The Outer Foot Shell Assembly

The Outer Foot Shell Assembly consists of the shell itself and a structural frame, called a brace, that fits inside the shell and attaches to it. The completed shell slides down over the top of the Outer Foot Drive. It's a good idea to have completed the drive assembly before going too far with the shell since you will want the drive to test the fit of the shell.

We'll start with the shell,
move to the brace,
then integrate the two,
and finish with the trim.

We'll begin with the Shell Assembly


parts have been marked for cutting bevels
part noted in red is part of the drive assembly.


bevels are cut on the 'underside' of some of the parts which is one of the reasons they are cut on a router table.
Left: this is how the parts will look when they are shipped to you,
above: note that the parts inside of the side frame are left in place. this is to reinforce the frame during assembly.
Below:  add tape over the cuts in the frame to reinforce the tabs.
Right: lay the shell parts out with the bevels down
Below & Right: carefully line up the edges of the panels and tape them together

fold the remaining edges together and tape, making sure the corners line up

this is a view from the inside of the shell
Note: the curved end parts have proved troublesome to print without using Brim. The parts supplied will have Brim still attached. The parts shown here have had their brim's removed. For more information on Brim see the 3D Parts Care & Finishing article.
Left: the curved end pieces fit into a slot made by the channel in the bottom plate and the filler strip. For a good fit you need to sharpen up the edges on the curved piece with a file.
Above: tape the bottom filler strip to the ends of the two shell end plates. It will 'droop' a little, put a piece of scrap under it to keep it flat while you are taping it.

turn the shell on end, use a weight to help prop it up. place the curved end in position, using the lap joint on the top edge to hold it in place.

with a light behind the shell you can see where the gaps in the joint between the curved end and the shell end are. working slowly and carefully, file the profile of the curved end to eliminate the high spots.
Left: with a little effort you can close most of the gap. It's not worth trying to make it perfect. Make it as tight as you can with a reasonable effort. Repeat the process for the opposite end

Above: With both ends adjusted for a good fit, put the shell on the brace and tape the curved end pieces to the rest of the shell. Tape the lap joint on the inside for extra support. Then remove the brace and (see below) glue the joints on the curved end pieces.
Left: run a bead of  Weldon #16 down the curved joint. If you get some weldon onto the bottom (flat) edge of the  curved edge  scrape it off as that part needs to be smooth to mate with the brace bottom plate.
Above: note the use of tape on the inside of the lap joint to hold the curved ends in place while the glue sets. take the opportunity to put a few drops of  Weldon #3 into the inside of the lap joint while the Weldon #16 on the curves is curing.
Don't use Weldon #16 yet. that will go on later!

Set the Shell Assembly aside for the moment and start on the Brace.
Below: this is the Brace bottom plate
Right: In addition, there are 3 Brace vertical components (2 outside, 1 inside) on the Drive structural panel
Left: there is a also 3D printed inside brace, which has to be assembled from components 
Above: Use the supplied pieces of filament as alignment pins. After dry fitting, apply glue and clamp.

Once the glue is dry the excess filament can be cut off

the 3D printed parts have an edge on the base of the parts where printing was started on the glass plate

there may also be excess glue that has oozed out. both need to be filed smooth.
Below:  I've clamped the  brace in a vise to make it easier to file. You need to file all edges
you should also file the nut trap and make sure that you can fit the nuts you will be using in before you assemble the brace to the shell. It's OK if the nut is loose, you can always glue it in place.

The brace bottom plate slides down on the guides channels on the drive. The corners in the bottom plate need some additional clearance.

Just sharpen up the corners until it's easy to slide the bottom plate over the drive without it catching.
With 20/20 hindsight, I glued the bottom plate filler piece in place when I should not have. You'll see it in the photos that follow. Pretend that it's not there. When I assemble my next shell I'll revise the pictures.


The shell is held in alignment by 4 vertical braces, 2 outside (triangular) and 2 inside (curved) that ride in tracks on the drive frame.

There is a lot of symmetry in the design. The motor extends into the inside of the Battery Box (on the inside of the droid). The vertical inside brace with the nut trap (orange here) goes to the rear of the foot so make sure it matches the shell you are working on.

After checking to make sure the brace slides on/off easily, turn it over on a hard surface. The tops of the braces line up with the top of the drive.
Left: Put just one or two drops of Weldon #3 where the braces are. be careful, you don't want to glue the brace to the shell!
Above: Once the glue has dried remove the brace from the drive and glue the braces from the top.

there are 4 Nut Traps to be installed onto the brace.

Dry fit the nut traps to make sure they sit flush with the surface of the bottom plate.
Left: I made pencil marks to guide  me when I put the Weldon #16 on to hold the nut traps in place.
there is also a nut trap on the inside of the shell, shown here upside down. the inside shell is drilled for nut traps in both locations. The other one is in the vertical brace. Choose the location accordingly!
Below: now would be a good time to install 1/4-20 square nuts into the nut traps on the brace. Here I've installed bolts to hold the nuts in place while the glue dries. Try to avoid getting glue in the bolt threads.
Left: At the same time, install a nut into the shell nut traps (both here and into the vertical brace (the 3D printed orange one)

Now that the Brace is complete, we can integrate it with the Shell.

With the brace inserted into the partial shell, mark the locations shown above. Mark on both inside and outside of the Shell.

 Since the Brace must sit flush with the top of the shell you want to avoid a glue "bump" when you glue the shell parts.

Flow Weldon #16 into the internal joints of the shell. Now you know why we put tape over the full length of the joints!

if you get glue into the areas where the braces go just scrape it out!

with the shell glue dry, put the drive back in and mark the location of the top of the vertical braces.

clamp the vertical braces in line with the marks. Glue from the inside using Weldon #3. Be very careful to not get the weldon on the outside surface!
Left: the vertical braces on the inside are lined up by usinf a bolt (orange) and using the open bolt hole (white)
Above: clamp the white vertical brace and then use Weldon #3 and then Weldon #16 to anchor the braces.

Once the shell joints are dry clamp the brace in place. Don't clamp too tightly as you can distort the frame. See later!

With the outside edge of the frame clamped, put the bottom plate filler in place (it's held by the spring clips) clamp the shell to the vertical braces and clamp the inside edges of the curved pieces to the brace.
Left: flow some Weldon #3 into the brace joints between the filler and the curved piece.  You are gluing both to the brace bottom plate.
Turn things over and run a bead of Weldon #16 into the joints of the shell marked by the RED fingers. Don't put glue into the joint marked by the GREEN finger (see below for why!)

remove the panel from the side frame and the smooth off the tabs

Insert the side panel and use it to check the spacing between the frame and panel. You should have equal spacing particularly at top and bottom. I managed to distort the frame when I glued it and the space at the bottom was visibly bigger!

This is the way the spacing should look! And yes this is the same shell. I had to "disassemble" the joint (remove the putty and cut the plastic cement out) and then re-glue it. 

Rather than clamp it, which might distort the edge, tape the side panel into the frame inside (above) and out (Below - left)
Above Right & Below: then tape the edge of the frame to the bottom along the edge.  Once taped securely, apply first Weldon #3 and then Weldon #16.Right: Once the glue has dried file down any rough edges.
Left: I'm using Squadron Green putty here to build up a smooth flat surface for the apron to attach to.
Use a little at a time (Left is too much) and smooth it flat while it's still wet. Don't go back once the putty is set as you wind up with a rough surface that you have to file/sand.
Below: This was my first attempt.  I should have known something was wrong when I got a broad bead of putty on the long edge and a narrow bead on the ends. When I filed the surface smooth I was also taking too much edge from the shell! See how much wider the white edge is.
Above Right: While you have the putty out remember to fill in the exposed end of the bottom plate and shell jointAbove: also put some putty on the curved end joints with the shell (remember the battery box covers most of those joints.)
Right: I use a sanding sponge to smooth the curved joints.

Finally we'll add the remaining finish trim
Below & Right: Temporarily place the shell over the drive. Notice that the slot in the shell end is deliberately cut short. I've got the drive clamped in a vise for support.
Carefully file the bottom of the slot till it's the same height & angle as the  drive channel. You don't have to get it perfect now as we'll come back after we attach the trim strips on the ends.
The rear trim strip has two locator holes. Use 8/32 screws as pins to align the trim strip on the shell end panel. Note that the strip is not symmetrical! make sure the edge lines up on top!
Clamp the trim strip in place, remove the screws and put a few drops of Weldon #3 in the holes to hold the strip in place.
Left: I found that I could use sockets from a socket wrench set as alignment "pins" for the front trim strip. Anything that won't be affected by the glue will work.
Again, clamp the trim strip in place and then apply some Weldon #3 to hold the parts in place.
Left: with the glue dry, put the drive back in
Below: file till the edge is flush
Left: the side panel has 3 trim pieces, each with it's own set of locator holes. (I'm not yet providing a half moon piece!)
Below: Put the pieces in position, clamp with a spring clip  (not shown)
Left: put in a drop of Weldon #3 thru each hole.
Above: The Apron consists of 4 3D printed parts.
Below: the longer apron pieces are quite flexible, and may be slightly warped. The warp will be removed during assembly.
Right: the two shorter apron parts have been printed using "support". Support was needed to hold up the surface where the notch is.

Left: You can see the boundary of the support on the edge
Above: use your favorite cutting tool to separate the support structure. It's deliberately flimsy and should separate easily.
Below: separate support from both ends, the top edge, and the wedge section.
Right: clean up any rough surfaces with a file. You can be agressive on the wedge area, it's on the inside of the apron.

You want the corners coming together only at the front edges so file down the rear edges of the corners.

I'm using corner clamps to hold the apron pieces together while gluing. Because of the shape, it will take 4 clamps to hold the pieces in position. Make sure the edges meet cleanly.

While the C-Clamps hold the pieces against the bottom of the corner fixture, adjust the clamps of the corner fixture so that the edge shown above is flat against the bottom of the fixture. Drip just a drop or two of Weldon #3 into the joint and let it dry.


once your joint is dry, remove it from the corner clamp. then run a bead of weldon #16 down the inside of the joint. don't get any on the top or bottom of the apron.
The finished apron!
I tend to glue the corners together in pairs. That way the Weldon #16 can be drying on the first set while the Weldon #3 dries on the second.  However you work, double check that you are gluing the correct set of joints. Trust me, nothing hurts as much as having to tear apart something you just glued because you made a LH corner and you needed a RH one.

The last step is to attach the apron to the finished foot shell.
First step is to line up the apron so that the slots in the short sides match the center channel where the drive slides into the shell. More than likely your apron will be slightly smaller than the edge of the shell. You want to position it so equal amounts of edge show on all sides. This is easier said than done. Once you have a feel for how much space that should be, start on the inside edge since that's the easiest location for clamps. Clamp the ends first, setting up equal amounts of edge as shown below. Then use the clamp in the center to match the spacing in the middle. Start by dripping Weldon #3 into the joint along the edge (not in the corners for now)
Below: When the Weldon #3 has dried, without removing the clamps, run a bead of Weldon #16 along the joint, Let that dry too.
Right: now move onto the other long edge. More than likely the edge will not line up but you can gently push it into position.

this time you have to clamp from the inside. Shift the apron so that equal amounts of the edge of the shell are visible on both ends.  Apply Weldon #3 and then Weldon #16. Let everything dry.

Moving on to the end you can see where the apron is bowing out slightly in the middle.

I didn't have a clamp long enough to reach that wasn't too big for the project so I improvised a clamp point in the middle.

Don't get carried away with the clamp, you don't want to break the apron off! There's no way to clamp down on the ends, so you are relying on the existing joints.
Repeat the Weldon #3 and Weldon #16 sequence and then do it again on the other end of the apron.

One of the things about prototyping is that it leads to changes. You might have noticed some pencil lines on the bottom of the brace in the instructions above. When I did the design I was concerned about strength over convenience. Having now assembled the actual parts I find that strength is less of an issue than I had originally thought and that convenice is important. In the next iteration the bottom will be modified so that the shell can be slid on or off without having to remove the motor from the drive!

Congratulations! Do it all a second time and you've got a completed set of Outer Foot Shells to go with your Outer Foot Drives!


Please send me comments, questions, or suggestions on making the design better to me at: fpirz (at) media (dash) conversions (dot) net


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