Turning Drum Thing

A ball mill is a turning drum, lying on its side. So is a concrete mixer, rock tumbler, bottle polisher, parts polisher, etc. Throw in some big rocks to get some smooshing action, and it's a sand muller, too. It seems that every time I look at it, I think of another use for it.

My plan is to use it to grind up some bentonite cat litter, and then mix it with sand and water to make green sand for metal casting. After casting, I'll use it to re-mix the sand, and clean the castings.

One major goal was to use unmodified 5-gallon buckets as the drum, as this would avoid having to pour materials into and out of the drum, and to replace the drum easily and cheaply if (when) it wears out.

The one that didn't work

Here is the first design.

The First Design.

It's made from a set of skate board wheels that I've had knocking around the basement for decades, a flat belt from a vacuum cleaner I pulled out of the neighbor's trash, and a old (and very cool) electric motor out of an old turn-of-the-century dental vacuum pump, all bolted to a piece of wood that was in the way when I put up new shelves in the pantry. The drum was an old plastic 5-gallon paint pail (not shown here - it sat between the skateboard wheels). Total investment: zero.

(No, your eyes are not deceiving you: the motor really is pink. No, I don't know why.)

This design turned the empty drum at about 80 rpm. It should have turned it at about 100 rpm, based on the diameter of the drum versus the diameter of the motor shaft. This tells you how much slip there was! When I loaded the drum with 50 pounds of sand, it didn't move at all. First the motor shaft slipped - a bit of electrical tape on the shaft fixed that. Then the belt slipped where it touched the drum. Electrical tape on the drum didn't increase the traction enough to turn it, so after a lot of messing around, I abandoned the design. Too bad, it was kinda neat. (My kids got a huge blast out of watching it going round and round and round and round... hypnotic. Rather like TV, only more intellectual.)

The one that DID work

I got to thinking: how can I maximize the traction between the drum and the belt? By putting ALL the weight of the drum on the belt. So I decided to suspend the drum by flat belts under a drive shaft - the entire weight of the drum would be on the belts, and on the drive shaft. Ought to work.

The Second Design.

The drive shaft is a piece of 5/8" steel rod from the hardware store. The bearings came from another hardware store (on sale for only $5 a pair!). Same motor and drum as before.

At first, I drove it with the same vacuum cleaner belt as before, running on the motor shaft and the steel rod. It worked quite well, but it broke after only 15 seconds of operation. Too much tension, I think, was required to keep the belt from slipping. I tried replacing it with a huge rubber band, but it stretched too much. I backed it up with duct tape, but it still stretched, tearing the tape (the first known duct tape failure in history, I think). Frustrated, I borrowed the belt off my table saw and found a 6" pulley at the hardware store. That worked quite well, and let me reduce the drum speed a bit more as well. The pulleys are sized more to fit the belt than to achieve any particular speed, but it now turns the drum at 32 rpm.

Closeup of the drive belts.

The flat belts I planned on using to support the drum turned out to be a) very hard to find, and b) expensive as hell. So, during a "proof of concept" test, I tried loops of very heavy twine running on electrical tape "tires" on the shaft. It worked great! They're easy to adjust, if you use a knot that doesn't jam - I used square knots backed up with half hitches. If you adjust the lengths just right, the "belts" don't wander around much, but any little change sends them crawling up and down the drum - eventually they both end up at the top, and the drum cocks sideways and jams. To fix this, I shortened the bottom "belt" a wee bit too much, so it tended to crawl off the bottom, and then added a block next to the drive shaft that kept it in place. The top "belt" isn't a problem, as the fins on the drum hold it in place.

It works great. Even loaded down with a full drum of wet sand, it doesn't slip at all. It's quiet (when full of sand, at least), and reliable, though it does toss a "belt" once in a while. And cheap: $5 for the bearings, $5 for the shaft, and $12 for the pulley. I suppose I ought to count the belt, too, at $14, though I very much overpayed for it.


If it's gonna see service as a ball mill, it needs balls. My first idea was to use steel balls - but they're heavy and expensive. Lead balls, like those used as bullets by the black powder muzzle loading crowd, are far cheaper, but they're even heavier and I'm afraid they'd contaminate the mix and fill my basement with lead dust. How about glass balls? They may contaminate the mix with bits of finely ground glass, but since glass is basically sand, and I'm mixing the bentonite with sand anyway, it doesn't matter. Glass is much lighter (I almost wonder if it isn't too light) and cheaper than lead or steel. Unfortunately, a Altavista search on "glass balls" didn't turn up much. Finally I thought to search on "industrial marbles" and found JaboVitro. They'll sell me 5,200 5/8" ungraded industrial marbles for $32, plus shipping. (My wife found out about this, and said "Five thousand marbles? You've lost that many?")

What turned out to work well and still be cheap (as in "free") were rocks. Plain old crushed granite rocks, ranging from 1" to 3", picked up from the driveway. A half a bucket of those, and it ground several pounds of bentonite cat liter into a fine dust in an hour and a half. (Tip: wear a dust mask when you open it!) Just don't do it while the wife is trying to take a nap, as it's pretty damn noisy.


Mixing is easy - just dump the stuff in and let 'er rip. It doesn't take more than a few minutes. It seems to mix best when the drum is about half full.

Polishing rocks, bottles, parts, etc

I haven't tried this yet, so I don't know how well it will work. For bottle polishing, I understand copper pellets are used. Polishing takes literally weeks of continuous operation, so before I give that a try, I'll find a more reliable way of keeping the belts on the drum.


© 2003 W. E. Johns