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Coarse finish from 1200 diamond flat lap?

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Author Topic: Coarse finish from 1200 diamond flat lap?  (Read 542 times)
tkcaz
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« on: August 16, 2015, 10:47:52 am »

I wanted to try to make the surface of a an agate cab flat, I mean glass-flat, and found that the softer discs I use on my Ameritool flat lap always gave a slight curve to the surface no matter how careful I was.  So I purchased a 1200 (twelve hundred) grit metal diamond topper, thinking that would stiff and just aggressive enough to flatten any curves on a partially-worked stone, but not so coarse as to set me back to the grinding phase.  But it's not working at all, and I am hoping someone might have some insights.

Whenever I get a new abrasive I first use a small polished glass floor tile (from sheets @ Home Depot) to test out the surface.  It gives me an idea of the actual abrasion level and also checks for defects before I get to my real stones.  Imagine my surprise when my new 1200 grit lap scratched similar to a 180 (one-eight-zero) grit belt!  I examined the flat lap surface carefully under magnification, cleaned everything and checked for contamination.  Everything looked fine so I tried again but got the same results.  Thinking there might be a break-in period I ran several different junk stones then started with a new tile, but again, same result. 

That can't be right, can it?  I've searched this forum and saw a thread from a few years ago (http://gemstone.smfforfree4.com/index.php/topic,13906.0.html) where Carol M had some scratching problems and the difference between paper-SiC vs. diamond-metal was mentioned, but I couldn't see any definite conclusions.  (But I now agree with Carol's "*&^%$ SCRATCHES" assessment. :-) )  Nor did anyone suggest that 1200 d-m would compare all the way down to 180 SiC.  Any thoughts on what's happening here?  What am I missing?
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Tim

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lopacki
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« Reply #1 on: August 16, 2015, 10:58:43 am »

As a seller of diamond flat laps for nearly twenty years I have heard this many times when it comes to 1200 discs. When the diamond is plated no matter how carefully there will always be areas where it mounds and even if you use a strong 10X loupe more than likely you can't see these areas. This is the main reason I do not use 1200 mesh diamond plated products, I sell them because people want them.

The best thing you can do is get a piece of hard agate about 1/4 inch thick, round the edge a bit on a coarser lap and then work it on the 1200, usually with a bit of time you can get the lap to stop creating those pesky scratches.

Hope this is of some help .......... All my best ........ Danny
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tkcaz
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« Reply #2 on: August 16, 2015, 11:35:28 am »

Danny, thanks for the info.  I'm glad to hear I'm not crazy.  You're right, I didn't see or feel any unevenness.  Is it only 1200 grit, or does that happen to finer (or maybe all?) diamond on metal?
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Tim

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« Reply #3 on: August 16, 2015, 03:49:52 pm »

I rarely use flat laps and normally use a Genie or Pixie, but I think this may equate to dressing new wheels by using a flat piece of agate to remove the bumps on the wheel. I usually spend several minutes on each new wheel.
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kennyg
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« Reply #4 on: August 17, 2015, 11:50:25 am »

I have found that no matter how flat the surface seems coming off the saw there are highs and lows on the surface I'm talking in the thousands of an inch. The only way I found to get around this was to paint the flat with a magic marker starting at the 100 grit stage and sand until all the ink was gone, and repeated the process all the way through the sanding up to the polish. It was amazing how much more sanding was needed at each stage. I am assuming that you are using flat laps if you are using belts on the early stages you will never get a flat surface.
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tkcaz
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« Reply #5 on: August 18, 2015, 10:53:48 am »

Well I have spent about an hour now using the disc with various junk stones to break it in, but nothing has really changed.  And I want to clarify it's not gouging as if it's contaminated, it's just cutting very coarsely.  Any feedback would be welcome.
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Tim

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« Reply #6 on: August 18, 2015, 12:57:59 pm »

Try plate glass as a grinding surface with sic grits as the abrasive. Random motions with a bit of water works fine. You will not get a polish this way but it is a good prep for polishing. I have used it on geodes up to20 inches in diameter as have many old timers. Any grinding process has a leading edge so that the edges of a flat are a bit tapered. It happens in faceting as well with gems with large table facets. Standard solution there is to use the cheater function of the machine  to bring the prepolish and polish in.
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