I addressed the friction issues by covering the drive shaft with a sheet of sandpaper attached with bike tube rubber at both ends. The drum is covered by the same rubber, hold in place by some adhesive paper.
I found a small AC motor in the trash: it’s old but slow and has lots of torque. It’s mounted on silentblocks so it’s particularly quiet. I enclosed the main capacitor and the switch inside a small PVC box. Using PVC and silicone tubing, I assembled some sort of pulley. The motor rests on two aluminium rails. These aluminium rails slide against other rails which are screwed to the board. Therefore, the motor can move perpendicularly to its axis, and it can be blocked by horizontal screws going through both rails. This setup allows me to change the motor-driveshaft distance to fit various belt sizes.
For testing, I loaded the drum with 200g of granulated sugar and 150 lead balls, then I let it spin for 2h. The motor remained cool, the belt showed no signs of weaknesses, the whole setup performed nicely. The granulated sugar was turned to very finely ground sugar, similar to icing sugar. I repeated this test with harder materials such as potassium nitrate and I got the same encouraging results. However, these white powders turned grey as the lead balls were obviously worn in the process.
I’m quite satisfied by this project conclusion: it appears that a ball mill for amateur work can be built from scratch with satisfying results.
After some research, I finally found suitable lead bullets. I bought 150 cal. 0.490 bullets, which are about 12,45mm in diameter. On the picture, the bullets on the left are new whereas the ones on the right were used for a few minutes in potassium nitrate.
After a few unsuccessful tests using PVC gutter pipes, I chose the fast and easy solution: using an actual plastic bottle. I picked up an empty bottle of paint remover gel, which is thick, strong and has a large cap. The inside was cleaned using paint thinner then degreasing soap.
The drum rests on two wheeled shafts coming from an old printer. They are fitted with skate-board ball bearings which are stuck on small aluminium blocks, screwed to the wooden base.
Once the setup was ready, I tried to rotate it manually. At this moment I understood a few important things:
- the loaded drum is heavy, approximately 2kg without the powder, which makes it very tough to spin. I’m pretty sure that the initial 12V DC motor won’t be powerful enough.
- the friction between the drum and the wheeled printer shaft is quite low, thus the drum doesn’t move when the shaft rotates.
For addressing this friction problem, I put a few rubbed bands cut from a bike tube around the printer shaft -> no difference. Then I covered the drum with the same rubber coming from bike tubes -> no difference.
Finally, I applied sandpaper on the printer shaft and rubber on the drum. This configuration gave me a good grip but it was of course wearing very fast. Moreover, the sandpaper was quite difficult to wrap around the shaft, maybe an adhesive antiskid tape would be suitable….
A ball mill is a device designed to grind and mix solid materials to small size particles. It’s a relatively simple machine: a horizontally rotating drum is loaded with a grinding medium (ceramic marbles for example) and a rough powder. While the drum is rotating, the marbles friction and shocks against eachother grind the powder very finely.
Different uses require different grinding media, the marbles hardness must be selected according to the powder hardness and some security issues must be taken into acount: the rotation speed must be kept as low as possible to prevent overheating of the powder and one must never use iron/steel marbles when mixing fuel/oxidizers mixes to prevent deadly sparks.
The following setup was built this evening. The drum, represented by a piece of PVC tube, rests on two shafts taken from an old printer. These shaft have rubber wheels that allow a smooth rotation of the drum. The shafts also have skateboard ball bearings to minimize friction and one of them is connected to a small DC motor through a rubber belt.
Both the motor and the drive shaft can slide to fit different drum diameters and to get the optimal belt tension.
So, I still have to mount all these pieces together, to build a strong drum and to find lead marbles as I expect to use this ball mill for potentially explosive mixes. Lead round bullets appear to be available at gun shops, I’ll check that in a couple of days.
When charcoal is used as a reagent, its quality can sometimes have a big influence on the overall performance of the chemical process. It depends on various parameters, mostly the wood type. Therefore, it’s sometimes useful to make its own charcoal for very specific applications such as pyrotechnics. Making charcoal is rather straightforward, it simply consists in heating wood without oxygen. This technique is named pyrolysis.
I put some pieces of wood in an empty tin can closed by a few layers of aluminium foil, pierced by a couple of little holes. These holes must be small enough as gases must escape from the wood (water vapor for example) and leave the container, but no oxygen must get inside the can, otherwise the wood burns.
The can is heated using a simple butane torch and the flame is applied on its bottom. After 10 minutes of heating, I removed the lid and discovered that the wood had turned black as planned, and that condensed liquids covered the can’s cool surfaces. An even heating from different directions would probably be more suitable. Moreover, an accurate control of the temperature is necessary to reach the desired texture. Texture parameters such as porosity and pore size distribution highly depend on the pyrolysis setup and have tremendous effects on the charcoal behavior when mixed with oxidizers.
Today I tested a few homemade fuses. Most of them required a potassium nitrate solution of 4 teaspoons in 200 ml of water.
1. a piece of cotton fabric dipped in the oxidizer solution -> bad
2. a piece of synthetic absorbing fabric dipped in the oxidizer solution -> bad
3. a piece of cardboard dipped in the oxidizer solution -> bad
4. some smoke mix powder wrapped with a piece of adhesive tape -> not so bad although the powder is not compact enough and therefore moves inside the tape and leads to an unreliable unsteady combustion.
I added several batches today and reached 8,5 kg of cooked smoke mixture in my mold which is now totally full. I slightly changed the technique described previously: I used less water, about 200 ml for 2,5 kg of powder. It makes the stirring harder but it shortens the cooking time as there is less water to evaporate. After removing the cake from the mold, I removed the damaged aluminium foils and wrapped it with a new one.
Considering its size, I’m not sure I’ll find a safe place to light it up.
Fun fact: the thermal inertia of the sucrose mixtures is impressive: 8 hours after unmolding, the cake is still slightly warm!
I’ve just added another batch, so the “cake” grew from 2,6 kg to 3,8 kg.
It is about 6 cm in height over a 23 cm diameter, so about 2.4 L which corresponds to a 1.8 kg/L density. This can be roughly compared to the the unpacked KNO3 powder density (1,2 kg/L) and the unpacked sugar powder density (0,9 kg/L).
This demonstrates that the cooked version manages to get more reagents in a smaller volume which leads to a higher efficiency.
After my previous experiments (here and there), I decided to move to the next level and make a big one, I mean a really big one!
Learning my lesson from the Pringles incident, I decided to cast the smoke mixture in a large steel pot covered with an aluminium foil so that I can remove the hardened mixture from the mold once it’s cold.
It’s of course highly risky to cook a big amount of smoke mixture at once, therefore, I decided to make small batches. Each batch consists in 1kg of sucrose and 1,5kg of potassium nitrate. About 300 ml of water are added to help mixing and after a long stirring using a kitchen robot, the 2,8 kg thick “paste” is divided in two parts for cooking. It makes me cook only 1,4 kg at once, thus I can keep the temperature and foaming under control.
The heating which is supposed to remove the excess water and melt some of the sugar into caramel must be very gentle and constant stirring is required. After one hour of cooking, the paste turns to light brown and one must be even more careful as overheating and eventual ignition are prone to happen.
Finally, after approx. 1h30, I can pour the warm mixture into the pot-mold covered with aluminium foil…And resume heating the second part of this first batch! After more than 3 hours of intense work, I end up with approx 2,6 kg of mixture slowly cooling down in my mold. According to the remaining volume in the pot, it should be possible to reach more than 7,5 kg of mixture once filled.
Today, I fired my biggest smoke bomb so far! It’s made up of a Pringles box filled with a 3 stages KNO3/sucrose propellant which weights about 1,8 kg. The 3 stages thing is not serving any purpose, it’s just the way I made it, as cooking 3 small batches was safer. At first, the box seemed to be a good casing choice: widely available, light, watertight,… However the testing lead to a pretty serious consequence.
So what happened? The first part of the smoke bomb burnt evenly as planned. But the casing containing and aluminium foil didn’t consume as expected and held the pressure created by the fast combustion. This lead to a pressure and temperature increase resulting in a runaway reaction that eventually shot some propellant in the air (see 00:42 in the video)! This propellant take off could have had serious consequences as I found burnt spots all around the firing range, up to 12m far from the smoke bomb!
The casing of my next smoke bombs must therefore be upgraded: I’m thinking about pouring the hot cooked mixture in a mold and remove it before firing. As far as positive consequences are concerned, one has to admit that this first prototype was quite impressive and proved the efficiency of cooked mixtures. The (huge) smoke cloud was pure white and smelled like caramel, the ground was of course locally burnt but no other damages were made.
I addressed the friction issues by covering the drive shaft with a sheet of sandpaper attached with bike tube rubber at both ends. The drum is covered by the same rubber, hold in place by some adhesive paper.
I found a small AC motor in the trash: it’s old but slow and has lots of torque. It’s mounted on silentblocks so it’s particularly quiet. I enclosed the main capacitor and the switch inside a small PVC box. Using PVC and silicone tubing, I assembled some sort of pulley. The motor rests on two aluminium rails. These aluminium rails slide against other rails which are screwed to the board. Therefore, the motor can move perpendicularly to its axis, and it can be blocked by horizontal screws going through both rails. This setup allows me to change the motor-driveshaft distance to fit various belt sizes.
For testing, I loaded the drum with 200g of granulated sugar and 150 lead balls, then I let it spin for 2h. The motor remained cool, the belt showed no signs of weaknesses, the whole setup performed nicely. The granulated sugar was turned to very finely ground sugar, similar to icing sugar. I repeated this test with harder materials such as potassium nitrate and I got the same encouraging results. However, these white powders turned grey as the lead balls were obviously worn in the process.
