My first Double thread

Where I apprenticed, there was a ring and plug gage that was an acme thread about 1" diameter with 4 starts; if you held it vertically, either member would thread off of its own weight, when the place went up for sale, I looked for it, but someone else had the same eye for it, apparently. I never saw anything in the plant that used such a thread,.
 
OCJohn said:
I feel like an idiot asking, but what is the purpose or common application for a double thread. My background is limited mostly to automotive and I don’t think I’ve ever seen one in the wild…
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There can be a number of reasons. I'm not sure why you'd want a large bolt with multiple starts. Linear leadscrew contraptions sometimes have it. In this case, you'd get a 4TPI (quarter inch per revolution) feed rate, but only have to cut the thread (machine time, cost, and reduction of the parent metal) that's equivalent to 8 TPI (eighth inch pitch), so literally only half as much diameter reduction. The most common application that I see in my little world is smaller fasteners in softer materials, especially plastics. Maybe holding your coffee maker together. Or your portable battery booster pack. Or your flat screen television. In automotive use you find them commonly (although not exclusively) in HVAC module parts, blend door actuator motor attachment, headlight and tail light assemblies, etc. Among the two start threads in those applications, probably a third of them have one "start" that's taller than the other. Like a high thread and a low thread at every other thread. Those form (cut, mangle) their own thread on the way in, are not prone to vibration, do not want a lot of clamp load, and the high/low thread design is really good at not blowing out the hollow bosses that are molded into plastic parts to accept the screw.
 
For the clutch on a steam donkey cable drum, I made a screw and nut of acme form about 3" diameter with 3 starts, pretty exciting, threading on the slowest speed into s short recess at the end of the thread! The not was all the way through the hole, so no problem, the screw was steel, the nut was cast iron.
I once saw a turret lathe that use a single start but high helix thread left and right hand on the same rod that could be engaged by a single lever in either helix as desired; it was used for rapid travel on the saddle(s); perhaps a bit off topic, but of interest perhaps? I remember the helix was maybe about 45 degrees,
 
Multiple lead screws are used in aircraft everywhere .
 
Hey, great work on your first thread of multi starts.

Regarding methods of clocking the other treads start. The method I was taught was to utilize the change gears. If you have a 120t idler, revolve the chuck forward until the stud gear's key way is at the 12 o'clock position, remove the stud gear, revolve the 120t idler its normal direction a tooth count a division of 120, slide on the stud gear back and tighten its retainer.

Now you can cut 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, and 120 start threads.

<<Self correction>> This only works simply if the Stud and Screw gear are Equal.
 
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Looks grate Jeff. How did you start the second thread in the middle of the first thread?
 
OCJohn said:
I feel like an idiot asking, but what is the purpose or common application for a double thread. My background is limited mostly to automotive and I don’t think I’ve ever seen one in the wild…
Click to expand...
There are several pieces of commercial kitchen equipment that multi start threads are used. I've done one on the bowl lift screw of a commercial mixer and one on a tortilla press screw. One was double start and the other a triple start.
 
Congratulations, really nice work. They were used extensively in fountain pen manufacturing in the 1930s-1950s, probably to make it easier to screw the cap on to the body. And, in pretty fine threads. I plan to try one on a pen in the next six months.
Tim
 
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