Finger Assembly

June 26, 2016

Read these along with the accompanying 8min how-to video, above. I give time offsets to make it easier to follow along. Original link:

1\. Cleanup and check (0:25)

Clear support material. Spot check to make sure everything printed well and fits together. The PLA parts should barely be able to squeeze tightly together. If the hinges are already loose, or so tight you cant force them on, something is wrong. You’ll smooth these later.

I use a 1mm bit to start with on the tunnels, and then smooth the elastic tunnels with a 1.75mm bit.

2\. Drill the hinge-pin holes (0:38)

Drill the pin hinge pin holes with a 2mm bit, on both knuckles and the middle section. The pins should push in smoothly and easily, but not sloppy.

3\. Smooth the hinge insides (0:53)

Using a needle file, lightly sand and smooth the the built-in washer on the insides of each knuckle. It should should still protrude, this minimizes hinge friction. You don’t want the joints to be sloppy, so just a little at a time.

Alternate working on the inside of the hinge and the sides of the middle segment just enough until they fit very smoothly together. The middle segment end with two holes faces the tip. The side toward the base only has one hole.

4\. Pre-assemble the hinges. (1:27)

Pre-assemble the 3 PLA parts and ensure that they all work smoothly together. Push the pins (or wire) through the hinge holes, ensure it’s long enough to make it all the way through both sides of the hinge.

It should take nearly zero friction to move the segments across their full travel – if you can’t fully articulate the finger using only gravity, then keep working at it with the files until you can – or else it won’t work well later.

5\. Install the set-screws (1:45)

Each of the two lengths of elastic has a knot on one end, and a set screw on the other.

You’ll need to get the set-screw holes just to the right size. Too small and the screw will be hard to turn, and can crack the plastic. Too big and it will slip out – a 1.75mm bit works about right for me. You might want to practice on a scrap piece.

6\. String the elastic (2:14)

For the tip, run it through whichever way is easiest. I sometimes use a small drill bit or length of wire to help me push it through. Tie a tight triple square knot on the end that protrudes from the bottom of the middle segment. Trim it neatly, and it should pull flush. For now leave a couple inches on the other side.

For the base, I use a double knot on the side coming out of the end of the middle segment, in the gap of the tip hinge. It will sink down flush. If it pulls through easily, try again with a triple knot. Then on the base the elastic will protrude in the socket area with a set screw.

7\. Tighten set-screw to create tension (4:15)

Put slight tension on the elastic cord and carefully tighten the set screws to hold in place. You’ll want the base knuckle just barely tight enough to pull itself fully closed. The tip should have slightly more tension, to compensate for the additional leverage that it has. trim all but a small amount on both sides, for later adjustment.

If you’re not using set-screws, you’ll need to tie knots on both sides of the elastic. I find it easiest to stretch to the desired tension and then mark the elastic with a bright paint marker. This way as you’re tying the knot you can get it just at the right spot.

8\. Install the hinge Plugs. (4:25)

I warm the hinge plugs on my coffee warmer for a while, making them soft and easier to squeeze into the ends to hold the pins in place. You should now have fully assembled hinges.

9\. String the fishing line “tendon” (5:23)

String the fishing line through the tip, down under the middle section, and back up through the base. Tighten the set-screw at the tip, leaving just a small amount for later adjustment. Leave 3 or 4 inches on the base side, to ensure you have enough to connect it to your wrist linkage.

10\. Test the action! (6:04)

At this point, you can snap the socket and tip on, and should have a fully articulating finger! Make sure it all works well before moving any further.

11\. Install the bumper. (6:09)

If you use NinjaFlex or FilaFlex (more stretchy) you can probably stretch the bumper enough to get it over the tip and on to the middle section – that stuff is almost indestructible.

Lately I’ve preferred the cheaper Sainsmart elastic TPU, but it is not as stretchy. I print it with the cut in the bumper, which you can separate carefully with an exacto. Then wrap the bumper around, and trim the plugs as needed until it fits together well. Then I use my soldering iron (set around 3/medium) to quickly weld the TPU back together.

12\. Install the linkage (7:19)

Clear the hole through the linkage, at least halfway, to make room for extra string. Make sure you’ve cleaned the set-screw hole and gotten one started. Feed the loose end of the tendon string into the linkage, and tighten the screw. Link the hook around a bracelet. You’ll have to find the right amount to allow you to fully close the finger with the range of travel you have available.