Ball mill questions
Retrieved: 12/28/2014
Last Post: 10/17/2005
I've got a few questions on ball mill design:
1. Is there an optimal spacing between the drive roller and idle rollers based on jar diameter?
2. What's the best way to ensure that both rollers are parallel to each other?
3. Most PVC end caps have a slight taper on the outside surface after a pipe has been inserted into the socket. What's the easiest way to create a nice mating surface between the jar and the rollers when using a standard 'Sponen' jar?
--
Andy H.
"Clean Up or Die..."
h***@yahoo.com 2005-10-16 02:58:03 UTC
Hi Andy, while Lloyd is certainly the expert on ball mills, I can tell you from personal experience that the degree to which the rollers are parallel to each other is not particularly critical -- tape measure accuracy in the placement of their bearings is usually sufficient.
Regarding the distance between the rollers -- same thing. You definitely don't want the rollers so far apart that the jar starts to sink between them, but you want them far enough apart that the jar sits securely on top of them. My guess is that spacing them at about the radius of the milling jar would be about right, but let's see what Lloyd has to say one this. If you have to go it on your own, then draft an image of the jars and the rollers and observe how they will play together.
Perhaps I'm a perfectionist, but I chucked both of my jars into a lathe and made a small cut just to true them up. However, I doubt that such precision is actually required. What you will likely need to do is put a travel stop on each side of the mill jar, so that if it starts to walk its movement will be limitied to the space between the stops.
My advice to you would be to construct a ball mill and then simpy try it out. If it works, then fine but if not, you simply have to fix the problem. Remember the sage words: "If it isn't broken, don't fix it."
Hope this helps.
Kindest regards, Harry C.
detonationfilms.com 2005-10-16 07:52:29 UTC
Putting a couple wraps of duct tape on the middle of the driving roller causes a "hump" in the middle. For some darn reason this causes all my jars to gravitate toward the center. Accidental discovery (I was taping up a tear in the roller padding) but a useful one. I'm not sure why it works, but it does.
Bob
Andy H.
Bob,
What type of jars do you use?
detonationfilms.com 2005-10-16 23:39:23 UTC
Couple kinds. The standard 4" ABS, of course, and that one wasn't a surprise because the end fittings mean it is narrower in the middle and would ride the "hump." But I also have a straight 4" aluminum pipe with end plugs that I use for crushing material and even that rides to the center. Like I said, I can't explain it, but it seems to work. I've taken off my edge rollers.
Andy H.
That's neat. Does the bump in the center actually drive the jar, or is it short enough to only be a centering device (i.e. the jar still rides only on its end caps)?
My problem is that the edges of the encaps are slightly higher than the rest of the surface (beacuse of the taper) and as such, only the ridge is in intimate contact with the drive roller. It's worked fine for last 5 batches of powder that I've milled, but I think the rollers are starting to slip. I notice a white layer of PVC coating the rubber hose in the contact area.
f***@cableone.net 2005-10-17 01:52:46 UTC
"Crowning" a roller to cause a belt (for instance) riding that roller to center itself is a practice with a long pedigree. I once was capable of providing a cogent explanation of the forces involved but ... always the "but" ... that bit of information has taken its leave along with several truckloads of other bits and pieces that my fuzzy head now deems as less-than-essential.
But be assured, creating that increased diameter at the center of a drive or idler roller can indeed center a belt (or other moveable load) riding upon it.
Leo 2005-10-17 06:02:29 UTC
Crowning is typically used with flat belts. My vacuum cleaner for example has a crowned surface the belt rides on at the beater-bar end. IIRC, the belt rides right on the bare metal shaft at the motor end. I don't know the scientific theory of why crowning works either. I think the rubber just tends to creep toward whichever side currently has a better grip, which would be the higher surface. If it rides over the hump it's similarly drawn back again when the better grip switches sides.
Leo
Andy H.
Another question, what do you use for roller padding? I'm using hard rubber 5/8" ID heater hose which has been giving less-than-optimal results lately. I suppose I need something, as Lloyd said, with more cushion. Foam rubber, latex tubing?
Frozentech 2005-10-18 08:21:04 UTC
Bicycle innertube doubled over a 2" idler roller in my case. Use the trick from Lloyd's book to get the tube onto the roller. The drive roller uses 5/8" heater hose same as you already have.
Mark
Lloyd E. Sponenburgh 2005-10-18 12:06:46 UTC
What do you mean by "less than optimal"? Is it that they don't drive the jar well, or that they wear excessively fast. Heater hose has plenty of cushion and friction for the job.
My original (first) mill built on the book design has the same hose on the rollers as when it was built. Oh, they're ugly and have shrunk (shortened), but they still work fine.
If you're having slipping problems, it's probably due to the direction you drive the jar (lift the heavy side -- in other words, have the inside surface of the driving roller moving in the UP direction), or due to excessive pinch because the rollers are too far apart, or due to a hard-turning "out" roller (the non-driven equipment roller).
If you're having wear problems, either your jar is very much out-of-cylinder in shape or still has some rough casting flash or defects.
Like I said, my oldest mill has some 1000+ hours of time on it, and all the parts are the originals. That's the mill that was in the Mg fire... I deliberately never fixed it so I could show folks what the fire did. It is charred, cracked, and really a mess... but still works just fine.
LLoyd
Andy H.
The problem is that the mill jar doesn't turn as well as it used to. The drive roller is spinning the correct direction, but now that you've mention it, there appears to be a problem with the idle roller not moving as nicely as it should. I build my mill based on the article at Kyle's site (pillow block bearings for each roller) and the bearings that I picked up for the project seem to be lubricated with a heavy weight oil that's not cooperating with the cool weather this fall. I'm off to the hardware store this afternoon to find an out-roller to replace the idle roller.
Lloyd E. Sponenburgh 2005-10-17 11:54:49 UTC
Post by Andy H.
1. Is there an optimal spacing between the drive roller and idle
rollers based on jar diameter?
About 60 degrees apart. That's stable without too much 'pinch'. Also,
always drive the jar so the driving roller lifts the heavy side, rather than
pushing down on the light side.
2. What's the best way to ensure that both rollers are parallel to
each other?
Cut a wooden spacer the right length, and use it to set both ends the same.
3. Most PVC end caps have a slight taper on the outside surface after
a pipe has been inserted into the socket. What's the easiest way to
create a nice mating surface between the jar and the rollers when
using a standard 'Sponen' jar?
If your rollers are cushioned, don't worry about it. Just file off any
flash and/or de-moulding tabs.
LLoyd
If your rollers are cushioned, don't worry about it. Just file off any
flash and/or de-moulding tabs.
What do mean by 'cushioned'? My rollers are 5/8" steel rod with
rubber heater hose pushed on to them. The rubber is fairly hard and
doesn't really 'give' when the jar sits on it. Should I switch to a
softer material?
Andy H.