Combined Drill and Countersink Bits:
Global Procurement & Engineering Guide

Plain Type (Form A) combined drill and countersink bits feature a straight 60-degree countersink cone following the pilot drill. It is the industry standard for general lathe center drilling and countersinking standard fasteners. Bell Type (Form B) features an additional 120-degree outer chamfer at the mouth of the hole. This "bell" profile protects the critical 60-degree inner center bearing surface against accidental bumps, burrs, or deformation during handling and subsequent grinding or turning operations.

Pilot tip breakage is almost always caused by three operational factors: excessive feed rate upon initial workpiece contact, misaligned centerlines between the tailstock/spindle and the workpiece, or chip packing inside the flutes due to insufficient peck cycle depths. To maximize tool life, operators should enter the cut at 50% of standard feed until the pilot establishes full contact, verify alignment using a dial indicator, apply sulfurized cutting oil or water-soluble coolant, and peck frequently when drilling pilot depths greater than 1.5 times the pilot diameter.

For stainless steel (such as 304, 316, or 17-4PH), heat-treated alloys, titanium, and high-tensile metals above 28 HRC, specify M35 5% Cobalt (HSS-Co) combined drill and countersink bits. Cobalt increases red-hardness, allowing the cutting edge to resist extreme thermal breakdown and work-hardening forces that rapidly dull standard M2 high speed steel. For mild steel, brass, aluminum, and cast iron, standard M2 HSS bits remain the most economical choice.

Selecting the correct size depends on workpiece diameter and shaft weight to ensure sufficient bearing support without weakening the workpiece end face. As a general rule: Size #1 is ideal for 1/8" to 5/16" stock; Size #2 for 3/8" to 1/2" stock; Size #3 for 1/2" to 3/4" stock; Size #4 for 3/4" to 1-1/4" stock; Size #5 for 1-1/4" to 2" stock; and Size #6 through #8 for heavy structural shafts exceeding 2" diameter.

Yes. Double-ended center drills feature symmetrical flutes on both ends, offering double the tool life per bit. When one end dulls, the operator simply flips the bit in the chuck. Once both ends show wear, the 60° countersink face and pilot point can be resharpened on a specialized tool-and-cutter grinder or automated CNC drill sharpener, provided the web thickness and flute geometry are accurately maintained.

Century Drill & Tool offers flexible global OEM and distributor packaging options, including custom vinyl pouches, plastic index sets, metal cases, retail-ready branded clamshells with anti-theft tagging, and bulk master shipping cartons. Our team provides complete electronic data interchange (EDI), custom laser marking on tool shanks, and barcoding for automated warehouse logistics.

Concentricity between the pilot drill and the countersink cone directly dictates the rotational center of the workpiece. If the countersink cone is eccentric relative to the pilot or outer shank, the workpiece will wobble slightly when mounted between centers on a lathe or grinding machine, resulting in run-out, out-of-round turning, and premature live center bearing failure. Century Built Blue center drills are precision CNC ground to hold concentricity within 0.0003 inches.