Lead Angle definitions

Lead Angle on Indexable Milling Cutter

In North America (where we grew up with inches and feet) Lead Angle is the angle formed between the cutting edge of the insert and a line perpendicular to the direction of feed (line typically parallel to the spindle axis). See Fig. A. However, In the metric world Lead Angle is the angle formed between the cutting edge of the insert and a line parallel to the direction of feed (line typically perpendicular to the spindle axis). See Fig. B But it's not as simple as millimeters vs. inches. It seems that the definition used (A or B) is more based on the background and/or target audience of who is "talking" than on whether the tool is dimensioned in inches or millimeters. I have found several catalog examples of metric dimensioned tools stating lead angle per fig. A; and several examples dimensioned in inches stating lead angle per fig. B. So - 2 cutters with exactly the same physical cutting edge angle (relative to spindle axis) can be defined to have different lead angles. It's prudent to verify the tool drawing if you desire a specific lead angle other than 0°, 45° or 90° (which translate to the same physical angle in both systems).

Lead Angle on Single-point Turning Tool

The lead angle, shown in Figure is not ground on the tool. It is a tool setting angle which has a great influence on the performance of the tool. The lead angle is bounded by the side cutting edge and a plane perpendicular to the workpiece surface when the tool is in position to cut; or, more exactly, the lead angle is the angle between the side cutting edge and a plane perpendicular to the direction of the feed travel. Tool holders should be selected to provide the largest possible lead angle, although limitations are sometimes imposed by the nature of the job. For example, when tuning and back-facing a shoulder, a negative lead angle must be used. Slender or small diameter parts may deflect, causing difficulties in holding size, or chatter when the lead angle is too large.

Tap & Thread geometery

Flute Lead Angle: Angle at which a helical or spiral cutting edge at a given point makes with an axial plane through the same point.

Thread Lead Angle: On a straight thread, the lead angle is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis. On a taper thread, the lead angle at a given axial position is the angle made by the conical spiral of the thread, with the plane perpendicular to the axis, at the pitch line.

Lead Angle Angle B is applied to screw threads and worm threads and is referred to as the lead angle of the screw thread or worm. This angle B is a measure of the inclination of a screw thread from a plane that is perpendicular to the screw thread axis. Angle B is called the "lead angle" because it is subtended by the lead of the thread, and to distinguish it from the term "helix angle" as applied to helical gears.

Lead Angle at pitch diameter (B) = helix angle of thread as measured from a plane perpendicular to the axis. tan B = L ÷ 3.1416E where E=pitch diameter (basic, maximum, or minimum) and L=lead of thread = pitch × number of threads

Note: The information provided on this page is believed to be an accurate representation of the referenced source material.
However, I take no responsibility for the correctness or appropriateness of it when applied to your situation.