Palm Harness for Knick's Prosthetic Finger by swoozle

Last crawled date: 3 years, 1 month ago

This is a palm harness to use with Knick's Prosthetic Finger (http://www.thingiverse.com/thing:1340624).
This harness is an alternative to the bracelet-based tendon anchor design that Knick uses. I found this to work better with two Knick's fingers (as I need), both in keeping the anchor tendons where they should be as well as a little more comfortable with the increased pull force that two fingers requires. It is, however, visually more obtrusive.
Needed:
Flexible filament (TPU, eLastic, NinjaFlex, etc)
Hard filament (PLA, ABS, PETG, etc)
M2.5 x 3 mm set screw
Buckle for 10mm wide strap
The harness and straps are designed to be printed with the flexible filament. Since you are making a Knick's Finger if you are interested in this, you already have the flexible filament. I used eSun eLastic but I'm sure that many others will work just as well. You might want to make the straps and harness thinner if a stiffer filament is used. eLastic is fairly soft, akin to NinjaFlex I think.
The harness can be used on either hand and also worn with the smooth (print surface) out or against the skin. It's up to your preference. Figure out what you want and then use the regular or reversed versions of the harness and palm straps. The palm strap end near the thumb is a little closer to the fingers than the little finger side.
Two wrist straps are needed. Length is not important as they are over-long and trimmed depending on your buckle setup.
One palm strap is needed. This length needed is dependent on your hand size. Probably the easiest way to size the wrist strap is to print the main harness, then place the harness around your hand and measure the distance between the harness ends (A second person makes this a lot easier). Resize the palm strap to suit either by simply stretching the appropriate axis in your printing software, or loading the model in Sketchup and editing.
The anchor block is designed to be printed with a standard hard filament such as PLA, ABS, PETG, etc. I used a clear PETG for the block in the picures.
The block needs a set screw. Again, this is the same M2.5x3 set screw needed for Knick's Finger so you are covered.
The buckle is up to you. I used a plastic buckle designed for 10mm wide straps. I'm not sure where to get those in the states (I'm currently in South Korea), but I would imagine that craft stores have this sort of thing. I think a watch band buckle could also be made to work well.
Assembly:
The palm and wrist straps are attached to the harness by melting the ends together. This is easily done with a soldering iron at low temp. Specific temp and technique depend on your filament. I found that a wider tip than normally used for soldering makes this easier. A paper clip can be wrapped around the soldering iron tip and then bent perpendicular to make a smooth, wider tip. This also keeps stray solder bits out of the melted filament. The bent section needs to be quite close to the iron tip to stay hot.
Take some time to practice on scraps before you dive into assembling the harness.
There are v-grooves and ridges designed into the ends of the harness and straps. Place the ends against each other and melt the ridges into the v-grooves. Flip the joint over and melt the flush mating ends together.
The palm strap is of course attached to either end of the harness to make a loop to go around the palm. The wrist straps are initially attached at one end each.
The buckle is attached to the wrist straps in the same way (looped through and then melted together). Or they can be left adjustable (as one of the ends is in the pictures).
Assembly of the anchor block is self-evident.