

It is important that the mirror is suspended. A piece of old carpet, a old towel, or a stack of old, wet newspapers will be just fine. This will prevent astigmatism due to uneven support of the glass disc...

To get a sphere segment in the glass disc, we need to rotate both the mirror and the tool. As randomly as we can.

Example : Make 6 strokes, rotate the mirror 40 degrees clockwise, and the tool 30 degrees counterclockwise. Make 7 more strokes, rotate the mirror 60 degrees clockwise, and the tool 20 degrees clockwise. Don't think in degrees, nor numbers, just spin it randomly.

Rough grit breaks up quite fast, You will know that the grit is used up when the loud , crushing grinding noise disappears. Add more grit, spray with water and continue. Rotate rotate rotate! Slowly.. you will see the first indentation starting to appear in the glass disc center... To see it and measure it, you need to rinse your mirror, and the tool from time to time. Prepare a bucket with water, and rinse your glass disc there. Do not rinse it in a sink or similar, because you will be calling the plumber very soon. The used grit and glass mud settles on the pipes like cement.