Sharpening Mortise Chisels

How does one sharpen the four-sided chisels used with the mortising machines? The bevel is on the inside.

Richard Jones: Use a mortise chisel sharpener. It’s a reamer with a pilot that slips up the inside of the tube— they look like countersinks for metal with a range of rod-like interchangeable ends to suit different chisel sizes. The auger is touched up with a triangular needle file or saw file.