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01-10-2011, 06:18 PM #1
Optical thread reference/standards?
I'm duplicating some lens adapters for theater "effects" projection ... and need to cut a fine internal thread to fit a German lens of probably 1970's vintage. The threads are so short it's hard to sight a good match even with my best B&S thread gages.
The major diameter is clearly 50mm.
The thread pitch is a tossup with the .75mm, .8mm and 32 tpi gages - I can only guess.
Does anyone recognize this as an obvious standard, that I can consult to do it right? Any other words of wisdom?
01-10-2011, 07:02 PM #2
This may be completely irrelevant to your particular lens, but the old German lenses I have for microscope use have M50 x 0.75mm threads. Never heard of 0.8mm pitch, but then I haven't messed with projectors of that size.
01-10-2011, 07:55 PM #3
I tried old and new Machinery's handbooks, no joy. I do know that threads for optical devices tend to be of a funny form so they don't get crudded up or tear out even with the fine pitches. You might have to look at 'em with some serious magnification to figure out what's going on.
01-11-2011, 04:39 AM #4
Any way to make a mold of them?
If so, look at them from above with a small handheld magnifying glass and scale devices.
01-11-2011, 06:00 AM #5
The Japanese? WW-II transit that I bought has a broken eye piece. I tried all the thread gages that I had and it seemed to be around 40 or maybe 39.
I talked to a local Instrument gentleman who suggested 40 tpi was common. I made one and it worked perfectly.
01-11-2011, 07:00 AM #6
Use an optical comparator to get the pitch, get it right the first time.
01-11-2011, 07:17 AM #7
Use an Optical Comparator to measure it and then calculate the mating threads dimensions.
I see Porschefix posted the idea first....and if you dont have a Comparitor, find one, companies that do a lot of threading usually have at least a 31.25 power machine with at least a 20 inch screen.
01-11-2011, 01:11 PM #8
Since you have a male thread , how about measuring the pitchdiameter with a micrometer.
Should be notably different for .75 and .8 mm pitch.
I have to agree an optical comparator would give you a most undisputable answer.
Another thought : if available time and material allow you could make some testpieces.
FWIW : in my years in a physicslab-machineshop all the even slightly optical metric threads were either 1 or .75 mm pitch.
01-11-2011, 01:49 PM #9
Folks, thanks for the replies and suggestions. Based on the above I'm sure it's M50 x .75mm.
01-11-2011, 01:52 PM #10
Who made the lenses? If it's Schneider, Rodenstock, Zeiss or some other known maker, it should be easy to look it up or inquire. I wouldn't assume the existing adapters are correct unless they were made by the lens maker. I'd take a cast of the lens itself, then cut some trial parts.
01-11-2011, 01:58 PM #11
Another vote for the optical comparator. Check the flank angles too. Whitworth (55°) thread angles on mm pitches is not unknown also 47.5° BA angles on whatever the maker wanted. Another gotcha is close fits on truncated forms, e.g. 55° thread but 60° depth. Outside the standard camera, enlarger and microscope threads it really is something of a lottery especially if thin tubes are involved although 1970's seem to be the shake-out era where things settled down into something fairly logical. I had some really weird and wonderfuls in the useful bits and special kit built round it boxes in my lab when I was a wage slave.