To: 
From: "Thomas A. Moulton" 
Reply-To: "Thomas A. Moulton" 


> ----- Original Message -----
> From: "Dale Eason" 
> To: 
> Sent: Saturday, June 21, 2003 12:39 PM
> Subject: ATM 16" inch rough grinding questions
>
|
> | Most sites talk about making sure the blank is flat.  I have a plate
glass
> 1.125" blank.  I have tested it for flatness by spinning a piece of
aluminum
> channel about 12 inches long on top of the blank surface.  On one surface
is
> spins freely.  On the other it will not.  This means that one surface is
> slightly  concave and the convex.  On the convex surface there are places
> were the end of the bar will raise from the surface about .02 inch when
the
> other end is pressed down on the glass.  I cannot make it do that on the
> concave surface.   I'm planning on using the concave as the back.  Do I
need
> to flatten this surfaces more?
>
>     Yes, flatten the back.  I'm currently refiguring a 13" Coulter mirror.
> It's 1" Pyrex, but it has some straight ripples across one side a few
inches
[snip]
>     And then it will be made convex.  I don't know what to tell you here.
> You may (if you're careful) be able to use a piece of ordinary window
glass.
> Try for a thick piece of course, and try for something more like full
size.
> It can be square.  It can be broken.  It doesn't matter.  But you only
need
> to get it close so it looks good to a straight edge.  You can go through
> grit sizes quick too.  You don't have to worry about scatches or leaving
> pits.  You can be down to 9 micron in an hour.  Then just leave it at
that.

Here's some more details on his blank.

Dale stopped by to pick up some grit from me and I took a look at the flatness...

Taking his straight edge there were positions where I could get the corner
of a sheet of paper between the glass and edge.

It was hard to see light between the glass and edge.

Do these very Slight variations need to be removed too?

tom

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