A mandrel for winding sheet metal has a generally tubular mandrel body extending along and centered on an axis and formed at one end with a plurality of radially throughgoing and angularly spaced apertures each having a pair of closed and axially confronting end faces, a spreading bar extending axially inside the body and having a respective pivot adjacent each of the apertures, respective rigid links extending through the apertures and having inner ends carried on the respective pivots and outer ends projecting outside the body, and respective segments outside the body and each pivoted on the outer end of a respective one of the links. Respective pressure plates are operatively connected at the one end between the segments and the mandrel body.