PURPOSE: To position a crankshaft in a designated location by arranging an indexing motor to be freely moved between a first position at which rotatable driving engagement of the chuck is achievable and a second position at which the chuck is free to rotate about the spindle.

CONSTITUTION: A first latch 106 is released from the engagement with a gear 72 to freely rotate a pot chuck 64 about a chuck spindle connected to a driving shaft 32 shifting in the axial direction. The indexing motor 76 is energized to rotate a crankshaft 24 supported on the pot chuck 64 until a crank pin 30 reaches a desired position with respect to a grinding wheel 28. After that, a second latch 122 is engaged with a gear 72 to keep the crankshaft 24 supported on the pot chuck 64 from being rotated to the driving shaft 32, and then the indexing motor 76 is moved to a position for separating from the engagement with the driving gear, whereby the second latch is released to freely rotate the driving shaft 32.