Center drill types, center drill angle – what is used where
A center drill is used for drilling a hole on the axis of a shaft to hold it between centers. It was also used to drill center holes on machining centers, but has been replaced by the spot drill (which has a simpler geometry) for this purpose.
Center drills come in two common types, Type A (DIN 333-A) and Type B (DIN 333-B), based on the center drill angle.
Type A forms a centre hole with a single 60 degree chamfer. Type B forms a hole with 2 chamfers, a 60 degree chamfer and additionally a 120 degree chamfer at the start of the hole. The 120 degree chamfer protects the start of the hole from damage and deformation that can cause inaccurate centering of the tailstock center.
There is also a type R (DIN 333-R) that has a radius instead of a chamfer. This forms a center hole that results in more holding accuracy because there is contact only at a single point, but the load bearing capacity of the hole (ability to withstand lateral forces resulting from cutting) would be less.
By the way, you’ll notice that center drill sizes come in preferred sizes, as explained in this post.
Text and pics. source Cadem NCycopedia: multimedia CNC training software.
