[ATM] Sharpie Test , Mating Spheres Blank/Tool Query...

[melockwo at uiuc.edu (Mike Lockwood)]

Satish,

Grinding MOT deepens the mirror by wearing away the mirror's center more 
than the edge.  So, the lines go away in the center before they do on 
the edge.  Especially with course abrasives, the lines will often NOT 
disappear on the edge in the course of one MOT wet.
Grinding TOT does the opposite.

If the mirror is too deep, don't work entirely TOT.  Try two wets of TOT 
for every one wet of MOT.  Then the lines at the edge will go away.
Even at 120, the abrasive grains are large and you won't have a perfect 
sphere, and it may take one wet of TOT and one wet of MOT to remove the 
lines.  This is usually what happens to me.

In addition to drawing lines, I would also suggest marking about 10 of 
the largest pits you can find - pits both in the center of the mirror 
AND near the edge.  Do this with the Sharpie, and really force the ink 
down into the bottom of the pit.  The ink will only go away when you 
have removed the pit.  I switch abrasives after two or three of these 
cycles - mark 10 big pits, grind until they're gone, repeat once or 
twice more.  Do a little extra grinding after this happens if you like.

Heed Ken's advice about the proper stroke length and watching for 
"uniform frosting".  I had several people in my mirror grinding class 
switch abrasives while there were still GIGANTIC pits in the center of 
the mirror/tool.

	Mike


Satish K. Pagare wrote:
> Hi List,
> 
> I am done with rough grinding for my 8" (planned F/6)
> and now moved onto #120. After a number of wets I
> did the Sharpie test on the blank. And the lines
> near the edges take longer time to go away. Basically
> Sharpie test failed. Going by my reference site
> at http://www.stellafane.com , if the lines at the
> edges are still there for sharpie test. Then I need to
> do more 1/3 center over center with MOT. But my main 
> worry after doing quite a few wets, with MOT and 1/3
> center over center, I have already overshot the
> sagitta
> and the sharpie test is still failing for me. 
> 
> I just want to clarify my doubt here. Is it possible
> for me to do TOT 1/3 COC strokes and flatten my 
> overshot sagitta as well as get the perfect mating
> sphericals of the blank and the tool?