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would apparently make it practicable to harden the sprocket wheels and then correct the distortion from hardening by grinding, whereas it would seem that the method of generating the teeth with a forming cutter as previously described, permits the making of soft sprockets only, the form of the teeth of which quickly change through wear. It may be that some method might be devised for the use of a forming grinder having a small hollow grinding wheel, such as a lap of tough metal and charged with abrasive material to revolve around the tooth as the sprocket wheel is fed past the grinding attachment, but it is probable such a plan would not be practical.
In presenting this method as a means of generating the sprocket teeth, I wish it understood that I am not in a position to vouch for it in that it is a theory that has not been proven. I have endeavored to present it as nearly as possible as it was explained to me, having myself added the suggestion for the use of the double rack as a means of eliminating the lost motion. It would seem, however, that it presents promising possibilities as the only practical method whereby we may expect under present conditions to be able to guide the film to prevent side movement, and it is to be hoped that it may lead to practical results and ultimately result in the adoption of a standard that will add substantially to further accuracy and efficiency in film projection.
Discussion
Following the paper, the chairman opened the discussion by emphasizing the necessity of the grinding operation. He also brought out, that the grinding operation might be limited to simply bevelling off the corners. Mr. Roebuck objected that this would leave a corner which would be dangerous to the film.
Along the line of grinding, Mr. Mayer suggested that the question of suitable grinding material could be easily handled, citing the Chemical Exposition at New York where a number of various moulded abrasive materials were shown,
Mr. Roebuck brought up another possibility, that of using a hollow grinding tool instead of a hollow mill, providing proper feeding could be arranged.
Here the discussion digressed to the question of the relative motion of the sprocket and cutter, Messrs. Mayer, Roebuck, Cook and the chairman taking part.
Mr. Roebuck brought back the discussion to the question of feeding a grinding tool to take care of the wear of the abrasive and suggested a method. Mr. Richardson objected that this would not be accurate, at least from the standpoint of production. Mr. Roebuck said that it would be practical from the mechanical point of view. Mr. Richardson added that it would be all right if the grinding proposition were taken care of as an additional operation, but doubted the accuracy if the sprocket were generated by grinding. Short discussion on the accuracy question by Mr. Richardson and Mr. Roebuck followed.
Mr. Victor offered a suggestion of generating by the use of a hollow tapered grinder or cutter and rocking the sprocket. This would approximate the generated tooth and would be accurate enough for practical purposes. Mr. St. John approved Mr. Victor's suggestion.
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