One problem that I have with our hands is that they bend the fingertips long before they bend at the palm. It's not just a little bit, if you hold a fingertip and try to keep it straight as you bend the hand, it will fight you. This is because there's no leverage on the phalanges. All of the force on the wire passes through the phalanges to the tie-off point on the fingertip.
This causes a problem for all of the fingers, but it is most obvious on the thumb. Here is a hand trying to hold a cup.
- 01 holding a cup.JPG (74.21 KiB) Viewed 2012 times
It can barely get around it. If you squeeze harder, you expect to get a better grip. Instead, the thumb curls inward, forcing the cup out of your grip entirely.
- 02 a tighter grip pushes the cup out.JPG (63.05 KiB) Viewed 2012 times
There are multiple issues here. One is that for an item this large, you don't really want the fingertip to bend at all. Another is that the gripper box provides adaptive grip between the fingers, but it won't redistribute the load on the thumb, even when you might want it to bend a different amount than the fingers.
The issue that I'm looking at now is that the fingertips and thumbtip have a massive amount of leverage pulling them down before the joint at the palm moves at all. Skip suggested that the angle of the wires could help with this problem. To test this, I made a little spacer that I could add to the hand.
- 03 A little spacer bar added in.JPG (71.21 KiB) Viewed 2012 times
With this, the wire is pulling on the phalanges from the bottom, instead of the top. That's the direction that we want in order to force it to go down. There are little feat on the sides of my spacer to hold it onto the palm, but the bar that the wire goes around is flush with the bottom of the hand. This is equivalent to adding a little bar to the palm, it's just much easier to print, and I can add or remove it easily for testing.
The first test was to see if the palm joint bends any sooner with the spacer than without. Here it is without the spacer:
- 04 bending without the spacer.JPG (67.79 KiB) Viewed 2012 times
and with it:
- 05 with spacer, no difference.JPG (63.06 KiB) Viewed 2012 times
There's no difference. When I pulled on the fingertip as I bent the hand, it felt a little easier to keep it straight, but not a whole lot. Bending the fingers around objects, even smaller objects, wasn't improved at all. The fingertip bends first, then stays that way.
The next improvised test was to flip the spacer over, massively increasing the angle of the wire against the phalanges.
- 06 extreme spacer helps.JPG (65.38 KiB) Viewed 2012 times
I can't show you any pictures of bending the hand using this arrangement because holding the spacer in place and using the hand took too many hands. However, this finally got some results. It needed an angle of about 45 degrees to get enough leverage against the phalanges to pull it down.
Getting that much angle into a palm will be tricky. Now that I've tested it, I think a combination of two things will get enough angle. 1) Printing out a palm with a little bar between the palm and the phalanges. The wire can loop around this and pull down on the phalanges. 2) Changing the geometry of the phalanges. Instead of having an open groove at the bottom of the base, I'm going to partially close it off. Not all the way, I've tried that before because I liked the look of it, and it makes getting wires through the groove difficult. However, closing if off about halfway up the channel should give the wire something to pull against much closer to the palm. The combination of those two things should give us the 45 degree angle that will close the phalanges much sooner.