Tool Wear: Insert Breakage

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.

Catastrophic Failure

What it is:

Insert breakage, or catastrophic failure, refers to the complete fracture or failure of the cutting edge and insert.

Why it happens:

This type of failure generally occurs during roughing or semi-roughing operations, and it is usually a sign of significant mis-application. The mis-application typically falls into one of two categories:

• The setup and/or approach with the tool is completely incorrect, or
• The selected insert is unsuitable for the application.

In either case, the forces exceed the insert’s bulk strength, resulting in complete failure and tool breakage.

How to Correct:

The first recommended step is not to immediately blame the tool, but to confirm that the machine setup and tool application parameters are correct. Catastrophic failure is often caused by a poor machine setup or a distinct lack of rigidity. Another potential issue is unintentionally feeding into a small section of the workpiece (or fixture), which can damage the tool before it even reaches the intended cut.

Thus, the recommended first step is a fundamental piece of machining common sense—critically examine the application and ask, “Should this work?” and “Are the setup and machine correct?”

If the setup is confirmed to be correct, then the next step is to recognize that the selected tool may not be strong enough for the level of feed and depth of cut in the application. In such cases, catastrophic failure can be addressed by either significantly reducing the cutting parameters or switching to a stronger tool geometry or a tougher grade. Alternatives include using a larger insert, a thicker insert, or a single-sided style insert.

Finally, be aware that catastrophic failure might be preceded by or even be a byproduct of another wear pattern. If an alternative wear pattern develops unnoticed, it can propagate to the point where the insert fails completely. In that situation, the problem-solving focus might shift to catastrophic failure when the actual cause and solution differ significantly.