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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Don't you hardware stores or hobby shops sell 4mm bolts? Oh well it's good to see your still at it. Don't forget a little bit of grease between your moving parts. A little bit of white petrolium jelly (vasalene) will do.

... I can get. Half a mm I can live with. A whole is getting fattish. A regular bolt would drive me to different plywood thickness.

Maybe further down the project, just not now.

I was looking at your rearangement of my diagram and unless I'm not understanding something, why not just drill a hole in the outer sheet for the head of the bolt to poke through?
the sheets slide over one another. The hole would have to be a slit.

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!

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

Mike

I know the tube is fiddly. Putting a mandrel inside while cutting and reducing the pressure while cutting is your best way to prevent deformation. A mandrel is a support that goes inside tube while it's being cut or bent to prevent deformation. A piece of steel fencing wire or a wire coat hanger perhaps would work well. A cheap set of jewellers files or hobby files will also be usefull to clean up the inside of the tube after cutting. This is how I would make my joints.

Brass_joint.jpg

In this cutaway diagram, the brass tube is just a little bit longer than the width of the two pieces of wood and washers to stop the nut clamping down on the wood when tightening. Now the nut clamps tight against the tube but allows free movement of the joint.

 

This leg is supposed to move flush against the flat surface of the robot's body.

tj-pivot-oddbotstyle-alt1.png

So at least one side of the bolt/nut need to be sunken into the wood. That is not feasible with 3.6 mm thick plywood. Not a totl sink anyway.

I am now considering other designs. Like a bolt that has an ultra flat head (a washer soldered on). Or much thicker plywood. Or maybe thickening the plywood where it is necessary (double layers?). Or Chris' original idea of using 1.6 mm steel wire, bent around and pressed into the soft wood. Or using a spring somewhere, somehow to provide the pressure I want. Or a combination of options.

Or forget about this plywood altogether and move on with a different meterial. Preferably one that Ponoko knows about.

Thanks Oddbot. Your drawings are always the prettiest. It's a pleasure mangling them!

Rik