Milling the thickness taper along the length of the key for one of the gears on the #1 gear shaft. This is to make the key a wedge fit into the ramped keyway in the pinion gears in order to eliminate any tendency for the key to roll onto its corners as shaft rotation is reversed. As a point of interest, the two keys for this shaft were no where the same thickness. The tapers were close, but thicknesses were way off.

Photo by Rick Brigger