[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: [ATM] Spherometers and sphericity

The SHARPIE test is misunderstood by many. 
   
  I originally coined the term back in the 70's while working at Kitt Peak National Observatory. One day we (Lynn Hepburn, several other opticians and myself) were discussing the subject *sphericity and how to determine* when it was noted that the pencil test sometimes left scratches from impurities in the lead. I suggested using a SHARPIE marker to put the lines on the glass and the SHARPIE TEST was born. It is the same exact test as the pencil test described in the old ATM series of books using a less abrasive marking medium.
   
  As most understand and describe the SHARPIE test, they put the marks on the glass and continue grinding until the lines are gone. This is NOT A SHARPIE TEST. Is is different by one very important distinction. If you are using the typical grinding strokes, YES, the marks will eventually disappear but... you do not necessarily have a sphere. You have merely managed to grind away all the marks.
   
  The real SHARPIE test is done with a very few, very short, strokes in several directions to determine where the glass is in contact with the tool. If you are fairly clean in your habits, it can be done dry with little regard to scratching except in the very finest grades of abrasives. 
   
  I usually will do the SHARPIE test at the intermediate grades by cleaning the disks, putting on the lines, add a drop of 5micron slurry and give some very short strokes for a few seconds. Longer strokes or longer times will reduce the effectiveness of the test.
   
  Ken Hunter
  Inventer of the SHARPIE TEST 
  (my one claim to fame)

Mike Lockwood <melockwo@uiuc.edu> wrote:
  
The pencil test is another option. Put lines on the mirror with a 
pencil, then gently rub it against the tool WITHOUT any abrasive. The 
two are in contact where the pencil lines go away or are disturbed. 
Again, this does not guarantee sphericity, but it's a fairly good 
indicator of it.

Mike Lockwood


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


       
---------------------------------
Be a better sports nut! Let your teams follow you with Yahoo Mobile. Try it now.
_______________________________________________
ATM mailing list http://www.atmlist.net/