The SAK CS:R Center Foot Assembly

There are several phases to the Center Foot Assembly. There is a list of parts and suppliers here and also with the drawing set. 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 download also includes .stp 3D drawings of the Apron and the Hex Nut Trap. (That note also appears on each of the drawings)

First, here are drawings of what the CNC panels look like.

This is a 20x24 panel of .125 thick parts.
It includes both Shell parts and Structural parts.

This is a 20x15 panel of .1875 thick structural parts

This is a 20 x 9 panel of .1875 thick wheel parts
A note on the packaging of the SAK CS:R Center Foot:
Styrene Droid Builders are usually on a budget. Not everyone wants to spend the money for VEX or Omincaster parts early in their build. As a result I made the Wheel mount parts an optional, extra cost, part of the package.

If you start with the Colson version, you can always purchase the Wheel Mount later on, unbolt the Colson mount and add the Vex or Omnicaster wheel.

Also note: While the wheel mount parts for the Vex and the Omincaster wheels are the same, once you assemble the Vex version you cannot add the panels to mount the Omnicasters. Same is true if you assemble the Omnicaster version.

The first step is to separate all of the parts from the sheets they were machined from, remove the tabs and flash, if any.

Bottom Plate Construction Issues
Structural Assembly
Center Strip Construction Issues
Foot Shell Assembly
Colson Caster Version
Vex Omniwheel Version
Omnicaster Version
Finishing Touches

Bottom Plate Construction Issues - There are some construction issues with the Bottom Plate best talked about before we are hip deep into the build.
There is a problem in mating the foot shell with the Bottom plate.

The Shell wraps around the Structural Assembly. The edges of the Bottom Plate have been tapered to mate with the shell.

This is a view of the Shell/Bottom Plate joint on the long sides of the Shell. The gap allows the parts to meet at the edge.

This is a view of the Shell/Bottom Plate joint on the short sides of the Shell. The Highlight illustrates the problem.
The bottom edges of the Shell parts and the Bottom Plate have both been cut to a 30 degree chamfer. That's the smallest angle that I can cut with a 'standard' router bit. Non standard cutters would be too expensive for me to use.

As a result the short edges of the bottom plate are going to have to be filed  slightly to create the necessary clearance. I'm bringing this up now because it will be easier to file the Bottom Plate before the structural assembly.
To deal with the problem We'll jump ahead and tape up the shell
Tape the center section of the side panels in place to improve the rigidity of the panel
Place the panels on a flat surface,  bevels down,  and tape the edges.  Make sure the top and bottom corners line up.
Below: Then fold the pieces in together to make half of the pyramid shape. Fold the Shell Top  down and tape.
Assemble the second half shell the same way.
Right: assemble the two half shells to make a full shell.

File the short edges of the Bottom Plate. Use the Shell assembly as a gauge.
When the Shell Sits down onto the edges of the Bottom plate you have a good fit.

Structural Assembly
This is what we're going to assemble in this section.
The pictures used are from the first prototype while the illustrations are from the current version (version 1).

Collect the parts needed for the first step

Load Plate - this part has changed, now has hole in center and holes for nut traps

Center Strip (note bevel ends face down)

 Bolt Panel. There are 4 of these .125 thick. Glue pairs of them together to make 2 bolt panels .25 thick - there is an additional hole not shown, make sure those line up.

note the bevels on the center strip, now visible.
Left: make sure the center strip bevels are down. note that the center strip extends beyond the edges of the bolt plates.

Let the glue dry before the next step. Time to collect more parts.

Shell Braces

Bottom Plate - this part has changed. The center has a circular cutout. the mounting slots for the Shell Braces have moved closer to the short ends

Insert the Shell Braces into the Bolt Plate assembly and then into the Bottom Place. Note the orientation of the bevel on the Bottom Plate.

Clamp to get a good bond between
the Shell Braces and the Load Plate. Let the glue dry before the next step.

Center Strip Construction Issues - There are some construction issues with the Center Strip that also need to be talked about.
This is a closeup view of the center strip in the Structural Assembly. It extends beyond the edges of the Bolt Panels. That was done deliberately. My earlier foot design had a separate, decorative, strip of plastic to bridge the gap between the Shell End panels.

This design eliminates that extra strip and uses the Structural center strip to bridge that gap.

However, that has a consequence. The shell has to be assembled in halves so that the "half shell" pieces can be put in place around the extended edges of the center strip.

That's why the note about not gluing anything earlier in the
Bottom Plate Construction Issues section.

Foot Shell Assembly:
You already have most of the work done. This time around you get to put glue on the parts!

Tape the center section of the side panels in place to improve the rigidity of the panel
Place the panels on a flat surface,  bevels down,  and tape the edges.  Make sure the top and bottom corners line up.
Then fold the pieces in together to make half of the pyramid shape. Fold the Top Strip down and tape. Right: repeat the process for the other half shell
With the panels taped together, it's time to do a test fitting to see how the foot shell fits over the structural frame.
Right: Make adjustments in the structural frame for fit. Tape the two half shells together around the frame once you are happy with the fit.
Turn the frame on it's side and glue the shell to the frame. You won't be able to reach all the areas that need to be glued yet.


Once the initial glue joints have dried, remove the tape holding the center sections of the side panels in place. Those openings will give you access to the rest of the shell's glue joints. Do a second pass gluing the shell to the frame.

Next, once the glue joints are dry, we will add the apron braces and the apron parts to the bottom of the shell

The Apron Braces are designed to offer more support to the apron pieces than the previous center foot design. They also provide an anchor to clamp the apron parts to.

The Bottom Plate is also a solid surface in this design. The previous design had needed putty to fill the void between the Shell and the Bottom Plate.

Note: make sure the brace pieces are lined up. The inside edges of the braces should be vertical. Note the brace marked in the pix is out of line.

Apply the 3D printed Apron trim parts to the Bottom Plate.
Note: Apron Braces are now cut shorter than the Apron to eliminate the need to file them lower so they don't show.

Depending on how your Apron was printed you may want to reinforce the corners by filling in the corner voids with an epoxy cement or similar product.

Colson Caster Version

In the 3 sets of assembly drawings shown next (Colson, Vex & Omniwheel) the red piece is the Load Spacer. It goes between the Wheel Mount and the Load Plate. I supply a piece of .1875 plastic with clearance holes cut. Those of you who like to experiment can try substituting a layer of resilient, shock absorbing, material to see if you can give your R2 a smoother ride.
For the SAK center foot we created a mount for the same Colson Stem caster we used in  our earlier center foot design. Why not use a plate caster instead? We did a design with a plate caster and then discovered none of the suppliers carried it. The stem caster, on the other hand, is available from several suppliers.

Above: Note that the Load Spacer is NOT glued to the Colson Mounting Plate.

Left: The Colson Mount, 3 Caster Spacers and the Caster Shoulder Spacer are assembled as a stack. Note that the recess cut into the Caster Shoulder Spacer mates with the rounded part of the caster. Glue the stack of parts together. Use the Caster Stem bolt to align the parts.

Once the glue is dry you can install the caster in the mount. Then bolt the mount and the Load Spacer to the Load Plate.

Not Shown: the Hex Nut Traps on the top side of the Load Plate are designed to hold Nylock (or equivalent) nuts. Builders have had bolts that were not secured with vibration proof fasteners fall out!

Vex Omniwheel Version

Assembly of the Vex Wheel Mount is simple.
Be careful when  you assemble the wheel mount not to get too much glue between the ribs. They slide in on the shell braces seen below. The notches were enlarged after the first prototype. Check the fit before you install the Vex Wheels.
Right: Install the Vex components into the Wheel Mount

Install bearings in the Wheels.

Use the spacers both between the wheels and also between the wheels & the Wheel Mount.
The Wheel Housing bolts into the bottom of the Center Foot. Use the supplied Wheel Anchor Spacers between the Wheel Anchors and the Bottom Plate. If you are trying out a shock absorbing Load Spacer then swap out the supplied Spacers for ones made of the same shock absorbing material.

Not Shown: the Hex Nut Traps on the inside of the foot are designed to hold Nylock (or equivalent) nuts. Builders have had bolts that were not secured with vibration proof fasteners fall out!

Omnicaster Version

The Wheel Mount for the Omnitrack Omnicaster Wheel Mount is assembled the same way as the Vex Wheel Mount except that the two Omnitrack Wheel Mounting plates are added to the assembly.

Note the orientation of the mounting holes. Make sure you install the plates as shown. It spaces the pair of ball casters symmetrically  under the Center Foot mounting point.

Finishing Touches:
Attaching the Center Foot to the Center Ankle

File a flat in the threaded spacer. You don't want to take a chance that one of the bolts will freeze up and you can't unscrew it because it's spinning around in the ankle.

Drill a hole thru the bottom of the center foot.

Above: And tap so you can insert a setscrew
Right: make sure the setscrew engages the flat on your spacer.

Once you think everything is anchored in place give the bolts a hard twist to make sure the threaded insert does not move.

There are 8 center Ankle Locks on the .125 panel. In theory, 8 of the parts glued together should fit in the 1 inch channel of the center foot. However, in practice, if the panel is only slightly thicker than .125 then 8 of the parts will not fit. Use the number of parts that works for you.

This is a cutaway of the structural assembly with the two bolt panels removed. The Ankle locks are designed to limit the range the center foot can pivot. It is held in place with a .25 thru bolt.

Congratulations, you've got a completed Center Foot for your R2!