Re: [ATM] Abrasives

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> So what you're saying is that the sequence should be
> #320, 25micron, #500, 12micron, etc., no matter which
> (either SiC or AlOx) is used?  My reference shows #500
> to be somewhere around 16microns.  So if these two
> (#500 and 25micron) were mistakenly switched, you
> wouldn't be doing that much damage-just not making
> progress?  And could you explain again why you prefer
> #500 SiC over #500 AlOx?  I go down to #220 SiC
> myself, and then make the switch to #320 AlOx, and so
> forth.



Well, there are many possible grit sequences.

Let's see what the forces behind them are:

1. complete fine grinding with a spherical figure
2. as little glass damage as possible: not only surface but also sub-surface
3. complete fine grinding as quickly as possible, including as few as grit
size changeovers as possible
4. complete fine grinding confidently, ie, as few as possible backtracks,
scratches, chipped edges, et al
5. no gross astigmatism (an issue with thin blanks)
6. control the radius of curvature or focal length
7. learn what acceptable 220, 500, 9 micron pits look like, learn how to
detect subsurface damage

Some realities of grits:

1. carbo causes subsurface damage ~ 3x the grit size
2. 5 and 3 micron aluminum oxides can cause scratches if the glass is
flexible, the wet is run too long, the wet dries out, or the fine grinding
is done in too rough a manner
3. grits come with a distribution of sizes, eg, 50% of particles are +=50%
size, with more professional grades coming in a tighter distribution
4. amateur obtained carbo contains a very wide distribution of particle
sizes

Some observations:

1. ending with a pro grade of aluminum oxide like MicroGrit 9 micron speeds
up polishing 2x over a poor grade of 5 micron aluminum oxide
2. adjusting the radius of curvature/focal length can be done quickly up
through 220 grit, even 320 grit

Minimum set of grits:

1. a grit to control radius of curvature and come close to spherical 
2. a grit to control coarse pits and subsurface fractures
3. a grit to prepare for polishing, removing anomalous pits and subsurface
fractures

Any grit sequence that accomplishes the above is fine, particularly if it
can be done in 3 grit sizes.  The higher the quality of grit, the fewer grit
sizes needed, and the faster fine grinding proceeds.

As I mentioned, I like 220 carbo, 500 carbo, and 9 micron aluminum oxide.  I
don't prefer 15-25 micron aluminum oxide because it takes a relatively long
time to remove the pits/subsurface fractures from 220 carbo.

Mel Bartels


_______________________________________________
ATM mailing list http://www.atmlist.net/