[ATM] Wedge: How much is too much?

[Douglas Hudgins <hudginsclan@mac.com>]

Hi Everyone:

This is my first post to the ATM list, but I have lurked on the list  
in the past and have benefitted from the collected wisdom in the  
archives for years.  Here's my quandry...

I'm working on a 32" BVC blank. The blank came pregenerated to an f/5  
curve, and at this point I have made a tile-on-plaster tool (26"  
diam.), mated it to the blank, and ground through #120 SiC.  Now,  
before I move on, I need to get the back of the blank whipped into  
shape.

Right now, the back side of the blank appears to have received a very  
cursory degree of grinding.  From the somewhat mottled appearance of  
the ground surface it appears the work may have been done with a hand  
grinder.  It is my understanding that BVC blanks are pregenerated by  
slumping on a mold, and that afterwards, the convex back side is  
ground (very roughly) flat, and that is consistent with the  
appearance of the back side of the blank.  Obviously, I want to clean  
up the back side by grinding it flat through #220.

All of that is no problem. My question is about wedge--specifically,  
how much wedge is acceptable in a blank of this size? Using a  
caliper, I find that there is about 0.015" difference between the  
thickest and thinnest points around the edge of the blank. Is 0.015"  
of wedge across a 32" blank acceptable, or should I be trying to cut  
that wedge down as part of dressing up the back side of the blank.

Thanks for any advice.

Doug Hudgins
Dumfries, VA
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