Re: [ATM] trepanning mirror

[Aki Lötjönen <aki.lotjonen@phnet.fi>]

Hi,

Gregorian 8 inch f30 sounds like exciting project!
I have tried two methods of making the center hole to a primary as follows:

With loose abrasives by using a biscuit cutter I have drilled from the
pre-curved mirror face through the back of the mirror. If the blank is
moulded the sides are more or less transparent and you can see when the
cutting is close to the back side surface. Then by adding finer abrasives I
have gently drilled through the surface. Maybe a few small pieces can get
loose from the edge of the hole. But that's the backside of the primary!
Patience and little force with the last millimeters. Also it's good to use a
piece of plywood under the blank, for example.

Secondly and lately, I have been lucky and got a possibilty to use a drill
press with an adjustable table. Ie. same kind of the fixing system as in a
lathe. What is the right name in english...don't know. :) Anyway, now it's
nice to drill to a half way and turn over the blank and drill until the
cutting meet the other one in the middle of the blank's thickness. No fair
of piece breaking! And the cuttings will meet each other accurately in the
middle of the blank if only the blank is round.

I have always drilled the center hole after grinded 220 carbo or so. Just
plaster the core back, seal with candle wax and that's it. Finalize
grinding, polishing and figuring.

Regards,
Aki Lötjönen


----- Original Message ----- 
From: "Wayne Hilliard" <woodycxd@adelphia.net>
To: <atm@atmlist.net>
Sent: Sunday, March 26, 2006 5:44 AM
Subject: [ATM] trepanning mirror


> Hi all it's been a while since I posted a question here but I have been
> lurking.
>
> My new project is a gregorian 8 inch f30. On a Springfield style mount.
> I want  to pierce the primary. I know that the traditional way seems to be
to
> wax or pitch a piece of glass onto the surface your going to break through
to
> avoid clamshell fractures etc. However I am lacking appropriate pieces of
> glass. I was wondering if casting the mirror into a mass of plaster of
Paris
> much the same way you cut a diagonal would work? I am only planning on
> casting onto the back of the mirror. Also when to do it. I am planning now
to
> do it before I finish rough grinding ala Ritchie and plaster of Paris the
> slug back into the hole to finish the mirror.. The other way is to
partially
> go thorough and finish the mirror with the last step being to very gently
> finish trepanning the hole. Right now I'm leaning towards the first way.
>
> Ideas comments?
>
> Thanks in advance.
>
> Wayne Hilliard
>
>
> -- 
> Wayne Hilliard         "If computers are making the world a global
village,
> KA1CXD FN32             and I can't figure out how to use one, does that
> woodycxd@adelphia.net   make me the global village idiot ?"
>                                                      Shoe
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

_______________________________________________
ATM mailing list http://www.atmlist.net/