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THE PROJECTOR 303
plied by manufacturers. The sprocket arm is then removed. The utmost care is required to avoid contact of star with casing, or with anything else. It is machined to 1/10,000 of an inch accuracy. A wholly invisible abrasion or dent may and probably will produce faulty results.
The sprocket is joined rigidly to its shaft either by small taper pins or machine screws. Usually some type of "pin ejector" is used to remove the pin's (See Fig. 137). The sprocket may usually be removed from the shaft by pulling with a twisting movement. If not. lay its hub (hub — not rim) on suitable hardwood blocks laid across partly opened vise jaws and drive the shaft out by tapping gently on the shaft end, using a light hammer and hardwood punch. Do not use a metal punch of any sort, or permit the shaft to contact anything, however lightly.
(52) To install a new intermittent sprocket, first clean the shaft very thoroughly and blow strongly through the hole it is to enter. Do not, under any circumstances, attempt to wipe it out. Having the shaft clean, cover its entire surface with high grade oil. Shove the sprocket on the shaft with a gentle, twisting motion. If it sticks, remove, reclean the shaft, relubricate and try again. It may take several trials but patience must be exercised. Only the small fraction of 1/10,000 of an inch of metal must be worn away. In any event proceed gently. Don't attempt to rush matters by using too much force. In replacing taper pins or holding screws, set them up firmly, but not too much so, lest the sprocket be strained and its rim thus warped out of shape. A 1/10,000 inch fault here means a fault that may be visible on the screen.
(53) In the event you do not care to purchase the pin ejector, you can make a V block as shown in Fig. \37. If covered with vaseline and protected from rust it will last a lifetime.
Aperture Tension
(54) When there is too little tension the screen image