The Twyman effect is caused by the introduction of stresses by coring with coarse grit, and that fine grinding and polishing the ground edges relieves these mechanical stresses. You seem to be saying that coring with coarse grit releases stresses in the glass which is exactly the opposite of what Mark and Eikonal described above.

Technically, grinding a polished surface induces stress, it doesn't "release" it. Conversely, polishing a ground surface produces strain of the opposite sign and direction. So a piece of float glass that is ground on one side bends towards the polished side, as the surface area of the ground side is higher than the polished side, basically, but it won't quite fit into the same space anymore. Polishing relieves this stress. It has nothing to do with anneal.

See for instance, https://www.ncbi.nlm...pubmed/21127579

In the Twyman effect (1905), when one side of a thin plate with both sides polished is ground, the plate bends: The ground side becomes convex and is in a state of compressive residual stress, described in terms of force per unit length (Newtons per meter) induced by grinding, the stress (Newtons per square meter) induced by grinding, and the depth of the compressive layer (micrometers).

I see Mladen cited the same paper - but it's not due to surface tension - that's the result of a liquid and ionic bonds.

If you take a polished glass plate and grind one side the polished side will tend to become convex, due to released surface tension (release of stress).

Anytime you induce a crack (or pit) in the surface of a piece of glass it causes stress, because the VOLUME of the material after it cracks now has to include the volume of the air space as well - and so the new volume only fits into the old space under compressive tension. Also, the polished side becomes **concave**, not convex...

My conjecture is that the traditional approach you described, coring all but the front 1/8th inch and then finishing off the hole after figuring, relies on the stresses introduced by coring being stable, and since most of those stresses would be introduced before figuring and before coring the final 1/8th inch, the effect on the final figure of coring the final 1/8th inch would be minimal. Also, chamfering the edge of the cored hole with fine grit would likely relieve much of the stress introduced by the last 1/8th inch of coring.

Right, but if you THEN polished the entire core edge you'd remove that compensated strain and end up worse off.

**Edited by mark cowan, 25 November 2016 - 08:30 AM.**