Fine Grinding
The mirror maker has his choice of three ways
of performing the fine grinding. The mirror may be ground on top of the tool, accompanied by a shortening of the radius; or the mirror may be placed on bottom with the tool on top, in which case the radius will lengthen; or the positions of the disks may be alternated, either with each charge of abrasive, or for half of the charges of each grade, and a constant radius can be maintained.
By these means the worker has control of the focal length, which
should be about 481/4" when fine grinding is completed. Polishing
will usually reduce this by 1/8" to 1/4".
As to stroke length, the one-third stroke is the longest that can
be used without deforming the surfaces. It will at the same time
produce the greatest change in radius, amounting, if used continuously throughout fine grinding in the same positions, to 3" or
4" or more. A shorter stroke, say about ½" in length, will produce
a minimum of change, perhaps of only an inch or two.
An advantage in alternating the disks is that greater freedom of movement is had, and equal abrasion is accorded to the edge of the mirror, where the least action takes place in the usual grinding-on-top. At no time should a rapid stroke be used in an effort to speed up the work, as this will almost certainly result in flattening the edge of
the mirror.
In any upside-down grinding, not more than a one-third stroke should be used, for a similar reason, except in a deliberate attempt to lengthen the radius radically. As he knows his focal length to a very close approximation, the worker can now select the stroke and method to be used with No. 120.
fine grinding part 2
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