Re: [ATM] Yet another hogging question (Bill Wheaton) - May 2

["Bob May" <rmay@nethere.com>]

Chris, start the next mirror with 80 grit at the largest.  If you
do a good job with the hogging, you can even start with something
smaller.  30 grit is something that I'd use to hog out a two
meter diameter primary if the pregeneration was bad.
I'd also try to just cast the tiles in the tool next time - no
epoxy needed.  Just don't put any tiles so they are exposed at
the edge of the tool.
Then again, I'm cheap and in a lazy hurry on all this.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: chris forder <camkit@mweb.co.za>
To: [ATM] Mailing list <atm@atmlist.net>
Sent: Friday, May 23, 2008 3:13 PM
Subject: Re: [ATM] Yet another hogging question (Bill Wheaton) -
May 2


> I have hogged out 8" - 14" blanks to saggita using a 230mm
angle grinder
> with a 6mm thick disk.
> The disk to be hogged is stuck to a turntable with 3 pieces of
Prestik
> (a kneadable white adhesive which
> never sets) and rotated at about 30rpm. Using a scything
action, with
> the angle grinder, gently stroke the surface,
> edge to edge, with the disk. The angle between the mirror blank
and the
> rotating disk should be about 15 degrees.
>  From time to time check the depth with a straight edge and a
small
> piece of metal or plastic of saggita
> depth. Once you reach the correct depth, stop the grinding. It
does not
> matter that the hogged out blank is not smoothe and perfectly
spherical,
> the point is you will have removed the bulk of the glass.
>
> I then make a cement (no sand) mix and cast the tool cutting
the profile
> with a piece of plastic cut to the correct radius.
> The tool is allowed to dry for 24 hrs and is then placed in
water for a
> couple of weeks. At this point the tool is almost
> like china and can be sealed with polyestr resin. Once sealed,
floor
> tile sqares can be fixed to the curved surface with
> an epoxy resin. When dry start your rough grinding with 30
grit. In an
> hour or so you will have the mirror and tool
> matching. from the on it's to 60 grit and the rest.
>
> No breakages have happened with this hogging method (touch
wood) and the
> amount of time saved makes it really worth
> while. As Thick glass is rather difficult to find in South
Africa I
> prefer to use a cement tool  and save the glass for other
> projects.
>
> I hope this helps you,
>
> Chris Forder
> Cape Town RSA
>
>
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>

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