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Parametric Proximal Phalange

by orgemd, published

Parametric Proximal Phalange by orgemd Dec 14, 2013
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Parametric Proximal Phalange by orgemd is licensed under the GNU - GPL license.

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This is a parametric version of the proximal phalange used for Robohand.

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 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 thickness of the tabs to tune the fit with other parts. There is also a support option to make it a bit easier to get a good print.

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 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 thickness of the phalange to make a tighter or looser fit into the other parts.

There is also a Support parameter that can be set to true or false. If true, the bottom of the print will be squared off a bit to make an easier print (because of less initial overhang). The parts will then need a little filing or sanding to round them back out. I found this well worthwhile since it made the parts print much more cleanly. I tried conventional support settings, and that did not work as well as this solution. If you don't want/need that extra support, just set the value to false.

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 4 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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An observation: When we created Robohand and kept all the fingers the same length it had to do with the anatomy of how a normal hand functions. When you close your own hand around an object do all your fingers stay the same length when the hand is closed? Regards Beth

Greetings Beth,

Thanks for the comment. When I initially looked at making these changes, I asked about reasoning for equal length fingers in a comment on thing 44150. I got no response then, so I appreciate your input.

It is interesting to hear that Robohand utilizes equal length fingers because it was considered closer to how real fingers close when grasping an object. Were there tests to compare the ability of equal length and distributed length fingers to grasp different shaped objects? When I close my hand, I do not find the fingers to generally close in an equal length fashion - though sometimes they are approximately so. The motion is close, for example, if grasping a beverage can. The way fingers close, largely because real fingers are individually controlled, is incredibly variable. It tends to be uniquely tailored to the object grasped.

I guess, at this point, I will be continuing to test the distributed length version, and will see how well that form can grasp different objects. My best guess is that neither equal nor distributed length is better across the board. I expect distributed length will prove better at grasping round objects, and equal length will prove better at grasping cylindrical objects. I guess we will see. Unfortunately, I doubt any simple closure method will prove ideally suited for all purposes.

Of course, if it proves that equal length fingers do just work better, I will still be able to benefit from this part. In that case, I will select a uniform length that will be a good average of the real phalange lengths of the person for whom I am designing it.