ATM 21" f/6 - Rough Grinding

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

All,

Here's the status & a couple questions on my mirror project.

  - Chamfered the edges, flattened the back of the blank.

  - Tried making a grinding tool with the "glue less" method described by
Richard Scwartz, but this didn't work out for me.  The plaster oozes in
between the tiles, and it was too difficult to open up the lines between the
tiles again.

  - Glued another mat of tiles over the first layer, using epoxy and the
method described by Mel.  The resulting tool looks good, but we'll see if
the tiles wind up staying put.  (This was a problem for me on my 10"
mirror.)

  - The tool is 14.5" in diameter and 4" thick, weighs about 40# (a beast!)

  - Rough ground for an hour last night with 60 grit, using a long W stroke
(center of tool moving out to within an inch or two of the mirror edge.
Working with tool on top, since I'm not comfortable with constantly handling
the mirror.

  Observations: At the end of an hour, have good contact on about 85% of the
tiles.  The wets are very short: either the grit is breaking down very
quickly, or it's being pushed over the edge of the blank very quickly, or
both.  Used about 1 salt shaker of grit.  At the end of the hour I could see
maybe 1/128" or less of concavity in the center of the blank.  That's with
averted vision, squinting, and looking over my shoulder!  Groan.  This is
going to take a while.

  Question: Will I see better progress by shortening the stroke?

  Question: On a number of occasions, the tool stuck (hard) to the blank at
the end of a stroke.  Does this indicate that the slurry is too thick or too
dry?  Should I be using more water?

To be continued.  Thanks in advance for your ideas.

- Ken Bertapelle