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Parametric Distal Phalange

by orgemd, published

Parametric Distal Phalange by orgemd Dec 15, 2013
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Parametric Distal Phalange by orgemd is licensed under the GNU - GPL license.

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This is a parametric version of the distal phalange used for Robohand. Really, it is a combination of the intermediate and the distal phalanges in one part.

I tried to make a Robohand scaled to adult size, and found a couple of areas I thought could be improved. First, I did not like that all of the fingers were equal in length - very unlike a normal hand. Second, the uniform length was too short - smaller than a pinky finger. Third, there was no ability to control the hole sizes, leading to some problems with holes that were too big or too small. Fourth, there was no ability to control the fit between parts, leading to too much lateral motion in some joints.

This version allows you to tweak the settings to make the fingers custom to your needs. It allows you to set the scale factor to match the other Robohand parts. You can then set the length of the intermediate portion of the phalange, the size of the holes for the knuckles, the size of the hole for the bungies, the size of the hole for the drive string, and the width of the slot that the proximal phalange fits into.

The ability to set the hole size for the knuckle on this part is pretty useful. You need to tap threads into that hole. As it scaled larger, I found there was not a lot of plastic to hold the tap.

This version currently only lets you set the length of the intermediate phalange. The actual distal portion is always the same. I thought of changing this, but decided it was diminishing returns. Most people do not have a lot of difference in distal lengths. If you set the intermediate lengths with this part, and then use my proximal phalange (thing 203208) to set that length, the finger lengths will be very natural looking - without worrying about the distal length.

This is a pretty hard part to print. My version is neither easier, nor harder to print than the original Robohand version. I can get them to print acceptably at this point. Let me know if you want to know my Slic3r settings. Keep in mind that the distal portion (which is the part that gets a little rough when printing) gets wrapped with string and dipped in rubber. That will cover minor print defects.

I have made a first test print of these - scaled to the actual finger lengths of the person for whom I am making the hand. They appear to work properly, but the hand is not complete and more testing is needed. For that reason, I am going to call this a work in progress for now.

NOTE: I have no medical training whatsoever, and make no warrantee of any sort regarding this part. I am providing it in the hopes that it will be helpful for others. If, however, you intend to use it, seek appropriate medical attention from a trained physician.


Open the .scad file and customize values in the PARAMETERS section.

Start by setting the scale factor to match the scaling used for any other Robohand parts. The sample .stl files I included used a 1.3 scale. Once the scale is set, set the length of the intermediate phalange. If desired, adjust the hole sizes (in mm) for different size screws, different size bungie cords, or different size strings. If desired, adjust the width of the slot that accepts the proximal phalange for a looser or tighter fit.

Once the values are set, use OpenScad to compile the object. Be forewarned that this takes a long time to compile. I do it on a laptop that is a few years old, and it takes about 2 hours per finger. Although that is a long time, I think it is worth it to have customized parts. It should be less time on a newer, more powerful system. You need a fairly recent version of OpenScad because of the dependence of this object on the Minkowski function that had bugs in some older versions. Once the file compiles, export it as an stl. You will probably need to use Netfabb to repair the stl. Then slice and print the repaired stl.

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Hi there
Thanks for posting this, I think is a great idea. I´m working on a hand for my nephew and would love chat to you about it, is there a way I can contact you over chat?