[ATM] Cutting a diagonal

["Russell Jocoy" <russjocoy@hotmail.com>]

   To all,

     As we all know cutting a diagonal mirror for a telescope becomes more 
difficult as the diagonal
diameter increases. Larger diameter diagonals need to be made with thicker 
glass.. we all know
this.. well most...  In the case of small diagonals the thickness is usually 
no more than 1/4",..
up to say 1.5" to 1.75" diameter?.
Since diagonals are strange animals , as far as the actual cutting and 
production of this oblong
optical pain in the butt, most of the time the 45 degree wedge/circle is cut 
by a "core drill"
or "trepan" at a 45 degree angle. In other words the flat glass is mounted 
at a 45 degree angle
to the "ground", and a round hollow "drill" of the desired diameter comes 
spinning downward to
cut a circle from the top to the bottom...  Sounds simple , doesn't it... 
Any on this list will tell
you that if they tried it ... It is not something they want to do again..
     Unless you have a machine that can "core" downward with a recurring 
supply of slurry (grit)
with no attendance for a few hours, you will spend way too much time at this 
process.
   Besides most buy diagonals from the pros who have the machines (computer 
controlled) are
available at reasonable prices, (well they used to be). I may be wrong but I 
used a core driller,
(a machine used to core out concrete for railing posts) to core (straight 
on, not 45 deg) a 1-5/8"
thick piece of pyrex... It took me 1.5 hours to break through to the other 
side. I was standing
on the drill with my full wieght, (it had diamond core cutters) and I also 
added 40grit carbo..
   My thought was that the bit was dull.... I put a brick under the same 
machine ..15 seconds and
a cut of 2 inches cored complete...I can't see leaving the machine 
unattended for 2 to 3 hours
to cut a diagonal .. This machine is borrowed from where I work and this is 
a slow process to say
the least.
    Well I had this brainstorm.. oh hell.....Using my angle grinder with a 
7.5" diameter diamond
blade I cut 2- 1 inch thick by 4 inch diameter diagonals today.
  The cutter is mounted with the blade vertically , about 2 inches from  the 
table (or base).
   I cut a 4 inch PVC pipe at a 45 degree angle on one end , and square on 
the other. The pipe
was then filled with a wood core( by grinding and sanding to fit), cement or 
other fillers are an
option in the future.. A base was made for the square end out of plexiglass 
about 1 inch in diameter
larger.  The top was sealed with polyurethane and I used contact cement to 
glue a felt pad to
this surface.   The glass disk ( already cut half way to oblong) was contact 
cemented to the felt
pad on the 45 degree slope,.. let it dry overnight....
       I fired up my cutter with lots of water, a respirator, earplugs, 
faceshield, and a painters overalls.
  Cutting in a circle, as close as possible to the PVC pipe, and rotating 
the homemade stand..
I was successful in creating (or imitating) the "core cut". If the cut was 
not round enough, I used
the blade to "grind" and smooth in a twisting motion to smooth out the edge.
       I ended up with two nice 4 inch diagonals with consistant measures on 
their circumferance (sp)
They both need edging and to be ground flat ( this Pyrex is real wavy) but 
total cutting time was
about 1.5 hours for both... The long cut on the major axis was about 1.625 
inches, glass was flying
everywhere....     Russ Jocoy....................................
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