A code like CNMG 120408 is not random. Each position names one thing: the shape, the clearance, the tolerance, the chipbreaker and fixing, the size, the thickness and the nose radius. Learn the seven positions once and you can read any insert in the drawer, brand-neutral, without reaching for a catalog.
Take CNMG 120408 apart one position at a time. The first four are letters that describe the insert, the last block of numbers is its size. Decode it once here, then jump to any specific insert below.
| Pos | Symbol | What it defines | In CNMG 120408 |
|---|---|---|---|
| 1 | C | Insert shape, set by the corner angle | 80° rhombic (diamond), the general-turning workhorse |
| 2 | N | Clearance (relief) angle | 0°, so it is a negative, double-sided insert |
| 3 | M | Tolerance class on size and thickness | Medium, general purpose |
| 4 | G | Type: fixing hole and chipbreaker | Hole plus chipbreaker on both faces |
| 5 | 12 | Cutting edge length (size) | 12 size, a ½ inch / 12.7 mm inscribed-circle class |
| 6 | 04 | Thickness | 4.76 mm (3/16 in) |
| 7 | 08 | Nose (corner) radius | 0.8 mm |
ISO 1832 turning insert designation. The same logic underlies the ANSI B212 code, with minor differences in the number block.
The first letter is the corner angle. Bigger angle means a stronger corner and more heat capacity but worse access into a profile. Smaller angle reaches into tight features but the tip is fragile.
| Letter | Shape | Corner angle | Where it earns its place |
|---|---|---|---|
| R | Round | no corner | Heaviest roughing, strongest edge, high feed |
| S | Square | 90° | Strong edge for facing and roughing where access allows |
| C | Rhombic | 80° | The workhorse: turning and facing with one insert |
| W | Trigon | 80° | Three strong 80° corners, single-sided, turning and facing |
| T | Triangle | 60° | Versatile, more corners, good with decent access |
| D | Rhombic | 55° | Profiling, more side clearance than a C |
| V | Rhombic | 35° | Fine profiling and tight detail, weakest tip |
The second letter is the clearance angle, and it quietly dictates your holder and your cutting forces before you look at anything else.
N (0°) is a negative insert: double-sided, so twice the edges, the strongest option, but it pushes harder into the cut and needs a holder that tilts it to create clearance. C (7°), P (11°) and friends are positive: single-sided, lower cutting forces, kinder to thin walls, small machines and gummy material. Position 2 tells you whether you are holding a roughing tank or a finishing scalpel.
| Letter | Clearance | Insert character |
|---|---|---|
| N | 0° | Negative, double-sided, strongest, highest forces |
| A | 3° | Positive, low clearance |
| B | 5° | Positive |
| C | 7° | Positive, common general clearance |
| P | 11° | Positive, common in finishing geometries |
| D | 15° | High positive, light cuts |
| E | 20° | High positive, soft and sticky materials |
The third letter is how tightly the insert is held to size on the inscribed circle, thickness and corner. You rarely change it without reason, but it matters when parts go straight from the tool.
| Class | Character | Use |
|---|---|---|
| G | Precision, ground | Tight tolerance, finishing and fixed-pocket tools |
| M | Medium | The default for general turning, pressed and sintered |
| U | Utility | Looser, as-pressed, economical roughing |
The fourth letter pairs how the insert clamps with whether it has a moulded chipbreaker. It is why a CNMG and a CNGG behave nothing alike on the same part.
| Letter | Fixing hole | Chipbreaker |
|---|---|---|
| A | Yes | None |
| M | Yes | One side |
| G | Yes | Both sides (so it suits double-sided negative inserts) |
| N | No | None |
| R | No | One side |
The number block is dimensional. Read it as three pairs: size, thickness, then nose radius.
| Block | Meaning | How to read it |
|---|---|---|
| 12 | Cutting edge length (size) | Nominal edge length in mm. 12 is the ½ inch / 12.7 mm inscribed-circle class. Bigger number, bigger insert and deeper cut. |
| 04 | Thickness | 4.76 mm (3/16 in). Thicker resists bending and breakage under heavy feed. |
| 08 | Nose radius | Hundredths of a mm: 08 = 0.8 mm. 04 = 0.4 mm, 12 = 1.2 mm, 16 = 1.6 mm. |
A bigger nose radius gives a stronger corner and a better finish at higher feed, because surface roughness scales with feed squared over radius. The cost is higher radial force and more chatter risk on thin or overhung parts. When a slender part rings, a smaller radius often settles it faster than dropping the feed.
Read position 2 first (negative vs positive), then the shape, then size and nose for the cut you are taking.
Every code below has its own page with the brand-neutral breakdown and equivalents across major makers, because the right insert might be a Sandvik number while your shop stocks Kennametal or Iscar.
Free, no strings: 8-brand grade cross-reference (PDF) · ISO material-group cheat-sheet (PDF)