This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Perhaps the last thing that would be thought within range of the amateur who lacks a full equipment of machinery is the cutting of gear wheels. The device shown in the sketch is very practical, and, with the most ordinary assortment of iron-working tools, will serve to turn out an accurate gear.
Ill: Details of Gear-Cutting Machine
No system of supports is shown, as they are easily supplied. A flat, square board, B, as large as can be obtained - 2 ft. on a side being the safest minimum - is used for a dial. A sheet of paper is pasted over the entire board and a large circle drawn on it. This circle is divided into as many parts as there are to be teeth in the gear. A depression is made with a prick punch at each division. A shaft, C, is run through the center, to which an arm, A, is firmly attached. A nail, X, is placed at the end of the arm so that the point can enter each of the punch marks on the periphery or circumference of the circle. A blank wheel, W, is attached to the shaft C, in the position shown, and resting on the iron plate or strap F. A cutting tool, D, works up and down in a slot in F. This cutter is held away from the blank wheel by the spring S, and moved up to it by the screw G, acting through the sliding member H. A stove bolt may be used for G, with the nut firmly fastened to the strap F. The cutter is actuated with a handle E, whose motion is limited by the pegs as shown. In Fig. 2 is shown a top view of the strap E, with a cross section of the cutter and the slot in which it works.
The operation of the mechanism is as follows: With the blank wheel in place set the nail N in one of the punch marks and move the handle E downward. This will make a slight cut on the wheel. Then give the screw G a turn or two, which will make the cutter take a deeper bite, and push the handle down again. This operation is repeated, screwing G constantly deeper, until it is stopped by the lock-nut J, which regulates the depth of the cut. Move the nail N to the next punch mark and repeat. The operations are very quickly performed and the circle will be closed almost before you know it.
There are several points to be observed. The accuracy depends upon three things. First, the slot through which the cutter passes must be tight-fitting, even at the expense of working hard. Second, the shaft C must fit tightly in the holes made for it through B and F, also the arm A must be firmly attached, the shaft being filed flat at the point of attachment. The blank wheel should be keyed on the shaft or securely fastened with a setscrew. Third, the nail N must enter the punch marks accurately. As the grinding circle is so much larger than the blank, any error here is greatly reduced in the finished wheel. Once in a punch mark the nail must not be allowed to slip until the tooth has been cut.
In practice, it will be found better, after each stroke of the handle, to give a quarter turn outward to the screw G, thus avoiding the scraping of the tool on the up stroke. The cutter can be made of any suitable steel with the cutting point ground to give whatever shape is desired for the tooth. The best shape for any particular purpose can be found by reference to a book on gears.
This cutter is adapted, of course, only to the softer metals. For experimental purposes a blank cast in type metal is excellent and will last as long probably as required. The mechanism here described will cut brass perfectly well and a gear made of this more durable metal will answer for a finished construction. Type-metal blanks commend themselves because they are so easily cast, any flame which will melt solder being sufficient, and the molds, of wood, plaster of Paris or any easily worked material, being quickly constructed. - Contributed by C. W. Nie-man, New York City.