docharber
has rocks in the head
Member since October 2008
Posts: 693
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Post by docharber on Nov 14, 2008 16:07:15 GMT -5
Recent threads about saving grit through much of the cycle got me thinking about what actually happens when SC grit breaks down. Does the grit splinter like diamond, exposing fresh cutting edges as it does so, or do the particles round off like garnet or alumina abrasive particles do? SC is very brittle and chemically the crystal structure is like diamond. Another question is whether coarse grit is made up of aggregated small crystals of SC that break apart into smaller crystals in tumbling, or is coarse grit made up of larger crystals? Anyone have any thoughts? Mark H.
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Post by connrock on Nov 16, 2008 15:53:29 GMT -5
the grit ,when new,has jagged edges for a better grind.As it wears down it gets smoother and smoother the longer you run it.
it doesn't crack or break it just wears down.
A s far as I know grit comes in spacific sizes.If it's rough grit (60/90 or 80) this means that the 60/90 is "supposed" to have grit from 60 through 90 and 80 is 80.
connrock
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docharber
has rocks in the head
Member since October 2008
Posts: 693
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Post by docharber on Nov 16, 2008 18:16:17 GMT -5
the fact that SC is abrading SC would lead me to think it does round off eventually- but that would seem to preclude using the same charge through much of the cycle. After a point, it wouldn't cut any more. I am centrally blind from macular degeneration and use closed circuit magnifier for reading and detailed work. It has very high power magnification- I can look into the proes of my skin- and I decided to look at the grit under it. The 60/90 grit particles appeared to be single crystals, and at high magnification appeared to be from 1/8" maximun dimension to about 2/9" across on my 20" monitor. the particles were generally elongarted and sharp edged. When I looked at the 150./220 grit, the maximum size was generally no more than 1/8 across on the monitor. There appeared to be a lo of overlap in size between the two grits and tremendous variability between the individual particles. I can't clearly see 300 or finer. After a couple of days I ll look at some SC grit from a recently started rough grind to see what happened. i need to get out more. Mark H.
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Post by connrock on Nov 18, 2008 4:58:17 GMT -5
After 7-10 days in rough stage take some of the slurry/grit and put it in a container of water with a drop of dish soap. Mix it all up and let it settle. Pour the slurry off slowly.Do this until all you have is clean water and left over grit. You can now comapre this with your new grit.
Interesting thread!!
connrock Ps:i need to get more often tooooooo!!
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drjo
fully equipped rock polisher
Honduran Opal & DIY Nut
Member since May 2008
Posts: 1,581
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Post by drjo on Nov 18, 2008 7:39:03 GMT -5
Or pour slowly thru a coffee filter or paper towel works too.
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Gem'n I
freely admits to licking rocks
Member since March 2008
Posts: 980
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Post by Gem'n I on Nov 18, 2008 11:25:31 GMT -5
From what I understand the process of separating grit into the various grit is done through various sieves (SP?) which establish the size distinction..ie, 80-220-320 etc. after larger grit is rolled and/or crushed. In tumbling this grit then is rolled by the action of the tumbler so the edges of the individual SC particle become more rounded as the process continues...the do not lose their hardness but do lose their cutting edge...this is very gradual and you reach the point of diminishing returns in a tumbler. For effectiveness then the particles would have to be re-crushed to expose the sharp edges once again and pass through the appropriate sieves to control size. The same applies to new sandpaper versus worn sandpaper...the aggression is lost after the cutting edges become rounded...once the become totally round all bets are off...just my 2c...Larry
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1Mark
noticing nice landscape pebbles
Member since November 2004
Posts: 91
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Post by 1Mark on Nov 18, 2008 12:04:24 GMT -5
SC tends to fracture into sharp particles. It loses its grinding effectiveness only after it has fractured into grit that is too fine for grinding work (it becomes a polish -- that's why you can let a load run "forever" without changing the grit and you'll end up with polished rocks). AO fractures less readily and is more prone to rounding, which is why it's so helpful on softer materials.
Mark
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docharber
has rocks in the head
Member since October 2008
Posts: 693
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Post by docharber on Nov 20, 2008 14:11:46 GMT -5
Yje comparison to sandpapeer is a good one. We know corundum (aluminum oxide) is not only hard but tough- without distinct cleavage and that as an abrasive it rounds off gradually to the point of being a burnishing rather than sanding or abrading product. Garnet sandpaper is also well known for losing its cutting ability fairly quickly because of grain rounding. Well, today I sampled some grit from a 5 day coarse grind and examined it under magnification. as before. The particles are a little more uniform in size, but overall not muh smaller that I can tell. . They are still sharp. with flat crystal faces reflecting the illumination light in distinctly squared and uniformly bbright reflections. This would not be the caase if they were rounding, as the reflections would have indistinct edges and variations in brightness. So far it looks like the grains are indeed fracturing if anything. If anyone has a gem scope at home, take a look. Mark H.
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ssinnock
off to a rocking start
Member since March 2012
Posts: 2
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Post by ssinnock on Mar 1, 2012 2:15:10 GMT -5
The Cloud is amazing. Perchance, I wondered what the erosion history of SC grit is duriing tumbling. Just 5 minutes later I find this 4yr old thread giving a pretty good answer. Ten minutes more revealed a journal article's abstract on cleavage in cubic SC. Amazing. (For the article just Google "A. N. Pilyankevich", lead author)
Anyway, thanks Mark, if you are still around. I am just an old geomorphologist intrigued by how things change while eroding, and tumbler erosion is a controlled-erosion experiment. Has anyone followed up on docharber's great start with microscopic studies of grit behavior through its "useful" life? From the journal article it appears SC cleaves, probably cubically, like sphalerite for most types. Much as docharber observed, erosion of a particle will approach an ellipsoidsal shape in homogeneous stress fields (sort-of like in a tumbler) dictated by relative bond strength along its crystal axes and "random" weakness planes inherited from its formation and subsequent history (the same as all other crystaline materials behave). Sharp edges will remain at all times, but average "gouge depth" of a particle (or set of particles) will decrease as such an equilibrium shape is approached. SC grit becomes, in effect, finer grained, even though particle size itself may not decrease as rapidly. As in all erosion environments, the sharp projections will break off first, rapidly reducing grit effectiveness to its "typical" condition. Such "rounding" and attendent "gouge depth" reduction will likley be an exponential decay process, much as the energy required to remove pits of greater depth increases exponentially because more mass need be removed to "flatten" (ellipsoidize, yuk, what awful verbalizing (and again of an adjective) of a noun) the undulating topography. I guess eventually the SC will break down to and beyond a fine polishing grit. The proper timing of grit replacement or replenishment then depends on your pocket book and patience. The longer you wait, the cheaper, but the slower the process. This is all theory assuming SC has cubic "sphalerite" like cleavage, but stronger bonds. I wonder if the data exist. Surely, grit manufacturers like Washington Mills have data on grit life cycles. Perhaps not, at least not available here. A microscope, some sort of classification system for "roughness", rounding, and particle sizes, lots of patience, and a little OCD are all that're required. I have the mental attributes but not the microscope or even a tumbler, which I could, and may soon remedy, but perhaps someone has already done it.
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Post by helens on Mar 1, 2012 3:03:04 GMT -5
Interesting. I have no idea about how the grit cleaves or rounds, but I, and anyone with a tumbler, can tell you that the longer you leave the same grit in the same batch without changing, the more and more polished your rocks become. Meaning the grit starts to behave like a much finer grade.
You see this fast with a vibratory tumbler. Leave say a 500 grit in place for 3 days and it's going to behave from that point forward (with softer materials) like a 1000 grit. Leave that for 3 days and it's going to behave like a 1500 grit or even polish. This doesn't matter for the finest grades, but for say the 120/220, it's why they recommend changing the grit every 24 hours so you continue to get the abrading action.
Logic tells you that saving the grit at the lower grades sort of defeats the purpose if it's becoming finer and finer on its own... ie., 120 grit becomes 1000 grit in a week or so... you can't reuse it, if saved, as a 120 grit again.
You should buy a tumbler, not only because it's of scientific interest, but because it's just plain FUN to watch rocks get smooth. I think it's some kind of visceral genetic coding thing. There is no logic to it, but it will make you happy. If you get one, you'll see:).
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