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Arm v2

by masnart39, published

Arm v2 by masnart39 Nov 16, 2015
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Arm v2 by masnart39 is licensed under the Creative Commons - Attribution license.

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These STL files are parts that I modified based on the original Enable RIT prosthetic arm. The RIT arm is being phased out of use, and these files are intended for interim use while new designs are being tested and approved. They are intended for those who are already skilled in the fitment and use of the original RIT arm. This design is not endorsed or approved by the Enable organization.

This version has a more sleek design that is less bulky and weighs just 303 grams completely assembled and ready for use. It incorporates a whippletree adaptive grip mechanism and a palm utilizing knuckle curvature and thumb relocation to further enhance gripping capabilities. The elastics are separated into knuckle and finger sections for adjustable finger closure rates.

Use of these items require careful implementation of sizing methods that may require consultation with a medical professional. By downloading these files, you assume all risk and responsibility for proper fit and use.

There is a PDF in the Thing files section listing my print settings and some assembly instructions.

The native files are available in SolidWorks 2013 format here: http://www.thingiverse.com/thing:1156821
These files are available in STEP format here: http://www.thingiverse.com/thing:1156799

Print Settings

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All files should be printed in an upright position, except the proxes, fingers, and pins. The only part that requires support is the palm - under the thumb. Most large parts printed upright will benefit from the use of a 10 perimeter brim with no offset or a raft. This depends completely on your experience with your printer and slicing software. Soon I will link to a document with all of my settings and instructions.

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When I download the files as STL, when I try opening them, a message pops up telling me that the file is invalid because of the Certificate Trust List. Do you know what it's about?

Thanks in advance.

I'm honestly not sure. The files have been on here for 2 years, with almost 5000 downloads, and no one has mentioned any issues. What program are you trying to open them with?

Hi All, my son is a high-level quadriplegic, meaning no control of his wrists or hands. Has anyone ever combined the wrist driven models that accept hands with this elbow driven model? A product like that would give 100,000's of quads a way to bring back grip that did not involve risky expensive and often unsuccessful surgery. (We are actively trying to decide which of these options to pursue for him and then I cam across this site.) I'll keep this short and happy to answer any questions. He CAN bend his elbows (as most high-level quads can). GREATLY appreciate anyone's help, advice, pointing me in the right direction, whatever comes to your mind.

Thanks - Dave Sternberg (jessesrecoverydotcom if you want to see who you'd be helping)

I would point you toward E-Nable--a volunteer network of printers, kitters, and helpers that I am involved with. The goal is for those with access to 3D printing the print prosthetics like this print for candidates who could use them. This is entirely volunteer base and not doctor/PT based.
If you have 3D printing access, you can use this guide to choosing a 3D printed design: http://enablingthefuture.org/which-design/
There's also a matching program through E-Nable http://enablingthefuture.org/enablewebcentral/ (this is where I am involved--being matched to print for people's requests)
Hope that helps.

I have completed the arm. It printed great, although I probably should have scaled it up a bit so that it will serve the child a little longer, but he should be getting a professional prosthetic in a couple of years, so this should serve him well in the meantime. One question, I am getting, is what are you (or anyone else) lining the forearm cup with? The plastic can tend to be pretty rough over time, so I am trying to figure out how to best line it. I have some silicone sheets that I can epoxy in, but am having a hard time getting them to nicely fit without a lot of seams.

I have an idea to pour some liquid rubber/plastic (Plasti-dip) in and swirl it around to coat the interior but am not sure of its durability as far as sticking to the plastic.

The recipients I have worked with have put a sock over their arm. I originally started out making the cup interior oversize to allow for a silicone mold of the child's arm to be inserted. During the very first fitting I was involved with, we just pulled a sock out of the drawer, and put it on. It worked so well, that I've just continued with that method. It can be washed frequently as well, unlike glued in padding. There are professional grade prosthetic socks available if that's preferable.

Ok. That makes sense. I tend to over-complicate things, haha. Thanks for the insight.

Hi! I am trying to make this arm but struggling to figure out what and how the tops of the fingers are strung. If you could help out that would be amazing!!!

There's a good explanation about stringing the actuator lines in a comment below. The elastics can be done two different ways.
One method uses two different strengths of elastic for a more realistic grasp. A light elastic at the fingertips, and a heavier elastic at the palm joints. This allows the fingertips to start to close ahead of the proxes. This is a more difficult method and is shown in the yellow palm in the photo link below.
The second method uses a single weight of elastic that is strung from the tip of one finger, looped through the palm, and through the next finger. This is simpler, but causes the fingertips to close last when gripping, and is shown in the purple/pink palm in the photos linked below.

There are photos in this link showing various stages of assembly, and the elastic routing for both methods.

I was wondering where the info was for the sizing of the parts and what measurements are needed? Thanks!


I'm building this arm for a kid here in Brazil, congrats and thank you for the great design!

I want to make this as durable as possible, and would rather use screws instead of 3d printed pins, mainly on the knuckle and thumb base joints, although I realized this designs is not compatible due to the curvature.

But I did some research, and found these pictures:


Where could I find this palm design?
Thank you!

I do believe that I still have that palm, but I will need to see if it matches up to the forearm connector. It will also need the original fingers as well.

I'm not sure that there is really cause for concern. There are a lot of arms and hands built using printed pins over the last couple of years. Enable has provided a few thousand hands, and I've not heard of any breakage issues.

It would be great if you still had the files, if not, its reassuring to know that the 3d printed pins work fine! Thank you!

This is excellent, thank you Nick!

One question though--I've printed out all the parts and when I put them together the thumb doesn't seem to come back very far, only to about 90 degress from the palm. Is this normal? From the pictures, it looks like yours comes back considerably further. Let me know if there's a better way to contact you with questions during the build!


Do you have assembly instructions available? Thanks!

Sorry, I don't have anything at the moment, but here is a description of how to run the activation strings:

I use 50 lb. fishing line for the finger actuation, and crafting crimps to secure the line. You can certainly use any of the other common string/line that others use.

I start by feeding the line from the tip of the ring finger all the way through the finger to the inside of the palm and out the opening where the forearm connector attaches, pulling an additional 12-18 inches of free line. Then go back to the finger tip and cut the line, leaving enough extra to either tie the line off, or squeeze a crafting crimp on the end of the line.

Repeat this on the middle finger.

Now take a piece of line that is 12" longer than needed to reach from the palm to the tensioner on the tricep cuff, and wrap it around the single post of the whippletree. Either tie it off or crimp it in place. This will be the pull side of the whippletree.

Now take the two lines that are routed through the middle and ring fingers, and wrap the ring finger line around one post of the two-post side of the whippletree, and the middle finger line around the other.

Next feed the line from the ring finger into the hole on the inside of the palm that leads to the pinky. Thread that all the way through the pinky, but leave slack in the line on the palm side for now.

Do the same with the line from the middle finger, but feed it into the hole inside the palm that leads to the index finger. Feed it all the way through the index finger.

Now you can pull the line feeding the ring and middle fingers out of the finger tips, which in turn pulls the whippletree to the front of the interior of the palm. Now tie off or crimp the lines at the fingertips.

You should be able to pull the single line that goes through the arm, and it will make the fingers work. I would suggest you try picking up or squeezing something to test the strength of your knots or crimps before assembling the rest of the arm.

Take an extra long piece of line and feed it through the tip of the thumb all the way through to the palm, and out of the opening where the forearm connector attaches. This should be long enough to reach 12" beyond the tensioner on the forearm cuff.

Now you can feed the two actuation lines, one for the whippletree, and one for the thumb through the palm to forearm connector. The line for the thumb should feed into the small hole at the bottom, and the line for the whippletree should feed through the small hole in the top. These lines will merge into one hole coming out of the other end.

Assemble the arm, feeding these two lines through the main hole of each part as you go. Tie or crimp the lines to the tensioners, and make fine adjustments by turning the screws. I would not cut the lines on the tensioner end until you are sure everything is adjusted and working properly.

There are some photos here that may be of some help:


My 3D printing class is printing and assembling this for a project. It will be given to a child in our school district. Thank you for the design!

You're very welcome! I would be interested in any feedback you may be able to provide regarding functionality, fitment, print-ability, etc.

Where would I find the assembly instructions? Or just use the ones at the Enable site? Thanks!

I'd love to get the native CAD files if they are available.

I've uploaded the SolidWorks version of these files here: http://www.thingiverse.com/thing:1156821
I've also uploaded STEP versions of these files here: http://www.thingiverse.com/thing:1156799
The STEP version has an extra right and left forearm with no internal cup so that you can make the internal shape to match your recipient.

Arm v2 - SolidWorks format
Arm v2 - step format