Drill point, countersink & counterbore
Drill point length for a given point angle and the total depth to reach a full diameter hole, plus countersink diameter and depth for flat head screws and counterbore sizes for socket head cap screws.
Drill point, countersink & counterbore
How much deeper the drill has to go to get a full diameter hole, and the countersink or counterbore for a flat head or socket head screw to sit flush.
Drill point
Countersink for a flat head screw
Counterbore for a socket head cap screw
Drill point length: L = (D/2) / tan(θ/2). A 118° point is 0.300·D, a 135° split point is 0.207·D. Total depth for a full-diameter hole is the required depth plus L.
Countersink: the diameter at the surface is the screw head diameter (max). Depth below the surface at the hole edge: (D_csk − D_hole) / 2 / tan(θ/2). Inch flat heads are 82°, ISO countersunk heads are 90°.
Counterbore: diameter is the conventional clearance for the head, depth is the head height plus whatever you want the head below the surface.
Head sizes: ASME B18.6.3 (82° flat heads), ISO 10642 (90° countersunk), ASME B18.3 and ISO 4762 (socket heads), counterbore diameters per common practice and DIN 974.
Head dimensions are maximums from the standards. Countersink a touch deeper than calculated so the head sits at or just below the surface. Through-hole sizes shown are normal clearance.
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