(ATM) Fergy's Mirror 2

[kferguson@aquilagroup.com (by way of Mel Bartels <kferguson@aquilagroup.com>)]

Well I found a (slightly ironic) solution to my grinding stand quest:

I was prowling the back room at work viewing every odd thing as a potential 
stand, when I found an old bench grinder thrown under a bench, it was on a 
stand, sort of like an "I" beam, but "C" in section.  I asked around, and was 
told I could NOT borrow it, but I could HAVE it...but only if I promised 
NEVER to bring it back...such a deal!  My friend Jesse Cox says "Don't look 
for a thing" (a barrel in this case) "look for a function." Good advice, now 
if I could just train the hardware store clerks on this concept!            

This thing is cast iron, ugly as sin, hell for stout, and only a few inches 
lower than ideal.  I fastened it down to a 4x4' square of 3/4" ply, with a 
couple of sugar pine 2x4 blocks on edge for spacers. 1' square of 3/4 ply on 
top, and I'm in business.

I thought that having the base big enough to stand on would eliminate need 
for weight.  Wrong! this only works for push direction, ply flexes too much 
when "pulling". So I stacked half a bag of sandblast media (OK,OK, "play 
sand"), 1' of rail (my "anvil") and a big cast iron vise around the base of 
the pier. Perfection!    

When the mirror is done, I will use this for MY bench grinder, which is now 
taking up valuable area at the end of my workbench.  No barrel to dispose of! 
(Bob Madden says I should just build a machine, but I am following standard 
advice to do at least one mirror by hand first)
         
It was only 8pm when I finished the stand (stopped at library on way home, to 
get Richard Berry's book...out,overdue, dang!) So, by skipping supper,  I got 
in a couple of hours glass pushing with the #60 grit.  Boy, is my tool 
wearing fast! No danger of running through, but the ceramic is obviously MUCH 
softer than the pyrex. I had to saw some epoxy out of the channels to keep 
them open. (I filled them with epoxy, because I was _sure_ they were too deep)

The sag. is developing nicely.  The edge is still nearly flat for about 1/4" 
in.  I first noticed this when it was about 1/2" wide band, so I am assuming 
this will come out as the center deepens, and am not too concerned.  Somebody 
please say something if this sounds like trouble brewing.       

I was suprised at how hard it was to use the template..I am familiar with 
"looking for daylight" from wood and metal working, but never had a 
translucent surface to "eyeball" before. The light coming under the frosted 
surface makes it look like daylight _everywhere_ to me.  I tried both wet, 
and dry,but no improvement.  Guess I can go by feel (when it won't "rock" 
anymore) , but I sure would feel better knowing just how close I am, how fast 
(or even "if") I'm progressing, or by how far I overshot.      

I cut my template from shiny Al flashing.  I wonder if painting it black 
would be an improvement?  Anybody know a trick here?

I think I will just build a spherometer and be done with it.  I have a good 
3/4 travel indicator which aughta be the ticket.  I am thinking of having 
three pins with rounded points, set on a 1" radius with the indicator in the 
center. Is there some problem with this design? (too big/small radius??).  I 
think I'd want a test indicator (or 1/10 mic) ,a reference flat, and ball 
feet if I was doing lenses, but this rig aughta be better than a template for 
ballparking RoC on a mirror. I think the ground table on my mill is close 
enough to flat for a reference for mirror work, but it also occured to me 
that I could gauge the tool, then the mirror, and take half the difference as 
my reading.  Any reason why not?
                                                    

All fer today..Clear Skies,
-Kevin