Letter by letter
The first four or five characters carry the geometry and the mounting system, and these are the ones that must match the holder.
| Position | Meaning | Common values |
|---|---|---|
| 1st letter | Insert shape | C rhombic 80, S square 90, T triangle 60, D rhombic 55, V rhombic 35, W trigon 80 |
| 2nd letter | Clearance angle | N zero, C 7, P 11, D 15, B 5; N is negative and suits double-sided inserts |
| 3rd letter | Tolerance class | M general, G precision ground, U semi-precision |
| 4th letter | Mounting and chip breaker | G cylindrical hole double-sided, M double-sided, T single-sided, W 80-degree single |
| Numeric 1 | Inscribed circle size | e.g. 12 = 12.7 mm, 09 = 9.525 mm, 16 = 15.875 mm |
| Numeric 2 | Thickness | e.g. 04 = 4.76 mm, 03 = 3.18 mm |
| Numeric 3 | Corner radius | e.g. 08 = 0.8 mm, 04 = 0.4 mm, 12 = 1.2 mm |
A worked example
CNMG 120408 breaks down as: C for an 80-degree rhombic shape, N for zero clearance which means the insert is used in a negative holder and can be turned over, M for a general tolerance class, G for a cylindrical hole with a double-sided chip breaker, 12 for a 12.7 mm inscribed circle, 04 for 4.76 mm thickness, and 08 for a 0.8 mm corner radius.
The practical consequences: this insert fits a negative holder, gives four usable corners on a rhombic shape, and has a 0.8 mm radius suitable for medium turning. Change the second letter to C and it becomes a positive insert that fits a different holder and cannot be turned over.
Why the tolerance class matters more than it looks
The tolerance class describes how closely the insert complies with its nominal dimensions, and it therefore decides whether the insert sits correctly in the pocket and whether indexing to a new corner changes the cutting position.
M class is general purpose and suitable for most work. G class is precision ground, which suits finishing operations where a small change of position between corners would show up in the part. Using an M class insert for a tight finishing operation is a common source of unexplainable size variation.
Corner radius and what it controls
The corner radius is the last number and it decides the strength and the finish at the same time. A larger radius spreads the load over more edge and produces a stronger corner with a better theoretical finish, but it also increases the radial force and tends to produce chatter on a slender part.
A smaller radius cuts more freely and suits slender parts and light finishing passes, but the corner is weaker and the finish is more sensitive to feed. Choosing it against the part stiffness as well as the finish requirement is what keeps a finishing operation stable.
Frequently asked questions
What does CNMG mean?
C is an 80-degree rhombic shape, N is zero clearance for a negative holder, M is the general tolerance class, and G indicates a cylindrical hole with a double-sided chip breaker. The numbers that follow give the size, thickness and corner radius.
What is the difference between CNMG and CNMM?
The final letter describes the chip breaker and mounting geometry rather than the shape. Both are 80-degree rhombic negative inserts, but they take different chip breaker designs and may suit different holders.
What does the corner radius number mean?
It is the last two digits in hundredths of a millimetre: 08 is 0.8 mm, 04 is 0.4 mm, 12 is 1.2 mm. A larger radius is stronger and gives a better theoretical finish, but increases radial force.
Can I use a positive insert in a negative holder?
No. A negative insert has zero clearance and relies on the holder's negative inclination to provide the clearance angle, so a positive insert in a negative holder would have no clearance at all and would rub.
Why does the tolerance class matter?
It describes how closely the insert matches its nominal dimensions, which decides how it seats in the pocket and whether indexing to a new corner changes the cutting position. For tight finishing work, an M class insert can produce size variation that a G class insert would not.