On two occasions I have had to perforate mirrors, once for an optical
flat for null testing and later Cassegrain primary mirrors. The usual
practice seems to be is not to cut the hole right through, but from
the back to within a small distance of the face of the mirror. The idea
of this is that the centre of the mirror is retained for testing purposes.
Also to prevent turned edge around the hole. Also in pressing the lap
a ridge is formed by the saw groove and has to be carefully removed.
After the mirror is finished the perforation is finished. As the cutter
breaks through there is the risk of chipping around the edge of the
hole. I however cut the hole from the face of the mirror to within an
1/8" of the back.
When pressing the lap, I place the mirror face up and lay a ring cut
from .030" nylon shim over the groove and place the lap on top for pressing.
This depresses a small section of the lap. I have not experienced any
turning of the edge around the perforation. After the mirror is polished
the centre plug is removed. To prevent chipping as the cutter breaks
through a piece of thin glass can be waxed over the break through area.
I used carborundurn when I perforated the optical flat blank, but purchased
a diamond hole saw for the Cassegrain primaries as I had a few to do.
Below is illustrated the set ups.
Fig. 1 illustrates how a centering plate is used to locate the hole
in the centre of the mirror blank. After the cut has started. this can
be removed. If using a diamond cutter a copius supply of coolant is
needed. I cut the holes on the Bridgeport using its coolant system.
If the hole is being cut with carborundurn powder, then a ring is waxed
onto the face of the mirror to form a dam for the powder and water.
Fig.4 and Fig.8 show the nylon shim ring placed over the groove while
pressing the lap. Fig.3 shows a piece of thin glass waxed to the back
of the mirror blank for completing the cutting of the perforation. The
cutter penetrates into this piece of glass and leaves a clean sharp
edge around the hole.
Fig.5 shows the design of the cutter for using carborundum. Notice how
the thickness of the tube is reduced behind the cutting edge and four
cut-outs made to allow powder and water to get down to the cutting edge.
Fig.6 is the chamfering tool for bevelling the edge of the hole. I did
this before the centre plug was removed. I used a spindle but the tool
can be worked around by hand.