When I used to mess with it, years upon years ago, the store-bought versions usually ran about 15 RPM, give or take. (It wasn't exactly what you'd call a "precision instrument"!)
Not sure I follow what you mean - Are you asking "Amount of grit versus amount of stones"? Or something else?
I wouldn't expect it to work real well. Granite is awful hard, and unless you're using some seriously tough (and relatively pricey) grit, it might take months to get any really noticable results. Go for agate or similar "not super hard" stones unless you want to be looking at literally months of spinning between grit changes.
Again, back in the days when I was doing it, anywhere from 4-6 weeks per grit-stage.
Be aware that "lapidary" covers one helluva lot more ground (no pun intended) than tumbling rocks...
And make sure you've got someplace to put it where noise is a complete non-issue... Especially if you're using a non-resilient barrel. even the rubber-barrel type make a LOT of noise, and unless you're going to have it on a timer to stop it at night (with a corresponding doubling, or maybe more, in "time to finish") you're likely to find yourself crawling out of bed to pull the plug 'cause it's driving you crackers...
Many years ago I built a rock tumbler out of half 3 gallon size rubber drums, 1 quart size and a 1/4hp motor with a v-belt drive to get it down to about 20-30 RPM. One drum is loaded with 80 grit carbide, the second with
220 grit, the third with 600 and the small drum an alumna polishing compound.. It takes about a month to progress a load through all three drums and the results are great when I could find decent material. The rocks need to be HARD or they will turn to mud. That rules out sedimentary and most metamorphic and small crystal igneous rock. That was the real problem. I live in a mainly sedimentary area with no decent rock other than some common gneiss and flint. Had to order agate, quartz, jasper, onyx and other minerals from out west and it just cost to much to keep the tumbler running. You will pay upwards of $3/pound for decent tumbling stone. I run maybe 3 batches a year.
If you are into geology you can do some weekend rock hounding and gather your own but it takes some experience to figure out what will tumble. We used to go to some old jasper and quartz mines up in North Georgia but now I just look for interesting specimens when I travel. I brought back about 30 lbs. of interesting rock from the British Virgin Islands last summer and about 1/3 of it tumbled reasonably well.
With a single drum be sure to clean everything really well as you progress through the grits. Even a small amount of coarser grit left in during the next stage can make you rerun that step and the longer they tumble the smaller they get.
Don, thank you, I added it to my bookmarks and am looking through it right now. Does not seem to discuss homemade tumblers as much (unless I missed something), but there is plenty of other info and pictures.
Yeah, barrel size IS going to be a consideration. I never did much experimenting in that area, though. "Fast enough to bounce things around inside, but not so fast the barrel spins by under the (near motionless) mass of rocks and grit, or everythign sticks to the side form the centrifical force" ought to be good enough :) Since you've got a variable supply, play with it until you get something that sounds about right.
Stones: Anything "hard, but not super hard". Agate is ideal, and usually works up *REALLY* nice. If you can get your hands on a source of petrified wood, try it on a *VERY* light load, in very short (check it at least 2-3 times during the first 8 hours or so to see how it's behaving - some tumbles well, some disintegrates) "bursts" with mild grit. Depending on the exact type you get, it can turn out anywhere from "WOW!" to "What a waste of time". Obsidian is too brittle - Tends to shatter and chip, rather than polish. You're not likely to have it easily available, but I'd strongly advise against trying to tumble amber. It can be done, but it's *DEFINITELY* too pricey to learn on, and you really need to watch it close (As in no more than a couple hours between checks) and use a *VERY* fine grit on it. I got some interesting (not impressive, just interesting...) results with a light charge of pumice. Volcanic Tuff was also interesting. Forget any sort of sedimentary rock such as sandstone, limestone, shale, or slate - it'll beat itself into sand (or mud...) in short order. Flint isn't usually very impressive, and takes quite a while to get anything to happen. Even if/when you do get results, it's usually pretty much a case of "Yeah, it's a smooth rock. So?" - Not particualrly interesting to look at. Marble is a "don't even think about it" concept. Granite can be made to work, and turns out fairly pretty, but expect it to take
As for grit, you're probably going to want to go with "proper" tumbling grits, which is likely to mean carborundum in various grades for early stages, and for final finish work, jeweler's rouge and/or super-fine alumina.
Oh, another one that just dawned on me to avoid: So-called "Petoskey stone" - It's a fossilized coral, most commonly found in the northern part of Michigan's lower penninsula. You can wheel-polish it *REALLY* pretty, but in a tumbler, it turns into gritty mud almost instantly.
Put some stones in it, and you'll hear it... Trust me!!!
When I was a kid, my tumbling gear got banished to the barn after the second night in operation... It had a rubber barrel, and it ran practically silent when empty. But put a load in it, and it was enough to wake the dead! On a really still night, you could still hear it thumping and clunking inside the house from clear out in the barn!
My dad used to take us to rock shops and clearings. He was quite inventive and like to take us out in the old English Ford. We would go fishing an, rock huntin' and bow shootin and plinkin. He taught me arrchery and on a fine falll day we set up a target at the school yard and he split an arrow like Willliam Tell. Claude Peterman was his name, rock tumbler. accountant, machinist taught me slide rules and measuring.
Ball mills (similar) are very dependent on diameter and weight of media to determine grinding action. Larger diameter mill means higher lift and drop, more energy.
Generally, grinding energy increases with speed until the bulk is free falling the maximum distance. Very little faster than that speed, the charge just stays on the wall and energy falls off rapidly. So there's an optimum.
But are a bit fragile for high energy tumblers. Keep it low and slow.
A doctrine fostered by a delusional, illogical liberal minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
The 'centrifugal force' will immobilize your drum contents if rotation is above M(r); M in RPM, R in inches
((2*pi*r)/(M/60) )**2 /R = (32 * 12)in/s**2
4*pi*pi*3600 *R/M**2 = 32 * 12
M = sqrt((4*pi*pi*3600*R)/384
M =19 sqrt(R)
so for 6" diameter drum,
M= 33 rpm is the guaranteed-too-fast maximum
and for very slow rotation, the drum might never lift a rock above halfway (which would mean the agitation is substantially less than optimal).
I'd guess the optimum rotation something like 1/3 of M, so 11 RPM would do. Other posters have said the typical value is 15 RPM. The exact value will depend on the fill material, size of rock, and on the crowding of the drum.
Interestingly, you can measure this; using a Kill-a-watt or similar power meter, look for the relationship between power usage and rotation rate; at the speed of fastest grinding, the power should be a maximum (though the maximum might be slight). The goal of a tumbler, after all, is to turn the electric power into material abrasion (and that 'wastes' the power).
I am somewhat surprised at your low RPM number for rotational force to overcome gravitational force, but you have a good point about maximizing power delivered to rocks. Since my power supply has both power and voltage gauges, calculating power is easy.
I also need to get different casters that Don recommended.