This standard specifies the type and size of the arc center drill-R type. This standard is applicable to arc-shaped center drill-R type for processing R-type center holes (see GB/T145).
1998GB/T 6078.3-1998 Centre drills. Part 3: Centre drills for centre holes with radius form--Type R. Dimensions
GB/T 6078.3-1998 Centre drills. Part 3: Centre drills for centre holes with radius form--Type R. Dimensions was changed to GB/T 6078-2016 Center drill.