How to Read a Carbide Insert ISO Code
If you've ever looked at an insert box marked "CNMG 120408" or "TNMG 160404" and wondered what the letters and numbers actually mean, the good news is that almost every carbide turning insert on the market follows the same international standard: ISO 1832. Once you understand the pattern, you can read the geometry of an unfamiliar insert code in seconds.
The first letter: insert shape
The first letter of the code tells you the insert's shape as viewed from above. Common shapes include C (80° rhombic), D (55° rhombic), V (35° rhombic), W (80° trigon), T (triangular, 60°), and S (square, 90°). A wider included angle, like the 90° square or 80° rhombic shapes, gives a stronger cutting edge suited to roughing. A narrower angle, like the 35° rhombic V shape, gives better access to tight contours but a weaker edge — better suited to finishing and profiling.
The second letter: relief (clearance) angle
The second letter describes the relief angle on the insert's flank — essentially, how much clearance the insert has below the cutting edge. N denotes 0° (a negative insert, which needs to be tilted in the holder to create a working clearance angle), while letters like A, B, C, P, D, E, F and G denote increasing positive relief angles. Negative inserts (N) are generally stronger and are often double-sided, giving more usable edges per insert; positive inserts cut with less force and are common for finishing or on lower-power machines.
The remaining letters: tolerance class and fixing style
The third letter sets the insert's tolerance class — how precisely its dimensions are held — and the fourth letter describes the fixing/clamping style, such as whether the insert has a mounting hole, a chip-breaker groove, or both. These matter mostly for insert-to-holder compatibility and are usually matched automatically once you know which holder or cutter body you're using.
The numbers: size, thickness and corner radius
After the four letters, the numeric portion of the code specifies the insert's size (edge length or inscribed circle diameter), thickness, and corner (nose) radius, in that order. So in a code like CNMG 120408, the digits after the letters break down into a size group, a thickness group and a corner-radius group. Exact millimetre values can vary slightly by manufacturer and insert series, which is why we recommend confirming exact dimensions against your existing tooling or drawing before ordering, rather than assuming a fixed conversion.
Why this matters when ordering
Understanding the code means you can compare inserts across brands by geometry rather than by brand name alone, and it means you can describe exactly what you need even if you don't have a manufacturer's catalogue on hand. If you're not sure which shape or geometry suits your operation, tell us your workpiece material, the machining operation (roughing or finishing) and your existing insert code if you have one, and we'll help identify a suitable match.