How to Determine Contact
Numerous bubbles collect between the disks during the grinding, with the larger ones at the center. As
the abrasive is broken down, the surfaces approach each other more
and more closely, and the bubbles become smaller. If those at the
center remain large, or if they are concentrated locally in that area, they indicate a gap between the disks there. In this event, when a charge of abrasive has been broken down very fine, so that at the edges the surfaces may be actually abrading each other, suction caused by the central gap will make the disks stick and cling.
Upon separating the disks, it will be found that the abrasive around
the edges is fine to the touch, while comparatively large grains still
remain at the center. When the rough-ground surfaces fit at every
point, that is, when they are spherical, they will pass back and forth over each other evenly during grinding and no clinging will occur.
The means just described for determining contact are entirely satisfactory in the coarse-grinding stages, and in fact are all the criteria needed to carry one safely through fine grinding. But the
mirror maker may feel more confident if he can have visible confirmation of what may be, in the case of the inexperienced, sheer guesswork. There are two ways of obtaining a visual check on the sphericity of the curve
How to determine contact part 2 |
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