I have made a set of mecanum wheels that were not for FIRST robotics but used in a high capacity application and the machining of them does not take too much work but more a good deal of time. I can speak more on the machining of the hubs than how to make the rollers because I used a neoprene rubber for the rollers with a nylon core and the neoprene was just too soft. As well as there are much better ways to make the rollers than how I did (CNC Lathe).
As for the machining of the hub I did it in a 3 step process where I started with a 6x6x1in block of aluminium and the first operation I cut out the profile of the wheel hub, the center bore for a FR8 bearing and a 6 hole pattern at 1.875". Then using the 6 hole pattern I used it to mount it to a fixture which stood the hub up vertically and I then used that to cut the fins and drill a hole that I would a #10-32 set screw in. I did 6 fins so that they would line up with the hole pattern and it just made life simpler. I rotated the part to cut each fin one at a time. Then I had another fixture which held the part at a 45 so that I could fillet the corners of the fins and drill a 1/4" hole for the axle of the rollers. The part was also rotated on this fixture for each fin.
I have also uploaded some pictures of the manufacturing process here:
https://picasaweb.google.com/j3ffr3y...eat=directlink
Hope that helps.