A good deal for $5. I have good-enough results for postwar Lionel and Marx axles by strategic light hammering with the axle laid on a heavy steel plate, wheels removed, and gauged by eyeball. That is for wheels that rotate on the axle, not the entire axle rotating. Don't hammer the axle where the wheel turns ! Old trains often have bent axles from hard play decades ago.
My steel plate is actually a clamp block from a rail curve lubricator, with shapes that allow me to position the axle bumps in a recess so I can work the full length of the axle. If you follow that. Something that the gadget above doesn't have.