Every cutting insert wears out. The key is knowing what the wear looks like, what caused it, and how to fix it. This guide covers eight insert failure modes in milling and turning. For each one, you will see what it looks like, why it happens, and what to change.
The 8 Insert Failure Modes
The eight failure modes are flank wear, crater wear, built-up edge, chipping, thermal cracks, plastic deformation, notch wear, and edge fracture.
Flank wear is the preferred type of wear. It is a sign of normal wear over time. The other seven point to a problem with speed, heat, grade, geometry, or setup.
Insert Failure Modes at a Glance
| Failure mode | What you see | Main cause | Common fix |
|
Flank wear
|
Even wear band on the flank face below the cutting edge | Cutting speed too high, or grade not wear resistant enough | Lower speed, use a harder coated grade |
|
Crater wear
|
Concave dip on the rake face behind the cutting edge | High heat where the chip meets the tool | Lower speed or feed, use a coated grade with an Al2O3 layer |
|
Built-up edge (BUE)
|
Workpiece material welded to the cutting edge | Cutting speed too low on gummy material | Raise speed, use a sharp PVD-coated grade |
|
Chipping
|
Small, irregular breaks along the cutting edge | Interrupted cut, vibration, hard scale, weak edge | Use a tougher grade and a stronger edge prep |
|
Thermal (comb) cracks
|
Fine cracks across the cutting edge, like a comb | Big swings in temperature | Use a tougher grade, keep coolant steady or run dry |
|
Plastic deformation
|
Edge that looks slumped, rolled over, or pressed in | Heat and pressure beyond the grade’s hot hardness | Lower speed and feed, use a grade with better hot hardness |
|
Notch wear
|
Notch at the depth of cut line or trailing edge | Work-hardened skin, scale, or oxidation | Vary the depth each pass, increase the lead angle |
|
Edge fracture
|
Large piece broken from the edge or nose radius | Shock load, chatter, chip jamming, hard inclusion, or brittle grade | Use a tougher grade, reduce feed and depth |
What Is Flank Wear?
Flank wear is an even wear band on the flank face, just below the cutting edge. It is the preferred type of insert wear because it shows normal wear over time.
Why it happens: Hard particles in the material rub away the edge, or the cutting speed is above the grade’s wear limit. The grade may also not be wear resistant enough.
Why it matters: Fast flank wear can cause poor surface finish or tolerances that vary.
How to fix it:
- Reduce cutting speed.
- Step up to a harder coated grade or pick a more wear-resistant grade.
- Check that coolant reaches the cutting edge.
- For work hardening materials, select a smaller entry angle.
- When machining heat-resistant materials, reduce cutting speed.
What Is Crater Wear?
Crater wear is a concave dip on the rake face, behind the cutting edge.
Why it happens: High heat and pressure where the chip meets the tool. At higher speeds, diffusion wear adds to the problem.
Why it matters: Too much crater wear weakens the cutting edge and leaves a poor surface finish.
How to fix it:
- Reduce cutting speed.
- Reduce feed rate to lower the heat.
- Use a coated grade with an Al2O3 layer or select a more wear-resistant grade.
- Select a positive insert geometry.
What Is Built-Up Edge (BUE)?
Built-up edge is irregular workpiece material welded to the cutting edge. It leaves a poor finish.
Why it happens: The cutting zone is too cool. This is common when machining gummy or sticky materials like low-carbon steel, 300-series stainless steel, and aluminum. A negative cutting geometry can add to it.
Why it matters: When the built-up edge tears away, it can chip the cutting edge.
How to fix it:
- Increase cutting speed.
- Use a sharp, PVD-coated grade, or another more suitable coated carbide grade.
- Select an insert with a more positive geometry.
- Improve coolant pressure.
What Is Chipping?
Chipping is a set of small, irregular breaks along the cutting edge.
Why it happens:
- Shock from an interrupted cut
- Vibration
- Hard scale or inclusions in the material
- A weak edge prep
- A cutting edge that is too brittle
- A built-up edge that has formed
How to fix it:
- Select a tougher carbide grade.
- Use a stronger T-land or hone, or another insert with a stronger cutting edge.
- Ramp or roll into the cut, and reduce feed at the start of the cut.
- Check rigidity and improve the stability of the setup.
- Increase cutting speed.
- Select a positive geometry.
What Are Thermal (Comb) Cracks?
Thermal cracks are fine cracks that run across the cutting edge in a comb pattern.
Why they happen: The insert goes through repeated swings in temperature. This is common in milling, in interrupted turning, and when coolant supply is on and off.
Why it matters: The cracks lead to chipping and poor surface finish.
How to fix it:
- Select a tougher insert grade.
- Run dry if practical. If you use coolant, keep the flow steady.
- Lower the cutting speed.
What Is Plastic Deformation?
Plastic deformation is when a cutting edge has slumped, rolled over, or sunk in. You may also see an impression on the insert flank.
Why it happens: Heat plus pressure goes beyond the grade’s hot hardness. This is common when cutting tough or hardened workpieces.
Why it matters: It leads to poor chip control, poor surface finish, and insert breakage.
How to fix it:
- Lower speed and feed.
- Select a harder, more wear-resistant grade with better hot hardness.
- Use a larger nose radius.
- Use a strong T-land.
What Is Notch Wear (DOC Line Wear)?
Notch wear is a small notch at the depth of cut line or on the trailing edge.
Why it happens: A work-hardened skin, scale, or oxidation on the material. It is a classic problem on 300-series stainless steel and superalloys. Cutting speed that is too high, or a carbide grade without enough wear resistance, can also cause it.
Why it matters: Notch wear leads to poor surface finish and a risk of the insert edge breaking.
How to fix it:
- Vary the depth of cut each pass.
- Increase the lead angle.
- Use an oxidation-resistant grade, or a more wear-resistant grade.
- Ramp into the cut.
- For work hardening materials, select a smaller entry angle.
- Reduce cutting speed on heat-resistant materials.
What Is Edge Fracture?
Edge fracture is a large piece broken off the cutting edge or the nose radius.
Why it happens:
- Shock load
- Chatter
- Chip jamming
- A hard inclusion in the material
- A grade that is too brittle
- Too much wear before the insert was indexed
- A grade and geometry too weak for the job
- Too much load on the insert
- A built-up edge that has formed
Why it matters: Edge fracture can damage more than the insert. It can also destroy the cutter body, the workpiece, or both.
How to fix it:
- Select a tougher grade.
- Use a stronger edge prep, or a stronger, thicker insert.
- Reduce feed and depth of cut.
- Improve chip evacuation.
- Secure the fixturing.
Frequently Asked Questions
Which type of insert wear is normal?
Flank wear. It is the preferred type of wear and shows normal wear over time. If it happens fast, it can hurt surface finish and make tolerances vary.
What causes poor surface finish?
Six of the eight failure modes can cause it: fast flank wear, crater wear, built-up edge, thermal cracks, plastic deformation, and notch wear.
Which failure modes come from too much heat?
Crater wear comes from high temperatures and pressure on top of the insert. Plastic deformation comes from a cutting temperature that is too high. Thermal cracks come from big swings in temperature.
What causes built-up edge?
A cutting zone that is too cool. It is most common on gummy materials like low-carbon steel, 300-series stainless steel, and aluminum. Raising the cutting speed is the first fix.
Which failure mode can damage the cutter body or workpiece?
Edge fracture. A large break at the cutting edge can destroy the cutter body, the workpiece, or both.
