The Swiss Army Knife

CS:R Center Foot

"Houston - We have a PROBLEM" - VEX Robotics posted this on it's Omni-Directional Wheels web page: "New for the 2019 FTC / FRC season, we've redesigned the 4” Omni-Directional Wheel to have a similar construction style to our 3.25” Omni-Directional Wheel. We decreased the number of rollers, to reinforce the body and improve the strength during sideways impacts. Don’t worry, it’s only a little wider than the current 4” Omni-Directional Wheel, and should still fit anywhere you were currently using a 4” wheel before". [emphasis mine].  Well, that's not quite true!!! The V1 Omni Wheel was 1.344 wide or 2.688 for the pair (not allowing for the spacer between them) while the V2 Omni Wheel is 1.53 wide or 3.06 for the pair (also not allowing for the spacer between them) and will be an impossibly tight fit in the 3.1 space of the SAK Center foot. [follow the links for copies of the original VEX drawings - they have removed the V1 drawing from their website]. (Added 04/22/19)

There are several options. First, if you like the V2 version of the VEX Omni Wheel, you can use just a single wheel set front & back. You'll need to revise the spacers on the bolt/axle to center the single wheel in the SAK mounting. It won't provide the same stability or smoothness that a 4 wheel center foot would have. Second, there are, naturally, other manufacturers of Omni Wheels. The discussion of alternate suppliers continues on the The World after VEX page.

Important Note for Customers who have purchased the SAK Center Foot from me. I will supply you, at no charge, the bearing mount parts you need to use the ServoCity Omni Wheels in the SAK Center Foot.

Original SAK web page follows:
In late 2016 I was ready to modify the design of my CS:L Center foot to bring it up to the CS:R standard (Scroll down to section on Center Feet Compatibility). Of all of the Droid parts that I provide the difference between the center foot apron and the outer foot aprons has always bothered be as 'not looking right'. While the changes look cosmetic from the outside, because the apron trim got taller (to match the outer foot trim) the structural mount would have to be changed to lower the caster position. That meant doing a full re-design anyway. In talking with PaulG (The Hobby Guy) about the redesign, he challenged me to create a center foot that could accommodate a VEX Omni Wheel based on work published by Kevin Holmes.  After designing a Shell that would accommodate the Vex Wheels, I realized that it could also handle the Colson caster used in my original design or could also be equipped with a pair of Omnitrack ball casters and so was born the "Swiss Army Knife" center foot!

This is the Colson Center Foot, with only 1/2 of the shell in place.

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.

This is the Vex Omni wheel Center Foot, with only 1/2 of the shell in place.

The image, like the one above, links to a 3d pdf file.

This is the Omnitrack Omnicaster Center Foot, with only 1/2 of the shell in place.

The image, like the one above, links to a 3d pdf file.

Disclaimer: Many of the images used here show a Half Moon mounted on the Shell. This is shown for illustration ONLY. Half Moons are NOT INCLUDED as part of the SAK CS:R Center Foot parts package.

Additional parts:  There is a list of parts and suppliers here and also with the drawing set. Note: if you are thinking of using my design for the outer feet then you might want to purchase the wheels at the same time. The larger the order the bigger discount you are likely to get and you only pay for shipping once. I have an aversion to installing bolts in holes threaded into styrene parts when the bolts are under stress. In order to attach the Center Foot to the Center Ankle I'm using a 3/8 x 1 in. aluminum spacer in the ankle and a pair of 3/8 shoulder bolts in the foot shell. While these are common parts, the 5/16-18 tap needed to tap the spacer is not. I can provide tapped spacers, with or without the shoulder bolts, at nominal cost (and a lot cheaper than buying the tap!)

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). I've also put together a set of assembly instructions to help you put the foot together.
 
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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