Locking mechanism for the mandrel collar on milling and drilling spindles

A tightening apparatus for a mandrel collar of a milling or boring spindle, said apparatus comprising:

• a spindle housing having a central bore therein, a circumferential groove around said bore and at least two spaced holes intersecting said bore;
• a mandrel collar extending into said bore;
• a pair of arcuate clamping elements retained by said circumferential groove within said bore, each said clamping element having a notch adjacent one end thereof;
• retainer pin means in each of said spaced holes engaging said notches to prevent circumferential movement of said clamping elements;
• and
• means for engaging the other ends of said clamping elements and forcing said other ends toward each other thereby gripping the outer surface of said mandrel collar, said engaging means and said retainer pins being disposed about the circumference of said mandrel collar so as to apply force on said mandrel on angularly displaced zones which are substantially opposite one another, thereby minimizing radial distortion of said collar and insuring a centered position thereof within said bore.