Re: [ATM] keep polishing or return to fine grinding?

[Mike Lockwood <melockwo@uiuc.edu>]

Kirk,

Mel Bartels wrote:
> We can really only guess as we cannot look over your shoulder.  Most likely 
> you are fine, and simply need to get good contact with the lap.  Less hard 
> pitch would definitely help.  Even the #64 might be a tad too hard for you. 
> Warm press by floating the lap in fairly warm water, then pressing against 
> mirror, then dunk lap in cold water to 'freeze' the shape.  Do longish
> strokes with tool on top.

Just be careful if you do this with a plate glass tool or mirror.  I 
have seen an 8" plate glass tool crack from thermal shock.  If either 
tool or mirror is plate glass (it looks green from the side), warm the 
lap and mirror gradually you slowly raising the temperature of the 
water that they are in.  Then press them.  Then let them cool in 
contact, or use cool water to cool the pitch only.

If both are Pyrex, you don't have to worry.  Dunk away.

I'll second the notion on the pitch - 73 is HARD.  Great for flats, 
not so great for an F/6 mirror.  You can try adapting your technique 
to get it polished, but if it won't polish out, then the fine grinding 
was likely not done properly.  Note what Mel said - polish tool-on-top 
(TOT) - this will work the edge more than the center.  If you have 
been working only mirror-on-top, that might explain what's going on.

Richard Schwartz wrote:
> Your problem polishing out the edge is common with beginners.   The most
> likely cause is that you were grinding with a solid glass tool and did not
> do any of the usual tests for surface quality while grinding.  (None of
> those tests are in any book on mirror making, but are commonly known to all
> who make mirrors.)  Besides flying blind, you wore also working crippled
> with a grinding tool that has no channels. 
> Finally, you need to burn all of your obsolete books from the time of Adolph
> Hitler, and all books by French Nazi collaborators.  Nazi is now obsolete.

Kirk, that is excellent advice followed by a little bit of Richard's 
humor.

> Of course I don't expect you to believe me or follow my advice.  Why am I
> typing this?

Because you are absolutely right, Richard.

Many underestimate the importance of fine grinding.  If done 
improperly, though, the damage cannot be undone by polishing.  If done 
properly, polishing goes much more efficiently.
	Mike L.

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