I’m trying to build an omni-wheel robot using VEX wheels and standard motors. I need help/ideas with coupling these wheels, with the VEX 0.125" square shafts, to motors with a 6mm shaft. Any help that can be offered would be greatly appreciated.
Wheels: http://www.vexrobotics.com/276-1902.html
Motors: http://www.lynxmotion.com/p-186-gear-head-motor-12vdc-301-253rpm-6mm-shaft.aspx
You may be able to use the couplers that come with the 2-wire motors.
I am not sure if this part does what you want it to. I think it does. But they are $6 a piece.
Half way down this page, FDM CA is how it’s listed. Looks like a 1/4" to 1/8" square shaft coupler to me. I can’t be sure though. Give them a call.
http://www.finedesignrc.com/hardware.asp
Do you have a motor mount mechanism devised as well?
That looks very similar to an ftc motor. what i would do is sandwich these
and i think screws will fit through the openings within the omniwheels. to reduce the need for maintenance, i would also install padding in between the hubs and omniwheel.
But just a little curious, why are you using that combination of parts?
Good question. More power is my theory. You can see the power curves from the link he provided. A bit more than a 393 for higher speed too.
A 12V motor - I wonder what the controller is on this guy - probably not a Vex cotrex. Arduino maybe? Care to give some hints Weep-Tes? A different competition? Or just for fun?
You would be absolutely correct in assuming that I am using these motors for their power. As for the wheels, I really just liked the way that they look. I also thought that VEX sold some sort of coupler to go from their standard square shafts to a standard DC motor shaft (since they do sell a similar motor on their website).
It is for my Bachelors final. It is a competition of sorts:
Myself and 4 others in my class are all building one. They will all have at least one hacked AirSoft gun on them, a wireless camera, and will be controlled wirelessly via our computers.
I will be using an Arduino Mega for the brain. We will be at least 250ft from our robots with no view of them. The end result will look we are playing a first-person shooter on our computers, while some distance away others can watch our robots that look like they are battling each other autonomously.
What couplers are you talking about? Do you have a website or any other info on them?
This is a pretty good idea. The outside diameter of the VEX shaft coupler is 1/4 inch (6.35mm) so find a shaft coupling that can accept the 6mm motor shaft at one end and the 6.35mm VEX coupling at the other.
You can buy the VEX part here
You could use something like this to adapt it to the motor, look on eBay as they have cheaper ones there.
I have a project where I am building a really fast, light-weight robot with non-vex motors. We have found the motors we want and the shaft diameter is 4mm. I want to connect this motor to the large VEX omni wheels.
I measured the vex shaft and diagonally the shaft measures just over 4mm. If I got a shaft coupler for a 4mm shaft, would the VEX shaft fit?
Might sound crazy, but hear me out. I do this for everything now it works pretty well as long as you get things nice and straight. Drill a .25 inch hole down the center of your wheel and then press a .25 inch hex shaft down the center. The fit is insanely tight and won’t budge at least under my testing.
If you want to connect this to a motor simply use a shaft coupler like Jpearman has posted and round one end of the hex shaft with a lathe to that specific size. 
I would rather not ruin the wheel.
Maybe something like the VCX30JLRG - Hub Adapter Kit for VEX Robotics 5" Wheels
could be Machined for the other size Vex Wheels…
I actually found these: 2pcs Flexible Shaft Coupling DR 5mm x 8mm D19 L25 CNC Motor Coupler Connector | eBay
It seems like I should be able to fit a vex shaft straight into the coupler. I can then tighten the set screws like on the vex shaft collars.
My school also has a metal lathe, but I have never used it. If these couplers don’t work, we will probably make the ends a little bigger.