Motor ideas for the Prosthetic Hand project

[Zak Smolen]

So I have been looking around at geared DC motors and found this one: https://www.sparkfun.com/products/12285. I would probably purchase the final motors from somewhere cheaper, but they had more data here than many other sites. They are available from many sellers in many different gear ratios.

Anyway, so the idea I discussed on Tuesday was a lead screw of some sort. I have looked around a lot, but I can't really find any reasonable screws/nuts. The few I found are like $10-25 a piece.

Does anyone have any ideas of where I could find something like that? I'm looking for lead screws with a diameter of 3mm or 1/4", or something small like that that I could connect with a coupler to the 3mm output shaft of the motor. 

I will need 5 sections, around 3" long, along with nuts to travel and provide the linear motion. I should be able to cut down a large screw into smaller sections.

Thanks,

Zak

[Ian Baillie]

1/4-20 threaded rod and nuts? I've seen it done before on some stuff, and its cheap.

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From: Zak Smolen
Sent: ‎6/‎9/‎2017 1:52 PM
To: ashevil...@googlegroups.com
Subject: {Asheville Makers} Motor ideas for the Prosthetic Hand project

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[Avi Silverman]

Figure out your threaded rod and how fast you want to pull first, then you can calculate how fast your motor needs to spin.

On Jun 9, 2017 2:25 PM, "Ian Baillie" <mountain...@gmail.com> wrote:

1/4-20 threaded rod and nuts? I've seen it done before on some stuff, and its cheap.

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From: Zak Smolen
Sent: ‎6/‎9/‎2017 1:52 PM

To: ashevillemakers@googlegroups.com

Subject: {Asheville Makers} Motor ideas for the Prosthetic Hand project

So I have been looking around at geared DC motors and found this one: https://www.sparkfun.com/products/12285. I would probably purchase the final motors from somewhere cheaper, but they had more data here than many other sites. They are available from many sellers in many different gear ratios.

Anyway, so the idea I discussed on Tuesday was a lead screw of some sort. I have looked around a lot, but I can't really find any reasonable screws/nuts. The few I found are like $10-25 a piece.

Does anyone have any ideas of where I could find something like that? I'm looking for lead screws with a diameter of 3mm or 1/4", or something small like that that I could connect with a coupler to the 3mm output shaft of the motor. 

I will need 5 sections, around 3" long, along with nuts to travel and provide the linear motion. I should be able to cut down a large screw into smaller sections.

Thanks,

Zak

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[Zak Smolen]

Yeh, motor speed isn't an issue. There were a large variety of gear ratios, and I can always slow it down with some sort of PWM. I picked that specific gearing based on the torque and speed achieved (about 1.5 rev/sec) which I totally guessed would be somewhat appropriate. Switching out the motor wouldn't be hard, and I'll only order one set of parts initially while I troubleshoot.

I need actual lead screws and nuts though, not just a threaded rod and nut. The nut needs to travel freely when the screw spins, possibly assisted by some sort of track, so that it travels up and down the length of the screw. Usually they cut the threads in a specific profile to help achieve this, for example I have seen trapezoidal and circular, and the nut is slightly looser than a standard nut to fit that diameter screw.

I'm open to suggestions for different motors and such, but overall I need to fit 5 of these adjacent to each other in a space the size of a forearm... so the smaller things are, the better off I'll be fitting it all in there. Also, we have to think about weight, so the smaller the metal parts are, the less weight we add to the arm. I could consider trying to model and 3D print them...

[Zak Smolen]

Ah, so I picked up a 6/32" rod from ACE and the normal nut that fit it actually spun pretty loosely. It's worth a try, at least for a proof of concept.

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[Bob Sinclair]

Could you maybe couple to a lead screw like this? It's the head moving motor from a CD drive: 2" lead screw and the mechanism has the travelling part...

[Will Eccleston]

You might also consider running your threaded rod over a wire wheel.  The zinc plating is very imprecise, (lots of micro-bumps and lumps) and if you brush it, the nut action will immediately become much more smooth.

[Zak Smolen]

That's actually what I originally tried. I had steppers that were slightly more robust than cd ones but with both commercial drivers and my own homemade drivers they didn't have enough pull. They are made for a near weightless shuttle, not for pulling loads. Steppers with enough power were way too large or expensive unfortunately. 

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