The Anatomy of a Drill Bit

Thanks to Rocky Mountain Twist for this excellent breakdown of the anatomy of a drill bit.

(A) TIP DIAMETER
Drill bit diameter at tip.

(B) SHANK DIAMETER
Drill bit diameter at shank end. Also includes 3/8″ reduction diameter.

(C) DRILL LENGTH
The length of drill bit from shank end surface to the leading edge of the primary chisel. (measured at the outer diameter)

(D) TIP THICKNESS
Thickness of the web at the point. (unless another specific location is indicated)

(E) RELIEVED DIAMETER
Diameter of the drill bit body that has been cut away to minimize friction from the hole.

(F) MARGIN WIDTH
Width of the non-relieved portion of the major diameter of the drill bit.

(G) CHISEL ANGLE
Included angle between the chisel edge and the leading edge of the cutting lip, as viewed from the front end of the drill.

(H) SECONDARY CHISEL ANGLE
Included angle formed by the intersection of the cutting edge and the face of the notch with the relief surface of the point, resulting in partial removal of the primary chisel edge. (applies to split point geometry only)

(I) POINT ANGLE
The angle included between the cutting lips projected on a plain parallel to the cutting axis and parallel to the cutting lips. In form this angle resembles a cone, but it departs from a true cone to furnish clearance behind the cutting lips.

(J) LIP RELIEF ANGLE
The clearance angle of the cutting lips. Measured by projection onto a plain tangent to the periphery at the outer corner of the lip. The lip relief angle is usually measured across the margins of a twist drill.

(K) SECONDARY CLEARANCE ANGLE
The included angle generated from the clearance operation for the secondary cutting edge.

(L) FLUTE LENGTH
Length of heliocal flute measured from outer edge of cutting lip to back leadout edge of the flute.

(M) CUTTING LENGTH
Length of relieved diameter measured from outer edge of cutting lip to back lead out edge of the relieved diameter grind.

(N) WEB
The central portion of the body that joins the lands. Extreme end of the web forms the chisel edge on a two flute drill bit.

(O) LAND WIDTH
Peripheral portion of the body between adjacent flutes.

(P) RADIUS IN NOTCH
Clearance angle of the cutting lips. Measured by faster, more efficient chip removal

For additional questions related to cutting tools, contact us.

Johnston has an experienced Cutting Tool Specialist who can evaluate your current operations and suggest product or process changes that will help improve your productivity.