Powder
Glass is hard, brittle and transparent. A single oversized particle leaves a scratch that is visible under oblique light long after the surface roughness elsewhere is acceptable. You cannot polish a scratch out with the same grade that caused it; you have to step down.
That is why optical shops judge powder differently from tool makers. A tool maker asks how fast the wheel cuts. An optical shop asks how many parts pass inspection before the pad needs dressing.
Build A Size Ladder, Not A Single Grade
Production optical polishing is normally a sequence, not one grade. Each step removes the damage left by the previous one, and the rule of thumb is that each step should remove at least the depth of damage created by the step before it.
| Step | Typical size | Purpose | Grade direction |
|---|---|---|---|
| Rough grinding | W20 – W40 | Shape and stock removal | HDY, HDG |
| Fine grinding | W9 – W14 | Remove coarse damage | HDY |
| Pre-polish | W3 – W7 | Flatten the surface | HDY, HDYP |
| Polish | W1 – W2.5 | Reach Ra target | HDY, HDYP |
| Final finish | W0.5 – W1 | Remove residual haze | HDYM, HDYP, HDP |
Real shops compress or extend this ladder depending on the starting surface. What they rarely do is skip a step, because a two-step jump usually leaves subsurface damage that shows up later as sleeks or as coating failure.
The Size Ranges Optical Work Actually Buys
Across optical, connector and cover-glass work, the grades that appear most often in purchase orders sit between 0 to 1 and 8 to 16 micrometres. Below that, connector end-face and final polish steps run sub-micron material such as HDYM at W0.5 to W0.1, or detonation poly at 0 to 1 micrometre and finer.
Two things decide where you start inside that band: the material and the tolerance. Crown glass removes faster than sapphire, so it tolerates a coarser starting point. A lens with a tight surface specification needs the ladder to start finer, not the final step to work harder.
Mono Or Poly For Glass
Both work, and the choice is usually economic. Monocrystalline HDY removes faster at the same size and costs less per carat, which makes it the default for rough and pre-polish steps. Polycrystalline HDYP or detonation HDP produces a shallower scratch pattern and is normally tested at the final step when the surface is the limiting factor.
A practical compromise many shops use: mono through the ladder, poly only on the last step. You keep removal rate where it matters and pay the poly premium only where it buys surface quality.
Slurry Behaviour Matters As Much As Size
Glass polishing is usually done with a slurry, so the powder has to stay suspended. Three things affect that:
- Fine fraction. A high D10 share keeps the slurry stable but slows the cut. A low D10 share settles quickly and needs constant agitation.
- Agglomeration. Fine powder that clumps behaves like coarse powder. If your W1 slurry scratches like W5, the problem is dispersion, not grade.
- Carrier chemistry. pH and surfactant choice change how the particle behaves against the pad and the glass. The same powder in two carriers is two products.
What To Send With The Inquiry
Five details shorten the loop dramatically: glass type, current step in the ladder, target Ra or surface specification, pad and carrier chemistry, and the defect you are trying to remove. If you can add a photo of the scratch pattern under oblique light, the answer usually arrives in one round instead of three.
Common Mistakes
- Jumping two sizes to save a step. The scratch depth from the skipped step stays in the glass and appears after coating or during final inspection.
- Blaming the grade for an agglomeration problem. If the fine powder is not dispersed, going finer makes the scratch density worse, not better.
- Comparing suppliers at different sizes. “W3” from two suppliers can differ at D90 by several micrometres, which is exactly the tail that scratches.
How To Verify
Ask for the PSD of the lot and a morphology image at the size you bought. Run a flat test on the actual glass type, count scratches under the same lighting on every part, and measure Ra at the same locations. Ten parts per cell, not three, because the pad changes during the first hour.
Next Step
Products: HDY W50 – W1, HDYP 0.7 – 40 um, HDYM W0.5 – W0.1. Application detail: optical glass grinding and polishing.
Related reading: mono versus poly and slurry and paste selection.