I set them up in a cheap import spindex, in a collet, and reduce them on a surface grinder.
Any grinder would work, so long as you can get a spindex clamped or mounted, and a way to "nudge" it toward the wheel with some control.
I use mist coolant running obviously wet, and you can't hog or it will grab and spit the rubber out of the collet.
However, it is a relatively quick process with steady gradual infeed or down feed. (depending on orientation of spindex to grinding wheel
36 AlO wheel, as open structure as possible, even induced porosity if available. Wheel hardness is irrelevent in this case, but mine are mostly I or J.
46 grit, same specs as above, will also work.