Magic Screw for Makerbot

by VeryWetPaint, published

Magic Screw for Makerbot by VeryWetPaint Dec 6, 2011
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This is a 'kit' for designing or building magic screw threads that will (hopefully) build on a Thing-o-Matic and similar hobby printers. This should enable you to build a bolt and nuts so that one nut tightens normally while the other seems to turn the wrong way as shown in this video (this is the new model data):


The ancestor model did not build accurately on hobby-type 3D printers because (I think) the geometry had atypical overhangs that could foreseeably flatten out during extrusion. Instead of fighting those overhangs, this Magic Screw version tries to exploit them by building overhangs in straight lines between supported walls. It's just a conjecture (I don't know if I'm right) but it's illustrated in my blog at http://mysd300.blogspot.com/2011/12/magic-screw-for-makerbots.html


Try building the three STL files as-is and see if they'll work exactly like the video.

If the model needs tweaking, or if you'd like to optimize the appearance of the trick threads, download and edit bwbolt.scad using OpenSCAD. Follow the instructions (comments) for some simple adjustments, or revise the whole thread profile as desired.

Be sure to share a derivative if you get a model that works better or looks better!

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I easily rotated the bolt in Simplify3D Creator. I accidentally printed this out with supports. Luckily, it didn't produce too many blocks, and I was able to cleanly remove them. Great! Fits absolutely perfect. My only request would be that the bolt be longer so that there is more play area to work with.

Edit the SCAD file and make yourself a derivative! You could make the bolt as long as you like. While you're at it, try to improve its appearance or build-ability. I don't own a Makerbot, and my SD300 printer is immune to overhang issues, so I can't do any more tuning for Makerbot users.

Printed perfectly on my Tantillus RepRap printer. :)

My only mistake was slicing the bolt with 15% fill the first time. It looked great, but the thread shaft didn't have a strong enough bond to the bolt head so it broke off when I tightened one of the nuts a bit too much. Reslicing/printing with 50% fill for the bolt fixed the problem completely.

Working fine! Thanks.

I only had to change the Bolt's orientation using the provided scad file, as the head was on top.

Printed the bolt in Faberdashery Crystal Clear PLA - no problem at all and did not require any additional cooling (which I was expecting owing to the small layer surface area as the thread builds up). The forward nut - printed in Electric Blue equally as good and both bolt and nut worked exactly as they should - smooth yet tight when the nut got to the bolt head. However .... the backwards nut in Fire Engine Red has come out rather loose. I can wiggle it up and down the bolt with a little bit of twisting - which is a shame as this was shaping up to be a first time win thing. (When this nut gets to the bolt head it just swivels around - cannot tighten it). Not a clue what parameters to change otherwise I would have another go at printing the backward nut. I am using a RepRapPro Huxley by the way.

Tried printing with bwd_off at 1.2 and this time I couldn't get the backward nut on the bolt .... basically not a clue what I'm doing! Anyone else got some settings to share?

The larger thread nut was a bit tight and the smaller thread nut was a bit loose, but both work (in opposite directions) on the bolt. Printed on a Thing-O-Matic MK7 HBP 220C/110C with a .35mm layer height.

Correction to an earlier comment... I printed this on an Orca from PLA with 0.20 layer height and the results were... odd. The threads on the bolt look good, but what happened was that the backwards nut would go in forwards. with just a little fiddling around. Forwards nut would refuse to go in at all and when I printed it with increased tolerance (tol = 0.3 in SCAD file), it would go in backwards (using quite a bit of force...)! Spooky!

I cut open one of the forwards nuts and it looks like the overhangs when printing the threads had curled upwards, thus distorting the thread profile. That's one problem at least. Any measurements I could take to verify why the threads didn't work as printed?

Where did you get the clear plastic? Is it ABS or PLA?

It's PVC, the only material my SD300 uses. There's a small selection of colors in addition to the transparent material. My blog has plenty of pictures showing how I put the transparency to use in various ways at http://mysd300.blogspot.com/http://mysd300.blogspot.com/

It uses the LOM build process, which is totally different from FDM, which is why I can only guess about what went wrong for Thing-O-Matic users. In LOM each layer is completely unaffected by the layers below it, so overhangs aren't an issue...or even a definable concept.