Homemade rock tumbler (made from refrigerant accumulator and DC gearmotor)

That's cool. I still haven't got round to building my rock tumbler, although I have now got the parts to build a variable speed tumbler using a special pulley which changes in diameter. It's not at the top of my projects list, though.

You might want to line the drum with rubber. I reckon that without any kind of lining the abrasive will wear away the drum fairly quickly. Rubber can deform when the abrasive is pressed against it, which is (I think) why it is pretty durable in this kind of application. A rubber lining should also make the tumbler quieter.

I bet your sons will love the results, even if they get tired of the wait.

Merry Christmas!

Chris

For me, it's easiest to control speed with variable DC voltage, since I already have power supplies that can do that, so it's no work and no time spent on pulleys, etc.

This was mostly a "project" to do with my son together, no dangerous parts, voltages etc.

I hope that a 1/8" (3mm) thickness of the drum (it wasmade to contain

I think that there is a good lesson here, which is that some things have to take a while and not everything brings instant gratification.

Same to you!

I'd be curious to see what the innerds looked like. Did you take a pic of what came out when your cut the end?

Also, the casters will wear out l>

There were two pipes inside. They ran parallel to the axis of symmetry. I managed to yank one out after rocking it right aand left. Another is still there, does not want to come out after similar rocking. I want to buy a cutoff grinder from HF to get at it. The walls are 1/8" thick steel.

As for the casters, they are made from solid steel and I lubricated them with machine oil. They are actually original casters from my shop crane, which I replaced with smoother casters. If they wear out, I will try to make something from sealed bearings. The nice thing about casters is that they self align everything.

Fun project. Just a note about DC PM motors for future reference: torque is proportional to armature current at any speed from rated down to stall. Reducing supply voltage does not reduce ability to deliver rated torque at rated current.

While your casters are wearing out themselves and the drum, look around for a couple of rubber-covered platens as used in some printers and as often found in surplus stores. Run a parallel pair of those, seated in pairs of sealed ball bearings, with the drum cradled between them. They'll last a long time and be a lot quieter.

Another possibility would be two or four urethane skateboard wheels with integral ball bearings. They're about $1.50 ea at surplus stores.

Don, thank you, I did not realize that. In any case, the amount of torque here is plenty for the application.

I tried to stall the motor, which was kind of difficult to do by hand, the current would go up to about 1A at 9v.

i i

Sorry Don, I do not understand.

A point to note is that my current casters are completely quiet, at least with low rpm.

This is a brilliant idea, I think that I will try to locate some skateboard wheels right away. We have a skateboard shop near out house. Maybe they have sensible prices, never visited it. Skateboard wheels should, hopefully, last for a while under relatively light loads.

thank you.

So do you have an end cap on it? If you don't, depending on the diameter of this thing, there are rubber end caps made to temporarily close off the end of PVC pipe.

Wes S

Wes, I do not have an end cap for it, good idea wrt those rubber end caps. I will check out home depot.

They'll slow down under load because of armature resistance. Load requires torque which draws current, IR drop in the resistance reduces the available armature EMF.

DCPM motors are neat because they are so easy to control. Their behavior is defined reasonably well by just two equations:

S = Kv*(V-IR) where S is speed, Kv is a constant, V is voltage I is current and R is armature resistance

T = Ka * I where T is torque, Ka is a constant and I is current.

Note that if you contrive a supply whose voltage rises with load current so (V - IR) becomes a constant, then the motor will exhibit constant speed (at any voltage) with load varying from 0 to max. Some motor controls, as from Minarik, work exactly this way. Power output drops at lower speeds because power = speed * torque, so it's good to match a motor to a load with gears or belt drive so that the motor's max rated speed matches the load's max desired speed if different.

Don, thanks, that's very helpful, I saved your post for my future reference. Yes, I like ease of control present in this tumbler, it is much easier for me than messing with belts, bearings, etc. The gear motor already provides sensible speed reduction and suitable torque.

You will likely need rubber of some sort. The first tumble with the coarsest grit is the longest...and most critical. Expect the rocks to wear 30% or more...as well as the drum. Actually, the goal is to keep the rocks rolling so drum wear might not be as bad as the rocks. If you are sliding, your speed is too low and you are not only getting flat spots but killing the drum.

The real trick is NOT to shortcut cleaning between grits. This is a tough one because you need to clean until you want to give up. Even one tiny chunk of coarser grit can completely screw the process. Unfortunately, you usually don't find out you missed a piece of coarse until you get past fine..hence you need to start all over.

Commercially, they use different drums for each different grit to avoid some of this problem. THe cleaning process of the load between grits is also quite agressive.

If you don't have plastic pellets for the polish run, you may want to look into getting some. They can really help a lot. The polish grits tend to cake easily and the pellets not only help heep it flowing but help the grit wipe more of the odd parts of the rock surfaces. This is one of those "voodoo" things though. Pellets can often make all the diffference but there are some types of stones and polish that don't like em.

Koz

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