[ATM] RE: ATM Grit Sequence for 8" Mirror

[mdholm at telerama.com (mdholm@telerama.com)]

This is what GotGrit.com includes in an 8 inch kit.  Sic means silicon carbide. 
WAO means white aluminum oxide.  CEO means cerium oxide.

SiC #80 1 LB, #120 1/2 LB, #220 1/2 LB
WAO 25 Micron 1/4 LB, 15 1/4 LB, 9 1/4 LB
CEO 1/2
Gugolz Pitch 1 LB.

Gugolz is a particular brand of optical grade polishing pitch.  There are other 
brands.  Any specifically intended for optical polishing should be good.  You 
don't want what is called "blocking pitch".  That is for a different purpose.  
The thing to know about pitch is that it comes in different hardness grades.  
Choosing the correct hardness grade is important.  You want one that is 
appropriate for the temperature of the working area where you will be doing the 
polishing.  Grade numbers for pitch vary by manufacturer.  Gugolz uses the 
following numbers:  55 is the softest, use it for temperatures below 70F.  64 
is suitable around 70 - 75F.  73 is good around 75 - 80 F.  82 is the hardest 
for use above 80F.

If your polishing temperatures will be outside this range, ask for advice on 
this list.  There are ways to modify commercial pitch to make it harder or 
softer.

I would suggest buying a whole kilogram of pitch for an 8-inch.  You may well 
make a mistake or wear out the first lap.  Then the extra will be very handy.  
Gugolz comes packed in 1 kilo containers anyhow. 

Many brands of aluminum oxide are now sold by micron size number instead of the 
old M numbers.  Some are sold using the same mesh numbers as SiC.

You want the sizes to decrease roughly by 1/2 for each grade.  I think that if 
one had the choice, #240 SIC would be a slightly better choice in the series 
listed above than # 220.  The jump from #220 SIC to 25 micron aluminum oxide is 
a bit big because the aluminum oxide doesn't cut quite as fast as SiC.  If you 
do use that sequence, spend some extra time at 25 micron.  SiC leaves damage 
deeper in the glass than aluminum oxide, and you want to make sure you get down 
through it all.

With modern abrasives, six grades are plenty for an 8-inch mirror.  Many mirror 
makers get by with only 5, using wider spacing than listed above.  80 is a good 
grade to start with, but 60 would work too.  9 micron is a good place to 
finish, but 5 works too.  Some atm's regularly use 5 micron aluminum oxide 
after 9. Other's do not.  Accidental scratching is more common at 5 micron than 
at 9.  Finishing grinding with 5 micron rather than 9 makes polishing go a 
little faster.  If you can not get CeO, and have to use rouge, it might be best 
to finish with 5 micron.

You do need a polishing agent.  It doesn't have to be cerium oxide.  Red 
jewler's rouge was the standard for a hundred years or more.  Cerium oxide does 
polish faster, and is cleaner to use.  Also, any cerium oxide sold for optical 
work should be clean and fine enough for good results.  Actual Jewler's rouge 
may not be clean and fine enough for optical work, but it can be easily cleaned 
up.  If you need to know how, just ask on this list.  Lots of atm's know the 
method.

Here is a chart that compares some standard grit size scales.
http://www.cs.rochester.edu/u/roche/rec.wood.misc/grit.sizes.html

Mark Holm
mdholm@telerama.com