Notch Wear at Depth of Cut: Causes and Prevention

What Is Notch Wear and Why It Matters

Notch wear (also called depth-of-cut line wear or DCL wear) is a localized wear groove that forms on the cutting edge at the point where the tool enters the workpiece surface. Unlike uniform flank wear, which progresses gradually across the entire cutting edge, notch wear concentrates in a narrow band (typically 0.5-2.0 mm wide) at the depth-of-cut boundary. When the notch reaches a critical size (usually VB notch of 0.6-0.8 mm), the cutting edge fractures at that point, causing catastrophic insert failure.

The Mechanism of Notch Wear Formation

Notch wear results from a combination of chemical, thermal, and mechanical mechanisms that are more severe at the depth-of-cut line than elsewhere on the cutting edge:

• Oxidation wear: At the depth-of-cut line, the cutting edge is exposed to air on the workpiece side. The high temperature (700-900 C) accelerates oxidation of the carbide substrate (WC oxidizes to WO3 at 600-800 C) and the cobalt binder. This oxidation is not present along the rest of the cutting edge, where the chip shields the tool from air.
• Work-hardened surface layer: The depth-of-cut line contacts the unmachined surface of the workpiece, which may contain a work-hardened layer from previous passes (especially in austenitic stainless steels and nickel alloys where work-hardened layers can reach 45-55 HRC compared to 20-25 HRC base material).
• Scale and decarburization: Forged and hot-rolled surfaces have a decarburized surface layer (0.1-0.5 mm deep) and iron oxide scale (FeO, Fe3O4, Fe2O3) that is harder and more abrasive than the bulk material.
• Variable chip thickness: At the depth-of-cut line, the chip transitions from full thickness to zero thickness, creating a stress concentration and promoting built-up edge formation that can tear carbide grains from the insert surface when it breaks away.

Materials Most Prone to Notch Wear

Notch wear severity varies significantly by workpiece material:

• Austenitic stainless steel (304, 316, 316L): The most notch-prone material due to severe work hardening (surface can reach 50+ HRC after one pass) and high chemical reactivity with carbide. Notch wear rates of 0.05-0.15 mm/min of cutting time are common.
• Nickel-based superalloys (Inconel 718, Waspaloy): Similar work-hardening behavior to stainless steel, combined with high cutting temperatures. Notch wear is the primary failure mode in these materials.
• Titanium alloys (Ti-6Al-4V): Chemical reactivity between titanium and the carbide substrate causes severe diffusion wear at the depth-of-cut line.
• Medium-carbon steel (1045, 4140): Moderate notch wear, typically not the limiting failure mode unless forging scale is present.
• Grey cast iron (GJL-250): Minimal notch wear due to the absence of work hardening and the short, brittle chips that do not weld to the cutting edge.

Prevention Strategies

1. Vary the Depth of Cut Between Passes

The single most effective prevention method is to change the depth of cut for each roughing pass so that the notch wear groove does not accumulate at the same location on the insert:

• Programming method: Use a stepped depth-of-cut pattern in the CNC program. For example, if total stock removal is 6 mm, use passes of 3.0 mm, 2.0 mm, and 1.0 mm rather than three passes of 2.0 mm each.
• Depth variation: A minimum variation of 0.5-1.0 mm between successive passes is needed to spread the notch wear across a wider zone on the cutting edge.
• CNC cycle support: Many modern CNC controls (Fanuc G71/G72, Siemens Cycle95, Heidenhain Cycle 811) support variable depth-of-cut roughing cycles natively.

2. Use a Lead Angle (Approach Angle) Less Than 90 Degrees

A lead angle of 75-85 degrees (using a 45-degree or 75-degree approach angle holder) reduces the effective depth-of-cut line load by distributing the cutting forces over a longer section of the cutting edge:

• Effective depth-of-cut: With a 75-degree lead angle, the effective chip thickness is reduced by sin(75) = 0.966, a minor reduction, but the depth-of-cut line contact is spread over 1/cos(75) = 3.86 times the nominal depth-of-cut width on the cutting edge.
• Radial force increase: A 75-degree lead angle increases radial cutting force by approximately 15-25% compared to a 90-degree approach. This must be considered for thin-walled workpieces.

3. Select the Correct Insert Grade and Coating

Insert grades with high hot hardness and oxidation-resistant coatings resist notch wear better:

• CVD Al2O3 coatings: Alumina coatings (8-15 um thick) are highly resistant to oxidation and diffusion wear at the depth-of-cut line. They are the preferred coating for steel turning where notch wear is a concern.
• PVD TiAlSiN coatings: For stainless steel and superalloy turning, PVD coatings with silicon content (TiAlSiN, AlTiSiN) provide superior oxidation resistance at 800-1,000 C compared to standard TiAlN.
• Grade toughness: A tougher substrate (ISO M20-M30 or S20-S30 for stainless and superalloys) resists micro-chipping at the notch zone better than a harder, more brittle substrate.

4. Reduce Cutting Speed at the Entry Point

Since oxidation wear is temperature-dependent, reducing the cutting speed by 15-25% specifically lowers the cutting zone temperature and slows notch wear progression:

• Speed reduction tradeoff: A 20% speed reduction in stainless steel turning (from 150 to 120 m/min) can double the notch wear life, from 10 minutes to 20 minutes per edge.
• Compensation: Increase feed rate by 10-15% to partially offset the lost production rate.

5. Apply High-Pressure Coolant at the Depth-of-Cut Line

Directing high-pressure coolant (70-150 bar) at the depth-of-cut line reduces oxidation wear by lowering the temperature and creating a physical barrier against air exposure:

• Nozzle positioning: The coolant jet should be aimed at the depth-of-cut boundary on the workpiece side of the insert, where air access causes oxidation.
• Effectiveness: Tests show 40-60% reduction in notch wear depth with high-pressure coolant compared to flood coolant in stainless steel turning.

Monitoring Notch Wear

Notch wear is measured as VB notch (the maximum wear land width at the notch location) per ISO 3685. Recommended action limits:

• Steel turning: Change insert when VB notch reaches 0.6 mm.
• Stainless steel: Change at VB notch 0.4-0.5 mm (notch wear progresses rapidly once initiated).
• Superalloys: Change at VB notch 0.3-0.4 mm to prevent insert fracture.

Conclusion

Notch wear at the depth-of-cut line is a distinct failure mechanism driven primarily by oxidation and work-hardened surface contact. It is most severe in stainless steels, nickel alloys, and titanium alloys. The most effective prevention combines variable depth-of-cut programming, oxidation-resistant coatings, reduced cutting speeds, and high-pressure coolant delivery. Implementing these strategies typically extends insert life by 50-150% in notch-prone materials.