- ISO insert codes have up to 9 positions, each describing one property, always read left to right
- The first letter (shape) tells you the included angle at the cutting edge, which drives edge strength
- The 5th and 6th positions (size and thickness) directly set the insert’s maximum safe depth of cut
- The code is standardized across brands — a CNMG from one manufacturer fits the same holder as a CNMG from another
- Grade and chipbreaker geometry sit outside this code and still vary by brand, even when the code matches
CNC Insert Identification
A CNMG120408 insert isn’t a random string. Every character has a job. Once you know what each position means, you can look at any insert box from any brand and know exactly what you’re holding.
A worked example: CNMG120408
Take the code CNMG120408, one of the most common turning inserts in any Indian job shop. Split apart, it reads like this:
C
Shape
N
Clearance
M
Tolerance
G
Fixing
12
Size
04
Thickness
08
Radius
That’s a rhombic 80° insert (C), zero clearance angle (N), medium tolerance (M), with a hole and chipbreaker on both faces (G), a 12mm cutting edge, 4.76mm thickness, and a 0.8mm corner radius. All of that, from seven characters.
What each position in the code means
1
Position 1 — Shape
The included angle at the corner. A wider angle (like square or 80° rhombic) gives a stronger, more durable edge. A narrower angle (35° rhombic, or triangular) gives better access into corners and profiles but a weaker tip.
C = 80° rhombic • S = square • T = triangle • R = round • V = 35° rhombic
2
Position 2 — Clearance (relief) angle
The angle ground below the cutting edge so the insert doesn't rub against the finished surface. Most general turning inserts use N (0° clearance), which is also the strongest and cheapest to manufacture.
N = 0° • C = 7° • P = 11° • B = 5°
3
Position 3 — Tolerance class
How tightly the insert's dimensions are controlled at manufacture. This affects how precisely the insert seats in the holder pocket, which matters more on finishing passes than roughing.
M = medium tolerance (most common) • G = ground, tighter • U = unground, looser
4
Position 4 — Fixing / hole type
How the insert is clamped into the holder — no hole, a plain hole, or a hole with a chipbreaker groove on one or both faces. This decides which clamping system your tool holder needs to use.
N = no hole • M = hole, chipbreaker one face • G = hole, chipbreaker both faces
5
Positions 5 & 6 — Size and thickness
The cutting edge length (or inscribed circle diameter, for round/square shapes) and the insert's thickness, both in millimetres. These two numbers, together with the shape, set the insert's maximum safe depth of cut.
12 = 12mm edge length • 04 = 4.76mm thickness
6
Position 7 — Corner radius
The radius ground at the insert's tip, in tenths of a millimetre. A smaller radius cuts sharper corners and generates less cutting force, but is more fragile. A larger radius is stronger and gives a better surface finish.
04 = 0.4mm • 08 = 0.8mm • 12 = 1.2mm
Positions 8 and 9 are optional and only appear when needed — an extra letter for cutting edge condition (sharp, honed, or chamfered) and one for cutting direction (right hand, left hand, or neutral). Most standard catalogue inserts only carry the first seven characters.
Common insert shapes at a glance
| Letter | Shape | Included angle | Best for |
|---|---|---|---|
| C | Rhombic | 80° | General turning, strong edge, good all-rounder |
| D | Rhombic | 55° | Profiling and copy turning |
| S | Square | 90° | Roughing, heavy stock removal |
| T | Triangle | 60° | General turning, versatile, 3 usable corners |
| R | Round | – | Contour finishing, high feed roughing |
| V | Rhombic | 35° | Tight corner access, profiling, low rigidity setups |
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FAQs:
1. What do the letters and numbers in a CNC insert code mean?
A standard ISO insert code has up to nine positions, each describing one property: shape, clearance (relief) angle, tolerance class, fixing/hole type, size, thickness, corner radius, cutting edge condition, and cutting direction. Read left to right, and any insert code decodes the same way regardless of brand.
2. How do I know an insert's shape from its code?
The first letter of the ISO code gives the shape: C is a rhombic 80°, D is rhombic 55°, S is square, T is triangular, R is round, V is rhombic 35°, and W is trigon. This letter alone tells you the included angle at the cutting corner, which drives how strong the edge is and how it accesses corners.
3. What does the size number in an insert code represent?
The size position gives the length of the cutting edge in millimeters for most shapes, or the inscribed circle diameter for round and some other shapes, generally expressed in an abbreviated two-digit form. It directly determines the maximum depth of cut the insert can safely take.
4. Is the insert code the same across all tooling brands?
Yes, for the most part. The ISO 1832 standard defines this coding system, so a CNMG insert from one major brand is dimensionally interchangeable with a CNMG insert from another, though grade and chipbreaker geometry, which sit outside the core code, still vary by manufacturer.
5.Why does the insert code matter for CNC programming?
The insert code determines maximum depth of cut, corner strength, and which tool holder it fits, all of which directly affect the cutting parameters a program can safely use. Getting the insert selection wrong is one of the most common causes of tool breakage and poor surface finish on a CNC lathe.
