Sharpening Drills

Improperly Sharpened Drills (unequal edge length, unequal point angles, inadequate relief angles) will either rub, drift and/or drill oversize.

The picture above is an illustration of a standard Twist Drill.

Regardless if you sharpen it holding it by hand or with a drill grinding machine, the results should be the same.

Grinding the relief behind the cutting edge:

Referring to the above picture, the drill is swung around the A-axis of an imaginary cone while resting in a support which hold the drill at one-half the point of angle B with respect to the face of the grinding wheel.

Recommended angles for Twist Drill sharpening:

Point Angle:

118 degrees for low and medium carbon steels.

118 degrees to 135 degrees for high carbon steels.

90 degrees to 140 degrees for Aluminum.

Helix Angle:

24 degrees to 32 degrees

Lip Relief Angle:

10 degrees to 15 degrees for low and medium carbon steels.
7 degrees to 12 degrees for high carbon steels.

Note: the lower values of these angle ranges are for large diameter drills and the higher values are for smaller diameter drills. For drills less than .250" diameter the lip relief angles are increased beyond the listed max....up to 24 degrees.

Drill Point Thinning or "Splitting the Drill Point"

The Chisel edge is the least efficient Operating surface on a trist drill point because it does not cut. The chisel edge squeezes or extrudes the work piece material.

Point thinning is desireable on large diameter drills and also on drills that have become shorter because of grinding due to usage. The thickness of the web increases toward the shaft of the twist drill, which increases the chisel edge. The above picture illustrates what a thinned drill point looks like.

Twist drill grinding by hand takes patience and practice. There are other types of drills such as Spade Drills that have different geometry. This post focuses on Twist Drills.