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Mar., 1942] PROJECTION DESIGN DEVELOPMENTS 271
In threading the film, the pad-rollers are opened and provide a gauge for setting the loop sizes above the film-trap and below the intermittent sprocket. The film is threaded tight over both padrollers, and the loop sizes are automatically set when the rollers are closed.
The film-trap and gate have been designed to fill not only the technical requirements for perfect picture reproduction, but also to provide the most desirable features from an operating standpoint. The film-trap is a complete assembly readily removable from the mechanism for inspection and exactly interchangeable without readjustment. It includes interchangeable film-shoes and studio guides, heattreated, hardened, and ground. The upper guide-roller is designed so that the guiding face is integral with the shaft on which it is rotating, and is ground on the same centers on which it is mounted in the film-trap. This construction assures perfect accuracy of the guiding face and eliminates the side weave that will occur if there is any eccentricity of the surface.
The film-trap is mounted to the main frame with three large screws, and is held in accurate alignment to the optical centerline by means of the large ground pad, which forms the base of the filmtrap, and two dowel pins so located that the complete trap assembly may be moved forward and back parallel to the optical centerline. This adjustment is provided so that perfect alignment may be obtained between the trap shoes and the faces of the intermittent sprocket.
A number of experiments have been made to determine the best relation of the alignment between the trap shoes and the sprocket. From jump measurements which were made, it seems desirable to set the shoes slightly ahead of the sprocket face so that effectively the film position under tension is exactly parallel to the face of the trap shoes.
The tops of the trap shoes are curved back away from the flat surfaces of the shoes, allowing the upper loop to be made in back of the centerline of the film in the film-trap. The reason for this will be discussed under the explanation of the film-gate operation.
The lens-holder and film-gate mounting is a complete unit, readily removable from the mechanism. As in the case of the film-trap it also is mounted to the main frame by means of a large accurately ground pad and held securely in position with large screws. The complete assembly is also located with two dowel pins so designed