Turning Insert Designation Decoded: What CNMG, TNMG and DCMT Really Tell You

You pick up a box of inserts and the label says CNMG 120408. Is it right for your holder, your material, and your finish? The answer is inside the code. A turning insert designation follows ISO 1832, and every letter and number tells you something about shape, strength, size, and nose radius. Once you can read it, ordering the wrong insert gets much harder. This guide shows you how to read an insert code step by step, using CNMG, TNMG, and DCMT as examples. Then you can choose from our range of turning inserts with confidence.

Quick answer: A turning insert designation is an ISO 1832 code. It reads in order: shape, clearance angle, tolerance, hole and chipbreaker, size, thickness, nose radius. In CNMG 120408, C is an 80° rhombic shape, N is 0° clearance, 12 is size, 04 is 4.76 mm thickness, and 08 is a 0.8 mm nose radius.

What Is a Turning Insert Designation?

A turning insert designation is the standard code printed on the box and often on the insert itself. ISO 1832 sets the rules, and you can look up the standard on the ISO website. So a CNMG 120408 from one brand matches the shape, size, and thickness of a CNMG 120408 from another. The grade, coating, and chipbreaker can still differ, so check those on the data sheet.

The code has seven main positions and two optional ones. Makers often add their own chipbreaker code after a dash, such as CNMG 120408-PM.

Why does this matter? Two inserts that look alike can differ in thickness or radius by a few tenths of a millimetre. That is enough to change the part size or the finish. The code removes the guesswork, so you can compare quotes from different brands on equal terms.

Positions 1 to 4 are letters. Positions 5 to 7 are numbers. Read left to right and the insert explains itself.

How to Read Insert Code: Shape, Clearance, Tolerance and Hole

Read the four letters before you look at any number. They tell you whether the insert fits your holder and your job.

Insert shape letters

The first letter gives the shape. The shape sets corner strength and how much of the part you can reach.

A negative, double-sided insert makes more usable corners. On a CNMG you get four corners. On a TNMG or WNMG you get six. On an SNMG you get eight. A 35° VNMG corner can fit tight spaces, but it chips fast. Use it mainly for light finishing.

Clearance angle letter

The second letter gives the clearance angle under the cutting edge. N means 0°. That makes the insert negative: flat-sided, usable on both faces and strong enough for roughing. Letters such as B (5°), C (7°), and P (11°) make a positive insert. It cuts with less force and suits small bores and low-rigidity parts. A negative insert needs a holder with a negative-rake pocket, so match it with the right negative-rake turning holders.

Tolerance letter

M is a commonly used mold-pressed tolerance class for general turning applications. It is cost-effective for everyday machining, whereas tighter precision tolerance classes like G (ground) are used when extreme dimensional control is required. Tighter tolerance classes are available when more precise dimensional control is required. Exact tolerance values can vary depending on the insert size and specification, so always check the manufacturer’s data sheet for the correct tolerance information.

Hole and chipbreaker letter

The fourth letter describes the hole and chipbreaker. G means a straight hole with chipbreakers on both faces, as on CNMG and TNMG. T means a countersunk hole with a chipbreaker on one face, as on DCMT and CCMT. N means no hole and no chipbreaker.

Insert Size and Thickness Code: ISO Insert Code Chart

The fifth position is size, and it causes most buying mistakes. For square and round inserts, the number is the inscribed circle (IC) in mm. For other shapes, it follows the cutting-edge length with decimals dropped. That is why a TNMG 16 and a CNMG 12 sit in very different holders.

The sixth position is thickness. Two digits give it, and T3 is a special case.

Thicker inserts take heavier cuts and more feed. CNMG 160608 is 6.35 mm thick against 4.76 mm for CNMG 120408, so it copes better with heavy and interrupted cuts. Size also determines the holder. A CNMG 16 will not fit a CNMG 12 pocket, even though both are 80° rhombic, so match the holder to the full insert code and not just the shape letters. For a deeper look at angles and grades, read our guide to insert size, angle and grade specifications.

Nose Radius Code: What 04, 08 and 12 Mean

The seventh position is the nose radius code. Read it as tenths of a millimetre: 04 is 0.4 mm, 08 is 0.8 mm, 12 is 1.2 mm, and 16 is 1.6 mm. This small number controls surface finish, edge strength, and the feed you can run.

Pick the feed based on the insert shape, the part material, the type of cutting, and how stiff the machine is. If the nose radius is larger, you can often run at a higher feed. If the radius is smaller, it is usually better for light finishing cuts. In all cases, use the insert maker’s stated cutting values for that exact job.

Set the radius first, then set the feed. For example, a 0.4 mm radius works well for finishing inside a long bore or on a narrow shaft. A 1.2 mm radius fits better when the part is rigid, like a forged blank with enough extra stock. If you are unsure, 0.8 mm is the safe middle for most CNC lathe work in steel.

CNMG 120408 Meaning, Plus TNMG and DCMT Decoded

Here are three common codes decoded side by side.

Quick Summary: CNMG = Heavy & Roughing/General Turning (Negative 0°), TNMG = Multi-edge Facing & Turning (6 corners, Negative 0°), DCMT = Light Cutting & Finishing/Boring (Positive 7°).

CNMG 120408 meaning: an 80° rhombic, negative, double-sided insert with a 12.7 mm IC, 4.76 mm thickness and 0.8 mm nose radius. It is the all-rounder for steel and cast iron. See this style in our CNMG 120408 inserts.

TNMG 160408 shares the negative design but uses a 60° triangle. It gives more corners and good reach for facing. The 60° tip is weaker than the 80° corner of a CNMG.

DCMT 11T304 is positive and single-sided with a 55° point. It cuts freely, which suits finishing, profiling, and small-bore boring bars. DCMT 070204 and 070208 inserts follow the same logic in a smaller size.

Optional letters can follow the numbers: F for a sharp edge, E for rounded, T for chamfered, and S for chamfered and rounded. Then comes R, L or N for cutting direction. A suffix such as -PM or -MF is the maker's own chipbreaker code. Makers such as Sandvik Coromant publish their own insert-identification pages if you want a second reference.

How to Choose the Right Insert Code for Your Job

Use the code in reverse when you buy. Start from the job, not the catalogue.

Example: roughing and finishing an EN8 shaft on a CNC lathe. You are turning a 60 mm EN8 shaft on a rigid machine. For roughing, choose CNMG 120408: strong 80° corner, 0.8 mm radius, negative holder. With a coated carbide steel grade, a starting point is 180 to 250 m/min, 0.20 to 0.35 mm/rev, and 1.5 to 4 mm depth of cut. For the finishing pass, switch to DCMT 11T304 on a positive holder. Start near 220 to 300 m/min, 0.08 to 0.15 mm/rev, and under 1 mm depth. These are indicative values. Follow the insert maker's data sheet and your machine's rigidity.

Buying checklist:

1. Match the holder. Check that the pocket suits the shape and clearance letters. Learn how to read turning holder codes too.
2. Decide rough or finish. Use radius 08 or 12 for roughing, 04 or 08 for finishing.
3. Match the material. Grade groups P (steel), M (stainless), and K (cast iron) are separate from the designation.
4. Check rigidity. Low rigidity calls for a positive insert and a smaller radius.
5. Confirm grade, coating, and chipbreaker on the data sheet.

Common mistakes

• Ordering by shape and size only and ignoring thickness.
• Pairing a negative insert with a positive holder, which loses clearance.
• Using a large nose radius on a long, thin shaft, which invites chatter.
• Assuming the same code means the same grade. The ISO code does not give grade or coating.
• Mixing up 04 and 08 radius in one job. The radius changes both finish and part dimensions.

Frequently Asked Questions

What does CNMG 120408 mean?

C is an 80° rhombic shape. N is 0° clearance, so the insert is negative. M is the tolerance class. G means a hole with chipbreakers on both faces. 12 is the size (12.7 mm IC), 04 is the 4.76 mm thickness, and 08 is the 0.8 mm nose radius.

What is the difference between CNMG and TNMG?

CNMG is an 80° rhombic insert with four usable corners and a strong tip. TNMG is a 60° triangle with six usable corners and good reach for facing, but a weaker tip. Both are negative, double-sided inserts that need a negative-rake holder.

What does the N in CNMG stand for?

The N is the clearance angle letter and means 0°. It marks a negative insert. Negative inserts are flat-sided and double-sided, so they give more corners and handle roughing well. They must sit in a holder with a negative-rake pocket.

What is the difference between CNMG and DCMT?

CNMG is negative, thick, and double-sided, built for heavier cuts. DCMT is positive with a 7° clearance and a 55° point, and it is single-sided. DCMT cuts with lower force, so it suits finishing, profiling and small-bore boring on less rigid setups.

How do I read the nose radius on an insert?

Take the last two digits of the numeric part. They show the radius in tenths of a millimetre: 04 is 0.4 mm, 08 is 0.8 mm, 12 is 1.2 mm, and 16 is 1.6 mm. A larger radius gives a smoother finish and a stronger corner.

Are CNMG inserts from different brands interchangeable?

An ISO code sets the part’s shape, size, and thickness. That is why the same code should match the same holder pocket. Still, the grade, the coating, and the chipbreaker can change from one brand to another. So look at the data sheet before you switch. Also, test the new brand on a noncritical job first.

Conclusion

A turning insert designation looks like code, but it is a short specification: shape, clearance, tolerance, hole, size, thickness, and nose radius. Read it left to right and CNMG 120408, TNMG 160408 and DCMT 11T304 stop being mysteries. Check the holder, match the radius to the job and confirm the grade on the data sheet. Ready to choose? Browse CNMG, TNMG and DCMT turning inserts at Jaibros and compare sizes, grades, and brands in one place.