Building a Ball Mill


By Jeff Thorsgaard, Thorsgaard_foundry@hotmail.com

Ball Mill Photo


Building the Machine

I built a ball mill from looking at a picture of one on the internet. It is simply a device that will rotate a container, filled about 1/3 full of a granular substance (I used Kitty Litter as a test), and a "grinding media" of some type, to reduce the granular to a powder. Here's what I did to build mine. This is not a plan, just an explanation of the course of action I took to achieve the end result.

It'd be best to procure some parts first. Look for a motor. I had an old 3/4 HP 1/2" shaft AC motor that already had a 1-1/2" diameter pulley on it. Next, find a pulley with a diameter around 6" or so.. then, a shaft.. say 24" long that'll fit the 6" pulley. Some 2 by 4's, some plywood or MDF, another shaft around 1/2 up to 1" diameter.. same length (2 feet).. A drive belt.. not too short, but not 32 feet, either, unless you have a big shop. Screws, bolts, C-Clamps, oil, some rubber hose that'll fit snugly onto the shaft with the big pulley. And, I had to find a vessel. I used a one gallon round plastic pickle jar. The type that industrial food places and restaurants use. It has straight sides, nothing fancy, and has a wide mouth screw-top lid. Then, I measured the length of it, to make sure I could make the drive rod long enough. Then, I simply built a frame out of 2 by 4's, and I screwed on a plywood base. I made it to be a good 4 inches in width MORE than the vessel or DRUM is, tall. That's 2 inches at least on each side!

The length of the frame is determined on where your motor will be at, and the length of belt you have. If in doubt, make it as long as is humanly possible (not 5 feet though, unless you have LOOOONG belts!) Mine is about 22".. There is an upright that is what holds the drive roller that needs to be added, which is what I'll describe next!

The uprights are simply chunks of 2 by 4's cut about as high as the drum is in diameter. They could be a little longer, doesn't matter. Before mounting these to the frame, you must drill out the holes for the shaft to run through. I used a 5/8" shaft, because that was my pulley hub size that I found. So I drilled out a 5/8" hole with a spade bit. I clamped the two uprights together, and drilled on the drill press to get them pretty straight, and plumb.

After the holes are drilled, start assembling your drive roller. As I said, I used 5/8" shaft because that's what I found in my junkpile. I wouldn't go any bigger than a 1" shaft, due to the drive ratios, which I'll describe later. The pulley was a 6-1/2" diameter with a 5/8" hub. I made the shaft quite a bit longer than needed, due to the fact that this machine is so simple, and requires no lock collars, or pillow block bearings whatsoever, and the shaft has a tendency to "walk" a bit -- which doesn't hurt anything at all. It'd be worse to be too short! Since the shaft size is 5/8", it was a simple matter to get some automotive "heater hose" that has a 5/8" inside diameter. The hose I cut to a length that is equal to the inside width of your frame, MINUS (2) 2 by 4 widths (=3"), MINUS about an inch for either side. This isn't too critical, but too short, and the shaft will "walk" too much, and too long, well, it'll simply bind between the uprights.

In other words -- you don't need any shaft collars since the hose retains the shaft well enough for you. And yes, by the way, wood is an acceptable bearing material, particularly when oil-impregnated! -- Tim

Say your frame is 17-1/4". 17-1/4" - 3" - 2" = 12-1/4". Just cut it at 12 inches for ease of measuring.

Now, you have to get the hose wriggled onto the shaft, which is MUCH easier said than done! It should be centered on the shaft. I used WD-40 lube, and still needed to use a little saliva (I know, YUK!) to get it to slide on, then had to use a 3/4" inside diameter pipe to hammer it on the rest of the way. Once installed, you can't re-position it - you'd have to cut it off, so get it on right away!

Now you can put the uprights onto the shaft and drill and bolt (I don't think screws will work here) them to the inside of your frame. Make sure to clamp them with C-clamps, and try to get them as plumb and straight as you can. Next, install the pulley on the end of the shaft that the motor pulley is on and tighten down its set screws. Now turn the shaft freewheel so you can see if the holes you drilled are straight. It should wheel pretty good. If not, unscrew the uprights, remove the pulley and ream the holes out a bit. This isn't a science...

To mount the motor to the "top base" is the next procedure. I took my belt, put it on the pulleys, set the motor onto the platform, marked the mounting holes, removed the belt and motor, drilled holes and used 5/16" bolts to mount it!

The Length of the box is determined by your belt size. (I used a spare belt.) On the end you decide to mount your motor on, make a plywood top base wide enough to mount said motor to and screw it onto the top. Next, Mount your motor so that the pulley shaft sticks out the end to clear the side of the frame, and go ahead and put the belt on. You can probably eyeball the pulleys paralell as well as you need to. One of the advantages of having a walking shaft is there's room for error, which is REAL good for me!

Now, all you need to do is put in another couple of uprights for the idler shaft, and you have this thing built! They are made the same way as the driver shaft, except that they aren't driven by anything other than the drum. To help get it fairly straight, just run the mill with the drum resting/rolling on one shaft and sort of hold the uprights with clamps, so you can set them to where they run best, then drill and bolt them on.

Now the shafts turn inside holes drilled into wood, so they need lots of oil. I simply drilled "oilers" into the tops of the uprights, down to the shaft hole. I just oil them once in a while and it works well. I think the holes are small enough to hold a little oil via capillary action, so it lasts longer.

Once it's running, you can add some bearings, wheels or whatever you think would help, to the uprights to roll against the screw cover of the jar. I added some double-sided sticky carpet tape to the roller and the drum, for extra traction, and it works very well.

Now, just add some granular clay, and some ball media -- steel balls are great if you have them; some use lead balls. I used marbles, and it worked fine. I only broke one marble, but glass is good to use as grog in a refractory anyways. Use at least 100 balls, 1/2" to 3/4" sized. Doesn't matter if they're the same size or not. Experimentation is KEY in a project like this. If you decide to build huge, with tractor-PTO powered drum, then 100 of any sized balls won't work. Think of their purpose...they have to roll over and through the granules to crush them, continuously. Too few balls will just bypass the clay chunks and it will take days, not hours to mill a batch!

Reduction Ratios

My motor is 1750 RPM AC 3/4HP. It has a drive pulley of 1-1/2" and my driven pulley is 6-1/2" diameter. The driven shaft is 5/8" diameter with a rubber hose which is roughly 1" in diameter. The final drive is about 6" in diameter, the plastic jar.

DRIVEN divided by DRIVE = MULTIPLIER
MULTIPLIER times RATED RPM = FINAL RPM
(This doesn't account for slippage, and you will have some!)

Keep in mind this also doesn't account for the belt actually running about 1/2" in from the outer rim of the pulley, so your number should be slightly high.

1.5 divided by 6.5 = .2307
.2307 times 1750 = ~400 rpm

The second set of "wheels" is the one inch roller, and the 6 inch drum.
Divided by 6.0 = .167
.167 times ~400 = ~67 rpm
67 RPM is about one revolution per second, which is a good ratio, for the stuff inside to roll about without being held by centripetal force.

Uses

After building this I wondered if I could have simply connected a 5 gallon pail to the face of a large pulley to achieve a "cement mixer" type of contraption. To do this, with a 1" drive pulley the driven pulley would have to be around 26 inches in diameter. Not real promising on finding one of those in my scrap pile, but You can have any contraption that will rotate a drum or bucket at a slow enough speed to where the stuff inside isn't held to the sides by centrifugal force.

It really is easier to build than to try to explain!

The main reason for building a ball mill was to grind up bentonite for a refractory component in a furnace I am building. I looked all over the place on the internet about how to grind granular clay to a powdery consistency.

Safety a.k.a. Standard Disclaimer

Finally, on getting side-tracked, and looking at some Pyrotechs websites, I discovered that they use ball mills with VERY special conditions to grind up chemicals, for fireworks and other incediary devices. I do NOT condone the use of this machine to reduce any explosive materials! YOU WILL LOSE YOUR LIFE, and PROBABLY anything else you hold dear in this world. THIS IS A MACHINE DESIGNED FOR GRINDING CLAY ONLY -- NOT explosives nor any types of food products!

I will NOT be held responsible for any misuse of this design, intentional or unintentional, resulting in loss of fingers, toes, hair, life, clothing, pets and animals' tails, impotency, etc... IF YOU LOSE ANY THING, IT'S YOUR OWN FAULT!

Copyright 2004 Jeff Thorsgaard, styled and edited by Tom and Tim, 01/2004.