Tool Wear: Thermal Cracks
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.
Thermal Cracks (Sometimes Called Comb Cracks)
What it is:
Thermal cracks appear as very small, numerous cracks that form perpendicular to the cutting edge. In early stages, they are often only visible under magnification. These cracks are sometimes referred to as "comb cracks" because they resemble tiny, evenly spaced furrows—similar to the marks left by comb teeth—along the edge line.
Why it happens:
Thermal cracks result from rapid and extreme temperature cycling at the cutting edge. They can occur during interrupted cuts in turning, but are more commonly seen in milling operations—where the cutting action is inherently all interrupted cuts. The repeated, rapid fluctuations in temperature generate high thermal stress, which leads to the formation of fine stress fractures or cracks.
How to Correct:
Several steps can be taken to reduce the thermal cycling and prevent crack formation:
- Adjust coolant application—either drastically increase it or turn it off completely. The goal is to maintain a more consistent temperature during cutting, minimizing thermal fluctuations.
- Reduce cutting speed to lower the upper end of the temperature range at the cutting edge.
- Lower the feed rate to reduce heat generation and thermal stress on the tool.
- Switch to a tougher insert grade. A tougher grade has better resistance to thermal stress and can better withstand the rapid temperature cycling conditions.
See our separate blog post on Wet vs. Dry Machining for more on this topic.