If you have a bench grinder, and a reversible, variable-speed 1/2" portable electric drill, you have a 'lathe' to sharpen centre punches with.
Chuck a centre punch's handle in the drill, and arrange it so you can spin the punch's point-cone against a rotating grinding wheel. You want the punch's rotation to be counter to that of the grinding wheel. The punch needn't be spun at very high speed; just fast enough that the grinding wheel's effect on the punch's point-cone is kept more-or-less uniform.
The pointy ends of centre punches are not always perfectly straight, or coaxial with their handles. This method won't work with a crooked or badly off-centre punch, but a true or near-true punch will sharpen up quickly and easily this way.
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