A rotary workpiece indexing apparatus including a spindle supported in a housing for rotation about a spindle axis and including a workpiece-mounting surface coaxially disposed at one axial end of the spindle. A spindle lock releasably secures the spindle against rotational motion relative to the housing and includes clamps carried by the housing in circumferentially-spaced locations around the spindle. The clamps are driven to respective clamped positions securing the spindle against rotational motion by clamping the spindle to the housing through the application of clamping forces at respective spaced-apart locations around the spindle.