Tag: robohand

Printable Prosthetics Fingers and OpenSCAD Design Tips

Solid finger tip for Cyborg Beast
Solid finger tip for Cyborg Beast

Above is my first attempt at designing a “solid” finger for the Cyborg Beast DIY printable prosthetic in OpenSCAD.1 The reason this is a “solid” finger is that I haven’t subtracted out any material to allow this partial finger to connect with anything else.

The problem with scaling (up or down) any design that requires fasteners and hardware is that when you do, the holes for the hardware are similarly scaled.  This leads to more post-printing work drilling holes to widen them or to find larger fasteners that won’t rattle around in too-large holes.

Thus, if the hardware consists of 3mm screws, the holes for the hardware should be 3mm no matter how much the parts are scaled up or down.  To make matters more interesting, not all holes in the model should be excepted from scaling.  The above finger tip has a plastic end that is supposed to fit into a mid-finger piece – and those parts should be scaled up or down according to the size of the overall hand.  Thus, some voids should be scaled2 and others not at all.3

I’m rather happy with how this finger has turned out so far.  It has most of what I understand to be the essential features of the Cyborg Beast fingertips, including little nubs along the finger pad to allow for gripping.  I intend to make this an option, in case a user would rather use something like Plasti-Dip to make grippy finger pads, rather than relying on printed plastic bumps.

However, converting a decent design into a parametric design requires a little more work.  The way I go about designing a parametric model is to first design one instance of the thing, in this case the finger tip.  My next step is to poke through the OpenSCAD code to locate those aspects parts that contribute to the models’ essential features – length of the finger tip, for instance.  Once I’ve found these bits, I then try to modify them so that I can insert different variables and arrive at sane variations on the model.

Wish me luck!4

Default Series Title
  1. If this is your first time tuning in, check out the prior posts in this series using the links at the bottom of this post [↩]
  2. Where parts meet [↩]
  3. Such as holes for hardware [↩]
  4. See, this is a post about finger tips and design tips!  Oh, man, I crack myself up! [↩]

Printable Prosthetics R&D Q&A FAQ: Part The Third – The Answering

This slideshow requires JavaScript.

Tuesday afternoon I had the good fortune to talk to Professor Jorge Zuniga of Creighton University regarding his insights on printable prosthetics, measurements of uneffected/effected hands, and various important design considerations.  Getting to talk to him really helped crystallize my understanding of the various measurements and the way in which the parts of the printable prosthetic1

  1. Design Ideals
    1. One of the design ideals of the Cyborg Beast prosthetic is to fashion a device that strives for symmetry with the unaffected hand.  Thus, all of the necessary measurements are taken from the unaffected extremity.  This serves two purposes.  First, it allows for the prosthetic to be similar in scale to the unaffected hand.  Secondly, the unaffected extremity tends to be, in most cases2 , slightly larger than the affected extremity.  The size difference may be due to the unaffected extremity being used more, and thus having more muscle mass, or due to the loss of muscle tone and muscle atrophy in the affected extremity.  Either way, a prosthetic designed using the measurements from the unaffected extremity should generally fit the affected extremity.  Since this particular prosthetic design uses velcro straps to fasten to the affected forearm, a prosthetic that is slightly too large can easily be adjusted to fit well by tightening the straps.
    2. Another design ideal is to create a core prosthetic design which works for the vast majority of persons.
  2. Critical Printable Components
    1. A rough sketch of the various parts of the Cyborg Beast prosthetic appear above as “Figure 2.”
    2. Palm.  This is the part that fits over the hand.
    3. Gauntlet.  This is the part that fits over the forearm, between the wrist and elbow.
    4. Four fingers, each comprised of two pieces.  The above simplified sketch only shows the fingers as a single piece.  Do not let my sophisticated drawings fool you.
    5. One thumb, comprised of two pieces.  Like the fingers, the thumb is comprised of two plastic pieces.
  3. Critical Measurements
    1. These measurements refer to the lines labeled in “Figure 1.”  All measurements relate to the unaffected extremity.
    2. F5.  This is the length of the forearm, from the interior of the elbow to the wrist.  While this could be measured along the side of the forearm, it very likely doesn’t matter.
    3. F2 (measured at 1/2 F5).  At a location along the forearm, half way long F5, the width of the forearm.
    4. H1.  This is the distance across the knuckles, from the pinky to the forefinger.
      1. When I lay my own hand flat on a table top, I perceive that an imaginary line drawn through my pinky and forefinger knuckles would end up being not exactly perpendicular to an imaginary line drawn from my elbow to my wrist.  More on this below.
      2. All of that is another way to say that I suspect H1 is not perpendicular to F5.
    5. W.  This is the width of the wrist.  Rather than being strictly measured from either side of the wrist, this measurement appears to best made using the endpoints of the H2 and H3 lines closest to the wrist.
    6. H2 and H3.  H2 is the length from the wrist to the pinky knuckle and H3 is the length from the wrist to the forefinger knuckle.
    7. All other measurements indicated might possibly be useful for refining the design, but they are primarily important for the Creighton University research study purposes.
  4. How Each Critical Measurement Informs Design
    1. F5.  Gauntlet length is not longer than 1/2 F5 and not shorter than 1/4 F5.
    2. F2.  Gauntlet forearm width is F2.
    3. W.  Gauntlet wrist width is W.  Theoretically, if the prosthetic’s palm is scaled up to accommodate the wrist width (W), the affected hand  should fit under and inside the prosthetic palm.
    4. H3 can be used to inform the relative lengths of the fingers to match the overall length of the unaffected hand.   This isn’t strictly required for a functional prosthetic.  As designed, the Cyborg Beast appears to use fingers of equal length.  However, the fingers could be scaled up or down along with the rest of the prosthetic hand.  Alternatively, and as will be discussed below, its possible that the fingers could be designed to be of different lengths.  Prosthetics for young children should contemplate fingers based upon slightly larger, 1-2cm, measurements.  The reason being that they quickly outgrow existing parts.
  5. Functional Design Considerations
    1. Thickness of parts is 3mm – 5mm, 20% fill.
    2. The wrist hinges should line up as exactly as possible with where the user’s wrist bends.  Additionally, the wrist hinge should be perpendicular to the line of the forearm/gauntlet.
    3. There should be about 1 – 2 mm of space between the hinge part on the palm and the hinge part on the gauntlet.  This allows a washer to be inserted for more fluid movement.
    4. Eliminate square corners when possible, as sharp edges can contribute to possbile injury.
  6. Cosmetic Design Considerations
    1. Using the unaffected hand for measurements also allows us to seek symmetry between the hands.
  7. Advanced Considerations
    1. Degree tilt to H1.  As mentioned above, it seems like the “H1” line is not perfectly perpendicular to an imaginary line drawn from my elbow to my wrist.  An educated guesstimate would be that there is a 9 degree tilt to this line.  While existing Cyborg Beast designs do not include this knuckle “tilt,” including this feature in future designs may allow the prosthetic to appear and function more naturally.  However, I don’t know if there’s any real ergonomic benefit to using incorporating this knuckle tilt.
    2. Different knuckle positions for fingers.  The Cyborg Beast has a knuckle “block” that positions the attachment points for all fingers in a straight line.  The reason for this is simple – it’s a lot easier to put one long screw through the entire knuckle block to secure and strengthen all four fingers at once.  At a recent e-NABLE meeting I had the chance to inspect a 3D printed prosthetic which used different knuckle positions for each finger.  Rather than all of the knuckles in a straight line, this model featured each knuckle at a different, and more natural seeming, position.  While this can appear more natural, I’m not sure there’s an ergonomic or aesthetic benefit.
    3. Different finger lengths.  Fingers are different lengths.  The Cyborg Beast, with all fingers having the same relative knuckle positions and same finger sizes, has a more mechanical look than might otherwise be possible.  I don’t know if there’s an ergonomic benefit to using different finger lengths, but this is certainly something to explore.

Based on the above, I think I’m ready to dive back into the OpenSCAD code and work out a parametric gauntlet, fingers, and thumb.  Stay tuned!

Default Series Title
  1. I’m basing my own designs off of his Cyborg Beast designs [↩]
  2. Let’s just choose the large and arbitrary percentage of 95% [↩]

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?
Default Series Title
  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 [↩]