Tool Wear: Oxidation Notch at Trailing Cutting Edge

This article is part of a 9-part series on tool wear and how to manage it. For insights into other wear mechanisms and how to address them, be sure to read the rest of the series.

Tool wear is arguably the most disruptive event—and a major cause of missed production time—on the manufacturing floor. It is also a key factor driving overall tooling spend. On most machines, production comes to a complete stop during tool changes. Additional time is lost when test cuts and offsets are made to reset the dimensional position of the cutting edge. Add inspection time and scrapped components due to wear or tool breakage—and all of this is before even addressing the time spent solving tool wear issues.

Of course, cutting tools don’t last forever, and while we can never fully eliminate the costs associated with tool wear, its disruptive effects can be reduced through an improved understanding of the various wear mechanisms—and by taking appropriate corrective actions.

Oxidation Notch at Trailing Cutting Edge

What it is:

A specific type of notch wear that appears at the trailing edge of the insert, just beyond the portion of the cutting edge that generates the final machined surface. This wear is a common cause of deteriorating surface finish and loss of dimensional accuracy on turned diameters. It typically occurs only in turning operations (not in facing operations or milling applications).

Why it happens:

Oxidation notching is caused by the physical mechanics inherent in turning. There’s a confined space between the rotating workpiece and the insert. As oxygen passes through this high-velocity, confined gap, it can effectively “cut” or erode the insert. The issue is more prevalent when using 80° diamond-shaped inserts (such as CNMG or WNMG styles).

If you look straight down at the top of an 80° diamond tool in a turning operation, you'll see the narrow space just behind the secondary cutting edge that forms the turned diameter. That tight gap creates the ideal conditions for oxidation notching to develop.

How to Correct:

This is a challenging wear pattern to address, but there are two primary approaches:

1. Eliminate the confined area: Open up the space behind the cutting edge to reduce the effect of high-velocity oxygen. This can be achieved by:
   • Using a narrower insert style, such as a 55° or 35° diamond shape
   • Employing a tool with a larger lead angle

   For example, a square insert used at a 45° lead angle creates a much larger open space behind the cutting edge, reducing the confined airflow.

2. Minimize the oxygen’s effect: Reduce the chemical and thermal interaction by:
   • Using an insert grade with an Aluminum Oxide (Al₂O₃) coating
   • Reducing the cutting speed, which slows airflow across the insert
   • Applying a high volume of coolant to flood the confined area and disrupt oxygen flow