The recess is put in the chuck, the external diameter is turned and the front face is trued up
A plywood spigot is screwed on (the screws have to be very near the centre to be out of harm's way)
This spigot is trued up but not cut right down to the disc – there is a danger of leaving a gouge mark
Hold the work in the plywood spigot – enlarge the recess until the disc width is right. True up the face
Attach a catcher disc as before and cut a spigot in it. If the old catcher disc is used, re-cut the spigot
Protect the edge of the disc with tape. screw in catcher screws as before. Remove plywood spigot