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Best Type and Source for Omni Directional Wheels?

I am currently in the process of compiling a shopping list for an omni directional robot a team and myself are planning to put together.
For this, wheels that go both forward/backward and left/right are needed.
From what I have found, there are two variations: Omni and Mecanum wheels. Due to our design, we will be needing Omni style wheels. 
However, doing some research, I haven't been able to find much of a selection! So far, robotshop is the only website that actively stocks them, and they sell two brands/types, one being VEX. 
Is there a generic name for these types of wheels? Where would I be able to find more/a different source for them? Why are they so expensive? If you've used an omni wheel, what type/brand was it, and what was your opinion on it?
The big problem is, is that we need to mount the wheels onto 5mm stepper motor axles. Half of the wheels I found on robotshop had square bores, and the other half used an interchangeable hub, none supporting a 5mm axle. Any ideas as of what to do here?
We would like to get four wheels, preferably for less than 50$.

kelseymh4 months ago

McMaster-Carr calls them "multidirectional acetal wheels" (https://www.mcmaster.com/#omnidirectional-wheels/). They sell 2-in (5 cm) diameter wheels for $16 each, close to your budget.

The ones you found from RobotShop are probably your best deal.

You should be able to make an almost trivial adapter using square rod and drilling or milling a 5 mm hole down the axis.

RocketPenguin (author)  kelseymh4 months ago

Never actually thought to check McMaster.

Yea, I am not as fond of those wheels that McMaster has, so I'll probably just stick with the robotshop ones.

The people of robotshop were actually kind enough to send me a 3D cad of the wheel, and so I think I'll just custom design an "adapter plate" to have the screws of an actual hub line up with the center of the wheel.

If only I had access to a mill :P I might try using a drill press, but will probably go with the adapter idea first.

Thanks for your input!