docharber
has rocks in the head
Member since October 2008
Posts: 716
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Post by docharber on Jan 16, 2009 20:17:37 GMT -5
I read in an eBay listing that mixed grade grits like the familiar 60-90 or 150/220 SCare (for unexplained reasons) best for tumbling, while the graded grits (single number designations like "300", "1000", etc.) are intended for lapping. Anybody have anty thoughts/comments on this assertion?
Mark H.
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Post by johnjsgems on Jan 16, 2009 20:38:47 GMT -5
Some lappers claim it makes a difference. It could be the grit breaks down more slowly on the laps than in the tumbler so graded grit my work better. Ungraded grits are cheaper so more economical for either application. On the better laps (like Rociprolap) 220 is used for a first step. 120/220 would be more aggressive than straight 220. Lappers also use 400 and 600 steps (generally). Tumblers use 120/220 and leave it in for a whole week. By then it is probably broken down to 400 or finer. There are tumblers that will use graded grits in 220, 400, and 600 and lappers that use all mixed grade grits. "We've always done it this way" rears it's ugly head quite often in lapidary.
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rallyrocks
fully equipped rock polisher
Member since November 2005
Posts: 1,507
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Post by rallyrocks on Jan 16, 2009 20:43:13 GMT -5
First of all, mixed grades are going to be less expensive to produce because for a given quantity of raw SiC you'll get more falling in a wider range than you will of a tighter graded grit.
In lapping you want to know that the grit size is consistent, so that the scratches being left at the end of each stage are all the same size and can be taken down uniformly with the following stage.
In comparison, with tumbling you usually run each stage long enough- at least through stages one and two- that the grit gets broken down to finer than any of it started out, so its not that much of a concern.
I'd be curious to hear how lappers using mixed grades get by, I suppose there's nothing stopping them but I'd think they are making more work for themselves that way.
You can spend more and use "lapping" graded grits for tumbling if you want to, but why bother when everyone seems to get great results using mixed grade grits in the first couple of stages?
Yeah, like John said...
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docharber
has rocks in the head
Member since October 2008
Posts: 716
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Post by docharber on Jan 17, 2009 0:05:57 GMT -5
I sort of get the point about lapping, but as I suspected it doesn't make muchsense to use expensive lapping grits for tumbling. Tghanks, guys.
Mark H.
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