Fine Grinding
When working with the mirror on top and using a short stroke, there may occur a slight lengthening, rather than the expected shortening of the radius. It would have been difficult and impractical to bring the surfaces into exact contact with No. 80 on account of
the coarse grain size, and a thin gap may have persisted at the center.
But as finer grades are used and broken down, the surfaces approach each other more closely, and a slight lengthening of the radius may thus take place through the wearing back of the edge zones of the mirror. After complete contact is established, however, the radius will begin to shorten.
The focal length now being quite exactly known, the worker can proceed with confidence. The radius can again be checked
upon the completion of each grade. As finer abrasives are used,
the quantity for a single charge should be reduced; for example,
just a pinch of emery No. 305 will suffice. In using carbo No. 400
and finer grades, the abrasive should be deposited at the center of
the tool (or mirror, if it is on bottom), a few drops of water added,
and the mixture puddled and spread over the surface with the finger
tip.
After each of these finer charges is thoroughly worked down,
the surfaces should be flushed clean of mud before applying the
next charge. Faster action is obtained in this way. With the last
two grades, the tiny bubbles that have always been present between the surfaces should be eliminated before grinding starts.
This is done by moving the mirror out over the tool, slowly rotating it the while; the bubbles will disappear over the edge, leaving nothing
but a thin film of water and abrasive between the disks. Only five
charges of No. 305 need be used, but the last one should be worked
out for about 15 minutes, with the mirror on top, adding water when
necessary. It should be possible to read 12-point type, held at a
distance of several inches, through a mirror ready for polishing.
If the tool is of the recommended thickness, pressure can be
used throughout the fine grinding, but beware of the disks sticking
together in the final stages. Evaporation may cause them to stick,
so if it is necessary to pause for a moment, the disks should be
separated.
Otherwise, keep them moving at all times, and if there is a suggestion of stickiness, due to evaporation, separate them immediately. As previously explained, suction from lack of contact
at the center will also cause them to stick. If this occurs, use a
carpenter's wooden clamps to separate them, or wedge the tool on
a bench, hold a block of wood against the side of the mirror, and
free it with a sharp blow of a hammer.
next examining for pits |
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