To: atm{at}shore.net
From: Dennis Rech 
Reply-To: Dennis Rech 



Hello Everyone,
Just for clarification, here is how I made my last vertical tile tool and
how I think that twin mirrors should be made. I covered the face of the
mirror with aluminum foil for protection. I set a 10 inch diameter by 2-1/4
inch tall metal ring on the face of the mirror.
I used a bunch of single one inch square tiles and ran the outside row of
tiles all around the perimeter of the ring RADIALLY to the center of the
ring. This made a one inch thick band of tiles around the edge. I cut a
strip of 1/8th inch thick by 1/2 inch tall machinist wax and made a ring of
wax strip that sat just inside of the radial row. I set a strip of tiles on
top of the 1/2 inch wax strip. This held this single strip of tiles 1/2
inch above the mirror face. I then wrapped a spiral coil of tiles inside of
the wax ring all the way to the center.
I then poured  a slurry of pourstone all over the tiles and wax within 1/4
inch of the top of the metal ring.
I wiggled each tile to make sure that pourstone reached all the way to the
bottom and left no voids to later trap grit.. I then placed a 10 inch
diameter 3/4 inch thick plywood disk inside of the metal ring and settled
it into the pourstone. The disk had a few 1/2 inch diameter holes drilled
into it to allow pourstone to flow up to the top of the plywood disk and
lock the plywood onto the pourstone tool.

The whole assembly cured about 10 minutes and I removed the metal ring and
washed the excess pourstone off the face of the tool. I softly scrubbed the
tool face with a stiff brush to expose the wax and tile faces.
The tool was then placed on top of my shop furnace overnight to cure completely.
The next day, I used a hot air gun to melt the exposed wax ring out of the
tool thus leaving a 1/8th inch x 1/2 inch deep channel in the tool. The
channel allows a vacuum break so the 10 inch tool can be removed from the
mirror without sliding it all the way off. I also seems to help grit
dispersal.

Grinding.
If making multiple mirrors of the same sag, I would hog the first one out
using edge to edge strokes with no overhang at first to establish the curve
and then perhaps change to a fixed arm position until the desired sag is
reached. Monitor the sag and decide if it will be reached before the curve
is established all the way to the edge. If it appears that this will
happen. Move the arm out a bit to increase the overhang. This will shallow
the curve. Repeat the sag checking with your spherometer until the proper
curve depth is reach at the same time that the mirror is ground edge to
edge.

Remove the mirror and set it aside.

Place the second flat blank on the turntable. The machine will need to be
set up with a large edge to edge motion and overhang. The tool is curved
and the blank is flat so only the center of the tool will touch at first.
With the long stroke and small tool contact area, the tool will scrub off
the curve of the tool fairly quickly. Every 15 minutes or so, readjust
(shorten) the stroke so the contacted area of the tool just about reaches
the mirror edge. Do not let the contacting area overhang the mirror. 
Within an hour or two, the tool and mirror will have each recontoured and
normal hogging can proceed. Keep hogging until your spherometer has the
same reading on both mirrors.

You now have two generated blanks and one tool of the correct ROCs.

In fine grinding, the ROC of the mirror and tool should not change. If it
does, you will need to adjust the overhang until you find a neutral
position. You can therefore completely fine grind one mirror all the way
through and then fine grind the other. After the neutral position has been
located with the first mirror, use it for the second.
There really is no reason to alternate mirrors.

Hope this helps.
Dennis

--- BBBS/NT v4.00 MP