ATM 21" f/6 - full size tool report

["Ken Bertapelle" <kenb@cqg.com>]

All,

Here's the latest development in my mirror project.  I reached a sagitta of
.21" over the weekend (target is .2165"), and decided to stop grinding with
the washer tool, and made a new tile tool.  I followed the method described
by Richard Schwartz and Bill T.  It worked out quite well.  I had had
problems with this method on a previous attempt mostly because of the
difficulty in mixing a large batch of hydrocal by myself.  The plaster sets
up quickly, and the stress of mixing a large batch and getting it into the
form made the job go badly.  The solution is to invest in a mortar mixing
paddle for use in an electric drill.  Mixing the mortar then only takes
seconds, leaving plenty of time for pouring the tool.

At the last minute I decided to make a full size tool.  This may turn out to
be a mistake, as the tool weighs 67 pounds!  But it looks very nice - the
tiles conform to the hogged out blank, and they are nice and clean.  Bill
T's technique of gluing the tiles to the plastic worked out very well for
me.

I'm now planning on finishing rough grinding with MOT and a COC stroke, as
described in Texereau's book.  This should get me close to a sphere and mate
the tool to the blank.  I was planning on doing most of fine grinding with
TOT.  But now that the tool has turned out so heavy, I'm wondering if there
are any dangers in doing so.  The mirror blank weighs 40 lb. and is 1.5"
thick.  Any comments on this question from the veterans out there?

TIA and clear skies,

- Ken Bertapelle