Film-Tech Forum ARCHIVE: The making of starwheel, cam and pin

This is topic The making of starwheel, cam and pin in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Erkan Umut (Member # 330) on 12-07-2003, 04:52 PM:

First of all, I thank you very much for answering and your time... [Smile]

I've visited some projector factories, and seen that the "concave cam guide surfaces" of the starwheel are evened manually with a razor-sharp hand tool during checking and assembling of intermittent mechanism.
I'm wondering why?, and what does cause for this process?

What are the materials (like steel, etc.) and what kind of machines (like lathe, milling machine, etc.) or machining are used during making of starwheel, cam and pin?

Posted by John Eickhof (Member # 323) on 12-07-2003, 05:11 PM:

Modern starwheels and cams are made of mild steel, then roughly machined on a CNC type lathe/mill
Then they are hardened by the case rockwell technique. Then they are finsh ground
on a rotary or radius grinder, then are finished by mechanical or hand lapping with a grinding compound untill they meet specs. The general tolorance of a starwheel and cam is about .005" This insures stable pictures when projected on a screen. The shafts are hardened and ground to the same specs. This method of manufacture has been in use for the last 100 years or so of projector making, and only has improved with the quality of machining equipment now available.

Posted by Greg Mueller (Member # 17) on 12-07-2003, 05:25 PM:

John
Not to dispute you as I have never made a star wheel, but maybe you meant .0005" (half thou) 5 thou is not very tight tolerance

Posted by John Westlund (Member # 913) on 12-07-2003, 05:58 PM:

I also have heard that the tolerance is .005" I would assume that you need to leave some room to let oil get in there. Someone please correct me if I am wrong.

Posted by Mark Gulbrandsen (Member # 72) on 12-07-2003, 07:32 PM:

"I've visited some projector factories, and seen that the "concave cam guide surfaces" of the starwheel are evened manually with a razor-sharp hand tool during checking and assembling of intermittent mechanism."
_________________________________________________________________

There is an answer to why it is done this way.....
Although it is the "old fashioned way" to fit things, hand scraping of the parts for proper fit is by far the best way to assemble a precision mechanism, and it is in itself considered an art. For instance, all "high end" machine tools have hand scraped ways. This is done for two reasons. 1. To provide the finest possible flat surface that can be produced...(even better than by surface grinder!). and 2., To provide scratches in the machined ways to hold oil for better lubrication. Hand scraped ways always outlast ground ways by a factor of about 10 assuming the machines are equally maintained. Two prime examples of this would be the ways on a Bridgeport Milling machine and the ways on a Hardinge HLV lathe. One has to see the ways on either of these machines to appreciate what it is all about. There was one projector that had a hand scraped main casting and that was the Kalee 21. Really a beautifully made machine with fit and finish that no American manufacturer has come close to yet.

As for the cam radius being hand scraped that you saw, you would find that the parts were not case hardened but you would have a much better fit of the parts in that movement and with the scratches left behind from hand scraping in the surface of the metal some lubricant would be retained and the movement may actually last longer. Nothing wrong with doing things that way at all as long as the person performing the scraping has learned the art. By the way the Brenkert movemetns were not hardened steel but soft, they last longer than the equivelent hardened and ground parts by at least a factor of 10. I'd bet the Turkish movenents are also then run in and lapped with a bit of lapping compound in the oil, guaranteeing a perfect movement.

While John may feel the art of making parts is getting better with technoology, I would flatly disagree. In this sort of case we are relying too much on the technology and too little on peoples actual ability to manufacture tight tolerance parts. Unfortunately the world is quickly loosing all the true artesians that could make super high quality stars, cams, and sprockets to retirement and or death. This is one of the reasons that Kinoton has shifted over to also making electronic intermittent machines. CNC doesnt guarantee you anything in the tolerance department with respect to these parts. Typical tolerance held to at LaVezzi is one ten thousandth of an inch on star wheels and cams. The final grind on the slot in a star wheel is actually done manually in a sound proof booth on special fixtures and the grinder operator has to be able to hear the wheel pass through the slot... Sometimes he can barely hear it at all aas the amount taken off is typically half a ten thousandth of an inch or less! The grinders used by LaVezzi for this are Studer air bearing grinders.

Mark @ CLACO

Posted by John Eickhof (Member # 323) on 12-08-2003, 09:55 PM:

Greg...Thanks for the correction...The specs are generally 1 to 5 hundred thousanths of an inch!!
Thus >0001 to .0005 is the numero I meant to say!!!! Later...

Posted by Mike Blakesley (Member # 26) on 12-09-2003, 02:34 AM:

See there? That's why John's quitting...his numbers are going to [bs]

Posted by Stephen Furley (Member # 1277) on 12-09-2003, 03:08 AM:

Five thou is more than a tenth of a milimetre, about the thickness of triacetate film, so this obviously couldn't be correct. I couldn't have said what the correct figure was, but it was obviously going to be a lot less than that.