How to Select CNMG Inserts for Steel Turning

How to Select CNMG Inserts for Steel Turning

CNMG is one of the most widely used ISO insert families for external turning, facing and general steel machining. Its 80-degree rhombic shape provides a strong cutting corner, while its double-sided negative design offers four usable cutting edges in a conventional application.

However, CNMG alone is not a complete product selection. The correct insert also depends on size, thickness, nose radius, chipbreaker, carbide grade, holder geometry and machining condition.

Understanding the CNMG Designation

ISO 1832 establishes the designation system for indexable inserts. Using CNMG 120408 as a common metric example:

The suffix following the ISO designation normally identifies the manufacturer's chipbreaker or geometry. The final grade code identifies the cutting-material system. A complete ordering description therefore needs all three elements:

Two inserts marked CNMG 120408 may have very different cutting behavior if their chipbreakers, edge preparations or grades are different.

Why Choose a CNMG Insert?

CNMG is well suited to external longitudinal turning, facing, medium machining, rough turning and stable-to-interrupted steel machining. It is a strong and economical choice when four usable cutting edges and a robust corner are required.

CNMG may not be the first choice when a component requires deep profiling or clearance behind a shoulder, cutting forces must be minimized on a thin or flexible part, a very small boring diameter is involved, or an extremely light finish needs a sharper positive insert. The holder determines the actual entering angle and working geometry.

Step 1: Select the Insert Size

Choose an insert large enough to support the required depth of cut and cutting load, but do not automatically select the largest available size. Larger inserts offer greater edge length and load capacity; smaller inserts can reduce insert cost and improve access in lighter applications.

The required edge engagement must remain within the insert and chipbreaker's recommended range. Confirm the exact holder, usable cutting-edge length and maximum permissible depth of cut before ordering.

Step 2: Select the Nose Radius

A larger nose radius normally strengthens the corner and can support higher feed, but it also increases radial cutting force and may promote vibration on slender workpieces or long tool overhangs. A smaller radius reduces cutting force but provides less edge strength.

Step 3: Select the Chipbreaker

Using a roughing geometry at very small feed may produce poor chip control. Using a finishing geometry in a heavy interrupted cut may overload the edge. Always use the published range for the selected JetEdge geometry.

Step 4: Select the JetEdge Grade

Both are fixed CVD-coated grades in the public JetEdge product system: JE8025P is positioned for medium/general steel turning, while JE8035P is positioned for roughing and interrupted cutting. Do not select the grade without the chipbreaker.

CNMG Selection Matrix for Steel Turning

This matrix provides a starting direction only. It does not replace the product-specific cutting-data range.

Quick Selection Flow

Step 5: Check the Toolholder and Setup

Verify the correct holder pocket and insert size, clean support surfaces, sound clamp or lever condition, minimum practical overhang, rigid component clamping, correct center height and stable coolant delivery. CNMG is mechanically strong, but no carbide grade can fully compensate for poor insert seating or unstable workholding.

Step 6: Start With Verified Cutting Data

Cutting speed, feed and depth of cut must be selected from the applicable JetEdge product data after confirming the exact steel, hardness, chipbreaker, grade, interruption level, coolant condition and machine stability.

Do not copy cutting data from an insert of the same ISO shape but a different chipbreaker or grade. The shape code confirms basic compatibility; it does not confirm identical performance.

Troubleshooting a CNMG Application

Information Required for a CNMG Recommendation

1. Complete current insert designation.
2. Toolholder designation.
3. Workpiece steel grade and hardness.
4. External turning, facing or profiling operation.
5. Finishing, medium or roughing requirement.
6. Continuous, lightly interrupted or strongly interrupted cut.
7. Cutting speed, feed and depth of cut.
8. Coolant condition.
9. Current wear pattern and desired improvement.

Conclusion

A CNMG insert should be selected as a complete system: insert size + nose radius + chipbreaker + JetEdge grade + compatible holder + verified cutting data.

For general CNMG steel turning, use JE8025P as the current JetEdge core starting grade. For roughing or interrupted machining where fracture resistance is the priority, evaluate JE8035P.

General selection guidance only. Final selection depends on the complete insert designation, workpiece material, hardness, toolholder, cutting condition and cutting parameters. Published cutting data for the selected JetEdge product takes precedence.

Technical References

1. ISO 1832:2017 — Indexable inserts for cutting tools — Designation.
2. ISCAR — Cutting Tools User Guide: ISO Designation System for Indexable Inserts.
3. Sandvik Coromant — Turning Tools: Indexable Turning Inserts.
4. Tungaloy — T9200 Series: CVD Grades and Chipbreakers for Steel Turning.
5. Noordin et al. — Performance of Coated Carbide Tools When Turning AISI 1045 Steel.