1001 Parcus Road, S.E. Huntsville, Alabama 35803
Fourteen years ago I made a decision to move up from ordinary workshop puttering to the realm of building a highly detailed and very challenging model of an Aultman and Taylor thresher where all features were as detailed and as accurate as I could possibly make them. This included having all operable elements work such as the feeder, elevators and straw racks, etc. Little did I realize at the time how much research and planning went into the building of such a model. The fabrication of such elements as cranks, pulleys and wheels each presented their own problems. In writing this article, I propose to share some of the techniques I have developed in this case, the construction of a model of a typical agricultural machine spoked wheel.
A spoked wheel may seem at first to be a simple item, however, rest assured, to make a small, strong spoked model wheel is not as simple as it may seem. While other model makers may have their own techniques, such as welding, casting or carving from solid stock, I feel that my procedure is easier and has something to offer the model maker.
In essence, my procedure is to cut and shape rims from an appropriate sized steel pipe stock, make up spokes in the form of long, slender screws, and shape, drill and tap hubs and assemble the wheels on a jig and grind them to give final shape and dimensions.
I admit that I have the benefit of a South Bend Model 10-K lathe with accessories that facilitates the procedures I use, however, a resourceful craftsman can improvise using jigs and simple tools such as an electric drill to accomplish the same end. Where there is a will there is a way.
The wheels that I needed were six inches in diameter, flat rimmed, with two rows of eight straight spokes and a hub which is illustrated by the accompanying photo and drawings.
The key feature of this procedure is that the spokes are screwed into the hub somewhat like a bicycle wheel is made. No welding or soldering is required.
The first step after research and planning is complete in the machining of the hubs. I recommend avoiding the use of iron unless you are extremely competent at tapping small diameter holes. Tap breakage is very troublesome if iron is used. I used aluminum which worked very well. Brass or bronze is quite good also.
I advise turning all four (or however many you need) hubs on the same billet at the same time which makes holding the diameters and shapes uniform by making it easier to generate the same contours on each of them. Space should be left between the individual hubs to allow for cut-off and final finishing of the ends.
After the hubs are shaped to one's satisfaction, the next step is to use a sharp-pointed tool bit mounted on the crossfeed to scribe very fine lines around the hubs where the rows of spokes are to be located. Then disconnect the power from the lathe and by improvising an indexing setup using the headstock gears the scribed circles can be divided off for the location of the individual spoke holes. Bear in mind that most spoked wheels have two rows of spokes with one row half offset from the other.
In my case I counted off a headstock gear that was divisible by eight I needed two rows of eight half offset for each wheel. Do not disturb the setting of the tool bit until all locations are scribed. I used a small Phillips screwdriver with the bit placed in the selected gear tooth notches, pulled back firmly but gently on the chuck until movement was locked, held the chuck firm, and worked the tool bit via the carriage adjustments across the selected scribed circles and marked off the hole locations. This was repeated until the circles were scribed all the way around with light cross marks. An accurate set of scribe marks now exists ready for center punching and drilling. No harm is done to the lathe and the need for an expensive indexer is avoided.
Now the individual hubs can be cut apart and finished being careful not to mess up the spoke hole scribe marks. I recommend completing the axle bores and hub end fairings at this time so that the hubs are completely finished before moving on to drilling and tapping.
In most cases spoked wheels have the spokes set at an angle so that the spokes are mounted much wider apart on the hub than on the rim. This provides rigidity and strength to the wheel. Therefore, when drilling the holes preparatory to tapping, the drilling should be at an angle around five degrees from square. A drill press can be set up by angling the table and using a V-block to do this accurately and easily. The lathe can also be set up to do it. Drill the holes all the way through into the axle hole. Doing this makes tapping much easier.
Upon completion of drillng the next operation is tapping. This requires a great deal of care and patience. A broken tap can take much of the enjoyment out of the project. A typical model spoke size will use a 4-40, 3-48 or 2-56 tap. These are more or less common standards. If a model is based on a 1:6 ratio with the full-sized machine using a inch spoke, the model spoke will use a 4-40 tap. The finer the threads, the better. The above taps can also be had in 4-48, 3-56 and 2-64 respectively. If the model is quite small other sizes such as 00-90, 0-80, 1-72, 1/16-64, 3/32-48 and British Association sizes 8 and 10 may be used. Small metric sizes are also available. There should be no problem of getting the proper scale size.
Be sure to use an appropriate lubricant when tapping aluminum such as 'Alumtap'® or some other solution to ease the strain on the delicate taps and to prevent seizing which is a problem in aluminum.
Now let's direct our attention to the making of the rims. My choice is to use either thin wall or regular pipe. If you use pipe make sure that it is acceptably round. Not all pipe is. The thick wall sizes work well when the face of the wheel needs shaping such as the wheels on late model Huber threshers.
Chuck a short piece in the lathe and true up the roundness a bit and cut off slices slightly wider1/16 inch than the finished width of the rim. The lathe cut-off tool works well and is far more accurate and easier than a hacksaw.
Now using the lathe chuck with the jaws set for inside gripping I prefer the three jaw universal chuck for these operations since its accuracy is good enough chuck the rings and lay out for the spoke holes using the same general technique described earlier for the hubs. Do not over tighten the chuck so as to distort the rings.
Turn out a round piece of wood that can be inserted into the rings to provide support during punching and drilling. With the piece of wood in place center punch the hole locations and drill the holes the size of the spokes so that a tight fit is assured. Drill these holes also at a slight angle to allow for the angle of the spokes.
Next use a larger drill and countersink the holes. Do not drill all the way through this time. For example, if the spoke is 3/32 inch in diameter, a 3/16 inch drill is used to countersink, since a 3/16 inch rod will be used to form the spokes.
The spokes are turned from round stock. Cold rolled steel is very good. In the example just given a 3/16 inch rod is carefully turned down to the correct diameter and length of the spoke. On the end opposite the threads turn a head to fit the countersunk holes in the rims. Thread the other end and cut off the spoke leaving about inch for a screwdriver head. Hacksaw a slot in the head to receive a flat screwdriver bit. Also do not cut too much thread on the thread end. Excess threads spoil the appearance of the finished wheel. Make all the spokes.
Assembly of the wheel is, in my estimation, easiest and best done on the lathe by using the universal chuck and the centers simultaneously. The chuck holds the rim firmly and accurately concentric with the hub which is carried between the two centers. The degree of accuracy is approximately .003 inch. The rim must be properly centered on the hub so the spokes line up properly. Now insert the spokes and gently finger tighten only enough to take out the slack. Inspect and make sure that all is well up to this point.
Now, using a flat bit screwdriver, carefully tighten the spokes a small amount at a time alternating from side to side and from row to row. Do not tighten too much at once or in series or the wheel will be distorted when it is removed from the chuck and centers. The tension must be kept as uniform as possible around the wheel. Mentally note or mark the spokes and tighten in a series such as 1,5,11,15,3,7,9,13,2,6,12,16,4,8,10, 14. Do not tighten all the way the first time. Make at least three circuits and do not over tighten so as to strip threads or break the spokes.
Remove the wheel and check for trueness. It should be excellent if the tightening sequence was carried out properly. If it is not true, loosen and do it again. Remember, the final 1/16 inch or less will be ground off in the finishing operation.
When you are satisfied with the trueness remove the excess screwdriver heads by jeweler's saw, mini hacksaw or by grinding. If you grind them off be careful not to heat up the wheel. From now on, heat is your enemy!
After the screwdriver head nubs are all removed and all the wheels are assembled, final truing (and if needed, shaping) is done on the lathe. I set up a tool post grinder with a fine wheel and mounted the wheel being trued on a mandrel chucked in the lathe. The lathe was set for a slow rotational speed and the tool post grinder was used to grind down the face of the wheel. The wheel thus became nearly perfectly round and the traces of the spoke nubs practically vanished. Finally the sides of the rim are ground to size. Warning: The rim must not be allowed to heat up during grinding. If it does the wheel will be ruined. A sponge soaked in water held against the rim works quite well to suppress the heat.
If all the above has been done carefully, you should now have a beautiful, sturdy spoked model wheel.
If you elect to use aluminum for your hubs I recommend that when the spokes are screwed in they be coated with Loctite or epoxy so as to prevent electrolytic corrosion with the steel. My first wheels are now ten years old and there is no evidence of corrosion. If you use brass or bronze Loctite will help keep the spokes from working loose.
Now inspect to assure that the spokes did not protrude into the axle hole area. If so, grind or file them off. Do not attempt to drill them out.
Lastly, the finished wheel should be thoroughly washed in dry cleaning solvent to remove oils and films. Do not touch them with your skin or oils will be deposited, causing poor paint bonding. Undercoat the hub, if aluminum, with a very thin coat of zinc chromate primer. Use iron oxide primer on the rest. After drying they are now ready for painting.