In a robotic tool coupler, a rotating cam member having a plurality of surfaces formed therein urges a plurality of ball members in one tool coupling unit radially to contact an angled surface in the other tool coupling unit. Further rotation of the cam member exerts a radial force through the ball members onto the angled surface. A component of that force is directed by the angled surface toward the opposite tool coupling unit, locking the two units together. The cam member may include a failsafe surface and/or a failsafe lobe to maintain the two units locked together in the event of a loss of power to positively actuate the cam member.