A broach looks like a thick saw blade and cuts by being pushed through a sleeve.  The brand I used was duMONT and you can see a picture here.

Apparently an Arbor Press is typically used to push the broach through the sleeve cutting a little bit with each passing tooth of the broach but on Nelson’s website he seemed to use a hydraulic press.  Since I needed one anyway to assemble the wheels and axles, I waited until one went on sale at Harbor Freight and used what seems to have become a weekly 20% off coupon to get one a great discount.

Here is the press and broach in action.

It takes too passes to cut the complete keyway.  For the second pass a shim is inserted behind the broach.  The final keyway size is 3/16″ x 3/16″ so each pass cuts roughly 3/64″.  It is fairly simple process but I was nervous that the press wasn’t pushing straight down due to the slop in the press.  A precision machine it is not.  Perhaps this is why an Arbor Press is typically used.

The finished keyway -

Definitely not your Grandmother’s broach.