fine grinding, 1/3 D W stroke

For fine grinding starting from 220 grit and finer, the normal 1/3D stroke used is shown in Fig 8. the mirror is centered over the tool and the stroke is a W-type stroke with a total lateral amplitude of ¼D (i.e. 1.8 cm on either side of the centre for a 15 cm mirror) and an up and down amplitude of about 1/3D (i.e. about 2.5 cm up, 2.5 cm down for a 15 cm mirror). It should be 1/3D on the average; some strokes should be a little shorter and some a little longer. No attempt should be made to make the strokes uniformly 1/3D in amplitude. The 120⁰ rotations of the blank above and the 45⁰ rotations of the blank below are of course continued. the wet now lasts longer, say for 4 or 5 minutes. At the completion of a wet the mirror and tool are cleaned as before and a fresh charge used.

Shankar