pivots

 Hi all,

You probably read my blog and know I am working on a design for robot legs inspired by the legs of Theo Jansen's Strandbeest.

Below is a picture of a plywood prototype on its way to become version 6.

The three layers of ply are connected in the pivot points by means of paperclip wire through 1mm holes. This is by far not strong and durable enough for the application I have in mind. This leg can easily carry a 2 kg load. But I need to improve the pivots first. As it is, the holes wear out fast and become very loose.

Any inspiration would be welcome.

tj-work_on_drawing3.jpg

You can see the black glass head of a pin sticking out in the center. This pivot hinges from the robot's chassis. I call it point €. At the far right is the point that is to be driven by the crank shaft. All the other pivots connect to leg parts only.

The pivot design I am looking for must be:
* tight enough to keep the flat parts snugly together
* without causing too much friction between the plates
* positioned with precision
* allow the rear side to be mounted flush to the flat robot body (so no screw heads sticking out)

I am considering some metal reinforcements of the holes through the plywood. Like hollow rivets or washers and bushes. I am also considering using a different material, like plastic or metal. But I am afraid that those materials will also wear out very rapidly.

I appreciate your inputs.


Update 16 feb 2009: For completeness, I include Chris' video on brass cutting. Thanks dude!

 

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May have used the one you have, on 3/8 (9.65 mm) copper tubing and even 1" (25.4 mm) galvanized pipe. I think they only cut well down to a certain diameter. Little ones like the K&S probably work better on the small stuff. I may need to get one soon, if I can find one. The K&S bins around here seem to be only metal, no tools, though there is a hobby shop or 2 that might have one.

I hope I've thrown myStanley blades away before they look like the one in your picture. Blunt blades cut people not stuff.

Mike

Wow, I just had one of those "I cannot believe that has been my problem all along" moments. Very reminisent of the "Ahah-firmware-erlebnis".

Here's the pictures that will save my all that typing:

tj-new-tools.jpg

I had a trip to the hardware stores today and bought all this and more. Notice: two types of blades, two different brands. Let's focus on the black-and-yellow brand.

tj-brass-blades-4-6-mm.jpg

That's right: each length of blade also has a different thickness of blade! Being the pseudo scientist that I am, I tried them both. I also took a close up of the cutting edges.

tj-compare_sharps.jpg

The picture does not do the sharpness any justice. But I can now report that the shorter, narrower blade is indeed much sharper. I think the photo shows that is has a very differently sharpened edge. Its edge is so much more smooth and refined.

And it cuts through brass tubing! YEAH BABY! Carpenter style!

Controlling the blade takes some skill though. I found it was easier to do on a stable worktop (unlinke my wonky bench). Also, clamping the blade in its intended knife helps me a lot to prevent the cut to "spiral out off control".

No video just yet. I am way too tired and, speaking of "razors" due to get cut.

Rik

tj-pivot-pt1-toolong.jpg

And I am getting the hang of this macro photography. Look at my prototype for a pivot bolt.

The base is thick walled 4 mm tubing. Which I threaded with an M4 thread cutter. I really like how pretty these little tiny grooves look! It awakens the crow in me. Shiny!!! CHRAWWH!

From left to right I added: a M4 washer and a bit of 5 mm tubing and soldered them onto the inner tube. I added the 5 mm tube, so that the washer would have something to sit on. The hole in the washer is not exactly a snug fit. The 5 mm bush will be "press fitted" inside a 5 mm hole in the first piece of plywood.

Then comes a loose washer, a loose 5 mm bush and another loose washer. These will turn freely around the inner pin and will fit the second piece of plywood.

The lock nut will be tightened just tight enough to give some pressure against the ply. Overall length is 48 mm.

Or that's the theory at least. First order will be to make one that is actually to size with the other materials. This prototype only serves to prove that fabrication is doable. Now I need to prove that I can make it much shorter: .5 + 4 + .5 + 4 + .5 + 5 + 1 = 15.5 mm overall. And that it actually works!

Still I am wondering if I just reinvented a nail, or a bolt, or a threaded nail?

Rik

tj_threaded_nail.jpg

Doable!

I bent this one though. The threads just look ugly! CHRAWWHfull!

And the bush won't slide over the nail's shaft all the way to the head, because there is a tool mark there from shaping the wire into a nail in the factory.

And no, I am not interested to read what "she" had to say about that!