From: Gerald Boutin 
To: atm{at}shore.net
Reply-To: Gerald Boutin 


Hi, I am into my first glass pushing project and I have bogged down at fine
grinding at 500 grit. I have done a lot of reading and made a lot of
mistakes along the way, so I know I am making some progress.

I am working on a Wilbell set of 8 inch pyrex blanks using the materials in
their kit. I fully expected to be quite inefficient throughout the steps
but I cannot figure out why I am having such a hard time getting rid of my
last pits at 500 grit. I would like to improve my grinding efficiency,
before I run out of 500 grit. The earlier grit sizes seemed to go more or
less how I expected, which was about twice as many wets as I have read
would be considered normal.

For 500 grit, I am doing wets of about 7 to 8 minutes of grinding 1/3 COC,
no additional pressure and about one stroke per second. I generally
alternate the disks with each new wet. I am pretty sure that one of my
earlier errors was having reduced the amount of grit between grit sizes too
much. I started SiC 500 grit using 1/8 tsp of grit per wet. I am now using
closer to 1/4 tsp per wet. However, progress still seems much slower than
it should be, or I would like it to be.

As confirmation, how can I determine what the optimum amount of grit is?
When using 1/8 tsp per wet, I never had any spillovers. Now, I am getting
about 3 to 6 spills over the edge of the bottom blank. Is this about right?

Likewise, how do I judge how much water is about right? I am generally
going for enough water to get more or less a thin slurry about the
consistency of milk. While grinding, I add a few drops of water if I notice
that I no longer see water receding as the top blank uncovers the lower
blank. (I may not have worded this last bit correctly, hopefully it conveys
some meaningful info.)

As further information as to my present status, I am up to over 50 wets of
500 grit, which works out to about 6.5 hours of actual grinding. The
concave blank has been pit free for the past 6 or so wets. I use a 10X
loupe and backlighting. However, I still have a couple more sizable pits on
the convex blank, the worst of which is near the center. My ultimate goal
is to produce a hyperbolic secondary, so it this blank that I need to be
pit-free. This pit is a triangular divot about .006 inches on a side and (I
think) about .001 inches deep at one end.

I have tried doing the last session of 5 wets using the concave blank on
the bottom as it seems to me that the other way around isn't going to work
the top of the convex blank where the pit is. Unfortunately, this is taking
the ROC farther away from my intended target, rather than nearer. Worst of
all, I could not see any noticeable reduction in the pit size.

So, any tips on how I can improve my technique and get on to my next challenges?

--
Gerald Boutin

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