Quick & Easy Black Powder Ball Mill
Here's Your Next Ball Mill
What is a ball mill? What is it used for in fireworks making?
From Wikipedia, http://en.wikipedia.org/wiki/Ball_mill, (this article contains an animation of what the contents of a ball mill do during operation):
A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls,flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on principle of critical speed. The critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus causing no further grinding.
At some point in this pyro-art adventure, we start to hear or get the feeling, that we need a ball mill to grind chemicals very fine or to make high-quality black powder.
This project will not attempt to be a comprehensive review of ball milling. For more detail, Lloyd Sponenburgh's comprehensive manual on hobbyist ball milling, "Ball Milling Theory and Practice for the Amateur Pyrotechnician" (BK0005), is considered the bible on this subject.
A Skylighter project, "Ball Milling 101," is an overview of milling based on Lloyd's work.
To fully understand the rest of this project, you should familiarize yourself with ball milling through one or both of those information resources. Both the book and the article above thoroughly address mill safety precautions. And you'll need to understand terms such as mill-base, mill-jar, milling-media, material to be milled, remote stopping, barricading the mill, optimum milling time, optimum media load, optimum material load, and optimum milling speed.
I remember 20 years ago when I was just starting out in this hobby, and the first ball mill I tried to use. I knew I needed a ball mill, and that it had to be a relatively small and inexpensive investment back then:
Typical Small Rock Tumbler
These small rock tumblers are readily available, popular, inexpensive tools for use in the rock-polishing hobby. Put some rocks and grinding media in along with some water, and after hours or days, out come nice, pretty, shiny rocks.
Unfortunately, I didn't have any good information on the possible use of such a tumbler for pyro ball milling, and the rubber barrel which came with the model I got promptly popped open when it was used half-full of lead milling media.
I have moved on from those first years, and now have several large and relatively expensive ball mills which suit my purposes as I work on larger fireworking projects.
Homemade Ball Mill with Two One-Gallon Mill Jars
So, the question becomes, what would I recommend for a beginner's ball mill to you right now?
Note: The milling media and jars, which are about to be described, can be used as dedicated jars and media for specific chemicals. For instance, I use one jar for milling strontium nitrate only; and I don't have to worry about cleaning it out for a different chemical next time. Having extra jars is a nice option and will save you a lot of time if you get serious about making fireworks.
A small-batch ball millLet's start with the assumption that a beginner pyro hobbyist needs a relatively small ball mill, that they either don't know how to or don't want to make one for themselves, and that they want that mill to be affordable.
If I was back in those shoes, this is what I would do.
Note: I could, right now, point you toward a high-quality, large-capacity, commercial ball mill for between $500 and $1000. And you'd also have to invest more money in the milling media for it. The following project ain't that one. The mill described in this project is a relatively small one, which has to be modified a little. It's a great starter and/or small batch mill. I've used the first of these mills that I got, with the original drive belt, for many, many hours of milling, with only a little oiling of the bushings. Standard length mill times are half an hour for individual chemicals, and 2-4 hours for high-quality black powder.
6-Pound, Dual Drum Ball Mill (Skylighter #TL5005)
Note: Some companies also sell similar, but less expensive 3-pound, single-drum mills. But they will not handle the weight that we will be loading into the jar. That weight will quickly burn its motor up. The 6-pound unit comes with 2 mill jars, but we will only be turning one jar at a time on that unit. That single jar, loaded with lead milling media, and material to be milled, will weigh 6 pounds, which is the weight the double-jar unit's motor is rated for.
Success or Failure? Many, many folks have tried to use small tumblers as beginner ball mills. And many have failed for one reason or another. They either didn't know the insider tips you are about to hear, or they didn't bother to follow them. The details and tricks which follow are critical to ball-milling success. If you follow my directions, you can and will be successful with your very first black powder or chemical grinding.
Tools you will needSnap-ring pliers: Get a pair of inexpensive snap-ring pliers like these. You will need them in some of the next steps.
- A Phillips-head screwdriver
- Needle nose pliers
- Razor-blade knife
Unpacking your new toyOpening the ball mill box reveals a user's manual, a mill base, two mill jars, and five extra drive belts.
Unpacking the Ball Mill
The ball mill jarsThe first thing that struck me when I first looked at these mills was the nice quality of the mill jars. These things are really heavy-duty, and have a very solid closing and sealing-lid assembly.
Parts of the Mill Jar
Removing the Outer Cap by Prying with the Washer
Once the nut and washer have been removed from the jar, the outer cap's removal is easy; just pry it off by inserting the washer in the groove between the cap and the rubber jar.
The inner lid can then be removed by pulling upwards on its threaded stud.
The Parts that Make Up the Inner Lid
Now is a good time to inspect the inner lid. Make sure the rubber diaphragm is stretched and seated correctly around the edges of the aluminum plate. Some of these lids are shipped with incorrectly mated plates and diaphragms, and the rubber has to be stretched out and re-seated.
The jars arrive coated with a layer of thick, waxy "stuff." I'd guess it is the release agent that is used to get the molded rubber jar out of its mold when it is made. I tried using the jars with this wax on them, and some of that wax does wear off from the inside of the jar into the material that is being milled. How much that affects its performance is anybody's guess. And the wax also wears off the outside of the jars and creates gobs of the wax on the mill base's rollers.
So, to avoid all that, I poured some lacquer thinner in a pail, and washed the wax off the rubber jars, scrubbing them inside and out with a rag wetted with the thinner. I then dried them with a clean rag. I observed all the safety precautions for using such a solvent:
- Work outdoors away from any sources of ignition
- Wear nitrile gloves
- Wear a respirator and safety glasses
- Allow the rags and jars to dry outdoors where explosive fumes cannot collect
The jars are closed and sealed as follows:
- Fill the jar with the desired contents.
- Seat the inner lid down into the top of the jar, where it sits atop the jar's inner lip. The jar's top edge may have to be pried gently outwards to allow the inner lid to slide down into its position.
- Pull the jar's top edge outward and "burp" the jar to relieve any pressure that was built up as the inner lid was inserted.
- Place the outer cap on the jar and push it downwards. The taper on the outer cap pushes the jar's top edge inwards slightly, crimping it inward around the inner lid.
- Put the washer onto the threaded stud.
- Screw the nut on snugly, but just tight enough to hold the outer cap on securely. Don't over-tighten the nut.
- A good option is to put a wrap of masking tape around the joint between the jar and the outer cap, as shown below. This tape goes on and comes off easily, and provides some insurance against the jar popping open during a mill run.
Securely Closing a Mill Jar
The ball mill baseThe "unpacking" photo above shows the mill base with its heat-radiating motor cover. This cover conceals and protects the motor compartment, while still allowing air to circulate through it, cooling the motor.
Remove that motor cover with a Phillips screwdriver. As you remove it, pay attention to how the two metal plates, the black plastic spacers, and the screws all fit together. Set the parts aside in a secure area.
Ball-Mill Base with Ventilated Motor-Cover Installed
Ball-Mill Base with Ventilated Motor-Cover Removed
The Ball-Mill Drive System
Looking inside the mill's motor compartment, you can now see the motor, with a small fan and pulley on its shaft. That small pulley is connected by a drive belt to a large pulley which is mounted on the unit's drive roller. The other roller is the idler roller which simply rolls along with the mill jar when the drive roller is turning the jar. Both galvanized-steel rollers are covered by pieces of clear plastic tubing which has a 1/2-inch OD (outside diameter).
Putting a mill jar on the unit, plugging it in, and flipping the switch on, reveals that in its current configuration, the mill turns the jar at about 60 revolutions per minute (RPM).
With the motor compartment open, the wiring exposed, and the pulleys and drive belt visible, too, please be careful when plugging the unit in and turning it on.
Mill Jar Running on Base in Original Configuration
Modifying the mill base for optimum jar speedAccording to commonly accepted milling guidelines, mill jars of this size should be turning at about 80-90 RPM for optimum milling efficiency.
I won't bore you with mathematical details, but the simple way to accomplish an increase of jar speed from 60 RPM to that optimal speed is to increase the drive roller's diameter from its current 1/2-inch diameter up to 3/4-inch diameter.
Fortunately this is easy to do. There is plastic tubing available which is 1/2-inch ID and 3/4-inch OD. A length of this larger tubing can simply be slid over the original tubing that came on the drive roller. Voila! A 3/4-inch diameter drive roller.
Home Depot sells that clear plastic tubing in 10-foot lengths for about $6. I've heard that some auto-parts stores sell similar tubing. It is important that the tubing fit snugly around the existing 1/2-inch OD tubing that comes on the drive roller.
I did try a tube from Lowes which had white-string-braiding embedded in the tube wall. That tubing was a bit too large in ID, and a little stiff and hard, too, so it did not work well.
Using a sharp razor knife carefully cut a length of this tubing the same length as the original plastic tubing on the drive roller. That should be very close to 8-inches.
Tubing to Enlarge Drive Roller's Diameter
Getting the length of new tubing onto the drive roller is simple if you follow the next steps precisely.
Unplug the unit.
Pull the drive belt off the small motor-pulley, turning the pulley as you do so. The belt can now be easily removed from the large pulley. This is a good exercise to learn: doing this, and the reverse of it, is the way you replace a worn or broken drive belt is with a new one, if and when that time comes.
Drive Belt Removed from Pulleys
On the end of the mill base, opposite the motor-end, you'll see two silver caps which cover the ends of the rollers as they project through that end of the base. Remove the Phillips-head screws which secure the cap over the end of the drive roller.
Metal Cover Removed from End of Drive Roller
Using the spring-clip pliers which were mentioned at the beginning of this project, remove the spring-clip which secures that end of the drive roller. You may need to install the pliers' correct-size interchangeable tips which fit the holes in the spring clip.
Note: Spring clips, especially small ones like these, are easily stretched too far open during their removal. Only spread the clip open just far enough to get it off the end of the roller shaft. Use a pair of needle-nose pliers to push the ends of the clip back together, to tighten it back up, before reinstalling it. When reinstalled, the clip should snap securely back in its groove in the roller shaft, and should not easily pop back out of that groove.
Spring Clip Removed from Around End of Drive Roller
You'll notice a black plastic bushing around the end of the drive shaft, projecting through the red end of the mill base. Remove that bushing and set it aside. You'll notice a little tit, or projection, on the bushing which fits into a slot on the side of the hole in the mill base. That tit, when it's in its slot, prevents the bushing from rotating when the drive shaft does.
Plastic Bushing Removed from Around End of Drive Roller
Loosen the Phillips-head screw which holds the large pulley on the motor-end of the roller. Notice that the screw tightens against a "flat" on the roller. Remove that pulley.
Large Pulley Removed from Roller Shaft
You'll now see that the motor-end of the roller has another spring clip, a silver washer, and another black plastic bushing on it.
Pulley End of Roller, with Spring Clip, Washer and Bushing
If you lower the free end of the roller, you can fit the spring-clip pliers into the spring clip to gently spread and remove it. The washer and bushing should also be removed. The drive roller may now be completely removed from the unit.
Spring Clip, Washer, and Bushing Removed from Drive Roller, and
Roller Removed from Ball Mill Base
Now slide the new piece of vinyl tubing over the original tubing on the roller. To make it slide on easier, smear a little spittle (yes, that's "spit"--accept no substitutes) and twist the new tube.
New, Larger Tubing Installed on Drive Roller
The drive roller can now be reinstalled on the mill base by reversing the steps above.
Install the pulley end of the roller (with the flat for the pulley) through its hole, with the free end of the roller sloping downward.
Install the plastic bushing, washer, and spring clip on the pulley end of the roller. Tighten the spring clip prior to its installation, if necessary, by squeezing its ends together with needle-nose pliers. Verify that the spring clip is securely in its groove, does not fall out of that groove, and rotates freely in it.
Install the large pulley on the drive roller, making sure the screw bears against the flat on the shaft.
Raise the right end of the roller, get it started into the other hole in the base, make sure the tit on the motor-end bushing is going into its hole-slot, and slide the roller as far as possible away from the motor end of the base.
Install the bushing, spring clip, and metal cover, on the end of the drive roller.
Reinstall the drive belt.
Optimizing the Ball Mill's RPM
Make sure the whole roller assembly now looks as it originally did, and rotates freely.
There you go: A larger, 3/4-inch diameter drive roller, simple as that.
Test-run the mill with a jar on it, and verify that the speed of the jar's rotation has increased to approximately 80-90 RPM.
Note: Sometimes the mill will not start turning the jar when the switch it turned on, and the jar is sitting on the mill. The motor is not a capacitor-start motor, so it can have a challenge starting from a dead stop with a load on it. If this happens, simply lift the jar and allow the drive roller to start turning, and then put the jar back down.
Test Run with Larger Drive Roller
Adjusting and lubricating the ball mill baseMake sure the unit is unplugged.
Put a drop of light machine lubrication oil, such as 3-in-1 oil, at the four points where the rollers enter the plastic bushings. Repeat this now-and-then to keep those bearing points lubricated.
Applying Light Oil to Four Shaft-Bushing Points
Ensure the two drive pulleys are aligned, with their centerlines in line with each other. This will minimize wear and tear on the drive belt. If that alignment needs to be adjusted, loosen the screw that holds the large pulley on the drive roller, and move that pulley in or out as necessary to align the pulleys. Then tighten the screw on the flat on the shaft. Verify that the motor pulley screw is tight, too.
Aligning the Large and Small Pulleys
The motor has four adjustment screws which allow it to be moved to properly tension the drive belt. This also prolongs the life of the belt. The belt should be tight enough between the pulleys that there is no loose slack in it. Loosen the motor adjustment screws, and slide the motor toward its side of the base to tighten the belt. Don't move it so far, though, that the cooling fan hits the side of the base.
Motor and Drive-Belt Adjustment Screws
The motor compartment cover may now be reinstalled using the four screws that hold it in place.
Loading and running the ball mill optimallyA few more tips will enable you to run your new ball mill safely, effectively, and confidently.
There are milling safety tips aplenty in the resources mentioned at the beginning of this project. But, most of all:
- Use a timer to stop your ball mill remotely after a mill run. You don't want to be around your mill as it's running in case of an accident. And the timer will stop the mill after your pre-determined optimum run time. Individual chemicals can be finely milled in about a half hour. Powerful black powder can be made with mill runs of between 2 and 4 hours.
- Use long drop cords. Unplug the cords at the electrical source, not at the ball mill, when you want to stop the mill.
- Run your mill remotely, outdoors, at least 150 feet away from people and inhabited buildings. Assume your mill will explode accidentally. Situate it and operate it as if that is a given.
- Barricade the mill with sandbags or concrete blocks to reduce the hazard of flying milling balls or debris in case of an accident.
- The mill motor is sensitive to heat. Don't completely enclose the mill so that air can continue to be circulated around the motor and the mill.
- Never mill flash powder, any sensitive composition, or any composition which contains metals.
Components for Effective, Safe Use of the Ball Mill
To load the mill jar optimally, fill it half full of a dense media such as 1/2-inch lead balls. 50-caliber muzzle loading bullets will work, but they are soft and will both wear down over time and leave lead dust in whatever it is you are milling. It's better to antimony-hardened 1/2-inch diameter lead balls, made specifically for pyrotechnic ball milling. They cost a little more than the soft, pure lead balls, but last much longer. 5 pounds (200) 1/2-inch balls will fill one of these jars half full, which is exactly what you want.
It is important to optimize your ball milling. That means, loading the mill jar with the optimal amount of grinding media, and chemicals. An optimal load will give you both the fastest grind and ensure that you'll get the results you want.
The ratios: This is the most important factor for ball milling success. Your mill jar should be 1/2 full of milling media, and 1/4 filled with chemical(s), with 1/4 of the mill jar left empty. Here's an easy way to get it right: first fill an empty mill jar one-quarter full of your chemical(s). With this mill jar, approximately 4.4 ounces (125 grams) of black powder chemical mixture will be an optimal charge. Approximately 6 ounces (170 grams) of a single chemical such as potassium nitrate or strontium nitrate will also be an optimal material charge. Then simply load the jar with your 200 1/2-inch lead balls (5 lbs.).
If you use less than an optimal load of milling media, you'll find that your black powder either does not have enough power, or may not work at all.
To separate the material from the media after a mill run, outdoors, gently empty the contents of the jar into a 20-mesh kitchen colander which is sitting in the top of a bucket. Careful "swirling" of the contents of the screen will result in the material falling through the screen into the bucket, with the media remaining in the screen.
Just pour the media back in its jar, and the material into a container and that process is complete.