Mecanum wheels are too expensive. Let's be honest--when the cheapest option is a set of 4 tiny wheels for $60, it's hard to justify the cost for basic hobby robotics. However, with the ease of access to 3D printing these days, I think it's high time to start working on high quality, cheap wheels that you can model into whatever you need!
I tried to keep this design simple, with as little hardware as possible. Due to lack of time, the hardware pictured is not actually the hardware I would recommend, I just didn't have 7 days to wait for my McMaster Carr order to come in!
Eventually, I will update the models to handle M3 bolts, so things will run more smoothly, but for now I'm out of time =(
All you will need is:
-48x 6-32 x 2" bolts (on McMaster-Carr pack of 100)
-3x M3x0.5 x10mm bolts (just buy a small pack at Home Depot)
-a can of Plasti-Dip (at Home Depot, 14.5oz) - $6.98
-a 7/32" drill bit and a 9/32" drill bit (these will be changed later when updated for the M3 bolts)
-Some way to 3D print the frame and rollers - $Variable
This sums to just under $20, the cheapest way I've seen to get mecanum wheels anywhere!
Step 1: Everybody Loves to Print
Whatever your method of choice, first thing is first: you've got to print out your frame and rollers!
I've linked STLs and the Fusion 360 files, for anyone who wants to modify their own interface to these wheels. Due to lack of time, I haven't been able to come up with a good all-around interface to go with a servo, so these models are currently designed to press-fit onto some stepper motors I got from a friend. Message me if you need help adjusting the model!
The print is simple, no support material needed:
-48 rollers (12 per wheel)
-two "Front Frame 1" models (the 1 and 2 models are mirrors of each other, as needed for a mecanum drive)
-two "Back Frame 1" models
-two "Front Frame 2" models
-two "Back Frame 2" models
Now prepping the prints is the secret to getting your wheels turning smoothly.
Step 2: Prepping the Rollers
Since so much rides on the rollers (the whole weight of your robot, in fact), making sure that they are in decent shape goes a long way in the end performance of your wheel.
Bore out the center hole with a 7/32" drill bit, to smooth things out a little on the inside.
Then, with the roller still on the bit, you can turn it against some sandpaper to smooth out any rough parts near the edges (my printer always puts a lot of extra material on the bottom layer...).
With that, you are primed and ready to make some use of your Plasti-Dip!
Step 3: Taking a Dip
Now for the fun part-stinking up your garage with Plasti-Dip.
Plasti-Dip is some amazing stuff! It leaves a very tightly-adhered coating of rubber, perfect for giving your rollers the grip that hard plastic lacks. The tricky part is finding a way to dip and dry them without messing up the finish!
First, you have to mix your Plasti-Dip up really well. A phillips screwdriver works fantastic, since the coating can be rubbed off later, once it has dried.
For actually dipping the rollers, I used some 6d 2" nails I had lying around, but paper clips could also be used. I'll post a picture of how to bend a paper clip later. The most important part is that the thing you use to dip the rollers can either be hung, or has a spot flat enough on the bottom to let the rollers sit and dry.
When removing the roller from the dip, there will likely be large drips forming on the bottom-make sure to wipe those off before you let the roller dry. This helps avoid interference-causing bulges.
When the outer coating is dry, you can do a second coat if you want, but one coat seems to do well enough for me so far. Anyway- when you are satisfied with your outer coating (after about 30 min of drying), remove the nail/paper clip from the center and let the roller fully dry.
There is one last set of boring to do before the final assembly.
Step 4: The "Boring" Part
To get the Plasti-Dip off of the ends of your rollers, simply rub them a bit on some sandpaper, it'll ball up the rubber and come off quite easily.
Then, remove any excess rubber from the center of your rollers by boring it out once more with the 7/32" drill.
Using that same drill, bore out each hole on the side of your frame with the posts and motor mount. Your screws with self-tap into these holes.
On the other side of the frame, the side with the three screw holes, use the 9/32" bit to create a clearance hole, allowing you to control the tightness of the screw on the roller.
Step 5: Makin' Them Mecanums
With all that "boring" stuff over, you can finally assemble your wheel!
Drive the three M3 bolts through the one side of the frame into the other. Orientation only matters to the point that the flat side faces outward.
Then you are ready to drive your bolts through the clearance hole and your rollers, self-tapping into the side of the frame with the motor mount. Once all screws are in, you are finished! $20 and some minimal effort later, you now have your own set of mecanum wheels, ready to be incorporated into any robotics application you can think of!
(note: this instructable is not even close to finished-I will continue to post pictures and further instructions specific to the other wheels, to incorporate M3 bolts, and have a general servo mount. Please comment any other changes you would suggest!)