Half a croissant, on a plate, with a sign in front of it saying '50c'
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Two threads

Just like one thread, only there's two of them...
  (+5, -1)
(+5, -1)
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A fine pitch thread in one direction (say right handed...but it could be left handed). In the other direction is a coarse thread.

This might allow a quick left turn to get the fastener in position, and then a right hand turn for tightening.

Ling, Jan 02 2012

Quick Release Nut http://www.jwwinco..../section8/gn6303.1/
[scad mientist, Jan 05 2012]


       So, you turn left-left-left to position the nut on the coarse thread. Then you turn right-right- right. What makes the nut follow the finer right- hand thread, rather than just unscrewing itself on the coarser left-hand thread?   

       I suspect there might be a way to do this, though. It's probably easiest to imagine the threads as being grooved "tracks" running diagonally on a flat surface, with the nut being a rectangular block with ridges engaging the grooves.   

       The difficulty is the swapping of the two threads; once the nut (or block) has engaged one set, I think it would tend to stay engaged.
MaxwellBuchanan, Jan 02 2012

       [bigsleep] maybe you're right, but I thought about it this way:   

       Tap the hole using a normal tap, then tap it using the left hand coarse tap. Would the new tap go in, and would it come back out? Would the first tap go back in and out again? My brain tells me: Yes.   

       Like wise with a die on a rod.   

       Now, putting the double tapped hole together with the double threaded rod is where my brain goes out to lunch.
Ling, Jan 02 2012

       Surely the threads _are_ point contacts (once you have threaded both bits both ways). The problem then is that the points that are ledt will go down either side of the thread - so you could never tighten it more than one bump either way.   

       I think the reversible-push- turny-screwdriver uses not a point, but a swivelling long thingy to mate with the thread. So the contact area is a length of metal which will only sit in one of the two possible grooves. To change direction you have to mechanically release the thingy and allow it to track into the other thread.   

       So this idea could work but you need one of the components to have mechanically shifting threads so that you can change from one to the other thread ratios engaged.   

       I like the idea of a shaft with multiple threads in each direction, and a nut with a tiny gear lever protruding from one side.
pocmloc, Jan 02 2012

       There's a clumsier but simpler way to do this. Make the bolt and nut with coarse threads. Then cut another finer thread on the outside of the nut, and put a fine-threaded nut on that. In other words, two concentric nuts, with the finer inner nut turning around the coarser outer one.
MaxwellBuchanan, Jan 02 2012

       I think you could do this with the rod or the nut, maybe even both, but with a standard tap, it won't engage both at the same time. That means that if you start the nut one way, it won't reverse.   

       It's vaguely possible that you could machine a thread that would accept both and have a small enough contact point that you could reverse the thread. It would tend to unscrew itself at the same time except in very limited positions.   

       In addition it would be so ridiculously weak that any significant pressure on the screw would deform it or destroy the thread.
MechE, Jan 02 2012

       [bigsleep]: us Yanks call it a Yankee Screwdriver, I assume because it either is a product of or appeals to our eponymous ingenuity.
Alterother, Jan 02 2012

       [Mech]- yeah, I was thinking something very similar to a 2-groove pawl thread, with the pawl-nut having very narrow engagement points, or even two points per groove, but spaced at an integer multiple of the pitches of both threads to prevent the nut from tipping. Obviously the fine thread pitch would need to be 1/2, 1/3 or some other integer fraction of the coarse thread pitch, so that you get regular alignment points. Also reversing the turn would need to be done at one of the points where the grooves align.   

       This would be interesting to 3D model, just to see how much thread land material is left over after the requisite cuts are made.   

       This would be a fragile artifact, but could theoretically be done. It'd probably make a great assessment tool for advanced machinists, or even 3D designers.   

       If you actually want it to work, just use concentric threads as per [Max]'s anno.
Custardguts, Jan 04 2012

       I don't see how to make double threading usefull, but I've seen something that might fullfill this need. It looks like a regular threaded nut, but in addition to normal tapped threads, the hole is drilled out larger at an angle. When the threaded bolt is at the same angle as the larger driled hole, it slides freely. When the nut is twisted to be perpendicular to the bolt, the remaining threads engage and allow it to be tightened. Pressure against the washer keeps the nut from tipping sideways, so it stays tight.   

       Ahh, I found a link to something like this.
scad mientist, Jan 05 2012

       Come to think of it, I have actually used one of those before. Maybe it was in a machine shop for clamping a piece down.   

       Tapered threads are another quick fastener, but they only work to a mostly fixed position.
Ling, Jan 06 2012


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