Wrong Way Nut

by VeryWetPaint, published

Wrong Way Nut by VeryWetPaint Oct 20, 2011

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Wrong Way Nut by VeryWetPaint is licensed under the GNU - GPL license.

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Two nuts travel opposite directions on the same thread! As seen on YouTube! http://www.youtube.com/watch?v=o-fPsvqjqZI

Update 27 Oct 2011: I've uploaded a new trick nut triplescrew_backward_floor.stl which eliminates superfluous slack and reduces the overhangs to where they might be buildable on a Makerbot.

The YouTube video inspired Thingiverse user msruggles to design his own "Screwy Screw" thing. The STL files below are the originals, which were used to make the video, which inspired msruggles to make Screwy Screw, which inspired me to upload the STL files below...oh gosh, I'm completely lost!

Don't know how it works? Obviously there's a trick to it, but I encourage you to try to figure it out before you "spoil" the fun by downloading the files. Try designing your own!

More discussion on my blog at http://mysd300.blogspot.com/search/label/screw


All units are millimeters.

  1. Choose the small bolt triplescrew_bolt40.stl or the long bolt triplescrew_bolt60.stl. To help camouflage the trick thread choose a transparent build material for the bolt, if possible.

  2. For the nuts, build both triplescrew_backward_floor.stl and triplescrew_forward2.stl. I think dark colors work best for the nuts.

  3. Thread the nuts onto the bolt by turning one clockwise and the other counterclockwise, unless you live in a Commonwealth Nation in which the second nut must be turned anticlockwise.

Additional observations:
•All parts have been confirmed buildable in LOM (on a Solido SD300) and FDM (on a Dimension 768) but your mileage may vary.
•In addition to the obvious trickery, these threads have an unconventional sinusoidal profile. msruggles used a more conventional 'vee' profile for Screwy Screw. I'm not sure which will work better for hobby FDM printers.
•The forward and backward nuts have the same travel per revolution (ie: they move the same distance but in opposite directions) but the backward threads on bolt and nut have 3 starts each. The forward threads on the bolt have 1 start while the forward nut has 2 starts to enhance its resemblance to the backward nut.

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Fun print! No cleanup or sanding at all required. I followed your suggestion and used natural PLA for the bolt, it really hides the trick better!

There is generus play in the model,+/- 1 mm or so, but both nuts spins fine.
I printed the right way (larger thread) nut with 0.2 mm layers and the rest with 0.1 mm layers. I was rather surprised by the play, bbuonomos classical bolt and nut was super tight! (it's at http://www.thingiverse.com/thhttp://www.thingiverse.com/th
ing:4880 )

I scaled it all up to get 75 mm bolt heads (x1,536 to be exakt), that way my printed wrenches fit. You can find them on http://www.thingiverse.com/thing:28527http://www.thingiverse.com/thi...

Wrenches for Large Threaded Bolts
by Jaknil

Interestingly, when I printed it on my ToM the wrong-way nut was too loose. I reprinted it at 98% and now it works fine. I guess the right-way nut will also work at 98% but haven't tried that so the two nuts I printed are of slightly different size.

Yes, this is why I want Very Wet Paint to decrease the ID for a TOM-version of the wrong-way nut.

Additional study suggests the STL includes geometry the Thing-O-Matic can't build accurately, nor could any other FDM printer without separate support material.

Each slice generates 3 scallop-shaped points, always unsupported because the threads are twisted from the layer below. Although the extruder can trace the exact path, the filament will merely form a straight line between the two points where it touches the wall below. Blindly reducing the diamet
er won't have any effect unless the whole contour is scaled down, and it wouldn't be accurate even then.

The better solution is to devise a thread profile that can be faithfully reproduced by the Thing-O-Matic (and other FDM printers).

Actually, I'd like Scott to make the screw's threads the proper size so that all nut/bolt heads are the same size. Scaling will only make one or the other larger.

ATM, they are the same size, but the bolt's threads are way too loose.

Another observation: the reverse nut's threads don't seem to hold the bolt at all, but the forward nut's threads, albeit a bit loose, do turn and tighten on the bolt.

The parts built accurately in LOM and FDM on two properly-calibrated commercial machines (Solido SD300 and Dimension 768) and the nuts positively engage. So I suspect there's something else going on here.

At first I conjectured your FDM slicers might be using too large a tool-offset. FDM can be affected by tool-offset error while LOM is inherently immune, so it was an appealing theory...but that theory doesn't square with Wim de Groot's experience: triplescrew was too loose for him, but Screwy Scr
ew was too tight. Calibration would've affected both models the same, albeit at different scales.

I've examined the mesh with VisCAM View and it tells a story: on the screw the reverse thread tapers to 0.17mm thickness where the overhang is 68° with nothing above or below. On the nut, the critic
al mesh zone only begins where the thread tapers below 0.37mm. (See images below)

So the hobby FDM undoubtedly just loses enough of that unbuildable geometry that the reverse nut doesn't mesh. In hindsight, my LOM machine was operating right at its limit (0.168mm layers) as was the Dimension, whi
ch built threads down to a single FDM layer by using its separate support material.

I tried remaking the reverse nut at 0.2mm and it still didn't catch. I think a version with the threads set to a smaller ID would work for MakerBots.

It can't be as simple as just reducing thread diameter because the portion of the threads that the Makerbot CAN build would be the wrong size: part of the thread might mesh but the rest of it would clash. Moreover, I couldn't guess the right tweaks because the model currently builds well on my machine.

No doubt it could be adapted to Makerbot, or a Makerbot could be tuned to build the model. (Or both) But it's not going to be as trivial as just slicing the model with default Skeinforge settings. This model pushes commercial machines close to their limits and it's extremely sensitive to small
deviations: an error of just 40µm made a difference between nuts that drag and nuts that spin freely like the video.

I'd like to pose it as a challenge for someone to adapt the model or devise a build procedure for Makerbot, so I'm working on an OpenSCAD version to use as a starting point. So far
I've managed to create functionally-equivalent nuts, but the bolt is giving me a little trouble. OpenSCAD can't do helical extrusions, but theoretically the sinusoidal profile can be matched by twisting a circle around an off-center origin.

Is there a consensus about the resizing for a Thing-O-Matic? I don't want to print it without some more guidance.

I printed the screwy screw bolt and nuts and the nuts were too tight.

The nuts had to be scaled to 102/3 %

With these triplescrew bolt ans nuts it is the other way around.
I guess the nuts have to be reprinted on my Ultimaker at a scale of 97-98%.