After a few frustrating day struggling with dead hard drives and faulty motherboards I'm finally back to paper craft. Just remember folks, back up, early and often. I was glad I had!
I'm happy to return to the crab project. The good thing about being elbow deep in backups and restores is the time for reflection as the progress bargraph creeps slowly across the screen. I've thought of a couple of changes to try out. First of all, I've decided to fix one of the claws rather than having both moving. The tendon now pulls against the larger claw. The other change is to the tendon placement.
Currently the tendon runs through the centre of the arm. Here's the problem. Because the tendon is flat against the joint there is no leverage. Pull the tendon and the movement of the claw is sudden and jerky. I've solved this problem in earlier projects by lifting the tendon from the centre line using triangular section blocks at each joint. That said, I'm always on the look out for ways for simplifying mechanisms.
How's this? I've woven the tendon under/over/under along the upper side of the arm.This keeps the tendon high up away from the hinge line thus increasing the leverage at each joint.
The result, smooth, even movement as the tendon is pulled and the arm raised.
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I've been thinking about a pose-able robot model for a while, it'll have bendy arms, mavable head, that sort of thing. I must have been thinking about it last night when I went to sleep because when I woke up this morning I had the claw all worked out. I know from experience that if I ignore these moments of lucidity they quickly evapourate so, just for today, I've set aside the escapement, my current main project, and spent the day putting together.... The Paper Claw!
I've made the file available for members to download for free in the usual place. Have fun!
Print the single parts sheet onto thin card. (230 micron / 230 gsm - is there a different US unit for thin card? I know how you like your imperial units) Score along the dotted lines and cut out the parts.
Fold up and glue together the main arm.
Assemble each of the three claw parts as shown above. Notice that the tab sticks out and the triangular section at the bottome is made from valley folds.
The three fingers completed.
Make the triangular based pyramid. This will be the arm end cap.
Fol round and glue the slider sleive.
Make up the handle as shown above.
Glue the tab on the first of the claws to the inside of the arm. Make sure that it is linied up accurately and is free to move back and forth.
Repeat the process for the other two fingers.
Fit the end cap into place. Make sure that you don't press it in too far as it's really tricky to get out. (I know, I tried it)
Thread the slider sleave over the arm.
Complete the prototype by gluing the long tabs to the triangular sections on the fingers.
That's it. Go out and take over the worls with your paper claw!
Back to the escapement for me; perhaps a coffee first :-)
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