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Two threads
Just like one thread, only there's two of them... | |
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.
Quick Release Nut
http://www.jwwinco..../section8/gn6303.1/
[scad mientist, Jan 05 2012]
[link]
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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? |
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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. |
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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. |
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[bigsleep] maybe you're right, but I thought about it this way: |
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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. |
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Like wise with a die on a rod. |
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Now, putting the double tapped hole together with the double threaded rod is where my brain goes out to lunch. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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In addition it would be so ridiculously weak that
any significant pressure on the screw would
deform it or destroy the thread. |
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[bigsleep]: us Yanks call it a Yankee Screwdriver, I assume
because it either is a product of or appeals to our
eponymous ingenuity. |
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[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. |
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This would be interesting to 3D model, just to see how much thread land material is left over after the requisite cuts are made. |
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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. |
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If you actually want it to work, just use concentric threads as per [Max]'s anno. |
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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. |
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Ahh, I found a link to something like this. |
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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. |
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Tapered threads are another quick fastener, but they only
work to a mostly fixed position. |
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