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32 PRINCIPLES OF CINEMATOGRAPHY
slitting machine consists essentially of a supporting bearing, to carry the film as supplied from the coating machine, two sets of rotating knives, arranged rather in the fashion of mangle rollers but accurately spaced to cut the film into strips of the required width and provision for spooling up the individual narrow rolls after slitting. Film base is a pliable material which is not entirely homogeneous. It is therefore necessary that the spooling operation be very accurately adjusted to avoid uneven pull on the slitting knives. When it is remembered that the width of cinematograph film must not vary by an amount greater than 0.002-inch (0.05-mm) it will be appreciated that the setting up and operation of a slitting machine is a very highly skilled piece of work.
We now have to consider a very important stage in film manufacture and one about which so much controversy occurred during the early days of motion pictures. Bearing in mind that the average cinema magnifies the picture in the film by as much as 350 times, it is not difficult to realise that any errors made when perforating such films would be immediately apparent to the audience. Figure 6 shows in outline the cycle of operations and the various parts of the 'head* of a typical perforating machine. Diagram 'A' shows the essential parts of the mechanism and, whilst different machines may vary somewhat in detail, all employ these basic principles. The 'pilot pins' are used to accurately locate the film both before and during the perforating operation. These pins are tapered over the region near their extremities to permit their easy entry into film perforations which may not be accurately presented beneath them. The 'punch legs' are usually cut from separate plates, but are 'ganged' together for final milling and grinding. These legs are shorter than the pilot pins and, being moved at the same time and by the same mechanism as the pilot pins, do not enter the film until it has been accurately located. The shuttle is best described by likening it to the 'under-foot' of a sewing machine since it is used to engage with the newly made perforations and to advance the film in readiness for the next perforating operation.
The sequence of operations is therefore as follows: firstly, the pilot pins enter the perforations, (b), Figure 6, and at the same time the shuttle commences to move to the right. The pilot pins then continue their downward motion and the punches make four perforations in each side of the film and whilst the shuttle comes to the end of its traverse, (c), Figure 6. The punches and pilot pins then move upwards, clear of the film, and the shuttle pins move into the perforations as shown at (d). The shuttle then moves to the left, carrying the film with it, as shown at (e), by an amount