Greetings once again from the lab,
I decided to try the beveling of the 8" down here at
the lab. So that I didn't do a fancy "insert" of text in the
pictures, I am including some notes you can follow for each slide. In the first
image, note the soft stone I am using to soften the edge of the tool. Since we
will use this at the upcoming event with the public, I wanted to soften the
edge to rid ourselves of the chance of chipping the edge of our Pyrex. Second
you can see this edge on the tool. On the third image, I wanted you to see the "rounded"
edge of the mirror our donator has originally created.
Now, I am not opposed to the idea of a rounded edge at
all. In fact I'd like to discuss this a bit more in our workshop. There is
strength to this round edge. When grinding the mirror, a hard edge will appear
on the upper edge of the bevel or rounded edge. The lower part of a rounded edge seems to me to be
impervious to any chipping. Am I wrong on this? The fourth image shows me
starting to cut a bevel on the edge. I am using a coarse diamond disc, that is
actually quite flexible. I try to hold the handpiece at a 45° angle. With a
16" mirror this may be a bit more difficult. I can kind of move the mirror
under the handpiece a bit with these smaller mirrors. Keeping this 45° is a
little more difficult moving just the handpiece over the mirror, but do-able.
The next picture shows the whole work top. I wanted to mention something Jerry
told me that is important. When grinding these, it is advisable to use a wet
grinding. For this, you can see the little dish of water behind. I can either
wet the surface of the mirror, wet the diamond disc itself or both. In the old
days, we had an attachment they made for dentistry to utilize compressed air
and a water canister to draw the water up a small hose to the nosepiece of the
handpiece. Today, they are more using this arrangement with high speeds. For
this, I don't think it is a good tool. These turn at about 250,000 rpm and are
for less "planing" stroke and more for spot grinding and facial
contouring. I also think the incidence of chipping is increased, especially
with pressure. I do want to add, you can see I have a hose in this picture.
It is a vacuum I use for grinding. Before I got this, I
found myself coughing as i ground my porcelain crowns. As time passed over the
years I realized I was actually working towards silicosis. With this vacuum
right next to the work, it literally eliminates the danger. A wet grinding set
up together with this vacuum would be better. Just get the sunlight to shine
into the lab late in the afternoon and see what we are working in. the dust is
silhouetted in the light. I am actually wondering why I have lasted all these
years!The last two photos show our finished edge. In the first you can see it
actually allows a fairly consistent bevel around the mirror. In the second I
wanted to show you that small chips were noticeable to me. Look in the area
that is most in focus and you will see them. I think this is within tolerance
and acceptable.
Hope you all enjoy this. I do not think anyone is using
this exact technique out there. Maybe the Newport Glass Works do, but it may be
a bigger machine doing the work. Next door to us is a glass company. I intend
to talk to the owner a bit on this. I have used a tool they have in back of
their shop to soften the edges of cut glass. Joe, it is a rather large wet belt
sander on a stand. Kind of cool actually (literally). It has a place to rest
the glass, but I wouldn't think holding a 16" against it would be as
accurate as what I am doing. This is better.
PS- Tom T. or Dmitrii,feel free to use this any way you
wish.
Tim the Tinkerer
see the photos attached to this email here: https://secure.flickr.com/photos/27241501@N03/sets/72157634222053744/
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