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THE REDUCTION PRINTING PROCESS 301
perforations and, once more, remain motionless for the period of the exposure. It should be realised that, although the 'dwell* which occurs at the end of each horizontal movement is equal to the exposure time, the actual exposure only takes place when the pilot pins have moved into the film perforations. This cycle of operations is so geared to the motion of the claw pins that the pilots only enter the film perforations at the moment when the claw pins are moving out of engagement and, after the exposure has been made, retreat from the film once more as the claw pins reenter at the top of their 'pull-down' stroke.
Three main stages in this cycle are seen in Figure 133; that at 'A' indicates the relationship when the film is engaged with the claw pins as described previously; that at 'B' indicates the moment of change over when the pull-down has been completed and the claw pins are moving out of engagement whilst, at the same instant, the pilot pins are taking control of the film and positioning it accurately in register with the aperture; diagram 'C indicates the condition during the exposure period and shows the pilot pins in full control of the film registration and the claw tips returning to the top of their stroke in readiness for the next cycle.
Fixed Pilot Pin Registration
The accuracy with which such a pilot pin system will control the position of any film during the exposing period depends, amongst other considerations, upon the degree of accuracy with which the sliding shaft supporting the pins fits the stationary bearings, and also the accuracy of the cam which controls the horizontal motion imparted to this shaft. A recently designed printing machine employs fixed pilot pins which are anchored to the mechanism plate itself engagement with either these pins or the claw pins being effected by oscillating the gate channel and, therefore, the film itself in a horizontal direction. The general layout of the 16-mm mechanism used in such a printing machine is seen in Figure 134, which shows a side elevation of the unit.
The main aperture plate supports two pilot pins, numbered T and '5', one above and one below the exposing aperture. These pilot pins are rigidly fixed to the main plate and do not move. The claw pins, numbered '3' and '4', oscillate in a purely vertical direction as indicated by the arrows. The film is guided through a gate formed by two highly polished metal plates which oscillate in a purely horizontal direction. The action of the mechanism is therefore as follows: -The film gate, as shown at 'A', has just moved to the left, and the film is located upon the pilot pins. The