The Box Beam Leg Assembly

While presented as an orderly set of steps you should realize that you can be working on several different parts of the assembly process at once. Just be careful, take your time, think about what you are doing and plan your work. The methods shown here are suggestions. I am by no means an expert craftsman. If you see a different way to do something please write me!
First read my note about .25 in thick styrene no longer available from my local supplier - 10/12/12

NOTE: these instructions were put together before I realized that the mounting for the Shoulder Hub was incorrect.
If you haven't already reviewed that material please do so before you start. Box Beam Legs Defect
 - 01/10/13

There are over 150 photos in the assembly instructions. I've added jump points so you can skip to the part of the assembly you are working on.
Familiarize yourself with the parts
Dry Fitting the Leg Top Inside Inner/Outer layers and Main Risers
Glueing the Main Risers in place
Assembling the Ankle Curve
Installing the Arc Risers
Prepare the remaining parts
Check internal joints & glue Inner box beam closed, add outer layers
Sealing up the legs
Putting the skins on edge of the legs
Putting the skins on the top of the shoulder
Last of the skins for the Ankle
Congratulations, you've got a finished Leg!

First step is to familiarize yourself with the parts. Only separate the parts you will be using from the sheets they were machined from, remove the tabs and flash. That way you'll have something to do (separating the parts) for the next step while you are waiting for the glue to dry on this step.

leg top outside inner and outer

leg top inside inner and outer


Above: Leg bottom inside, outside and inner .25 thick components
This is now a set of two .125 panels that you will have to glue together.
Left: Leg Risers and Ankle Curve components
With the change in shoulder hub mounting, the parts marked now have notches for clearance. Pay attention to the orientation of the notches
Left: Leg Skins

Not Shown: Leg Skins for the Shoulder

Note about Skins. These parts are deliberately cut oversize (.080 in width, .5 in length) and are intended to be trimmed to fit (or filed to fit ) after being glued in place


Assembly starts with Dry Fitting the Leg Top Inside Inner/Outer layers and the four Leg Main Risers. Additional parts are shown to illustrate the fit.

Here is how those notches create a recess for the shoulder hub to fit into. Unfortunately, the slots for the cross braces in the long risers are facing down so you cannot assemble this way.


The Main Outside Risers (left/right) have pockets that have to be on the outside


Above & Right: Begin your assembly on the top, inside, inner layer of the legs. Pay attention to the location of the cross brace slots and the notches on the risers.


As a consequence of the notches, the long inner risers and cross braces have been flipped over. the instructions and pictures that follow were put together before the change and as a result show the assembly being done on the top, outside, inner layer (the longer layer that has the leg mounting bolt).

you still have to do the 'dry fit' process, but the sequence will have to be slightly different to accomodate the notch change.



I've added the wiring slot that Dave Everett included in his latest Leg update. These parts are the same for both legs. Turn them over to swap between left and right legs (see below)

The Hole in the Leg Bottom Plug has to line up with the wiring channel.

Turn all of those parts over when you assemble the 'other' leg.

The final inner piece will cover the channel.

Inside view of Leg Bottom Plug. If you havn't given any thought about your wiring for the foot drive, take a moment to think about what wires you will need to thread thru that hole. You may want to enlarge it. You can drill it out later if you need to. wires will have to be threaded UP from the bottom of the leg.

Left: It's important that the Leg Bottom Plug be tight against the
end of the Main Inside Risers. 
Use a 3/8 Bolt to hold things in place while you test the fit.

 Make a mark where the top/bottom edges should be so you can align to the mark when you glue. It's better to be too far down as you can file the edge of the riser when you fit the bottom plug. If there's space you'll have to shim it.

Time to dry fit the Leg Top Inside Inner piece.
Note the "inside" written in marker. That's so the Under
Shoulder Pockets wind up in the right places.

You should be able to get the ribs to align with the slots relatively
quickly. You want to be able to clamp before the glue sets!
Try this a couple of times. It gets easier with practice.

Here's what the Under Shoulder Pockets look like.
Note: the slot on in the Main Riser has been removed.

Both top and bottom need to be on the "inside"
Critical Important Alignment Issue

It is possible to insert the risers backwards!!!
Make sure all of the cross brace slots line up.
Make sure the under shoulder pockets are on the outside.
I suggest using a marker to put in alignment marks.

Notice anything wrong here?
Yup! I went and put my alignment marks on the WRONG
ENDS! - To make things even worse, I glued it up that
way! DON'T MAKE THIS MISTAKE!!!

Next steps will be to actually glue the risers in place. I'm working on a piece of Formica counter top in these pictures. It's impervious to the solvent glues I'm using in this assembly (Weldon). If I get any excess glue between the part and the counter top I can slide a putty knife in between to pry them apart. A razor blade cleans excess dry glue off the counter top and a little fine sandpaper removes counter top scratches.
There are edge covers at top and bottom of the legs.
Draw some guide lines to help you when you run glue lines.

Run a bead of glue on just one line at a time.

Insert the riser.
Make sure it's the correct one and going in the right direction!

Repeat for all 4 risers. Remember, you are only gluing one edge of the riser.

Put the Leg Top Inside Inner piece in place and clamp.
At  this point what I've shown is ONLY gluing the Main Risers to the Leg Top Inside Outer piece. There are more internal parts to be put in place and glued.


While you are waiting for the glue to dry on the Inner Leg Assembly here is something else to work on.


These are the Ankle Curve parts. Note that one of the ribs is
larger than the other two.

Glue the larger one in the middle.
  Left: Glue the two rings in place.
Not shown, clamp or tape things in place and set aside to dry.



Next prepare the Ankle Curve End piece.

The corners of the slot were cut with a 1/16 inch bit.
you may want the corners square. File Carefully.
Use a file with a 'dead' (smooth) edge for more control.

Glue the cover piece using the inscribed alignment marks.
It fits in the notch on the center rib so alignment is critical!

Go to the scrap left from the riser parts.

Cut out four peices, trim the ends off.

We'll use them on the edges of the Ankle Curve

To provide a larger area for the skin to glue to.

Left: Glue the Ankle Curve end piece in place. Center it on the ankle curve.Note that the ends stick out slightly.
It's designed that way. Glue the 4 skin blocks in place as well.

There will be some rough areas where joints were glued and
the risers extend threw the ribs (including the back, not shown).
Make sure you sand these areas smooth to get good bonds
when the assembly is glued to the Leg


Next we'll fit and install the Arc risers at the top of the shoulder, the under shoulder detail parts and the cross braces.


There are 3 Short Arc Risers, 6 Long Arc Risers,
12 Cross Braces and two sets of Under Shoulder Details

There are 3 short Arc Risers (for clearance around the bolt holes) make sure they are in the positions marked.



Left: This is the under shoulder detail. This part has been re-designed. There is no longer a tab into the main riser, or the slot in the riser for it to plug into.

Left: You may want to shave off the corner of the edge with a sharp razor knife for a better looking pocket.
Left: make sure the cross braces fit flush with the top of the main risers. This one is riding up on a bead of glue.
I numbered the Cross Braces so when I glue them I know I am putting them back in the slots they were test fitted to in the same orientation.
Test fit the Leg Top Inside Inner piece (again). There are more pieces to line up this time. Again, it gets easier with practice.
Right: double check the fit at the Under Shoulder Details. Everything should fit flush.

When you are happy with the fit, glue the Arc Risers, Cross Braces and Under Shoulder details in place.

While you are waiting for the glue to dry, you can prepare the remaining parts. Not much left now!

There are two Leg Bottom Inside pieces. One is .25 thick (made from two .125 peices) the other .125 thick. 

Apply glue and clamp together.

I used a bolt, and a clamp to hold the Leg Bottom Inner parts together.
Remember to reverse the parts when you do the other leg!

Ankle Curve: The two outside ribs, and the skin blocks, are cut oversize so that they can be shaped to fit the contour of the ring.


File the edges down so they match the curve. The skins, when we get to them, will have a larger surface to bond to.

Like the ribs, the Ankle Curve End assembly is oversize.
Right: File the rounded edge so it lines up with the ring.


With the glue dry on all of the internal components you have one last chance to inspect the internal joints before they are sealed up. Add glue to any joint that seems to need it. Be generous, you won't be able to go back and add more later.

Left: I drew guide lines so that I could get the glue in the right places. Note that it now includes the tops of the cross braces.

Apply glue. Mate the two parts and clamp everywhere.

(Before) With the glue dry we can prepare for the outside layers. There will be some glue residue on the internal box beam. That has to be removed for a good fit.

(After) I typically use a medium grit sandpaper (80-120) in an oscillating sander to remove glue residue and any rib edges that stick up above the surface.

There are slots in the outer leg surfaces (just placed for photo) and a matching slot in the leg riser but no slot in the edge of the inner layer

Very carefully cut the edges , aligning the saw with the existing slots.

the slot in the riser is only .025 (we'll add .04 with the skins)

use a needle file to remove the plastic on the edges.

Apply your favorite cement, mate outer layer with the inner box assembly and clamp. Note the use of the bolt to insure alignment.

The outer layers are intended to be structural as well as contribute a smooth finish. You need to clamp the full area to insure a good cement bond.
Repeat the process with the Leg Top Inside Outer layer.

Note the use of 1/4 inch bolts this time at the shoulder joint for alignment.

Once you have the outer layers glued it's time to start sealing up the legs. We left this part to last so that the exposed edges could be used for clamping.

Glue in place the Leg Bottom Inside assembly (the  two pieces that you laminated together earlier.) and clamp for a good structural joint.


Check the fit of the Ankle Curve Assembly. 


Glue the Ankle Curve Assembly in place. Use a 3/8 bolt for alignment. Clamp for a strong structural joint.


check the fit of the edge covers at the top of the legs.

These parts are undersized in width and should extend slightly on the under shoulder end.


check the fit of the edge covers at the ankle

These parts are also undersized in width and should extend slightly at the bottom of the ankle.


Next we'll put the Bottom Plug on the end of the Leg

Make sure the Bottom Plug fits tightly against the ends of the risers and is flush with the end of the leg. The part is
oversize so it extends out on both sides.



Make sure the wiring channel lines up with the hole in the Bottom Plug. Remember to reverse this on the other leg.


Glue and clamp for a strong joint.



Here's the edge piece at the shoulder end.


File it down to match the angle of the under shoulder details


Here is the ankle edge and the bottom plug detail.
Left: file both extended edges down to match the angle.
This will give lots of area for the skins to glue to.

Left: The Ankle Bracket is in two pieces that sit on the top
edge of the ankle. Make sure the edge is smooth.
File off any high spots.



Both pieces have sharp
inside corners.
File lightly for a better fit.

Make a mark so you know the joint is in the right place.



Clean up any glue seepage that might interfere with the
skins.
Note the tight joint between the parts.

The final assembly step is wrapping the edge of the legs with .040 styrene.
The top of the shoulder gets two layers while the rest of the leg gets only one layer.
We begin with the lower part of the Under Shoulder Detail and work downward.


Start by sharpening up the inside corner of the
Under Shoulder area.
Left: before Above: After

Also sharpen the inside corners of the skin piece.
Left: measure, mark and cut the ends to length.
Then measure and trim the end by the leg slot.

Glue in place, tape while the glue dries.

Above: File the top and bottom edges being careful not to
cut into the leg surface.
Right: If necessary, trim the part of the skin at the slot end.
Update Pix (show slot in side)

We continue with the upper part of the Under Shoulder Detail.

Sharpen the inside corners of the skin piece.
Left: before Above: After

Pix Out of date
File the extended edge of the plug to match the angle
of the under shoulder detail

measure, mark and cut the ends to length.

Above: Check the fit then glue in place
Right: Tape while the glue dries

File the edges flush with the surface of the shoulder.
Be careful to keep the filed edges straight!

Left: Trim off the tab on the end and then tape the
skin in place.
Above: Turn the leg over and mark the edge using the
slot as a guide on both sides

Connect the edges together, Trim on the line.
Above: Check the fit, trim again if necessary

Left: When the fit is correct, glue the skin in place
and tape till the glue is dry.

At this point we're going to jump to the top of the shoulder even though we're not done with the lower part of the leg.
We do that because the top of the shoulder is done in sections and gets two layers of skin.
We'll work on the lower part of the leg while the glue on the sections at the top of the shoulder dries.


tape one end and wrap the rest of the skin around to
 get a feel for the fit.

Put a mark in the middle of the strip and a mark on the
shoulder to make alignment easier.

tape one side securely then turn the shoulder over
only glue about one third (1/3) of the piece at a time.

With the glue in place tape the just glued part securely
making sure the overlap on both sides is centered

We've taken off the tape on the part that was not glued for
this pix. Now put glue on the next 1/3 and Left: tape again.

Now we've got only 1/3 remaining unglued.

Apply glue and tape securely. I tend to turn the leg
over to use it's own weigh in addition to the tape.

Left: don't worry if some glue oozes out
Above: File the edge of the skin flush with the surface

Repeat the process for the second layer of skin.

I switched from Weldon #16 (syrup consistency) to
Weldon #3 (water consistency) to get a better bond
betweeen the two flat surfaces.

Once the second layer glue is dry file the edges off.

While shown as the final steps, you can work on the skins for the Ankle portion of the leg at the same time as the top of the shoulder.
Just let the glue set for a while on one end before starting on the other.

Trim the tabs on one long side.
Center the end skin and glue in place.

Update Pix
File the edges down. File to match the angle on the corner.

Update Pix

Update Pix
Remove the tabs on  one of the long edges. When you glue this piece in place line that edge up with the edge of the ankle curve piece. That edge will be the joint between the side and the curved skin. Work carefully.

Glue the corner pieces in place.

Note that the corner extends down onto the bottom of the ankle. you can also align the edge it's your choice where the joint falls

Next we'll wrap the skin around the bottom of the ankle. Like the top it's done in sections

Test the fit first

Above and Right: Glue one side wait till the glue dries

Left: check the fit, trim the end so you get a clean butt joint.
then glue in place
Above: If you were careful, your joint will come out better than mine.

Left: Trim the tabs off the top edge. Mark a centerline. Check the fit.
Above: Mark the edge locations and trim both sides. Take equal amounts from both sides if you can.

Above: tape down, check the fit
Right: glue down half of the skin.

Above: glue down the other half of the skin.
Right: file the curved edge to match the curve. Be careful not to gouge the flat part of the ankle with the end of the file.

Congratulations, you've got a finished Leg!

Now you get to do it again!


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