HSS Hand Tap 3pcs Set M3x0.5mm Pitch
Serial Taps - How they work
Serial Taps Sets (designated as Rougher, Immediate and Finisher) actually vary in thread diameter, unlike conventional tap sets (Taper, Second and Plug) where the difference between each one is the length and the taper.
Because the major and pitch diameter of the "rougher" is smaller than the "intermediate", and "intermediate" is smaller than the "finisher", each tap removes a smaller volume of metal resulting in a extremely smooth thread.
Excellent for tapping deep holes and tough metals such as stainless steel.