rota broach cutters

[roofman]

Stop looking for other reasons. It's the size of the cutter. Chatter is a result of lack of rigidity.

also can happen by the tool not getting held tight enough...i think this paticular cutter the OP is using is for another type of arbour fixing and not the regular mag drill that has 2 sunkin cap head screws that holds against 2 flats on the broaching cutter

 

[Jim Davey]

It does have the grub screw (weldon) fixing. It's flex in the drilling machine caused by the torque reaction from the bigger cutter. Use a holesaw or a proper broaching machine.