The Box Beam Legs

I have designed a one-piece leg-ankle assembly built as a box-beam structure. There's a clear center section down the middle of the leg to route wiring between the body, battery boxes and feet. It's made from a double layer of .125 styrene with an internal eggcrate construction style box beam for strength. Not shown in any of the 3d drawings is the wrap of .040 styrene that goes around the edge of the legs. There are two layers on the curved top of the leg where it gets the most abuse (and has the least support) and one layer everywhere else for cosmetic reasons.


This is the full box beam leg (with only the edge skins missing)

The image links to a 3d pdf file, click on it if you want to be able to rotate the image around and examine it from other angles. (Note, the 3d.pdf file opens in a new window. If you have problems with the 3d feature you may have to upgrade to the latest version of Adobe Reader).

After 'activating' the 3d mode by clicking on the display
select a part by left clicking on it (the part will be highlighted)
then right clicking brings up a window.  Follow the sequence
-> part options -> part render mode -> transparent
to make the outer parts transparent and the inner
details visible.

Or you can use the views below which also link to .pdf files.

this version has the back removed to show
the Shoulder Hub Mounting Details

The Ankle Curved Section is built on the outer layer of the Ankle Surface. Each of the images below links
to a 3d pdf file, click on it if you want to be able to rotate the image around and examine it from other angles.


When I did the design for the Box Beam Leg I made a mistake in understanding how the Shoulder Hub was supposed to mount on the surface of the leg.


This is the way I designed the Shoulder Hub opening, thinking that the Hub was supposed to be mounted on the surface of the leg.

I'm now aware that the Hub is supposed to mount flush with the leg surface as is shown above.

There are several consequences to this design mistake.
1. The mounting hole in the center of the leg is too small.
2. The risers (internal ribs) in the area of the mounting hole have to be notched to provide a surface for the Shoulder Hub to sit on.
3. The bolts on the 3-Leg Fixed Shoulders (Or 2-Leg if you have those) are too long and have to be trimmed.
4. The Shoulder Hub itself is too deep to fit into the space and has to have part of it's bottom edge removed.
The design drawings have been updated (now on version 4) to reflect the changes in the parts. I have made a template, and instructions, for my customers that purchased and assembled legs to help them make the necessary changes. The template marks a set of .125 holes that are .03 inside the finished .385 diameter of the correct hole. Use the template to mark the hole locations, drill the holes, remove the excess plastic & slowly enlarge the hole to finished size. The template is here. An alternative to using the template, for those of you who have already purchased Shoulder Hubs, would be to use the Shoulder hub as a template, tracing around it's outside edge. Customers who had not yet assembled their legs were given replacement parts. There are instructions here on shortening the Fixed Shoulder bolts. Finally there is information here on the changes to the Shoulder Hub.

As with any design, there are limitations to what can be reasonably accomplished by CNC machining. For example, cutters are available for 30, 45 and 60 degree angles. If you could find one for the 35 degree angle of the Ankle Curve section it would be prohibitively expensive to use. You might have noticed the red lines in the Ankle Curve with Skin drawing above. Those are the edges of the ribs 'sticking' thru the skin. I had a choice. I could make them the hight of the rings but then you'd only get support on the edge. Instead I made them higher. They will have to be filed down to match the contour of the rings. See the Assembly Instructions for the highlights of what you'll have to do by hand.

The design drawings are available as both Autocad .dwg files and Adobe .pdf files download drawing files with the understanding that the drawings are copyright to Media-Conversions and are not to be used commercially. (That note also appears on each of the drawings). I've also put together a set of assembly instructions to help you put the legs together.

Here's what a finished leg looks like.



From time to time I've seen comments about how strong Styrene Parts  really are. The precise answer is "I don't know".
So I went and set up one of my Box Beam Legs (the second one I built) supported only on each end, and I stood on it, in the middle.
The long answer is that it will hold 160 Lbs!
Might Hold more!!



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