Measurements Required for DIY 3D Printed Hand Prosthetics

Cyborg Hand v7.0

Cyborg Hand v7.0

I’ve recently embarked upon a quest to create a parametric version of the e-NABLE prosthetic designs.  I’ve chosen the “Cyborg Beast” as it came highly recommended and I had the good fortune to meet one the main designers.

I have a habit of diving headfirst12 into a project I know absolutely nothing about and learning just enough to be dangerous as I go.3 Even if the results aren’t what would be called “successful” under normal circumstances, they do tend to be entertaining.

I generally get started by asking a ridiculous amount of questions.4 I have some guesses, but no concrete answers to the below.  If you know, I’d greatly appreciate any comments or replies.  Here’s a bunch to get us started:

  1. What are the minimum required measurements to create a suitable prosthetic, such as the Cyborg Beast?
    1. Knowing the minimum required measurements would allow a designer to better create a parametric design.
    2. The Cyborg Beast instructions refer the builder back to the measurement instructions for the Snap Together Robohand by Michael Curry aka Skimbal. These instructions indicate that all you need is the measurement of the width of the hand, where the hand is held flat with the fingers together, at the widest point on the knuckles.  Based upon the ratio between the subject’s knuckles and the stock Robohand knuckle block, all of the parts for the model are then scaled up or down.
    3. The ease of reference, the entire set of instructions for the Snap Together Robohand are as follows:
      1. Measure the length of the individual’s knuckles across the back of the hand

      2. from the index to pinky finger. (Example: 85mm)
      3. Add 5mm to your measurement to account for the thickness of the gauntlet.

        (Example: if the individual’s hand measures 85mm knuckle-to-knuckle, add 

        5mm for a total length of 90 mm).

      4. The knuckle block in the files you downloaded is 65mm. Divide your result by 

        65. (Example: 90/65 = 1.38).

      5. Multiply the answer times 100 to get a percentage. 

        (Example: 1.38 x100 = 138%).

      6. Scale all the parts of Robohand by this percentage before printing. This can b

        e done using the ‘Scale’ tool in Makerware.

    4. Are there any other measurements, besides the width of the hand at the knuckles, required to create a suitable custom prosthetic?
  2. How do these measurements inform a customized prosthetic design?
    1. Scaling all parts equally makes sense for a “snap together” design where all the parts, including the fasteners, are sized together.  When one is using stock parts (such as screws, elastic cord, and nylon cord)), this approach can end up requiring the builder to do a lot of post-printing work widening holes or trying to find wider screws.
    2. Other than scaling all parts equally, based upon knuckle measurements, is there any other modifications to the printable design required in order to create a useful prosthetic?
  3. How accurate do these measurements need to be in order to create a suitable prosthetic?
    1. Do the measurements need to be down to the micron?  Is within about 1mm or so good enough?
  4. For each required measurement, is it better to round it up or down?
    1. If the only required measurement is the width of the knuckles at the widest point, I suspect that it is probably better to round this figure up, rather than down.  I believe it would be much easier to add a little extra padding or tighten the velcro strapping a bit more.
  5. What are the important structural features of the Cyborg Beast?  As in, what parts, dimensions, and part relationships are absolutely critical to its proper function and fit?
    1. I’m very very weak in this area.  I just don’t know which parts are “load bearing” and are so critical to the function of the device that I should make special efforts to replicate them in my design.  Any suggestions here are greatly appreciated.
    2. I suspect that the critical functional features include part thickness (especially where separate parts meet – for strength and durability), the height and length of the “outcropping” on the back of the wrist which appears to provide the mechanical advantage which causes the fingers to constrict, and the tightening block on the gauntlet.
  6. What are the important design features of the Cyborg Beast?  As in, what parts, dimensions, and part relationships are critical to the suitability of this model over others?
    1. Again, I’m incredibly weak in this area.  I suspect that the overall organic shape to the model is one of its most stand-out features.  However, I would invite more informed comments and observations.
  7. What parts of the Cyborg Beast are the most improved?
  8. What parts of the Cyborg Beast are most in need of improvement?
  1. Almost willy-nilly, if you will. []
  2. You will, won’t you? []
  3. I imagine this is what it is like to learn to fly. []
  4. If you don’t believe me, feel free to peruse this site where you fill find literally thousands of words on the smallest design variations on the smallest parts for a drawing robot []

4 Responses to “Measurements Required for DIY 3D Printed Hand Prosthetics”

  1. Peregrine Hawthorne says:

    I haven’t ever used a Cyborg Best, but I can speak for the Talon and I think some if that carries over, as they have the same common ancestor.

    For measurements, you mostly need the width of the palm, width of the wrist, and the width of the forearm, as eek as the distances between those measurements. Small children have a much smaller difference between their wrists and forearms than an older individual, and the MechEn needs to taper and flare accordingly.

    Thus far, the primary load bearing portions of the hand has been the knuckles. Before we based them up, I would break then at an alarming rate.

    Rounding up us generally a good idea, particularly for a child that would grow into it, but also for an adult that wouldn’t be growing anymore. A looser fit is generally more comfortable, and guides more room for padding and customization. Note, I’m using a far more robust gauntlet than the Cyborg Beast is designed to take, comprised of thick leather and silicone padding, so that might throw off some of this, but I hope some of it could be helpful.

    If I missed anything, feel free to ask! I may have just not seen it on my little phone screen.

  2. David Orgeman says:

    I don’t have direct experience with that particular hand, but I do have a fully parametric design for the hand I made for my sister. (Also an OpenSCAD design.) I find the standard knuckle measure to not be close to sufficient for a good hand. It is one needed measure, but only one. My script makes a complete hand from a series of measurements – but you might be able to get by with less. I take knuckle width, distance from wrist hinge point to knuckle, wrist width, wrist thickness, distance up arm that the gauntlet should go, arm width at that point, arm thickness at that point. I also take measurements of each proximal and intermediate phalanx. You could maybe skip the finger measurements and use a general approximation. I think it is better with custom measures – and if you are going to go through the trouble of making a highly customized hand, why not go all the way – but your milage may vary. I find measures to 1mm are good enough, and I round up on everything. Although I measure the fingers, I would reduce each by a couple of mm. My reason, however, is because my hand is for a user with a fully working thumb. That thumb is slightly smaller than the one on her other hand. It would therefore make sense for the fingers to also be smaller. If I was going a hand with a thumb, I probably wouldn’t adjust those measurements.

  3. MakerBlock says:

    @David: Thank you so much for your input! Are your designs available anywhere for download? Github or Thingiverse, perhaps?

  4. David Orgeman says:

    I put version 1 on the eNable share. I would not use the full version 1 though. I have parametric fingers, gauntlet, tuners, and a version 1 knuckle block on Thingiverse. Look for stuff by orgemd and you will find it. In about a week I will put out the full Second Degree hand. That will be a much more polished version and much better OpenSCAD scripts. I just want to wrap up a build to make sure it all works. That is almost done. The gauntlet just finished printing this morning. My current version, however, does not include a thumb – so the full hand is only useful for those with a functional thumb. It still has good elements that other designers could use, and some of the parts like fingers or gauntlet could be used as is. I intend to add a thumb after I am satisfied with what I have provided my sister. It should not be too hard to add it – just not there right now. When others have needed a thumbless design, I have made my in-progress files available. If you are OK with waiting about a week, they should be public by then. If you have a more pressing need, email me and maybe I can get you them a bit early.