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showing the tooth with formed circular ends that is now being used in their film printers, and also showing the method used by them in forming the teeth to fit the perforations in the film. While satisfactory for forming the teeth on the large size sprocket used in their film printer, the process as previously stated would be objectionable for a sprocket wheel of approximately one inch in diameter as used in the projector, because at the base of the corners of the teeth an unfinished projection would remain, unless at the ends of the teeth the cutter would be advanced to the point where the flange of the wheel would be recessed to an extent that would be objectionable.
In making the teeth with ends formed by their present process, as shown in the accompanying drawings, a hollow mill or forming cutter is used. The mill operates through a bushing to center it, for the prevention of vibration and to make it possible to cut on only one side of the center. The sprocket wheel should be indexed for each tooth
Fig. 4 Forming cutter with conical
opening.
the same as in the regular operation of cutting the teeth on a gear cutter. The opening in the end of the cutter used for forming the teeth is made conical to fit the form of the tooth, as shown in Fig. 4.
As the width of the teeth are made less than the width of the perforation, the ends of the teeth are milled on one side of the center and then the sprocket is indexed .012 of an inch and the ends are milled on the other side of the center.
As a means for "generating" the teeth for use in a projecting machine, Mr. Howell has advanced the following theory, which as I understand it, has not been demonstrated to the point of determining definitely that it is practicable, but it seems to present encouraging possibilities. In this as in the preceding process, a hollow end mill type of forming cutter is used, the opening in the end of the former being cylindrical instead of conical as in the preceding form of cutter.
The hollow end mill for this process may be mounted in the spindle of a bench lathe, the cutting end being supported preferably in a bushing, while the sprocket wheel together with its index may be carried on a gear cutting attachment for the bench lathe. To perform the "gen
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