[ATM] hogging out

[Mike Lockwood]

Frans,

Frans Franssen wrote:
> Thank you for your reactions to my question. I think I will follow Mike's
> advice to alternatively grind MOT and TOT, because I am familiar with that
> way of working. The mirror's material is Schott glass from Germany, sort of
> pyrex.

Keep a good bevel on both blanks, and when grinding MOT don't let the 
mirror "tip".

> Taking Bob's advice I would probably have to make a plaster tool for
> the fine grinding stages. Working with an iron flange or pipe end cap seems
> too risky to me, although I have noticed that  lots of people hog out their
> blanks that way. Seems to me that method is for more experienced mirror
> makers. I'll let you know how I get on in the course of next year. Thanks
> again for your help.

All of these techniques work.  I don't consider the pipe flange or cap 
to be risky.  They can save the cost of a glass tool, since you can 
hog out the curve with a pipe cap and then grind with a tile tool - no 
glass tool needed.

If you wish, you can do hogging, grinding, and fine grinding entirely 
with your smaller tool.  If you choose to go this route, read on.

After hogging, make sure you find a stroke and TOT/MOT balance of work 
that gives even grinding action - roughly the same amount of wear in 
the center and in the outer regions.  That will help you get a 
complete grind with each abrasive, right down to the last fine 
grinding size, and help keep the mirror spherical.

If, during grinding or fine grinding, you inspect the blank and see 
more large pits at the edge than the center, or vice versa, you are 
not getting even action.  This can happen more easily with a 
non-full-sized tool, but it happens quite often with full size tools 
and inexperienced operators, too.  :)

Grinding with a subdiameter tool is actually a great exercise for 
those who want to learn more about wear and inspection of the ground 
surface.

	Mike Lockwood