Adapt Your Ball Mill to Make a Star Roller


Any serious pyro hobbyist will want to use round stars for many of his display items. There are a few sources for purchasing these essential components, such as at some of the larger pyro events in the U.S. But the dedicated artist will eventually conclude that the art of making them himself cannot be avoided for long. Rolling them by hand is a painstaking and arduous task. This line of reasoning brings us to the conclusion that a star rolling machine will greatly enhance one's ability to produce round stars in prodigious amounts. In fact, I have heard complaints from some of my pyro cohorts that the greatest disadvantage of building such a machine is that the consumption of pyro chemicals multiplies tremendously because round stars become so easy and fun to make. Well, assuming that most of you can live with this kind of aggravate, I offer this simple idea to adapt a ball mill so that it may perform the function of a star roller machine.

CREDITS: The general concepts for building this kind of a star roller machine have been around for some time. If my memory is accurate, I believe the first person to invent them was named Fielder. Knowledge of these ideas first came to me from the description and pictures at the Connecticut Pyrotechnics Association star roller page. Another source of inspiration has been our good buddy Bob Svenson, who previously gave me the ideas for the star plate page. He has implemented these star roller concepts in a first class, professional looking design. If any of you are looking for a great description of a stand alone star roller machine, be sure to obtain the publications of his project when it comes out.


The common way to make a star roller machine requires a DC motor with a speed controller of some kind to allow adjustment of the machine's revolutions per minute. This requires a considerable amount of expense and expertise to acquire the components and build the controller. Being the thrifty (pronounced "cheapskate") guy that I am, I looked for a way to use the drive system that was constructed for my ball mill ( described in detail on another page ) to serve the purpose. I reasoned that since my mill jars rotate at about the right speed for rolling stars, the adaptation shouldn't be too elaborate. Therefore, I started with the finished ball mill pictured here. Now, the concept for a star roller is fairly simple. All you need is a rotating drum which has vibration added to the rotational motion. This greatly aids in causing the stars to cascade and mix as they roll. This star action leads to more uniformity of the finished stars due to even distribution of the solvent and star composition.
The heart of the adaptation is the mounting arm which uses two 1/2 inch, self aligning, pillow block bearings to mount a shaft which supports the roller bowl. The shaft is simply a 1/2 inch by 8 inch hex head bolt which was purchased at Home Quarters. The bearings are the most expensive part of this assembly. They cost about $10 apiece at Grainger. The mounting arm is a length of pressed fiber board that is about 3/4 inch thick.
The mounting arm is anchored at one end to the ball mill deck by a set of hinges. The hinges allow the arm to rotate so that the roller assembly can move up and down at the other end of the arm. These hinges are merely a set of 3 inch Stanley hinges which are available at most hardware stores. They have a little bit of play in them which allows the arm to move slightly in an undesired plane. A better choice might have been a 6 inch length of piano hinge or some other higher precision hinge which allows very little movement of the hinge pin.
A roller wheel was cut out of another piece of pressed fiber board with the use of a router. It is mounted on the shaft so that it comes into contact with, and is driven by, the small cam wheel at the bottom. The diameter of this wheel will determine the rotational speed of the rolling bowl. With the 6 inch wheel shown, the resulting RPM of the rolling bowl is 70. If some degree of RPM adjustability is desired, roller wheels of different diameters could easily be made to accomplish this. The current RPM of 70 is about right for my wide mouth rolling bowl, but may be too slow if a smaller mouth stock pot is used for the rolling bowl.
The cam wheel is where the vibrational motion is introduced into the system. There are two very different approaches to accomplish the vibration. The simplest and easiest method is pictured here. The picture shows a 1 inch length of 5/8 inch I.D. heater hose placed over the end of the steel shaft to act as a good frictional drive surface. A one inch long, number 6 sheet metal screw is held firmly in place by the rubber hose. This forms a lifter cam which will lift the roller bowl assembly every time the screw rotates under it. The screw is easily inserted under the rubber hose by screwing it in with a screw driver. The tip of the screw should be dulled with a file so that the screw won't try to bore into the rubber as it is screwed into place. The other method for creating vibration with this wheel is a little more elegant, but requires more effort to make. A short length of 5/8 inch wooden dowel or metal shaft could be attached to the end of this drive shaft instead of the rubber hose and screw assembly shown. This would be attached with an offset which is equal to half of the desired displacement of the roller bowl assembly. The rubber hose sheath would again be placed over the offset wheel. The resulting displacement curve for this method would be a gentle sinusoid which oscillates positively and negatively relative to the center of the rotating drive shaft. On the other hand, the displacement curve for the lifter cam, which was described first, would contain more violent positive spikes only. It is my opinion that the lifter cam method is more efficient at producing cascading motion of stars for a given displacement, but it makes the whole roller machine want to walk away because of the more abrupt spiking of the vibration.
This picture shows the spring which is used to maintain contact between the cam wheel and the roller wheel. One end is permanently anchored to the roller assembly arm with an eye bolt. The other end is attached to another eye bolt which slips into one of several holes which are drilled into the wooden frame of the ball mill deck. This allows adjustment of the spring tension by the choice of the hole the eye bolt is pushed into. It also allows easy disconnection of the spring when the roller arm is disengaged during milling.
The rolling bowl is a stainless steel salad bowl which was purchased for about $6.00 at a kitchen utensil store. These bowls usually have a pattern of circular marks in the bottom which aid in locating the exact center where the mounting hole is drilled. A single nut on the hex bolt is sufficient to attach the bowl securely and allows it to be easily removed for cleaning or when not in use. These stainless bowls are so smooth that stars sometimes tend to slide instead of roll along the surface. I have found that a little sanding with a fine grade of sandpaper will add enough texture to the surface to alleviate this problem.
When using the star roller, the deck of the ball mill is elevated at one end so that the stars will stay in the bowl. This elevation is accomplished with a support that folds out from under the deck. The support is secured in the extended position by tightening the wing nuts on the bolts which attach the support to the deck frame. It is helpful to add some weight to the elevated end to stabilize the whole contraption while in use. I simply hang one of my mill jars with its charge of lead milling media from the deck frame. It is quite amazing how much difference the added weight made to the operation of the machine. By greatly increasing the inertial mass of the milling deck, much less vibration was absorbed by it. Much more of the vibration is transferred to the roller bowl where it is desired. It also helps to run the machine on a concrete floor instead of a flexible wood shop floor or on soft ground.
A handle has been attached to the other end of the milling deck to facilitate lifting this end of the deck to pour finished stars out of the rolling bowl.
Here is a shot of some stars being tumbled in the rolling bowl. My digital camera doesn't do a great job of stopping the action, but, hopefully, you can see that the stars bounce as well as roll when the rolling bowl is in motion. This encourages the stars to cascade over each other instead of remaining in the same position in the pile. Forcing the stars to mix around avoids the problem of larger stars gravitating to the top and continuing to grow at the expense of the smaller stars. Thus, size uniformity remains fairly good. I have added a short barrier lip around the mouth of the bowl to discourage some of the more energetic stars from escaping. It is constructed from aluminum flashing and is held in place by a few spring clips which are easily removed when it is time to pour the stars out of the bowl.
When the star rolling is done or when the machine is used for milling, the elevator stand is folded under the deck frame and the roller arm is disengaged by rotating it out of the way. This dual use of the same drive mechanism works well for me, but it is not as ideal as having two separate, single use machines. Hopefully, this project illustrates the basic principles of a star rolling machine well enough that the reader might be inspired to construct his own machine using them.


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