Tool Wear: Chip Hammering

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.

Chip Hammering

What it is:

Chip hammering is the irregular chipping and deterioration of the insert in an area away from the primary depth-of-cut zone.

Why it happens:

Chip hammering occurs exactly as the name suggests: as the chip exits the cutting zone, it "hammers" the insert outside the immediate cutting area. If left uncorrected, this repeated impact gradually knocks small pieces off the insert. The wear typically starts slowly, but accelerates over time as the chip continues to strike the same area during each pass.

This phenomenon is especially common in rough boring operations, where large, robust roughing chips are generated. Because the operation takes place inside a bore, chip evacuation is limited, increasing the likelihood that chips will deflect and strike the insert surface.

How to Correct:

Chip hammering can usually be addressed using one or more of the following techniques:

1. Change the feed rate – Altering the feed rate changes the chip’s trajectory. Think of this like tuning the ground effects on a racecar in a NASCAR pit: changing the dynamics alters the line taken through the turns—similarly, a different feed rate may cause the chip to exit the cutting zone differently.
2. Change the lead angle of the tool – For instance, if using a 0° or –1° lead angle boring bar (common with triangular inserts), try switching to a –5° lead angle boring bar (more typical with 80° diamond inserts).
3. Change the chipbreaker geometry – Using a different insert geometry alters chip formation and may prevent the chip from striking the insert surface as it exits.
4. Run the tool "upside down" – In lathe operations, rotating the tool 180° and reversing spindle direction effectively runs the tool “upside down” compared to the current process. This takes advantage of gravity to alter chip flow and is often a very effective fix.
5. Switch to a tougher grade – While not addressing the root cause, this solution can often solve the problem as a tougher grade can better withstand the chip hammering effect.