How do the 3D printed fingers work?

[Tyler Scofield]

So at work I am baffled by a problem to bend a flat plate (3mm thick max) using string where the string is encased in the plate. So naturally I 3d printed some plates and attached a string but found as suspected that the plate would only bend when the string had some angle > than a few degrees getting progressively easier to bend approaching normal.

This led me to researching how other similar designs accomplish this and ultimately my question. I have seen many pictures but not a very good write up on how the fingers bend in the 3D printed prosthetic, when the string bending them is parallel with the surface. It appears that most of the designs do it this way so the strings aren't all over the place which makes sense.

The only way that I can think of is that the fingers are already bent and the string pulls them erect but it appears to work the opposite. So if anyone could shed some light on this I would be grateful.

 

[Mech_Engineer]

Your tendon (string) needs to have some small distance between it and the joint that will flex. As long as the string is pulling on one side, it creates an unbalanced moment and the joint will flex.

Like this:

 

[Tyler Scofield]

So by distance between tendon and joint this distance is:

1) Vertical off of the joint
or
2) Some distance inline but away from the joint?

In the picture above the tendons appear to be inline and separated by a small distance say 1 inch. But the force applied is parallel to the knuckle which in my mind will pull the finger into the knuckle not necessarily causing rotation. Obviously it does rotate in real world so is this due to variations in material properties causing it to not have a perfect inline force or am I still missing something?

My last thought on how this rotation occurs is that there is a delta height from before and after the knuckle, which would create some angle say an exaggerated 30 degrees which could impart some force = Fsin(30) to create the moment. I think this explains the rotation best, thoughts?
Thanks again.

 

[Mech_Engineer]

The distance you're interested in might be considered the vertical distance of the tendon off the joint. The joint of an artificial finger (or a real one) will have a center of rotation; the distance between the joint's center of rotation and the tendon creates a moment which causes the joint to flex.

The extra tendons you see in the image above are for controlling side-to-side motion of each finger, in addition to tendons for flexion and extension of the fingers.