Jacobs Chuck | Model Engineer
And run a file or stone around the snout to knock off any high spots before pushing it into the ring. Sometimes there are burrs there, especially around the three keyholes. When you disassemble it, the nut will be in two pieces and look as if it is broken. It is made that way: in one piece with weak points and then split to facilitate assembly. If you cannot find any visible cracks, and do not want to proceed straight to disassembly, make up a clamping collar to go around the ring, squeeze it up tight and see if the wheelspin disappears.
Source: model-engineer.co.uk