CNMG Inserts — Grades, Chipbreakers, and Cross-Reference

Compiled 2026-04-04 · 40 chunks, 15 posts · turning · inserts · cnmg · carbide · indexable · lathe

Summary

CNMG inserts are 80-degree rhombic turning inserts with negative rake geometry, designed for medium to heavy roughing and finishing operations. The "C" denotes the 80-degree diamond shape, "N" indicates negative clearance angle, "M" represents manufacturing tolerance class, and "G" specifies the insert type with hole and chipbreaker. These inserts are workhorses for external turning, [[face-milling]], and general machining across a wide range of materials from [[aluminum-6061]] to [[4140-steel]].

Insert Geometry and Specifications

Standard CNMG Sizes

  • CNMG 120404: 1/2" IC, 1/6" thick, 0.4mm corner radius
  • CNMG 120408: 1/2" IC, 1/6" thick, 0.8mm corner radius
  • CNMG 120412: 1/2" IC, 1/6" thick, 1.2mm corner radius
  • CNMG 160608: 5/8" IC, 1/4" thick, 0.8mm corner radius
  • CNMG 160612: 5/8" IC, 1/4" thick, 1.2mm corner radius
  • CNMG 190608: 3/4" IC, 1/4" thick, 0.8mm corner radius
  • CNMG 432 (Imperial): 1/2" IC, 3/32" thick, 1/32" corner radius

Corner Radius Selection

  • 0.4mm (0.016"): Sharp corners for precision finishing, light cuts
  • 0.8mm (1/32"): General purpose, balanced strength and finish
  • 1.2mm (3/64"): Heavy roughing, maximum strength
  • 1.6mm+ (1/16"+): Extreme roughing in tough materials

Speeds and Feeds by Material

Carbon Steel (1018-1045, 4140)

Manufacturer recommendations:

  • SFM: 450-650 (uncoated), 650-950 (coated)
  • Feed: 0.008-0.025 ipr
  • DOC: 0.050-0.300"

Real-world machinist experience:

  • SFM: 350-500 for roughing, 500-750 finishing
  • Feed: 0.015-0.035 ipr (forum users report success up to 0.050 ipr with rigid setups)
  • DOC: Up to 0.500" proven in [[4140-steel]] with CNMG 432 inserts

Stainless Steel (304, 17-4PH)

  • SFM: 200-400 (watch for [[work-hardening]])
  • Feed: 0.012-0.025 ipr (must maintain consistent feed to prevent work hardening)
  • DOC: 0.050-0.200"
  • Critical: Use positive chipbreaker geometry, flood coolant mandatory

Aluminum (6061, 7075)

  • SFM: 1200-2500 (uncoated preferred to prevent built-up edge)
  • Feed: 0.008-0.020 ipr
  • DOC: 0.050-0.400"
  • Shop tip: Sharp corner radius (0.4mm) prevents smearing

Cast Iron

  • SFM: 350-600 (dry cutting preferred)
  • Feed: 0.010-0.030 ipr
  • DOC: 0.100-0.500"
  • Ceramic inserts excel for continuous cuts at 800-1200 SFM

Carbide Grades and Applications

General Purpose Grades

  • Sandvik GC4225: Versatile PVD-coated grade for steel 150-350 HB
  • Kennametal KC5010: Uncoated for aluminum and non-ferrous
  • Iscar IC907: CVD-coated for steel roughing operations
  • Seco TP1500: Tough grade for interrupted cuts

Specialized Grades

  • Ceramic (Al2O3): For continuous cuts in [[cast-iron]] at high speeds
  • Cermet: Precision finishing in steel, excellent surface finish
  • PCD: [[aluminum-6061]] and non-ferrous at extreme speeds (3000+ SFM)
  • CBN: Hardened steels above 45 HRC

Chipbreaker Selection

Light Chipbreakers (M, F chipbreakers)

  • Feed range: 0.005-0.015 ipr
  • DOC: 0.020-0.100"
  • Applications: Finishing, thin-wall parts
  • Materials: Aluminum, brass, light steel cuts

Medium Chipbreakers (MM, MF)

  • Feed range: 0.010-0.025 ipr
  • DOC: 0.050-0.200"
  • Applications: General turning, semi-roughing
  • Most versatile option for job shops

Heavy Chipbreakers (R, RR)

  • Feed range: 0.020-0.050 ipr
  • DOC: 0.100-0.500"
  • Applications: Roughing, heavy material removal
  • Essential for [[chip-control]] in deep cuts

Cross-Reference Chart

Sandvik Kennametal Iscar Seco Mitsubishi Corner Radius
CNMG 120404-KM CNMG 432-K68 CNMG 120404-TF CNMG 120404-F1 CNMG120404-MS 0.4mm
CNMG 120408-KM CNMG 432-K68 CNMG 120408-TF CNMG 120408-F1 CNMG120408-MS 0.8mm
CNMG 120412-KM CNMG 433-K68 CNMG 120412-TF CNMG 120412-F1 CNMG120412-MS 1.2mm
CNMG 160608-KM CNMG 544-K68 CNMG 160608-TF CNMG 160608-F1 CNMG160608-MS 0.8mm

Common Problems

Chipping at Corner

Causes: Feed too low, speed too high, improper grade selection Solutions: Increase feed rate, reduce speed, switch to tougher grade (uncoated or TiN)

Built-Up Edge (BUE)

Causes: Low speeds, improper coolant, wrong grade for material Solutions: Increase SFM by 20-30%, use sharp uncoated inserts, improve coolant flow Forum insight: "In aluminum, BUE is death - go uncoated and fast"

Poor Surface Finish

Causes: Worn insert, [[chatter-vibration]], incorrect corner radius Solutions: Check for 0.003" flank wear limit, increase rigidity, match corner radius to feed rate

Insert Breakage

Causes: Interrupted cuts, excessive DOC, work hardening in stainless Solutions: Use tougher substrate, reduce DOC by 50%, maintain consistent feed in stainless

Shop Floor Tips

Setup Recommendations

  • Tool height: Set exactly on centerline for proper chip evacuation
  • Approach angle: 15-45 degrees reduces cutting forces
  • Workholding: Minimize overhang - every inch of stick-out requires 25% speed reduction

Insert Indexing Strategy

  • Rotate at first sign of flank wear (0.003" typical)
  • Save sharp corners for finishing operations
  • Use worn corners for roughing in softer materials
  • "One corner for aluminum roughing, save three for steel finishing"

Coolant Application

  • Flood coolant for stainless and high-alloy steels
  • Mist for general steel turning
  • Dry cutting preferred for cast iron and some tool steels
  • Critical: Consistent coolant application prevents thermal shock

Power Requirements

  • Roughing with 0.300" DOC, 0.025 IPR requires minimum 15 HP
  • Calculate: HP = (SFM × Feed × DOC × 300) / 33,000 for steel
  • Many forum users report successful heavy cuts on lower-power machines by reducing SFM

Troubleshooting Quick Reference

  • Stringy chips: Increase feed rate or change chipbreaker
  • Chip packing: Improve coolant flow, check tool clearance
  • Dimensional issues: Check for [[tool-wear-diagnosis]], verify rigidity
  • Vibration: Reduce overhang, increase speeds through critical frequencies
  • [[turning-basics]] — Fundamental lathe operations and setup
  • [[insert-selection-guide]] — Comprehensive guide to choosing inserts
  • [[wnmg-inserts]] — 80-degree positive rake alternative
  • [[4140-steel]] — Specific techniques for common alloy steel
  • [[aluminum-6061]] — Machining parameters for aluminum alloys
  • [[chatter-vibration]] — Diagnosing and solving stability issues
  • [[chip-control]] — Managing chip formation and evacuation
  • [[toolholder-selection]] — Matching holders to insert geometry