Category Archives: DIY Prosthetics

Printable Prosthetics: Brainstorming

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An illustrative hand

An illustrative hand

My overall designs for a printable parametric hand are still far from done.  And, yet, I’ve come a long long way since jumping headfirst into the realm of open source prosthetics a little more than 30 days ago.12

Forgive the digression, dear reader, before I return you to considerations of prosthetics. After visiting the Asian Art Museum today with the family, I am feeling particularly inspired to discuss dualities.3 I find I am an often-inspired person. This is a very charitable way to describe myself by what would otherwise colloquially and clinically be considered ADHD. When I am inspired by a new topic, I tend to jump right into it – reading voraciously and trying to learn as much about it as I can. When this happens, I also tend to set aside whatever thing I was most recently working on. This means that recently I’ve done little work on drawing robots (big and small) and a multitude of other small projects that would otherwise be just amazing. However, such inspirations/distractions are not only external to a project – but can also be very much internal to a project. Consider, for instance, feature creep – the adding of ever more features to a project, usually at a faster rate than which features are resolved and refined. In order to combat this aspect of my nature, the wanting to add more and better features, I have developed a coping mechanism. To prevent myself from falling down the rabbit hole of features and improvements, I jot them down someplace – either in a blog post4 , in an email to myself, or in a notebook.5 I find that once I’ve externalized and memorialized an idea, I can continue working on a project unfettered and undistracted these other ideas.

To this end, and in the spirit of open source ideals, I will jot down some ideas while I have them:

  • How large and how small are prosthetic designs typically scaled?
    • I wanted to have a range of sizes for which my designs were optimized.  My guess would be no smaller than 85% and not much more than 160% of the size of the existing  Cyborg Beast.  Jorge Zuniga was, again, patient enough to discuss this with me.  His estimate of a range would be between about 105 – 150%.
  • What is the diameter of the Chicago Screws typically used in the creation of a Cyborg Beast?
    • From the retailer’s website, it appears the “barrel” diameter is 0.2 inches, or 5.08mm.  I’ll need to make some adjustments to holes for the Chicago Screws in the designs.
  • How important is hyperextension of these fingers?
    • The designs of the Cyborg Beast include fingers that can bend “backwards” very slightly.  Each finger joint includes a “stop” at the back of the joint.  While certainly useful, I question their necessity.  I previously designed a connection system for printable snap-fit parts ((For use in an equally noble project)) that connect very tightly and/or bend with a user specified degree of movement.  The point with me mentioning these parts is that the “stop” used at the back of each knuckle and joint in the Cyborg Beast may not be necessary at all.
  • How necessary are metal Chicago Screws to strength and durability of the hand and fingers?  
    • Before you laugh, consider this question – what is the weakest point of any given finger which uses a metal Chicago Screw when having to deal with lateral forces?  I would postulate the weakest points would be those thin plastic parts surrounding the Chicago Screws themselves.  Thus, even though the hand incorporates metal pins, I have to wonder just how much strength they are providing to the overall device.  It would be easy to conceive of a plastic prosthetic hand that was so small that there wasn’t a lot of plastic around each metal screw6.  In such a case, the weakest points would be plastic surrounding the metal parts.  Extending this conjecture, of what use are metal fasteners to a design that is primarily plastic?  The best guess I can offer is that they allow reliable and smooth operation.
  • Work on proportional fingers
    • In designing the fingers, I worked to be able to make them customizable in several different ways.  The user may specify whether the fingers have the “star grip” pads, whether the finger should be slightly shorter or longer, and scale the finger up or down – without distortion to the hardware and cord channels.
    • I need to add at least three additional options to these parametric designs.  The designs should include the option to add “mouse ears” and easily removable support structures.  Additionally, the design should also allow the user to change the diameter of the finger.  I did implement this, somewhat, in part of the design.  Without implementation throughout the entire design, these partial attempts aren’t helpful.
    • In creating the fingers shown above, I adjusted their lengths to conform to the measurements of my own hand.7 Next time, I think I would also measure finger diameters.
    • I think I should create a way to prevent finger parts from being mixed up accidentally while printing.  A possible solution is to include “mouse ears” with each finger – but embed an identifying mark in each mouse ear to label the parts.
  • Ideas on making a better parametric palm
    • The palm should be redesigned so that the fingers, at the appropriate lengths, would fit into it.  I designed the fingers quite a while since working on the palm.  I haven’t had a chance to ensure the parts would mesh well without adjustment to the scale.
    • On an entirely different note, I have an idea to redesign the entire palm.  By carefully placing deformed spheres, I was able to design a palm.  Using a similar process, I subtracted out a void for the user’s hand.  The result is a palm with an uneven thickness throughout.  Uniform thickness isn’t necessarily an interesting or useful goal.  That said, it could lead to a reduction in unnecessary plastic.  If I were to redesign the palm, I could design the internal area first8 – and then use the “Minkowski” function to create a uniformly thick shell around the internal form.  The bottom would have to be sliced off and the original internal area would need to be subtracted from it.
  • Ideas on making a more realistic hand
    • My designs so far are based primarily on the Cyborg Beast, with some minor changes.  The “Flexy-Hand” appears to be very organic and realistic.  It also features flexible printed connections between each finger segment.  Additionally, each finger is comprised of three segments – rather than two like the Cyborg Beast.  Interestingly, since the flexible connections between segments allows the hand to return to an “open” position, the hand only requires five tension cords – rather than five tension cords and five elastic cords.  The fingers appear to not have any “stops” behind each joint.  I have to wonder how having three segments to each finger impacts the function.  Does it allow the hand to better grip things?  Does it make the hand less sturdy?
  • Masculine/Feminine hands
    • One well-intentioned comment to my latest designs is that they are “pretty.”9 While I accept the compliment with the spirit in which it was given, it immediately made me wonder – is the hand I designed “feminine?”  Then it occurred to me that with more design effort, I could make “feminine” and “masculine” version of these hands.  I think the primary differences would be two-fold – thinner fingers and a less “hefty” palm for a more feminine version and a thicker and perhaps more “blocky” palm for a more masculine hand.
  • New developments
    • There have been a number of interesting and new developments and experiments of late.10 In no particular order, these ideas are:
  • Discussions with a 7-year-old
    • A few days ago my daughter and I were jotting down some ideas in my sketchbook.  As we did so, she saw some of the notes from the e-NABLE meeting on 3/21/2014 – including several sketches.  We discussed the problem – affordable, customized, and comfortable prosthetics.  We talked about amniotic band syndrome, how fibrous amniotic bands affect fetuses, and the different ways in which these bands can cause11 deformities to single fingers, whole hands, and a range of changes in between.  I explained how Mr. Jose Delgado Jr. had a $42,000.00 myoelectric prosthetic, the problems he has with that prosthetic, how and why he prefers his $50.00 printed replacement, and how for the price of his one prosthetic people could make 840 more prosthetics.12 She asked, “Why can’t someone use a stump to operate a hand?” I replied that this was exactly how these prosthetics worked – and I drew a few simplified sketches of the Cyborg Beast.  Her next question was, “Why can’t it move side to side?”  I said that Mr. David Ogreman had designed such a prosthetic.
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  1. My first concrete step was going to an e-NABLE meeting in San Francisco on 3/21/2014. [↩]
  2. The above picture is slightly misleading.  I haven’t confirmed that the fingers I’ve designed will properly fit into the palm that I’ve designed – or that the thumb would work at all.  Thus, the picture is partially a parlor trick and partially an indication of where I hope to take this design. [↩]
  3. Many of the gods and goddesses in Eastern religions embody dual natures – creation/destruction, life/death, etc [↩]
  4. In one of my several different blogs. Besides, what could be more ADHD than having 3+ blogs?!? [↩]
  5. You may not find this as amusing as I do – but I probably have about four different sketch/notebooks. [↩]
  6. Say, only 1mm [↩]
  7. From pinky finger to thumb, the non-scientific measurements from knuckle to finger/thumb tip were 77mm, 102mm, 106mm, 92mm, and 72mm [↩]
  8. Using the deformed spheres and hull trick [↩]
  9. Thanks Erik! [↩]
  10. I don’t even know why I’m saying “of late” when I’ve really only been involved a little over 30 days.  I guess becuase these developments are new to me? [↩]
  11. Please forgive my lack of a more politically correct term.  If you’ve got a better or more sensitive phrase, please let me know as I will gladly adopt it [↩]
  12. She wanted to know if he could get a refund! [↩]

Printable Prosthetics: A Mock Hand

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More progress on a fully parametric prosthetic design

More progress on a fully parametric prosthetic design

Here you can see the fingers placed appropriately with the palm. I’ve used the same “finger” designs to add a thumb.

For right now, this is just to demonstrate the progress so far.  Theoretically, the only thing left to do is crank out an appropriate gauntlet to bring the entire design together.  In reality, there’s still a fair amount of work to do.  The design for the prosthetic palms was… not elegant.  Also, I want to create a separate (but very similar) design for the thumb.1 It is possible that if I improved the finger designs, I might be able to get away without designing a separate thumb.

In the meantime, I think it looks good and would probably be functional as-is.

Onwards and upwards!

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  1. I feel like the thumb should be stubbier, so I’ll go back and adjust the designs accordingly. [↩]

Printable Parametric Prosthetics: Design Features

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Parametric fingers - different lengths, same scale, with no distortion to hardware

Parametric fingers – different lengths, same scale, with no distortion to hardware

I’m not ashamed to admit it – I’m proud of these parametric designs like few of my other designs.  I’ve worked to make this design as customizable and organic as possible.  The two modules that define each finger can be customized in two important ways – they can be lengthened1 as well as scaled up or down – without any distortion or change in the size of the holes for the hardware, elastic cords, or tension cords.

Parametric fingers - with grippy bits

Parametric fingers – with grippy bits

Being able to lengthen2 the finger segments is important because it allows the user to create fingers of different lengths, as normal fingers are of different lengths, all without having to actually scale the fingers to different sizes and without causing a change in each finger’s diameter.

As I’ve discussed in earlier posts, being able to scale the parts up and down without distortion to the hardware holes is important because it allows users to use standard hardware throughout different designs.

For now, it’s back to work on the parametric palm to ensure a proper fit with these parts.

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  1. Or shortened [↩]
  2. Or shorten [↩]

Printable Parametric Prosthetics and OpenSCAD: Parametric Optimization

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Picking parametric fingers

Picking parametric fingers

This post is intended as a set of “guidelines” to creating a parametric design in OpenSCAD.

Last Sunday afternoon was spent working out a parametric design for printable prosthetic fingers.  Using the OpenSCAD function “hull” it’s relatively easy to crank out a nifty organic appearing design.  Admittedly, you have to have a working knowledge the basic union/difference/intersection function first.  However, once you do it’s really quite easy.

The feature of the design I’m most proud of is the “nail” part of the finger tip.  I designed the “nail” by using the OpenSCAD function “intersection()” on two cylinders.  The little “nubs”1 consist of a small cube, rotated so a corner is pointed straight up combined, with an identically situated cube rotated slightly.

When I’m designing something to be parametric, I usually don’t really start out designing it that way.  I first strive to create a form in OpenSCAD that resembles closely the thing I wish to design.  Then, I poke through the design code looking for those elements that are related to the design aspects I’m interested in changing based on parameters.  Once located, I replace those parts of the design code with variables that can be specified when the module is called.  I realize this is kind of a “high level” description of my design process for parametric things, but it’s still the best description.

Since last Sunday I’ve really done a lot with the design.  Some simplifying and a lot of improvements.  In the next post I’ll go over these features.  I’m really excited to show these off.  :)

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  1. Meant to give the finger grip and texture [↩]

Printable Prosthetics Fingers and OpenSCAD Design Tips

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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

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  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

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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!

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  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% [↩]

Printable Prosthetics R&D Q&A FAQ: Part 2 – The Wondering

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In order for me to design an OpenSCAD parametric model that can be adjusted to work for more people, I need to get a better handle on the necessary measurements and how they effect the final design.  Below is my understanding of the necessary measurements and how those measurements necessitate changes in the final prosthetic.

  1. What are the design ideals, besides functionality?
    1. While answering a different question, Marc Petrykowski suggests, “My goal as the designer and printer is to make the hand as near perfect as the other hand so it feels the same to the body and brain, thus they will respond with the effected hand like it was their real non effected hand. Also as stated above, the degrees of flexion and extension and the size/length of the fingers are all incorporated into the final design before the printing the hand.”
    2. Thus, all other things being equal, he tries to craft a hand that is as similar as possible to the non-effected hand.
  2. What are the main parts of the Cyborg Beast?
    1. I’ve drawn a picture with the main features of the Cyborg Beast printable prosthetic.  There are really just a few parts – the palm, the “gauntlet,” fingers, and thumb.  The gauntlet fits over and is secured to the forearm and is connected to the palm by two hinges.  The palm goes over the user’s effected hand and is connected to the fingers and thumb.
  3. What are the necessary measurements?
    1. Marc Petrykowski has provided a set of photos to demonstrate the various measurements.  They appear to all be in millimeters.  Please forgive my layman’s description of these various measurements.  Measurements are taken of the effected and non-effected sides so that a prosthetic can be made that will fit the effected side, but have similar characteristics to the non-effected side.
    2. Flexion angle.  This would be the maximum angle of movement from holding your hand out and then bending the hand at the wrist towards the inside of the wrist.  An example is pictured above as “Figure 1.”
    3. Extension angle.  This would be the maximum angle of movement from holding your hand out and then bending the hand at the wrist away from the inside of the wrist.  An example is pictured above as “Figure 2.”
    4. Knuckle width.  This is the width of the hand at the knuckles.  In Figure 3, you’ll see this as “H1” and “h1.”
    5. Wrist width.  This is the width of the hand at the wrist.  In Figure 3, you’ll see this as “W” and “w.”
    6. Hand measurements.  I’ve identified these as “H1 – H3” and “h1 – h4” in Figure 3 above.
    7. Forearm width measurements.  I’ve identified these as “F1 – F3” and “f1 – f4” in Figure 3 above.
  4. How does each measurement inform the design?
    1. Again, this is merely my guess, impression, or understanding of how each measurement results in a design change.  For the purposes of these diagrams, I’ve assigned each measurement a letter or letter/number combination.  When applicable, I’ve differentiated between the effected (lower case) and non-effected (upper case) hands.
    2. Hand Measurements (Figure 1,blue and green)
      1. Knuckle width, non-effected hand, “H1”.  This is necessary to creating a prosthetic of the size that will match the non-effected hand.
      2. Knuckle width, effected hand, “h1”.  This is necessary to creating a prosthetic of the size that will fit the effected hand inside the palm.
      3. Wrist to pinky knuckle, “H2” and “h2,” the purpose of which is to ensure a prosthetic that will fit the effected hand inside the palm.
      4. Wrist to middle finger tip, “H3” is the overall length of the uneffected hand.  The purpose of this is to create a prosthetic of roughly the same size as the uneffected hand.
      5. Wrist to index finger knuckle, “h3” is for making sure the prosthetic palm will fit around the effected hand.
      6. Wrist to middle3 finger, “h4” is for making sure the effected hand will fit inside the prosthetic palm.
    3. Wrist Measurements (Figure 1, orange)
      1. Wrist width, “W” for the non-effected hand and “w” for the effected hand.  The purpose of the effected hand measurement is to ensure a good fit between the prosthetic palm and the effected hand and the purpose of the non-effected hand measurement is to allow the prosthetic palm to match the non-effected hand more closely.
    4. Forearm Measurements (Figures 1, purple and red)
      1. Various measurements from “F1” (and “f1) just below the wrist to “F4” (and “f4”) which is the width of the elbow. As best as I can tell, these measurements are to ensure a good fit of the “gauntlet” on the effected forearm.
      2. Elbow to wrist, “F5” on the uneffected arm and “f5” on the effected arm.  I’m not sure what the purpose of this measurement is, but perhaps it is to ensure the effected arm with prosthetic is roughly the same length as the unaffected arm.
    5. Angle Measurements (Figures 2, 3)
      1. Somehow the flexion and extension are incorporated into the design.  I do not know how these settings inform the design.
  5. How accurate do these measurements need to be?
    1. Within 1mm, rounded up would be best.  Thanks to Peregrine Hawthrone and David Orgeman for the input.
  6. Questions begetting questions
    1. If you’ve ever made one of these prosthetics, please let me know if there’s anything I’ve gotten wrong.
    2. It appears the measurements effect the design as follows:
      1. Measurements “h1, h2, h3, h4 and w” dictate the size of the palm.  The ratio of the increase/decrease is then applied to all the finger bits.  The measurement “H3” is used to adjust the size of the palm and fingers on the effected arm.
      2. Measurements “f1, f2, and w” dictate the size of the gauntlet.
      3. The additional measurements on the corresponding uneffected arm could be used to make the prosthetic over the effected arm appear more like the uneffected arm.
      4. I’m guessing the other unused measurements (“f3, f4, f5”) are used as part of the Creighton University research study, to measure the physical changes in the extremities before, during, and after use of these prosthetics.
    3. How does the flexion and extension change the design?
    4. Have you printed the Cyborg Beast designs I’ve uploaded?  What are your thoughts?

Thanks for reading and helping!  Comments appreciated!

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Printable Prosthetics R&D Q&A FAQ: Part 1 – The Questioning

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Top down view, extension view, and flexion view

Top down view, extension view, and flexion view

Marc Petrykowski of Creighton University was kind enough to provide some additional practical experience and information about the university’s research study into printable prosthetics.  For anyone looking to dive into this project, I’m cross-posting the information from the e-NABLE Google Plus group here.1 I’ve adjusted the formatting slightly, but otherwise everything below are Marc’s words.

  1. What exact measurements do you need (e.g., hand-length from where to where?) 
    1. Below are photos of all of the measurements I use for designing a hand.2 Yes it does seem like a lot, but all of them are needed to ensure the best custom fit for the hand we make for the user. When making a custom hand, it is important to make the 3D printed hand as similar to the non effect hand (fingers, width, length, etc). Each hand also has to be custom because of the size (length, width, height) or the stump. Some are very tiny and some are much bigger, so that also plays a big role when you have to design a hand. There are two photos that are measuring angle of flexion and extension. Those are important to see how tight or how loose the hand has to be for the power and strength of the individual and to make the hand as functional as possible.
  2. How do you get them from scans etc.
    1. Scans from our 3D scanner are in the format .STL which can be imported into programs such as blender (Shown below). Then I can lay it into the preexisting hand design and see an image of how it will fit, including the gauntlet size. If there are further changes to be made, I can do it all in blender before the print.
  3. How do you apply those measurements to your model
    1. As stated above, the measurements matter for the size of the hand. You can’t have a hand that is much smaller then the opposite hand, but you also can’t have a hand that is too small or large for the stump. Everything has to be customize depending on each case. This is where the designing takes the longest. My goal as the designer and printer is to make the hand as near perfect as the other hand so it feels the same to the body and brain, thus they will respond with the effected hand like it was their real non effected hand. Also as stated above, the degrees of flexion and extension and the size/length of the fingers are all incorporated into the final design before the printing the hand.
    2. And if, as +Jorge Zuniga suggests, ALL parts can be pre-printed, I’m hoping you guys will take the lead in helping us make it easy.  (As easy as buying shoes at a shoe store)
    3. This is possible because all of the redesigning and redoing of the measurements are all done in blender. Remember, if you resize a finger to a certain percentage, then you have to do the same for the rest of the fingers, thumb, phalanges, palm, and the gauntlet. That is how you can print everything off as one complete print.
  4. What are the pre-printed unit descriptors and dimensions small medium large XL? Narrow/wide?
    1. Pre Print units are based off of the measurements and how you converted them in blender. Instead of having small, medium, large, XL, etc. I have converted that into being resizing percentages. I use makerware since we use makerbots (2x and 2) so for a hand such as below picture measurements, I would classify that as a small or (110-130%), an extra small would be closer to 100-110% (which is super small like a 4 year old or so), a medium hand will be around 130-150%, a large will be around 150-170%, and an extra large is around 170%-190%+. Again, the percentage matters from the sizes that correspond the non effected hand as you want to make it as close to the other hand as possible.
  5. Does a Medium finger always go with a Medium hand?  If not, what’s the deal?
    1. Yes, whenever you print a certain size of one finger, you do that same size for the rest of the print. For example, if I printed a palm at 110%, I would have to make the fingers, phalanges, thumb, thumb phalanger, and the gauntlet all at 110%.
  6. Where are  the models for printing S M L XL hands or fingers, etc..
    1. The problem is that there are no models. Each hand is supposed to be custom depending on each case scenario.

I have several follow up questions, but I’ll leave those for the next post.

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  1. The Google Plus group is private and you have to request an invitation, freely given, to be included.  While discussing discrete issues is easy enough, without the ability to quote original text, a detailed multi-issue open design discussion is very difficult. [↩]
  2. Note:  I’ve removed the photographs.  I am not sure I have permission to post these pictures publicly. [↩]

OpenSCAD Intermediates: How to Make Complex Organic Shapes

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Cyborg Beast OpenSCAD prototype

Cyborg Beast OpenSCAD prototype

OpenSCAD tutorials for the MakerBot blog.  In that OpenSCAD tutorial series I covered the basics of the OpenSCAD interface, how to make 2D forms, how to make some basic 3D forms, how to position those forms in 3D space, the different ways to combine forms, how to create mashups of one or more existing STL’s and OpenSCAD forms, how to use modules to reuse your code to make your life easier, how to extrude flat 2D forms into 3D forms, and how to fix design problems.  One of the last tutorials was on how to make organic looking shapes using OpenSCAD.1  However, I have a few design tricks left to share.  A little over 18 months ago I left off the series suggesting as new topics.2 There’s one particular “trick” I am using a lot as I work on designing a printable parametric prosthetic. This trick is somewhat easier to explain using pictures.  Suppose you wanted to make a shape that looked something like a “jack,” but you wanted it to have curved surfaces at the center.  Let’s see what happens when we try to use the “hull()” command.  Do do this, we’ll make a sphere at the center and put eight more spheres around it.  The code for this example is basically irrelevant, but I’ll provide it anyhow.

sphere(r=10);
rotate([0,0,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);

There’s a much easier way to create the 8 “orbiting” spheres, but that’s another post unto itself.  :)  Here’s what the above code will create:

Nine little spheres (I named one of them Pluto!)

Nine little spheres (I named one of them Pluto!)

Now, let’s use the “hull()” command to wrap around these spheres.

hull()
{
sphere(r=10);
rotate([0,0,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,0,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
rotate([0,180,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10);
)

That code will make this:

Nine spheres to... a box?

Nine spheres to… a box?

The result looks nothing like a jack! It looks more like a box with rounded edges. The limitation with the “hull()” command3 is that it connects all the outside points from the various shapes.  The result is more like what the objects would look like if you covered them in plastic wrap – but not what they would look like if you tried to use shrink wrap.4 However, our goal is to get a jack.  How should we go about this?  The same way we eat an elephant.56 We need to use “hull()” multiple times7 to connect the central sphere to the eight surrounding spheres.

hull() { sphere(r=10);
rotate([0,0,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,0,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,0,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,0,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,180,0]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,180,90]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,180,180]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }
hull() { sphere(r=10);
rotate([0,180,270]) translate([100,100,100*pow(2,0.5)/1.5]) sphere(r=10); }

The result would look like:

Much better!

Much better!

By breaking the overall design into pieces, you can use the “hull()” command to connect pieces of the design to one another in a seemingly organic fashion.  Here’s a set of pictures of my most recent work that uses these design tricks.

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  1. Full list here:

    1. OpenSCAD Basics: The Setup
    2. OpenSCAD Basics: 2D Forms
    3. OpenSCAD Basics: 3D Forms
    4. OpenSCAD Basics: Manipulating Forms
    5. OpenSCAD Intermediates: Combining Forms
    6. OpenSCAD Intermediates: Mashups
    7. OpenSCAD Intermediates: Modularity
    8. OpenSCAD Intermediates: Extruding 2D Objects
    9. OpenSCAD Intermediates: Fixing Design Problems
    10. OpenSCAD Intermediates: How to Make Organic Shapes
    11. OpenSCAD Design Tips
    12. OpenSCAD Design Tips: How to Make a Customizable Thing

    [↩]

  2. These are, in no particular order:

    • How to sketch an object with OpenSCAD
    • How to easily make regular solids – other than cubes and cylinders, like hexagons, pentagons, octagons, etc
    • How to easily make symmetrical solids
    • How to easily make irregular, but symmetrical solids

    [↩]

  3. I almost typed “problem,” but in this case it probably is just a feature [↩]
  4. That’s the best analogy I can come up with [↩]
  5. One bite at a time. [↩]
  6. It’s such a damn shame when a cool domain name is taken – and there’s nothing there.  Such as eatanelephant.com [↩]
  7. 8 times [↩]

Progress on Parametric Printable Prosthetic

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So, I’ve been hacking away at an OpenSCAD sketch of the Cyborg Beast 3D printable DIY prosthetic.  At the moment I’m working on designing the left palm.1 I’m reasonably happy with the version so far.  I’ve also managed to separate out the cutouts for the hardware from the design of the hand itself.  The benefit to doing so is that it should later be possible to scale the hand up or down, but keep the hareware cutouts the same size.

It’s easy to print a hand, fingers, and thumb scaled to 110%, but finding Chicago screws that is 110% may be more difficult.  There’s still a LOT of work to do here, but I think this is a good start.  Below are the same three views of the two versions (OpenSCAD parametric and original Cyborg Beast).

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As I’m going through and replicating the design aspects of the Cyborg Beast, more design issues crop up:

  1. The grooves where the fingers fit into the palm are either tapered or flared in a non-uniform manner.  To get a better idea of this by reviewing the two pictures below.
  2. The holes for the elastic cord to cause the fingers to return to the open position do not appear to be of uniform diameter, uniform distance apart, or equally centered in the knuckle blocks.

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I can’t tell if these are critical design features2 or just design elements that don’t provide additional function or utility.  If you happen to know, your input would be greatly appreciated.

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  1. And, really, once the left palm is done it’s an easy to mirror this part to make a “right palm.” [↩]
  2. I’m guessing not [↩]