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298 RICHARDSON'S BLUEBOOK OF PROJECTION
of cam ring C, in which position it is evident star B cannot be rotated. It must, in fact, remain perfectly still until the two surfaces are released. (36) In this condition the movement is said to be "on the lock."
Remembering that cam D rotates continuously in the direction indicated by the arrow, it is evident that presently pin D will engage with one of the slots in star B, and in that instant surfaces E and C will be out of contact and star B free to rotate under the control of pin D. Study the diagram and you will see how it works.
(37) As pin D enters the star slot it moves the star around. The movement is very rapid, but nevertheless by relation the star and its intermittent sprocket (mounted on its shaft) are started slowly and the speed gradually increased to maximum, which is about two or three times the normal 90 feet-per-minute movement of the continuously running film. The star and intermittent sprocket then slow gradually to a full stop, whereupon surfaces E and C re-engage, locking part B immovably. The picture is projected and the cycle of action is repeated.
Constructional Details
All reputable manufacturers use only the very finest materials in constructing their intermittent movements. The shape and diameters of the sprockets and the dimensions and spacing of the sprocket teeth are all measured in ten-thousandths of an inch. Any sprockets, stars or cams failing to come within two ten-thousandths of an inch of the correct measurements are rejected.
(38) All intermittent parts are finished by carefully controlled, precisely accurate grinding processes. (39) In modern intermittent movements the cam and star are always hardened, but the intermittent sprockets sometimes are not because it was discovered that hardening causes undercutting in the form of a notch in the metal at the base of the sprocket teeth. Teeth not hardened wear off smoothly.
(40) Faulty functioning of the intermittent move