For rough turning—where the goal is to remove material quickly while maintaining tool life and stability—the ideal insert typically has the following characteristics:

1. Insert Shape & Geometry

   • Negative-Geometry Inserts (e.g., CNMG, DNMG):

     • Thicker edge and stronger cutting edge to withstand heavy depths of cut and interrupted cuts.

     • Often used with a wiper-style chipbreaker (e.g., “R” or “T” chipbreaker) to facilitate efficient chip control under high feed rates.

   • Positive-Geometry Inserts (e.g., TNMG, VNMG) with a “U” or “S” chipbreaker:

     • Can also be used for lighter roughing, especially on less rigid setups or lower horsepower machines.

2. Insert Grade & Coating

   • Carbide Grades:

     • Look for a grade specifically formulated for roughing—often denoted with an “R” (e.g., K…R, P…R in ISO systems). These grades balance toughness and wear resistance.

     • A tougher substrate (higher cobalt content) tolerates heavy cutting forces and resists chipping.

   • Coating:

     • PVD (Physical Vapor Deposition) TiAlN or AlTiN: Provides good adhesion and heat resistance at higher cutting speeds.

     • CVD coatings (e.g., TiCN + Al₂O₃) are often favored when you need extra wear resistance on cast iron or high-alloy steels.

     • Multi-layer coatings (e.g., TiCN + Al₂O₃ + TiN) can further extend tool life in severe conditions.

3. Chipbreaker Style

   • For heavy-roughing, choose a “T” or “R” style chipbreaker (often called “roughing chipbreaker”).

     • These breakers are designed to lock the chip close to the insert, breaking it into smaller curls even under high feed and depth-of-cut.

     • They reduce the chance of large, tangled chips and help stabilize the cut.

4. Recommended Insert Examples

   • CNMG120408-RM or DNMG150408-RP

     • Negative geometry (“M” or “P” roughing chipbreaker) for very tough steels and interrupted cuts.

     • Coated with TiCN + Al₂O₃ for a balance of toughness and wear resistance.

   • TNMG160404-RP or VNMG160412-UT

     • Positive-rake roughing inserts with a robust chipbreaker—better for lighter to moderate roughing when machine rigidity is not ideal.

5. Application Tips

   • Depth of Cut (ap): Rough-turning typically uses ap ≥ 2–5 mm (depending on material and machine power). Ensure your insert has enough edge strength for the selected ap.

   • Feed Rate (f): Use a higher feed (e.g., 0.25–0.40 mm/rev for medium-carbon steel). Match the chipbreaker’s capability—“R” styles can often handle f ≥ 0.30 mm/rev.

   • Cutting Speed (Vc): Start with a moderate speed (e.g., 150–200 m/min on steel) and adjust based on tool wear and surface finish.

Summary

• Best Shape: Negative-geometry (CNMG/DNMG) with an “R” (roughing) chipbreaker for maximum edge strength, or positive-geometry (TNMG/VNMG) with a robust chipbreaker for lighter roughing.

• Best Grade & Coating: A tough, PVD-coated carbide grade (e.g., TiCN + Al₂O₃) designed for roughing.

• Why It Works: These inserts resist chipping under high feed/depth conditions, break chips reliably, and maintain edge stability, giving fast stock removal and longer tool life.

By selecting a negative, roughing-chipbreaker insert (CNMG/DNMG-R) in a tough, PVD-coated carbide grade, you’ll maximize material removal rates and minimize downtime during heavy roughing operations.