This post is a work in progress, updated when possible... to maybe include some plans if there's interest ;)
Read below to find out what a ball mill is, some theory and the story behind this design, or jump to Creating the shoe box ball mill
Early on in pyro’s experience, it seems perfectly fine to grind chemicals by hand in something like a pestle and mortar. Although this will usually give an okay result it’s time-consuming, a fair bit of effort, only produces small quantities and the end product is typically not of a uniform particle size. The solution is to mill your materials for a lot longer than any sane person would choose to grind by hand – this is where a ball mill comes in.
Read below to find out what a ball mill is, some theory and the story behind this design, or jump to Creating the shoe box ball mill
Early on in pyro’s experience, it seems perfectly fine to grind chemicals by hand in something like a pestle and mortar. Although this will usually give an okay result it’s time-consuming, a fair bit of effort, only produces small quantities and the end product is typically not of a uniform particle size. The solution is to mill your materials for a lot longer than any sane person would choose to grind by hand – this is where a ball mill comes in.
A ball mill is one of those bits of equipment that seems unnecessary at first, but in truth, it's pretty much an essential piece of equipment for any pyro looking to achieve the best results with the least effort. For those that don’t know, a ball mill is a rotating container (usually cylindrical) which contains the materials you want ground down into a fine powder plus milling “media”. This media is typically spherical (hence the name “ball” mill) and as the container spins around its axis the media tumbles about inside, smashing into the materials you want grinding. Over a period of several hours, the media pulverises the material into a very fine and consistent particle size. Click the video below for a good example of the process.
This process serves several important functions - You get a product which is very thoroughly mixed, with a particle size which is both very small and consistent throughout. As the process is automated it means that although it’ll literally take hours, it leaves you free to do other things during that time.
Ball mills tend to be quite expensive to buy (if you can find one!) so some people buy “Rock tumblers” instead, as they are similar. Again these tend to be quite pricey and aren't really designed to grind materials, their rotation speed isn't usually fast enough for this. There are lots of people who’ve made their own ball mills and uploaded pictures and videos of their creations. These range greatly, from large mills powered by old washing machine motors, to power drills spinning small plastic drinks bottles via direct drive. I'd avoid using drills, because although they develop high torque they're not designed for prolonged use.
In choosing to make your own mill one of the most important points to identify early on is what your specific needs are going to be. With this in mind, you can design something suitably small, efficient and cheap whilst still getting the job done.
So for me, this meant having something that is:
· Mains driven – as it’ll be used for hours
· Needs little assembly
· Is cheap to make
· Cheap to operate – so no big, overpowered motors
· Compact
· Would produce about 100g of milled powder in each milling session.
That last point is quite important. My mill is focused on making small batches of powder at a time. This is partly because I don’t require much powder in any one session and also to comply with local laws. Only producing a relatively small amount of powder means the entire unit can be small, which makes everything easier.
There is quite a bit of debate as to what the “best” ball mill media is. Steel, ceramic, brass and lead are all popular choices of media material and each with their pros and cons. For me, the “best” media is that which is going to be the safest to use. Again, there is debate here as well. In short, if you’re milling isolated substances that will pose no risk of igniting then pretty much any of the media listed above will be fine. If you are milling compositions, however, that’s when it’s very important that you take every reasonable effort to safeguard against a mill explosion, therefore having a none sparking media is a must. After all, who wants their ball mill to turn into a spinning claymore!
Lead and brass will not spark and is therefore considered to be the standard milling media for use when milling compositions such as BP. Lead in particular is cheap and easy to get hold of in a variety of ball sizes as it’s sold as catapult ammunition. Lead is quite weak as metals go, and will degrade with use. You’ll rarely need to replace the media unless you’re milling something especially hard but it will result in a trace of lead being mixed into your compositions. If you mill something white in colour, for instance, it’ll come out a little grey once you’re done. Lead is toxic so it’s something to be aware of, but as long as you’re not breathing in the fine mill powder or the smoke from the powders you subsequently burn, (and you wash your hands when you've handled the media) it shouldn't represent a great threat.
The other variable in your milling media is its shape. Some people prefer small cylindrical bits of media rather than balls but this again is a source of debate. Ball media is the standard and is perfectly good at doing the job, it’s also far more readily available so it’s what my mill uses. I found a multi-pack of catapult ammo on eBay, supplied as a tester pack to gauge what size of ammo you like to use. This pack had a total of 64 balls, in 4 groups of 16, at sizes 8mm to 20mm.
The mill container I’m using is a plastic jar with an airtight plastic lid, the kind used to hold sweets and condiments.
When using the mill you want to half fill the milling container with media, and then fill half of the remaining space (i.e. a quarter of the containers original volume) with the materials you will be milling. This should provide you with close to optimum milling efficiency and reduce the time the milling process will take. It’s generally thought that it’s more efficient to mill two smaller batches than one large batch, so having a compact ball mill is fine for most light hobbyists.
The power drive I chose to use initially was from an old desktop printer. This seemed ideal as it already had a geared motor plus the rollers with rubber support wheels on them. After putting my mill container on the rollers with the media inside I tested the mill rotation speed required using a variable resistor to control the motor speed.
What you’re aiming for is a speed of rotation where the media will be drawn up the back of the container before falling down onto the rest of the contents, this is the Optimal speed, which is approximately 65% of the critical speed. Critical speed is the rpm at which the media will be held to the side of the container continuously due to centrifugal force. This again will depend on the size of the media you’re using and the container but is typically about 60-90 rpm.
Having tested my mill, certain points became clear. The motor required the full 24 volts that the variable resistor would allow, and this still was not quite fast enough to achieve the optimal speed I was after.
The next move was to either get a more powerful motor or to modify the process to make the slightly underpowered system work. Whilst wanting to maintain a compact size and keep things as simple as possible, I chose the latter.
The simple solution was to create small ledges inside the mill jar, which would help carry the media and material up the back wall. As an added measure, I spent just a few minutes hand milling the materials to start the process of breaking them down, before adding them to the mill jar. Ball mills are most efficient when the materials are already quite small, so this step helped the mill past the early inefficient stage of breaking larger pieces down.
With these additions, it was time to test the mill again...
The results were good. In a little over 3 hours, I had an extremely fine grey powder the likes of which I simply could not create by hand. Burnt in a test line, this new powder produced a satisfying "whumpf" sound as it flashed out of existence, instead of the "hiss" and "fizz" that I was used to with a slower burning powder.
I'd mounted everything on a cardboard box lid, and although this served its purpose it wasn't an efficient use of space. It was also very cumbersome having the variable resistor wired to the motor and separate from everything else (it's an old style unit, outdated in its design).
Having made these test runs I decided to make a purpose-built unit that others could replicate, rather than scavenge parts and try to make everything work. Armed with a better understanding of the requirements, I set about making a true "shoe box ball mill".
To make the construction easy I opted to use a glue gun. Other glues will work of course but a glue gun makes the process simple and quick, forming a strong bond with hardly any drying time. Additionally, unlike other glues, hot glue doesn't seep into the cardboard which could deform it.
A strong 12v geared motor was purchased. The motor was deliberately chosen to be small, have an appropriate RPM (80) and develop high torque - advertised as 5kg.cm. Torque is a measure of turning force, expressed as Force x Distance. Kilograms are a unit of mass rather than force, but the crude unit "kg.cm" is often seen to try and convey the power of the motor. In this case, the idea is that if you had an arm attached at 90 degrees to the motor shaft, with an anchor point at 1cm, it would be as if a 5kg weight were pushing down at that point. In this expression, torque is kg x distance, so for the same torque, if you double the distance to 2cm, you halve the force to 2.5kg, and so on.
For the safest and easiest option, mains power was supplied to the motor with a standard 12v mains transformer plug. I purchased one which came with a socket, to easily plug in the power and not to have everything hardwired in all the time - as I also want the option to easily use the plug with other items.
You may already have some of these items, but if not then here are some useful links for your convenience.
***To be continued...
Having tested my mill, certain points became clear. The motor required the full 24 volts that the variable resistor would allow, and this still was not quite fast enough to achieve the optimal speed I was after.
The next move was to either get a more powerful motor or to modify the process to make the slightly underpowered system work. Whilst wanting to maintain a compact size and keep things as simple as possible, I chose the latter.
The simple solution was to create small ledges inside the mill jar, which would help carry the media and material up the back wall. As an added measure, I spent just a few minutes hand milling the materials to start the process of breaking them down, before adding them to the mill jar. Ball mills are most efficient when the materials are already quite small, so this step helped the mill past the early inefficient stage of breaking larger pieces down.
With these additions, it was time to test the mill again...
The results were good. In a little over 3 hours, I had an extremely fine grey powder the likes of which I simply could not create by hand. Burnt in a test line, this new powder produced a satisfying "whumpf" sound as it flashed out of existence, instead of the "hiss" and "fizz" that I was used to with a slower burning powder.
I'd mounted everything on a cardboard box lid, and although this served its purpose it wasn't an efficient use of space. It was also very cumbersome having the variable resistor wired to the motor and separate from everything else (it's an old style unit, outdated in its design).
Having made these test runs I decided to make a purpose-built unit that others could replicate, rather than scavenge parts and try to make everything work. Armed with a better understanding of the requirements, I set about making a true "shoe box ball mill".
Creating the shoe box ball mill
To make the construction easy I opted to use a glue gun. Other glues will work of course but a glue gun makes the process simple and quick, forming a strong bond with hardly any drying time. Additionally, unlike other glues, hot glue doesn't seep into the cardboard which could deform it.
A strong 12v geared motor was purchased. The motor was deliberately chosen to be small, have an appropriate RPM (80) and develop high torque - advertised as 5kg.cm. Torque is a measure of turning force, expressed as Force x Distance. Kilograms are a unit of mass rather than force, but the crude unit "kg.cm" is often seen to try and convey the power of the motor. In this case, the idea is that if you had an arm attached at 90 degrees to the motor shaft, with an anchor point at 1cm, it would be as if a 5kg weight were pushing down at that point. In this expression, torque is kg x distance, so for the same torque, if you double the distance to 2cm, you halve the force to 2.5kg, and so on.
For the safest and easiest option, mains power was supplied to the motor with a standard 12v mains transformer plug. I purchased one which came with a socket, to easily plug in the power and not to have everything hardwired in all the time - as I also want the option to easily use the plug with other items.
You may already have some of these items, but if not then here are some useful links for your convenience.
***To be continued...
