How does one machine a toothed wheel for a film sprocket such as this:
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That item is a stack of several cylindrical components, with the toothed portion apparently a slice out of a long toothed stock piece, somewhat like the sprocket stock you can get to make gearbelt pulleys. Would this toothed stock just be cut from round stock using axial cuts from a custom bit, incremented with a spin indexer?
I also have an interesting specimen, a 4-inch diameter metal sprocket that appears to be an exquisitely fine cast version. Is that possible?
Production units are probably cut with an internal broach. To do a one-off or small quantities, you don't need a custom milling bit--a straight end mill will do it. The tooth sides can be straight, although curved would be better (circular, no need for involute as on a geartooth). That's where you'd need a custom cutter, easy enough to make using a single-bit flycutter. You just mill the teeth, and the arc between them, in stages on a rotary indexing table after turning the profile on the lathe.
"exquisitely fine cast" would probably mean investment cast, using a sacrificial plaster-type mold where the mold cavity is formed with a wax pattern. Or die-cast, using a metal mold (most likely on a production item). You can do some pretty exquisite casting with sand, too.
I don't think they were cheap then either. I ran the projectors at the local movie house while I was in high school (about 68 or 69). The one cardinal rule was "don't drop the sprockets or the Cinemascope lenses"
My mom taught me that if a microscope cost more than $500 you could carry it with one hand. Above that, you had to use *two* hands!
I think the cinemascope lenses probably required about four people to get enough hands.
Jim
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Same with the Army. I worked on Pershing missiles. The inertial platform was a "2 man carry" rule even though it only weighed about 35 pounds. At least you could share the blame if you dropped it.
We make similar items all the time. If making only a couple, use a tapered endmill and do some drawing and calculation first to try and get an idea of the proper taper and tooth size. After milling on an arbor (or chuck if the shoulders give you room to grip and keep it true), you can use a wire brush or similar on a grinder to abrasive machine the teeth to a more curved profile so they will release properly. Finally, do your best to polish the tooth faces so that they will not abrade the film.
There are better ways to get the proper tooth profile but probably not worth the hassle unless you want to do it as a "learning" project
Koz
Richard J K>How does one machine a toothed wheel for a film sprocket such as this: >
A short, curious story that relates to this: Back around 1969, Life magazine's photo department bought about one or two hundred thousand (no exaggeration) indexed, glass-window plastic slide holders. The relationship of the sprocket holes to the frame area was fixed; there was a single prong on the molded slide holders that engaged one sprocket hole, and getting the full frame in the window depended on the camera having a uniform relationship between the sprocket drive and the window. Besides having a
*uniform* relationship, it needed a *matching* relationship, one that matched the relationship in the slides.
No one thought much about this until they discovered that the department-issue Nikons all had different sprocket-hole relationships. They were uniform, but each one was different, and practically none of them had the *same* relationship. Apparently the sprocket drives on Nikon F's weren't set in any particular relationship to the gears.
Anyway, they tried to take the slide holders back but it was no-go. Nikon couldn't, or wouldn't, supply the sprockets that they wanted. So they had Marty Forscher (Professional Camera Repair, then on 42nd Street) machine new sprocket wheels for all of their Nikons.
It cost them a significant bundle. I think somebody lost a job over it.
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