Re: ATM 13" SCT long

[PFAFF2@delphi.com (Robert C. Pfaff)]

As I have received several questions on the core drilling of
the primary and biscuit cutting the large corrector I will try
to cover the core drill project questions today. Will try to
describe the biscuit cut project by tomorrow. 

Aart writes:
>
>Here are some questions about how you cored your glass:
>
>Did you go half-way from each side to avoid chipping the back end?



The hole in the primary was cut from the front side to within
.200" of the back. (plug left in )This was done in the fine grinding stage.
I covered the mirror surface with 2" plastic pipe wraping tape
to protect the surface. 

>
>Like you I was going to use a hole saw with the teeth ground off and the
>sides slotted.  Did you grind the teeth all the way down to the body of the
>cylinder? 

The hole saw is a bimetal HS Starrett. The teeth were ground completly
off. I just spun it against a grinding stone.

  
>How many slots (and what do the slots do anyway)?

I cut 4 tall slots in the 2.75" saw that almost reached to the top. 
4  more small slots were cut in between (1/4" high) A total of 8 slots.
The slots alow the grit to fall in the groove and leading edge of the
slots and grit tent help in the cutting. I first tried 4 slots
but found the 8 cut faster. Rasing and lowering the hole saw now and then
helps get the grit down the groove also.
The slots in the steel hole saw can be cut easily with a carbide
cutoff disk on a table saw.


>One reference I have says the cutting edge should have more kerf than the
>rest of the corer to prevent excessive widening of the core away from the
>cutting edge by the captive grit (probably it's safer too, or does the
>coarse grit keep the gap open enough?).  Was your corer straight sided?
>Did the core end up with straight sides?

The hole saw was about .053" thick. The groove was about .100"
So binding was not a problem as the grit does cut somewhat on the sides.
Also the hole saws do not run that true. The saw is straight sided
but hole does  have slight taper. (.005")Because of the set of the
teeth of a hole saw the wall diameter is under sized. My hole came out .020"
over size in diameter. 

>
>Is a hole saw kerf okay or too thick/thin?

Don't know if there an answer for that question.

>
>Most books say to use thin wall brass or something soft like it.  Did you
>have any trouble with the hard steel?

Soft material might cut faster because the grit tents to become imbedded
in it. The hole saw was used because it is ready made and they come in
increasing sizes by 1/8" up to 4".  The cost is around $14. The arbor
would cost another $19 so I make one from a 5/8" bolt for $1.00
Cutting speed is about 3/4" per hour (new Pyrex). The spindle speed was 130 rpm.
I tried higher speeds but the glass got so hot the water would boil
and the grit did not go down into groove as well. A 10 lb barbell weight
is hung on the down spindle handle of the mill-drill. The drilling
then becomes semi-automatic. 

>Did you chamfer the edge of the hole during the cut?  Inside of the hole as
>well as the outside of the plug?  How did you do this?

The hole chamfer was made after the cut. I did this with a Dremel grinder
on both the mirror and plug. A suggestion from a friend 
which might be better is to turn a cyclinder with a wall thickness
greater the the groove and chamfer ,make the edge of the wall
with a 45 deg V. Then use this as grinding tool against the mirror groove. 
>
>Did you reinsert the plug for figuring the primary?  With plaster?  Any
>trouble?

As stated the plug was held in the mirror blank by .200" of uncut glass from
the back side.

Trouble?
This is the long story. Hope Mel does not kick me off the list<G>
The first try I filled the groove with water topped with string and
bees wax (wax was just bellow surface). At the end of polishing I had a fair
sphere.
There was some depression around the hole/plug. I think the chamfer was
a bit sharp and may have been cutting the lap. Not wanting to grind a
wider chamfer
for fear of a loose grit at this stage I filled the groove level with bees wax

BIG MISTAKE:-( After 1 hour of polish I now had a giant hole. I tried 
every machine stroke, scubbed the lap with soap and even cut the center of
the lap out. Nothing worked. The bees wax had become imbedded in the center
of the lap

Then I got a scratch ,so second try. The groove was  ground with a wider
chamfer and filled with plaster (Ultracal) to the top. Fined ground
starting with 12 micron. A new lap was made to replace the lap from hell,
20 hour a machine time again and a about 40 min of touch up,
it looks good. There still was about a 1/4" deppression around the hole.
I am  still happy camper.

The plug was bored out from the back side now. NOW the big retest with plug
removed. Well to my surprise the depression was even gone !! 


Just as a note does CeO eat plaster? After a the fine grinding
the plaster was still level with the mirror. After the polish the
plaster was below the surface by about .050". The was even all the way
around the plug groove. Strange.

End of story
Bob Pfaff