New threaded rod driven arm

I have finally come up with a solution for an easy reliable coupler you can make out of plastic. The attached STL file is a 3/32 to 1/4 coupler. It is 1/2" long, and 3/4" diameter. It uses 6 #4 nuts that are forced fit into the slots around the hole. This is so that the set screw does not strip out the plastic. 6 #4 X 3/8" set screws go through these nuts and hold the shaft in place. They can be adjusted so that both shafts are perfectly concentric. I have tugged on the shafts, and the set screws hold them very tight. I could not pull them out of the coupler. This should work for all of the motors on the printer. I think it can be reprapped, so try it out. It can be adapted easily for other bore sizes, so if you want a different size, just tell me.
By the way, pics are attached.

The library gnomes thank you, galaxyman7.

It can be adapted easily for other bore sizes, so if you want a different size, just tell me.
Hmmm ... want to upload the cad file?

http://reprap.org/wiki/Easy_Reliable_Coupler#Files

Also, you may want to add RepRap-style trapped nuts.

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

What format should I upload it as?

What format should I upload it as?

Probably your favorite native cad file format, and perhaps .step for interoperability?
(You'll probably have to zip the cad file, to upload it.)

Also, start lurking in reprap-dev.

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

galaxyman7, folk are following it:

[reprap.org] :

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Talk:Easy Reliable Coupler
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Hi galaxyman7 Thank you for sharing. The Easy Reliable Coupler is a great idea. Please correct me if I’m wrong, I think it would work better if the 6 holes provided for the screws go through the 3 slots provided for the nuts. Tony

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

Ok here is an updated STL model, along with a .STEP file. I tried an IGS file but it was too big. By the way I am not very good at editing wikis, so I have no idea how to change the pictures and ect..

Hmm looks like a job for scad and a parametric design. So that it can be scaled to suit differing shaft and bolt sizes by just changing a small number of parameters.

Are'nt smart parts great.

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Necessity hopefully becomes the absentee parent of successfully invented children.

I think you might be able to simply edit the STL or STEP file by hand and change the size. It might be encrypted or something though, so who knows.
By the way, I made some motor holders for the GM3. I have attached the STL and STEP files.

I have updated the wiki page for the robotic arm. It now includes some of the part files and pictures.
It can be found here:
[reprap.org]

I have built almost all of the mechanical part of the arm. I have replaced the plastic parts on the arm with plywood, which makes the arm much more stiff. It now has both of the arm motors attached along with the threaded rod and nut holders. When I hook it up to 12V it runs great, and smoothly moves the arm. The last thing I need to do is tension the x-axis lead screw so that it doesn't wobble when it spins. I need to create some kind of thrust bearing assembly. This should reduce wobble and let the arm travel back and forth smoothly. Hopefully I can get around to uploading the rest of the part files along with a video and some pictures.

I just wanted say that I was looking for an open source Arm 3D printer design saw your link I think the developers should have tried this concept first its way more versatile & although complex on conceptual spec engineering much simpler to "DIY" truly democratizes technology. Keep up your good work man you have the next gen idea.

Sorry I haven't been on in a while, but anyways...
I am having trouble with the motors on the arm. The assembly kind of wobbles as it moves. I think it is because either:
1. The coupler isn't connecting the motor to the rod very well. The GM3 motor has a strange shaped shaft so it is hard to get concentric with the threaded rod.
2. The nut holder and/or motor holders aren't held to the pivot bolts well enough.
3. The motors are just crap?

Anyways, I am going to try making some more couplers, and then make some new holders for the motors and nuts so that they fit snugly onto the pivots. Hopefully this should get rid of the wobble. I should have a ton of free time soon, from Christmas to April.

It may be possible to "true" the GM3 coupler/shaft by running the motor against a rigid fixture using abrasives, or a heated cutting tool. This is assuming the pressures are low enough that this doesn't worsen the situation by wrecking the bushing/bearing.

If the plastic itself doesn't want to play ball, then the coupler itself might be made of a material which will. Polyester resin, or machinable wax come to mind. Going 4-jaw chuck with some set screws sounds like a true PITA. Finer the pitch, the easier it is to make fine adjustments of course.

Do you have a dial indicator or dial test indicator? It's always nice to have feedback.

Unfortunately I have no dial indicator. I just made my drill press into a make shift lathe, so I might be able to make some aluminum couplers. I was also thinking I might be able to drill out a coupling nut and use that. I have done that with my milling machine and it works quite well. I will work on this after finals are over in December, then I will have plenty of time. Meanwhile I will keep trying to think of easier solutions.

I am thinking since threaded rod is pretty slow, and the x axis needs to move faster, that I should convert over to belt drive. The arm can keep the lead screws, since the mechanism will go 3 times as fast as the motors are driving it.
Also, I have been thinking about GM3 motors, and I really don't want to deal with them anymore. Their shaft is strangely shaped, the speed isn't very constant, and they aren't rigid. I have also heard that they wear out really fast. I am thinking that small stepper motors would save me a lot of trouble with encoders and programming.

Has there been any more development on this Robot arm repstrap design? I thought it sounded interesting, and a robotic arm could be fairly portable and still give a relatively good build size (possibly in exchange for accuracy, but still).

how do I get 2 parallel threaded rods to be driven in tandem, with just a single turning spindle or wheel etc all help will be most appreciated ... cheers!

The most common answer you will get is with a timing belt. You could also use chains or gears. Theoretically you could use string or a cable but you would need a very large spindle to hold all the string you will need to have a predictable position.


Here is the QU-BD RPM. You can see that it uses one z-stepper but has 4 screws.