[ATM] How round and how square should it be (blank

[Roman Toledo <rtoledo2002@yahoo.com>]

>The sides merely need to support the front of the mirror and can
>be at about any angle.  Some people (and companies) build
>telescopes with a cone shaped backside for light weight with the
>edges of the blank being a lot thinner than the center of the
>mirror.
>As to roundness, the blank should be reasonably round so having a
>blank that is 1" different in diameters is a bit much but you
>don't have to go to 0.001" roundness to be considered round.
>Grinding the front surface won't be affected by the oblateness of
>the blank so don't worry about that.  The sole effect of being
>oblate is that the Airy disk won't be round but rather oblate in
>the same ratio as the difference in the mirror.
>Bob May

         Thank you,  Bob for the encouraging words.
  I worked on it some more yesterday with a hand stone grinding down
the high spots.

 I cut a 3/4" mirror 2 weeks ago, it was my first and I did not do so
good, that was with a 7" diamond blade.
 I tried it on my table saw (crappy little one, but it got me through
my 800 square foot room addition I did 8 years ago) it had too much
play and vibration on the blade /shaft and kept chipping chunks.
 I had better but limited success with a circular saw as I could
control the feed and water better (well insulated).  with this 1" piece
of glass I really wanted to be more carefull as you know this glass is
not made anymore, so I "tought" a 4" diamond blade on my Makita grinder
would work better, and since it's a smaller diamter the cuts would be
tighter, but again since it was not mounted to some solid jig, it
vibrated too much and also broke chunks.

I got a new burst of energy and decided that the best one so far with
my limited resources was to try many small  lines at a time until I got
close to a circle, it took me a couple of hours, but I ended with a 13"
and a 14" mirror.

I will be keeping you, Richard, Thomas and a few others abreast of my
new adventure.

oh yes, last night was my very first visit as a member of the  laas.org
 club and got to meet a bunch of very knowledge guys and gals. I took
my son as I got a family membership and showed my  "frankestein cut"
mirror, I was pleasantly surprised as a couple of the old timers told
me it could be better, but it was ok to start on it.  so that's what I
did this afternoon started my rough grind, spent the morning getting 2
bags of hydrostone, and trew away the  Quickrete post cement I made
(junk)  sorry to carry own so long, but just  excited.  I'm sure you
remeber your first ;)

I got my autistic son interested in grinding one too, he felt confident
when we left there,  I have a  6" pyrex coming for him tommorow  from
James Mulherin, and I'm going to get him into this too, the kid needs a
new hobby besides computers and "trying" to make 3d games

again sorry to ramble on, just excited!
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