To what extent is material removed? 

The removal of material does not significantly change the glass surface. Machining is carried out within the permissible manufacturer's tolerances. The permissible tolerances of the nominal thicknesses for glasses are specified in standards and processing guidelines.

 

An example of this is DIN EN 572 Part 2 float glass.

For 8 mm float glass, a nominal thickness deviation of 0.3 mm permitted. Common depths of glass scratches vary between 0.02 and 0.05 mm. Grinding the scratch does not adversely affect the glass surface.