Re: [APD] Clear for Life Aquarium Problem -- grinding or polishing?

[Wright Huntley <whuntley at verizon_net>]

I suspect some of you are confusing grinding and polishing glass with 
proper polishing for thermoplastics. I believe those two processes may 
be pretty different.

I'm familiar with the former, having run several precision glass and 
optics shops, but you guys who have done acrylic auto finishes are way 
ahead of me there on plastics.

With glass, there are two utterly different processes, grinding and 
polishing. [We'll forget etching, for now. OK?]

A hard, shaped tool (e.g., cast iron) is used to roll grit across the 
glass (usually in liquid suspension for cooling) to do grinding. As the 
abrasive rolls, it chips out tiny pits in the glass, but doesn't make 
real scratches. Once the glass is fully pitted and shaped, a smaller 
grit can be run over it to make the average pit size smaller. In 
telescope making, this is known as working down through the grinds to 
get a fairly smooth surface.

For the final polish, the hard tool is replaced with a soft one (pitch 
or felt) that traps the particles and makes them slide across the 
surface. A thermal and chemical reaction happens that causes those long  
"sleeks" to flow closed and actually make like a microscopically melted 
surface. Where one might use a harder, inert material for grinding, like 
silicon carbide or diamond, the polishing compound is often a little 
softer, chosen for its chemical properties and could be jeweler's rouge 
(iron oxide) or a rare-earth oxide like cerium oxide. When the size of 
the polishing scratches is considerably smaller than a wavelength of 
light, the surface appears totally smooth, with very little scatter.

We used diamond turning of soft metal mirrors to make very nearly* 
indestructable mirrors for Star Wars back in the '70s. Those too were 
made with a rigidly held single diamond, making grooves so small they 
scattered very little light. The scatterometers we developed then were 
very helpful, later,  in controlling the polishing of silicon wafers for 
making semiconductors and lightly texturizing hard disk platters so the 
heads wouldn't accidentally bond to them, but could still fly low enough.

Wright
_____________________
* Didn't always work all that well. Even liquid-cooled copper mirrors 
rarely survived the first time the laser fired. Scatter has to be truly 
zero for some energy to not be absorbed by "the other parts of the 
groove." It never is zero, but they sure looked a helluva lot cleaner 
than those Clear-For-Life tank pics. :-)

-- 
Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net - 760 872-3995

 "One in four Katrina victims were white people; so if this was a racist conspiracy, it wasn't a very good one."  -- Ann Coulter
                 
http://www.self-gov.org/wspq.html

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