Making a Ball Mill – Part 2

Well it’s been a long time since I last mentioned my ball mill. I had a rather unsuccessful first try, and got a bit annoyed with it so didn’t even post anything about it. I’m now well into Ball Mill v2 and so thought I’d update on where I’ve got to. The motor and power supply I posted about earlier are still fine and will be using in my new mill, it’s just the mill itself that has cause some problems.

My first attempt was constructed of a wooden base with two upright panels. I mounted four ball bearing rings into holes in these uprights, with two hollow aluminium tubes running through them as my rollers. One of these was flush with the bearing at both ends, while the other protruded from the other side of the wooden panel, and would be used as the drive shaft.

I really should have planned it a little better but I was being too keen and building it on the fly. Firstly it was very difficult getting the bearings perfectly flush with the wooden upright, which is necessary to get a smooth rotation of the shafts. In the end they proved hard to turn by hand, so the motor was going to have a really hard time. In fact the wooden uprights rocked back and fourth by a millimeter or so as you turned the rollers, showing that everything was far from perfectly aligned.

Next problem was how to drive the rollers, and getting the right speed. I had initially planned to use a belt drive from the motor to the roller, but this proved too difficult without access to any proper bits for this. I tried using a hoover belt, but there was no grip without a proper slotted shaft of the motor and roller. Also speed was an issue, the motor has an internal gearbox which gives a nominal at 180rpm, and I need to spin my ball mill container at about 60-70rpm. Taking into account the roller shaft diameter and the diameter of the container it was going to spin way too slow. Again, I hadn’t really planned this in advance until it was built!

Ball Mill v2So a few weeks back I started on version 2, with a bit more planning this time. The first change was to make it all from metal to make it more sturdy, hopefully have higher precision, and also look a lot cooler too! I started by calculating the diameter of rollers I would need to get the optimal rotation speed, which handily was pretty much exactly 1″, plus 2.5mm that will be some kind of rubber on the rollers or container to give it good grip. I got hold of some solid aluminium bar for this, but into two pieces, and the very handy Stuart Hammond turned down the ends for me and centre drilled one of them for the motor shaft to be connected directly to.

Next was the bearings, this time I wanted them to have some flexibility so that they could align themselves to the rotation of the rollers, even if they weren’t perfectly in line. After looking through lots of options, I settled on some from Simply Bearings which came in adjustable pillow blocks, so the exact positioning had some movement until they were finally bolted down.

I decided to bolt the motor directly to a base place, with some angle aluminium to give it a sort of face plate for better stability. I figured that this would giver better stability for the motor. The base plate is a 250mm x 500mm x 4mm aluminium plate (also ordered online). I also got some aluminium flat bar of two specific thicknesses that are used as packing plates to bring the centres of the bearings up to the same height of the motor shaft, to try and keep everything on the level.

After accumulating all the pieces of the last few weeks I finally got round to drilling and assembling it all today, with the help of my Dad’s pillar drill and various other bits and pieces. The drilling proved a bit of a nightmare because of the size of the base plate, I couldn’t drill the centre of it as the drill couldn’t reach, so I had to do that one by hand with a power drill. It was incredibly fiddly drilling some the others too, as I drilled the packing bars and base plate at the same time, all clamped together, to give perfectly aligned bolt holes.

Only one small problem, I measured something slightly out, and the motor spindle doesn’t line up with the bearing opposite it! Oops, this means I can’t yet add the main drive shaft until I remove and reposition the motor. I’ll have to turn the bolt holes into longer slots so that the motor can ne moved a few millimeters. This might actually prove beneficial so that I have some adjustment to get the best alignment.

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