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Re: [ATM] Sharpie test: a little help?

As the "inventer of the Sharpie Test", I can help you out on this one. 
   
  This test is a modification of the "Pencil test" mentioned in the ATM series of books. When I worked at Kitt Peak National Observatory in the early 1970's, we were talking one day about the pencil test and how the graphite, even though is is extremely soft, would often produce scratches when used. I happened to have a SHARPIE Permanent Marker in my pocket protector (NERDSVILLE!). I pulled it out and suggested we try the SHARPIE and the SHARPIE test was invented.
   
  It should be done at the end of each stage of FINE GRINDING before proceeding to the next size of abrasive to verify that the curve on the tool and mirror match exactly. This happens when the tool and mirror are either SPHERICAL or FLAT. All other situations will leave some indication of non-contact.
   
  Clean off all the residual abrasives of the current grit and proceed as follows.
   
  1. Mark the mirror with several crosses, X's, or whatever you need with a SHARPIE permanent marker to get a reasonable coverage of the surface. 
   
  2. Carefully place the two disks together, one on top of the other. Do a few VERY SHORT strokes in different directions, 1/2 inch is a good size for most mirrors.
   
  3. Separate the two disks and see where there is contact evidenced by disruption of the SHARPIE marks. The undisturbed marks show where there IS NO contact between the tool and mirror.
   
  This test can be done DRY or with the current or smaller grit as you choose. Just make sure that you have a uniform grit distribution of grit if any is used otherwise the results will be inconclusive. I do the test with no grit and a sprinkle of water. If there is ANY INDICATION of a particle between the tool an mirror, STOP immediately, separate and clean the disks again
   
  Long strokes or continued length of stroking time will, of course, get rid of all the marks. I had one guy tell me that he had to resort to using a very long stroke to finally get the sharpie marks off at the mirror edge... This is NOT the SHARPIE test.. this is a WET using the SHARPIE as the abrasive.
   
  The SHARPIE test takes less than 30 seconds to accomplish after cleaning the disks.
   
  From the description, it sounds like you need to use SHORTER COC strokes to remove glass at the middle zones to get rid of that hole and lowered edge zone.
   
  Ken Hunter

Ross Sackett <rsackett00@yahoo.com> wrote:
    >snip<
  
Thus is bugging me because the reason I gave up mirror
making before was a nasty big central bubble that just
didn't want to grind out; now I think I know how to
handle it, but I would much rather not get into that
situation again.

Ross


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