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Author Topic: Looking for advise on lapping slabs  (Read 550 times)

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CHILELAP

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Looking for advise on lapping slabs
« on: June 21, 2022, 08:09:36 AM »

Good morning. I`m looking for some more advise about polishing slabs o a 18" flat lap, regular lap, Not vibratory. Bought one used and cheap and reconditioned it with a 3 phase motor and a VFD, which allow for slowing down to any RPM from the motor`s original 1350 RPM.  We tried using it with loose grit but it`s too messy and have to wash out each time when changing grit. Anyway, bought a set of 18" diamond disks (80, 220, 325, 600, 1200 and 3000) and set up a drip system. Am used to working with smaller pieces on a 6 wheeled Genie and a 8" flat lap also, using same grits as above. After a 4 hour session of trying to lap several slabs of different materials, it`s been frustrating and results have not been good. Main problems are not getting the scratches out enough between different grits and not getting a shiny finish, even after 3000. Anyway, here are some specific questions:     

1.) What RPM do you recommend for this case?  Same RPM for all disks of all grits?  Same RPM for all stones?
2.) Is it OK on a larger lap like this to try to lap several different stones on the same disk in the same session? (Yesterday, we were lapping 4 to 6 " slabs of crystal quartz, lapis, serafinite, obsidian and even tried to polish the faces of some hard cut in half geodes.) We assumed that this was OK and that we were not contaminating as there was a a good drip on the disks. We cut and polish all kinds of stones everyday on a Genie and never have contamination problems. But maybe on a 18" flat lap it is different.
3.) I was thinking also that maybe we should be more thoroughly washing the slabs and other materials between the different disks, to avoid any possible contaminations.

Any advise would be helpful.  I have been excited to put this machine to use after time and $ to set it up.  But so far its been quite frustrating.  Thanks.   

     
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irockhound

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Re: Looking for advise on lapping slabs
« Reply #1 on: June 21, 2022, 10:40:39 AM »

I think people with more lapping experience than I will chime in with better advice but I see a flaw in your thinking to begin with.  When you compare grinding different hardness stones on a genie they are one at a time.  If say: you were grinding a soft stone on the lower part of the 280 wheel at the same time you were grinding a hard stone higher on the same wheel, the harder stone would contaminate the lower softer stone.  The grit even with the drip on a lap doesn't move straight to the edge, it slowly rotates outward carrying you debris from your quartz across your seraphinite or obsidian.  If you've ever cut an agate with a druzy center pocket on a genie and those mystery scratches always appear near the quartz, same issue.  I would first lap like materials.  Rinsing the slabs between lap changes would be helpful also, can never be to clean.  If I had a speed change on a lap I can see speeding up on final polish and maybe even final grits.
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CHILELAP

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Re: Looking for advise on lapping slabs
« Reply #2 on: June 21, 2022, 10:56:04 AM »

Thanks Steve forthe great advise! 
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lithicbeads

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Re: Looking for advise on lapping slabs
« Reply #3 on: June 21, 2022, 01:28:42 PM »

Flat laps are a very slow way to polish slabs. A few people have gotten good at polishing them in big vibe laps  but most can't.In the olden days dry sanding was the fast simple way to do it as long as you maintain a not too hot slab. You can use double sided tape  to hold the slab down and use a water dedicated angle grinder set up and it is straight forward. The best is a recent invention  the bull wheel grinder polisher which I believe Kingsley - North sells. The results are astonishingly good with an experienced cutter at the helm.
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55fossil

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Re: Looking for advise on lapping slabs
« Reply #4 on: June 21, 2022, 04:56:21 PM »

    Are you putting any weights on top of the slabs?  I often had problems with lighter weight slabs not staying close enough to the plate or oscillating and not getting a perfectly flat face. You really cannot see it but after experimenting with putting weight on top of the slab I found my results greatly improved.
    I always run multiple pieces of similar hardness if there was room on the wheel. I put each slab inside a rubber pulley to protect the slabs from bumping each other.
   good luck
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CHILELAP

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Re: Looking for advise on lapping slabs
« Reply #5 on: June 21, 2022, 06:22:27 PM »

Thanks for all the replies.  Just to sure it`s clear, mine is a regular flat lap and Not a vibratory model. 
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CHILELAP

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Re: Looking for advise on lapping slabs
« Reply #6 on: June 21, 2022, 06:45:03 PM »

Thanks again for the replies.  Any recommendations for RPM for this 18" flat lap?  Also, as I can vary the RPM on this machine, would you vary the RPM according to the grit (higher or lower) or according to the type of stone?     
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vitzitziltecpatl

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Re: Looking for advise on lapping slabs
« Reply #7 on: June 21, 2022, 08:18:49 PM »

You'd probably want to run your coarse grits at a higher rpm for removing saw marks and heavy scratches.

Just for comparison, the Richardson's buffer we have turns at about 145 rpm. It's a 12" leather final polishing head. We use it with either cerium oxide or aluminum oxide.
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