From: Dale Eason 
To: atm{at}shore.net
Reply-To: Dale Eason 


This is for Bob May and others with experience hogging mirrors by hand.

I started hogging my 16" F5 last week after flattening both sides. 
The tools I used are shown at
http://doeason.home.comcast.net/grinding_tools.jpg.  I started with a 2
inch pipe cap not shown but quickly switched to the 2" diameter steel
cylinder shown on the far left because it weighed more.  This tool worked
quickly but only made a hole about 4" wide. When I got down to a sag
of  .060 I realized my stroke was probably too short and too much Center
over Center.  I had made a sphereometer and it showed the hole I had dug. 
I switched to the stepped pulley since it had a larger diameter and used
chordal strokes and got hole within about 1 inch of the edge according to
the sphereometer.  But then I couln't get the hole to deepen, I just
couldn't figure the right stroke.

If you remember I had started to make a 12.5 inch hydrostone tool cast
against another mirror with the same desired ROC  and covered with Nickels.
 Far right in the picture.  I figured that it was time to try it.
 It worked very well using short center over center
strokes.  It has brought the mirror to a sag of .175 just .015 from the
desired depth.  It has also made a very spherical mirror except...  there
is this ring around the outside were it has not done much grinding.
 I have watched it shrink from about 1.5 inch to about
1/4 to 3/8 inch.  That is good because I'm trying to reach the edge just as
I reach the desired depth.

I have used the sphereometer on the mirror and it is everywhere the same
ROC except for this edge.  See picture of mirror and the edge ring at
http://doeason.home.comcast.net/mirror_edge.jpg.  I have measured how much
the edge is down.  I can stick a shim .007 inch thick under the ball of the
sphereometer when it is at the edge and get it to read the same as the rest
of the mirror.  So I guess the edge is down by about .007 inches.

The correct sphereometer (r=1.635) reading for my ROC should be .0085. The
mirror is currently at .008.  The sag is .180 but will be .2 when complete.

I had planned all along to switch to a tile tool for fine grinding.

Now after all that (I hope some are still with me) my question.    Should I
switch to a tile tool now or
wait until I get the too shallow curve all the way to the edge?

Dale Eason



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