In this special class of pattern work, the greatest accuracy and care must be taken, not only in building up the rim of the wheel, but in fitting and placing on the rim the blocks out of which the teeth are to be formed, and most of all in laying out the teeth regularly and accurately on the tooth blocks, A pattern for a gear wheel whose teeth are carelessly made is almost worthless, the time lost in chipping and filing for the purpose of correction being too great to allow the use of such a pattern.
It is common practice in some pattern shops to build the pattern with the teeth stock fastened to the rim permanently, and having the teeth cut in a gear-cutting milling machine. To insure greater accuracy and smoother running gears, it is now the custom in many shops to have the wooden pattern made in the form of a blank without teeth, from which a metal pattern is cast. This cast pattern is turned up and placed in the milling machine where the teeth are cut and spaced with accuracy and to the exact form of tooth required. This metal pattern is used without draft. This method of making gear patterns, however, is expensive, and is used only when many wheels are to be cast of the same size and number of teeth from the same pattern, and, as in the case of pulleys, the wooden pattern is still used for all special sizes of gears.
At its best, the cast gear can never compete with the cut gear for smoothness of running and the efficient transmission of power. The modern machine practice calls for machine-cut gears, and consequently the cast gear is only for certain classes, as slow-moving machines where considerable backlash can be allowed, and when the teeth can be of such size as to be molded easily. For these reasons, the present-day pattern maker rarely ever gets so far as to cut the teeth of the gear pattern. However, several methods of constructing the arms, rims, and teeth sections of these patterns will be considered, and a few hints given as to the best methods of construction.