PURPOSE: To easily perform the positioning of a crankshaft for a corresponding spindle by performing the positioning of each shell body for the corresponding spindle by sockets of double two-plane bodies, in a grinding machine in which the rotary shaft of the spindle is matched with the axis of a crankpin to be ground.

CONSTITUTION: Each shell body 16 is mounted on a spindle 4 by two two-plane bodies. A first two-plane body is constituted of a vertical surface P at a right angles to the axis of the spindle 4 and a vertical surface P1 forming an angle a from 75° to 80° to this plane P. The other two-plane body is demarcated by the surface P and a surface P2 which forms an angle β from 75° to 80° to this surface P and cut this surface P along a horizontal ridge A which is orthogonally crossed with a symmetry axis X-X of a machine. Also the surface P2 is orthogonally crossed with a symmetrical surface passing the axis X-X of the machine. The two-plane body P-P2 exactly determines a location of a shell body 16 for the symmetrical surface of the machine and the two-plane body P-P1 determines a distance of the shell body for this surface. Therefore, the positioning of the shell body is accurately performed.