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Post by tntmom on Mar 16, 2011 21:10:03 GMT -5
Hello, As most of you know I am still pretty new at cutting rocks. I recently have started working on a piece of variscite which is a brand new material for me. I am using a dremel and diamond wheel because it was already sort of pre-shaped. Problem is that I am getting undercutting in a soft area on one of the sides. I'm having a really tough time trying to fix it. What do you guys do to stop, fix or prevent undercutting? This is the first stone that has undercut on me besides a piece of jade that I tried to tumble ![::)](//storage.proboards.com/forum/images/smiley/eyesroll.png) . Thank you for your help! ~Krystee
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Post by mohs on Mar 18, 2011 0:18:03 GMT -5
Hope in this discussion that some one will please define what undercutting is? I have a vague idea and probably doing a lot of it.. ![:)](//storage.proboards.com/forum/images/smiley/smiley.png)
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chassroc
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Post by chassroc on Mar 18, 2011 8:14:33 GMT -5
I believe undercutting happens when you are working with stone that has different hardness in distinct areas. The softer area teds to grind away quicker that the arder area and you are left with uneveness and/or depressions.
I do not know, other than being very careful, how to work these areas. I'm sure some of the lifers like Ralph or Jake or Connrock will have some specific advice and knowledge. Charlie
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Post by Toad on Mar 18, 2011 9:15:55 GMT -5
Just going by what I read here, but doesn't using a lighter touch help? A solid agate you can really press hard into the wheels and it will be cut uniformly, but a jasper hematite mix for example will need a lighter touch against the wheel to keep the hematite from grinding away faster than the harder jasper.
I think...
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Post by akansan on Mar 18, 2011 10:39:46 GMT -5
Not sure if there is a trick beyond a careful touch in the softer areas. That's the route I've always taken. If there IS another trick I'd love to hear it as well, lol.
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Wolfden
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Post by Wolfden on Mar 18, 2011 11:10:41 GMT -5
I use a really light hand and faster movements . oh ... and a watchful eye lol
Wolf
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Post by jakesrocks on Mar 18, 2011 11:35:47 GMT -5
I think Wolf has nailed it. A light touch, and keep that rock moving.
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Post by stoner on Mar 18, 2011 11:37:37 GMT -5
Yea, it is softer areas being pulled out of the rock, as in some black dendritic agates. Dendritic agates are the worst because there is no cure for the dendrites pulling out. In stones like Variscite or other stones with soft spots, it's been said already, a light hand over the soft spot and a heavy hand over the hard stone next to the soft spot to equalize the levels. It can be done, but it usually isn't worth the effort because the stone will have polished and dull spots. I usually only attempt it in Boulder Opal and Gem Chrysocolla and other expensive stones.
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drjo
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Post by drjo on Mar 18, 2011 13:00:22 GMT -5
Wow! Everybody's got it right. To finish off this process, a slower rpm speed on the wheel or lap and extra water to keep the stone clear of any residue (harder rock grit that wears away the softer part while grinding) will help immensely.
Dr Joe
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Sabre52
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Post by Sabre52 on Mar 18, 2011 18:58:46 GMT -5
Mixed hardness stones are one of the things I hate to work but if I have to do it, as folks have said, a light touch and keeping the stone moving. I also have better luck when the diamond wheels are more broken in and worn as opposed to fresh sharp wheels. A guy I talked to at a show once who specializes in cutting jade, which often will undercut, says diamond is the problem because it grabs and pulls more than silicon carbide belts or wheels and to avoid undercutting he uses only very worn silicon carbide belts for sanding his jade. His jade cabs were fantastic, mirror polished with zero undercutting so I think he knew what he was talking about.....Mel
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Post by Jack, lapidaryrough on Mar 18, 2011 19:54:29 GMT -5
Lapping black rock thunder eggs. our any thunder egg. i use 220 grit on laps. helps a lot. to prevent under cutting the agate an the rhyolite. And Mexican Coconuts, bad for under cutting, on a Bull wheel or disk & Lapps.
On the wheels i would use 220 grit diamond our finer.
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stefan
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Post by stefan on Mar 18, 2011 21:21:01 GMT -5
Worn disks or belts help a lot. I only do my boulder opal on worn sic belts in the grinding stage. Some materials will undercut no matter what you do. Those I usually just give up on and toss em into te tumbler. Sometimes u just can't beat ma nature
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Post by tntmom on Mar 19, 2011 0:45:16 GMT -5
Thanks everyone, I really appreciate the feedback. I don't have sic belts or disks but do have the little sic sanding 220 disks that came with my dremel. Never used them because I bought diamond bits. Maybe I will give that a go and see if it works. The little sic sanding disks are paperbacked and quite flimsy, definately would be that "light" touch though for the soft area. And... if that doesn't work then it will go back in the tub for another day!
~Krystee
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Post by mohs on Mar 20, 2011 15:02:38 GMT -5
Thanks for the explanation I think I have a painful example of undercutting ![](http://img.photobucket.com/albums/v497/stedmo/f2f2f2f2/undercutheartre.jpg) It’s a pretty deep soft area and tried buffing it out perhaps I should try sanding it out? Ed
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