Ball Mill Mods and thoughts
Entry posted by Swede
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When I first joined APC as a member, I designed a ball mill that was as simple as I could make it, with the thought that it could be replicated by just about anyone, via plans or a kit. While in appearance it was fairly crude, it has done an outstanding job, and the basic design is sound.
In the server transition, it seems that there has been a bit of loss or mixup of the blogs - or maybe it is just me. Anyway, the ball mill consisted of two shafts, four bearing blocks, 2 sections of 3/4" plywood, and the longitudinal supports were done by using 1/2" or 5/8" stainless all-thread rod and an assortment of washers and nuts.
Most guys use pillow block bearings to create the two rotating rods, one driven and the other what is called an "idler." One of the problems I noticed right away is that the idler bar was not rotating well. The bearing inside the pillow block is a bit stiff, and often the PVC jar would simply rub rather than rotate the shaft, even when the shaft is covered with a soft rubber or PVC tubing section.
Here is a typical bearing block assembly, in this case, the motor-driven bar:
http://www.5bears.com/firew/nbm01.jpg
One of my favorite sources for this sort of hardware in the United States is a place called Burden's Surplus Center. They have everything you need, including motors, to make a nice ball mill.
Some time ago, a guy on Passfire posted a link to a conveyer-roller element... a steel tube with some very light bearings that would make a perfect idler bar. Think of the roller bars when you enter an airport and put your bags through an X-ray machine. It would make sense that these things would be available by the thousands, and cheap. They are.
Surplus Center Conveyor roller
If you do the math, you are replacing $20 minimum worth of material for $4, and it will do a better job for you. I decided to retrofit my ball mill for one of these conveyor elements.
One of the three idler roller elements from Surplus Center. Note the tapped hole:
http://www.5bears.com/firew/nbm02.jpg
There are any number of ways to mount these. What I will show is one way, but there are easier ways that do not require machining of any sort. I will leave that up to you.
The first step was to make two mounting brackets with slots for the conveyor roller center shaft:
http://www.5bears.com/firew/nbm03.jpg
I made the hole pattern in the aluminum blocks the same as the original bearing block, so no more drilling of the plywood end plates was needed. The bearing block bolts to the plywood end sheets using the original pillow block holes, and the conveyor-roller stub end drops right into the slots as seen:
http://www.5bears.com/firew/nbm04.jpg
Note that there are TWO slots. By moving the roller from one slot to the next, jars of differing diameters can be handled.
The conveyor roller is far superior to the original setup. There is no rub, no hesitation, and less drag.
http://www.5bears.com/firew/nbm05.jpg
http://www.5bears.com/firew/nbm06.jpg
Some notes on that motor mount: The ball mill uses two long sections of threaded rod (called "all thread") to provide structural support. The one at the bottom/forward was cut intentionally long, so that a coupler nut (essentially a very long nut) could be used to then attach another segment of threaded rod. A small piece of 3/4" plywood was cut for the motor mount, and a section of PVC tube that slides over the threaded rod was attached to the front of this motor mount using what are called Adell clamps. Any method that firmly glues or attaches the PVC tube to the motor mount plate will work. This creates a pivot point, and the weight of the motor itself provides belt tension. In engineering this, I made the driven shaft especially long so that I could mount more than one pulley at a time, with differing diameters. By sliding the motor mount right or left, I can hang the V-belt into any available pulley, giving differing RPM's.
http://www.5bears.com/firew/nbm07.jpg
One final modification are additional holes in the end plates that allow me to move the idler bar plates to different locations. This, to me, is important. Sometimes, all I want to mill is a small lortone-style jar. At other times, I want to mill a large jar full of BP. In this picture, the bracket plates are tilted to create additional spacing between the bars, allowing a jumbo jar to be mounted:
http://www.5bears.com/firew/nbm08.jpg
Summary - these conveyor belt elements are a great deal. The ones that I purchased are cross-drilled and tapped, which means I can thread in a set screw that allows vertical adjustment of the bar end. This is especially helpful to adjust the mill so that the jar does not aggressively travel one way or the other. In all cases, some sort of stop or wheel must be positioned so that if the jar does travel (and it always does) it will not rub against fixed hardware at either end.
I hope this gives guys some ideas for mills of their own. I cannot overemphasize the need for a durable and powerful ball mill, jars, and media, for a successful pursuit of pyrotechnics. It is much better to research and build your own than it is to buy one. The money saved will be significant, and the performance improvement dramatic.
Stay green!
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