Having recently aquired a 6" optical flat I thought it would be interesting to checkout the granite placemats and chopping boards now stocked by the supermarkets. These are 15 to 20mm thick and small enough and light enough to be kept in a drawer when not in use.
Although the working surface is polished, it is only polished sufficiently to give a shine. There's severe "orange peel" surface disturbance - not good enough for optical interferometry checks. However the surface is ideal for the capillary/surface tension method.
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Show the test results on two 15mm thick placemats. The 6" flat was supported at the top end with an 0.005" shim to give an airgap slope of about 5/6000. The equal thickness capillary edge displays a 6000/5 amplification of the flatness error - approx 1" per 0.001" Each plate was tested at 0deg and 90deg and showed errors of less than 0.0004".
The tests were repeated on a 300 x 400 x 15mm chopping board.
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The first two show the 0deg and 90deg results The third is a repeat of the 90deg result but with an 0.001" shim.
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Is the result using an 0.001" slope on a 12" length of 2" x
1/2" plate glass (ex shop window display shelf)
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Is an 0.005" slope test on a 12" x 18" surface plate. Although the lower faint capillary edge is fairly visible by eye, the contrast is too low for satisfactory photography. Both Meths and ispropanol (rubbing alcohol) gave similar results but a marking out fluid which appears to be heavily dyed alcohol gave excellent contrast. The snag is that the higher viscosity results in a long settling time.
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The fist JPG is an 0.005" slope test. The second shows the beginning of an 0.001" slope test. the third is the result
1/2hour later. The results show up the comparative roughness of a ground and scraped surface.
Jim