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!

 

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
You can mangle all you want but I find violent games and alcohol are a much better way to relieve frustration :p

Don't you need to lock that nut on somehow? I seem to rtemember this from my Meccano days.

Mike

It should lock against the tube, being brass it is softer than steel and should bind reasonably easy but some thread lock would not go astray or even a dab of hotglue.

rik, oh brotha,

Man I could have saved you some time... Here's how you cut that brass tube...

Use a brand-new razor blade - the kind you would use in a retractable utility knife. Place the blade on top of your tube on a nice, flat table. Now roll the tube back and forth (with the blade) with gentle pressure on the blade. This is the same idea as the tubing cutter but without the rollers pinching the other side of the tube. Simple, -razor blade and roll it back and forth on a table. Be sure to "stay in the groove" to make sure you are not cutting a spiral. This will make a perfect cut and it won't "mushroom" into the center! --Also those metal cutting blades must be leaving burrs.

A trip to my armory resulted in a plethora of blades that could be regarded as utility blades or even as razor blades. Notice from my appearance (elsewhere, some other day preferably) that I have not used the first one in this picture for quite a while.

tj-tubecutters.jpg

Top to bottom, loosely translated from "Dutch common names":
"scraper" (sold as actual razor: Gilette Sensor, has its blades upsidedown in this photo, that's how long it's been),
"Stanley knife" with spare blade (scary utility knife, retractable version),
"small breakaway knife" (hobby knife also by Stanley),
"big breakaway knife" with spare blade(s) (bigger hobby knife, Stanley ripoff),
"pocket knife" (better not carry this in your pocket in crowded places like airports, more of a camping knife),
"Jokari" (big ass wire stripper: the wire has a big ass, not the stripper).

Something in your Walter post tells me you are probably talking about the second tool...

tj-scary-blade.jpg

 

I speculated about the blade and went with the scary type. Here is a result.

tj-brass-cut-ctc-method.jpg

Notice:
1) no stain, yeah!
2) spiraling, booh!
3) slight burring, oh well...
4) no mushrooming, yeah!

Some lessons learned:
1) this still is a job that will drain all the power from your fingers and fore arms;
2) do not subject your good tabletop to this method (use a pizzabox desk protector);
3) move the tube, hold the knife still - I started out by "riding the knife on the tube" like a clown would walk on a barrel;
4) there is no shame in starting a good groove with the original cutting tool;
5) although this may not be necessary any longer, now that I know to hold the knife still;
6) some brass saw dust will collect in your work area - mind your electronics!
7) a lathe might replace the (strained) left arm, maybe a cordless drill on low speed?

Thanks Chris! YodaMan!

 

 

That might help some, haven't tried one though.

http://www.hobbylinc.com/htm/k+s/k+s296.htm

Definately a better looking one than the one I ended up with.

The original store tried to sell me a K&S cutter, but could not find it in their cluttered display cabinet. And the second store sold me the one with a metal roller. Which looks so much more coarse than the one you're ponting out:

k+s296.jpg

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 made this practice piece. Each corner has a different pivot.

tj-pivot-all.jpg

 

This one has a piece of 4mm Ø, thick walled, brass tube running through bare holes in the plywood. Actually it is 5/32 inch. Dutch model builders stores buy their stuff in Chicago apparently.

tj-pivot-4mm.jpg

 

This is "5 mm" Ø, thin walled, brass.

tj-pivot-5mm.jpg

 

Cutting brass at these sizes is not very easy. I bought a tubing cutter, but it compresses the tube while cutting. The deformation makes it hard to fit the 4 mm into the 5 mm tube. Which is exactly why I bought three different sizes to begin with. Here is a 4 mm piece sleeved through two 5 mm bushes. I found that the outer tube does not deform (much) when I put a piece of the thinner tube inside it while cutting. But that will eventually lead to damaging the inner tube, making it impossible to slide off the tiny brass ring from the inner tube.

Cutting brass with my jig saw worked, but it costs me a blade per cut. Maybe I am not using the right blades....

tj-pivot-combo.jpg

 

And finally a plastic rivet. I bought a 100 of them at consilium design on e-bay.

tj-pivot-plastic.jpg

The preliminary verdict

Hard to judge now. None of the brass solutions I made hold the pieces together, yet. I did not fit a screw or bolt through them. The combination of small and wider tube as a pin though bushes works very well. The pivot turns very smoothly. But the fabrication is hard work: six cuts per pivot.

Perhaps if I used the inner tube as a split or "bifurcated" pin, I could use that to hold the plates together. I suppose I could complicate fabrication further by cutting into the end of a 4 mm tube with a small hack saw or a wire cutter maybe.

The cheap plastic rivets really are not made for this task. I had to weld the ring on with the hot glue gun, or it would slide off slowly with each movement of the pivot. Also the head and ring of these rivets are too big to sink into the 4 mm plywood.

This is tough with the plywood only 1/4" thick. Also you have some at 2 layers, some at 3 layers and some with the vert. pieces... Hmm. Just off hand, I would say 1/16" I.D. brass tube with 1/16" piano wire through the inside. The brass tube will be the sleeve while the wire is the hinge pin. On the inside the wire can be bent to a 90 and pressed into the wood .And on the outside a coller (with an allen key set-screw) can keep the wire in place. The company that sells the little collars is called "Dubro" and all model airplane shops carry it.

Little_Guy_1_006_0.jpg

Here is a tail wheel I made with this tube/wire thing. In this case, the brass tube was soldered to the little copper bracket you see, but this kind of configuration should work for you.