1001 Parcus Rd., S.E. Huntsville, AL 35803
In my visits to steam shows, I have had the pleasure of seeing many fine models of old engines and machinery, of special interest to me was the several model threshers I've came across since I'm building one myself and hope to build more in the future. I've seen many approaches to the fabrication of the belt pulleys ranging from using solid sleeves to soldered, built up, spoked pulleys.
There are indeed many methods that can be used. A good method is the 'lost wax' or investment casting procedure; however, not many modelers have the necessary expertise to make use of this technique. Beautiful castings can be made if they do.
The method I settled on produces pulleys that look very much like they are cast; however, in my estimation, it is much easier. It does require the use of a lathe, however. A small lathe such as a Taig Micro Lathe should serve just fine for this procedure and they do not cost an arm and a leg either. I use a 9inch lathe on which I've made pulleys from one half inch in diameter up to six inches both straight spoked and curved spoked and solid web with and without lightning holes. This procedure will also serve just fine for making spokes in gears, sprockets and hand wheels. For gears and sprockets I purchase the correct size solid units from a supplier and finish them myself to shape.
I will assume that the research has been completed and that the scale dimensions are established. I use aluminum in my discussion although other metals may be used. Prepare a billet of aluminum of sufficient size to make the pulley. This can be prepared from either round rod stock or from flat sheet stock. The round rod stock is ideal if you can get it. Turn it to the correct outside diameter and leave it uncrowned for the time being. If flat sheet stock is used you may have to turn it first on one side and then on the other unless the stock is thick enough to make the pulley with metal left over. Many of the pulleys on my model are on inch wide. I started out with a one inch thick octagon of aluminum which I chucked with one fourth in the chuck and three fourths out to be turned down to near the final diameter. Since one is not likely to get a match, when you rotate the piece, finish the final pass on the out side diameter when you prepare to crown the pulley. Turning a piece in this manner involves working the lathe bit very close to the chuck so extreme care is advised to prevent gouging into the chuck. Also, do not inhale any of the aluminum dust if it is created.
Two alternate methods may be used if the pulley characteristics permit. One is to turn the inside of the pulley down first and rough out the hub. Then chuck onto the hub and turn down the blank using very light cuts. The second is to bore the shaft hole first and use a mandrel again use very light cuts.
Regardless of the procedure selected, the idea is to turn the blank down to an uncrowned pulley with a solid web of the correct thickness for the spokes. The hub should be turned to final dimensions and bored for the correct shaft size. If you want a keyway or setscrew hump on the end product, leave an appropriate allowance and carve or mill away the excess metal with hand chisels or with a rotaty tool. More on this later. I found it convenient to grind some special lathe tool bits to facilitate the turning of the insides of the pulleys, especially the smaller diameters.
The very last thing to be done, and it is very important that it is the last thing, is the shaping of the spokes. All carving and machining should be done first to preclude distorting the spokes or breaking them. Once bent they can never be satisfactorly straightened out, plus they may be cracked.
With the solid web finished and the hub turned and/or carved to shape and bored, drill and tap set screw holes if they are required. Broach or file in the keyway if you want a key-way. Cole's Power Models of Ventura, California, offers a broach kit for model makers that can broach keyways down to one sixteenth of an inch wide. I elected not to use keyways in my project.
A method I have developed for carving the keyway hump is to turn the hub down to a diameter that includes the hump. Mark off on the end of the hub the width and depth of the hump. I mounted a rotary tool on the lathe carriage and used a small diameter cutter (apply Tapfree or Alumtap or other suitable lube to the cutter from time to time to prevent galling of the cutter teeth) to mill away the excess metal. Using light cuts, apply the cutter to the hub and slowly rotate the work against the cutter by hand rotating the lathe chuck. If properly done, a smooth, round hub with a set screw or key-way hump is the result. Finishing touches, if desired, can be made free hand. Be sure to wear a face shield or safety glasses since the rotary tool casts off a barrage of wicked, needle-sharp shavings and slivers. The cutters that come with the tool that are intended for wood carving work quite well on aluminum and if they are not overheated they will last long enough to cut away several ounces of metal.
The hub should be finished off to final size, shape, etc. at this point before proceeding to the next step.
Next, using very light cuts, crown the face to the desired shape. Make sure the crown is centered or the belt will run off to the high side. You will probably end up with a sharp vee in the center. For added realism, sand off the sharpness in the center to create a gentle rounded effect.
Now then, we are ready to tackle the spoke work. How well this is done can make or break the project. Wash the pulley in a solvent to remove any traces of oil and grime and allow it to dry. It should be absoulutely free of any oils and dirt in order for the next step to work well. Draw a template on ordinary white paper to the same dimensions as the machining. See the illustration. Draw the spokes using a compass, protractor and drafting aids such as French curves if you have them. Carefully cut out the center of this template so that it fits down over the hub without distortion. Cut the outer side so it will slip down inside the rim without distortion. Try the fit to see if it slides down onto the web without bulging or distorting. If it fits well, back it off and coat the inner side with glue and press it back onto the web and let it dry thoroughly. Do not use wide masking tape to make this template since the heat of filing will cause the adhesive to melt and the pattern will be torn away during the removal of the excess web.
After the glue has thoroughly dried, drill a hole between each spoke to admit a saw blade or file. Make this hole as large as possible without cutting into the spoke markings. Different modelers will probably have their own ideas on how to remove the rest of the unneeded web such as a jeweler's saw, rotary tool, jig saw, etc. I've tried most of them and have found that a round file such as a chain saw file works as well as anything. The coarser the teeth the better. Aluminum tends to gall in fine tooth files. Again, coat the file teeth with a cutting aid to help prevent galling. A very light coat will suffice.
File out the excess metal being careful not to use too much pressure on the file since this may distort the pulley. When the pattern shape is attained peel off the remnants of the template and round off the spoke edges with a narrow stripe of abrasive paper or emery cloth or use a rotary tool. Take it slow and easy and you should end up with a beautiful pulley.
You may want to check them at this point for balance and trueness. I've speed tested mine to about 25,000 RPM and they had excellent balance and trueness. However, please don't do this unless you are thoroughly knowledgeable of the dangers involved. Face and eye protection is important since they may explode at that speed. I used a tool and die grinder and started the test at a slow speed and gradually increased the speed up to full grinder RPM. If a pulley should break during a test the lopsided piece that remains on the grinder could shake the tool to pieces if it isn't shut off immediately. So be prepared for a quick shutdown when running a speed and trueness test.
An illustration of spoke template patterns for a curved spoke and a straight spoke pulley. These are cut out and glued onto the web and serve as a guide for cutting the spokes. I feel that it is far easier and much ore accurate than using bluing and scratching the designs directly on the metal or any other direct marking. If a mistake is made, merely draw another until it comes out right. This is not so easily done on direct marking.
Since most old machinery pulleys were made of cast iron, you can achieve a similar surface effect on these pulleys by a gentle sand blasting or by using a rotary tool with a small ball shaped tool bit and let it 'diddle' over the surface lightly. After they are painted, they look as if they were cast.
Once again, thoroughly clean the pulley and spray on a very thin coat of zinc chromate primer. If you can see through the coat easily so much the better. Thick coats of zinc chromate do not adhere well. After this coat is thoroughly dried allow at least a day paint to suit. The primer will dry dry to the touch in a few minutes but it should be allowed to cure for a much longer time to accept paint better.
I am toying with the idea of writing a follow on article dealing with model Rockwood fiber pulleys, however, I need to hear from the readers if there is an interest in these. So long and good luck on your projects.