Originally published: 2006
This project will show you how I built a ball mill for the process of making finely ground powders. They are often used for the production of high performing black powder and other pyrotechnic compounds. (Note: the manufacture, storage and transportation of pyrotechnic compounds without official licensing may not be legal in certain countries). I have shown how I built a ball mill and how I made my own lead ball grinding media for demonstration purposes only.
Ball mills are machines which are used to reduce the granular size of powdered compounds and chemicals; its use is also a highly efficient method to mix/blend multiple substances together. This is an important requirement for good pyrotechnic compounds, the finer the grain of a pyrotechnic compound and how well it has been mixed with other constituents determines how efficiently the compound will combust.
Ball milling pyrotechnic compositions is a very hazardous procedure. Care and the correct materials and procedures must be followed when running the mill. You should never mill any shock sensitive compounds such as flash powder. The grinding media within a ball mill must be non-sparking, common grinding media used are lead balls or ceramic media. Chrome steel or glass grinding media should never be used. Milling should always be performed outside away from buildings and the public in case of accidental ignition of the compound being ground.
Two 32cm x 19cm x 1.2cm ply wood boards.
110mm Ø, 250mm long PVC pipe.
110mm Ø, PVC pipe end caps.
Four 10mm Ø, 130mm long threaded bars.
Nuts, bolts and small wood screws.
High torque electric motor.
Four caster wheels.
Two PVC pipe brackets (diameter similar to motor).
25mm Ø, 30mm long aluminium bar (to be lathed for motor belt wheel).
Computer mouse ball or similar diameter ball.
Lead (flashing, tubing or scrap).
Butane gas torch.
Stainless steel ladle.
Construction of the ball mill
The first thing I did was to construct the tumbler barrel. This allowed me to deign the rest of the mill around the size of this barrel. I used a 250mm long length of 110mm Ø PVC pipe as the main part of the barrel. the end caps I made myself with a vacuum forming machine at my school because of the expensive price of commercial end caps.
The next part was to build a platform on which the barrel could roll. For this I used four castor wheels bolted upside down on a sheet of plywood. The gap between the wheels must be about the right distance apart so that the barrel can roll freely without the possibility of rolling off the casters. A large slot was also cut out of the centre of the platform (see picture) which would allow a motor belt to pass through it and around the barrel.
The next part was to mount the motor to the underside of the same board of plywood. The motor shaft has to be placed in the centre of the plywood platform to allow a rubber belt to pass through it. To mount the motor to the plywood platform, I used PVC pipe brackets which fitted the motor almost perfectly. I used some rubber inner tube wrapped around the motor where the brackets would be clamped to add grip and to stop the motor spinning in the brackets when in use. I screwed the brackets down using small brass wood screws and a couple of washers. The mounting turned out to be surprisingly sturdy and ridged.
The motor shaft had to have a wheel/pulley attached to hold the rubber motor belt. I used a short length of aluminium bar for this. I drilled a hold straight through the centre of the bar with the same diameter of the motor shaft. The pulley needed a grub screw to grip and hold the pulley in place on the motor shaft. This was done by drilling a hole perpendicular to the shaft hole on the pulley and then threading and counter sinking the hole. A short section of thin inner tube was stretched over the pulley to add grip.
Now that the platform and motor mount have been finished, a base is needed. This was simply another piece of plywood of the same dimensions of the platform used before. Four holes in the corners of both plywood boards were drilled. Then four threaded bars of appropriate length were bolted to the base board (as shown in photo) the top platform could then be bolted to the threaded bars as well.
The next thing to do before the ball mill was completed was to make a motor belt. I used a length of bike inner tube. The belt needs to be under a fair amount of tension when in operation to take this into consideration when performing the next step. Determine the length needed to pass around the barrel and motor pulley by simply wrapping it round by hand and marking it, making sure there is tension in the inner tube band. Now using rubber solution found in nearly all puncture repair kits, adhere one end of the tube into the other following the instructions for using the rubber solution carefully.
Due to the high costs of pre-made lead balls for grinding media, I decided to make my own lead balls by casting them. A ball mill should be filled approximately a third to half full with grinding media, and that is equivalent to about 50 to 80 lead balls for my mill. The first thing I did was to mould some wet clay into two rectangular blocks. I then needed a ball pattern which I used a computer mouse ball – one of those old track ball mice. I pressed it into the centre of the clay blocks and formed the clay tightly around the ball. I inserted it slightly deeper than half way and then removed the ball. The reason for making the depressions deeper than half way is that to make clean flat surfaces I would later sand the faces flat which would require some extra depth to ensure the final void was spherical. I used a thin wooden dowel to make depressions for an inlet channel and air an escape channel, making sure each block was a mirror image of the other.
I then baked the clay in an oven at a low starting temperature of about 80°C for about an hour to slowly evaporate the water out of the clay. This avoids cracking the clay due to heating it too quickly. After being dried I turned up the oven to full power at about 220°C for about 2 hours (this isn’t actually hot enough to properly cure the clay into pottery but it does set it hard enough for use as a casting mould).
After the moulds had cooled, I sanded the faces of each block flat and also sanding back until the depression became as close to hemispherical as I could. I glued a sheet of sandpaper to a flat sheet of MDF board to provide a flat sanding surface. The ball used as the pattern was placed in the depressions to align the two mould halves and the outside edges of the moulds were scribed to enable me to realign them without the ball pattern.
To cast a lead ball I set up the two mould halves and gently clamp them together using a clamp and some packing board. I placed a few scraps of lead in a cooking ladle and using a butane torch melted the lead in the ladle until it was completely molten (this is clear when the molten lead rolls around freely with high surface tension and does not wet the ladle surface). The molten lead was carefully poured into the mould through the inlet channel until the mould was filled. The air outlet channel should allow the air/gases in the mould to escape easily to avoid bubbles/void defects in the ball casting.After waiting a few minuets for the lead to solidify the mould was parted gently to reveal the casted lead ball. The excess lead from the inlet and vent ports can be easily cut off and filed smooth.