How to Read an Insert Designation

By Bradley Taylor · August 2026

The first time somebody hands you a box marked CNMG 432 and tells you to find another one like it, the code looks like noise. It is not. Every character is a field in a standard system, and once you know the fields you can read any turning insert in the crib without a catalog. The system is ANSI B212.4 on this side of the ocean and ISO 1832 everywhere else. I will walk through CNMG 432 one position at a time, because once you can read that one you can read most of them.

The letters, one position at a time

The first letter is the shape. C means an 80 degree diamond, a rhombic insert with two usable corners at 80 degrees each. That corner angle is the reason C shapes are the general turning workhorse. The point is strong enough to rough with and sharp enough to turn into a shoulder, so one insert handles facing and turning in the same tool.

The second letter is the clearance angle, the relief ground below the cutting edge. N means zero degrees of clearance. The sides of the insert are square to the top face. That sounds like a defect until you remember the holder itself tips the insert forward to create clearance at the work. Hold that thought, because this letter matters more than it looks.

The third letter is the tolerance class, and M is one of the common ones. It tells you how tightly the maker held the inscribed circle and thickness. M class is a molded insert with moderate tolerances, fine for general turning. Ground periphery classes like G hold the size much tighter, which matters when you expect a fresh corner to land within a few tenths of the old one. For most turning, M class is plenty.

The fourth letter describes the top and the hole. G means the insert has a hole for the clamp screw or lever lock and a chipbreaker groove molded into both faces. Both faces is the useful part. A CNMG is double sided, so those two 80 degree corners exist on the top and the bottom, four usable corners per insert. A single sided insert with the same shape gives you two. When you are paying per corner, that letter is money.

Then the numbers

The 4 is the size of the inscribed circle in eighths of an inch. Four eighths is 1/2 inch, so a CNMG 432 fits any pocket cut for a 1/2 inch IC 80 degree diamond. The inscribed circle is the largest circle that fits inside the insert outline, an odd way to state a size but one that works for every shape.

The 3 is the thickness in sixteenths of an inch, so 3/16 thick. Thickness has to match the seat and shim in the holder, which is why you cannot swap a 43 size insert into a 32 pocket and expect the clamp to reach.

The 2 is the nose radius in 64ths of an inch. Two 64ths is 1/32. Nose radius is the number you will actually change most often, because it trades finish against edge strength. A bigger radius wipes a better finish at the same feed and shrugs off interruptions, but it pushes harder on the part and chatters sooner on thin work. The old rule that feed should stay under about half the nose radius is a decent starting point, and the speeds and feeds calculator will get you in the neighborhood from there.

The same insert in metric

Order the same insert from an ISO catalog and it shows up as CNMG 120408. The letters mean the same things. The numbers switch to millimeters and get more literal. The 12 is the edge length of the insert in millimeters, approximately, for this shape and IC. The 04 is the thickness, 4.76 mm rounded into the code. The 08 is the nose radius in tenths of a millimeter, so 0.8 mm, which is the metric twin of 1/32 inch. Nobody converts these at the machine. You just learn that 432 and 120408 are the same insert, and after a while the pairs stick.

The other shapes and where they live

C is the 80 degree diamond and the default answer for general turning and facing. D is a 55 degree diamond, and that skinnier point reaches into profiles and undercuts the C shape cannot follow. T is the triangle, three corners of 60 degrees, common in simpler and cheaper tooling. V is the 35 degree diamond, the finish profiling insert, with a point slender enough to trace tight contours and weak enough that it will remind you not to rough with it. W is the trigon, which looks like a triangle that went to the gym, giving you three corners at roughly 80 degrees, so it cuts like a C with an extra corner. R is round, the strongest edge of all, at home in heavy roughing and in cuts where the whole radius does the forming. The pattern behind all of it is simple. Sharper points reach more geometry and break easier. Blunter points last longer and get into less.

Why the clearance letter picks your holder

Go back to that second letter. An N insert with zero clearance only works in a negative rake holder that tilts it to create relief, and because both faces present the same way, N inserts can be double sided. Letters like C, with 7 degrees of relief, or P, with 11, are positive inserts. The relief is ground into the insert itself, so it sits flatter in a positive rake holder, cuts with less pressure, and is single sided. Negative rake for rigidity, corner count, and heavy cuts on solid setups. Positive rake for small parts, thin walls, and machines without much backbone. The letter is not trivia. It decides which holders in the shop the insert will physically work in.

What the designation does not tell you

Everything above is geometry. None of it says a word about what the insert is made of. Grade, meaning the carbide substrate and the coating the maker put on it, is a separate code on the box, and it matters just as much as the shape. The same CNMG 432 comes in grades for steel, for stainless, for cast iron, and for interrupted cuts, and putting the wrong grade in the right pocket will burn up an edge as fast as any geometry mistake. The same warning goes for the extra letters after the standard code, things like PM or MF. Those are chipbreaker styles, and they are pure vendor language. One company's roughing breaker has nothing to do with another company's code that looks similar, so read the catalog for those and do not assume.

So the box tells you two stories. The standard part, CNMG 432 or CNMG 120408, tells you exactly what shape of carbide is inside and which holders it fits. The grade and the breaker suffix tell you what the maker thinks it should cut. Learn to read the first part cold and treat the second part as a conversation with the catalog, and you will spend a lot less time squinting at little gold triangles in the crib.

As always, this is general practice, not a spec. Prints, customer requirements, and the governing standard win every argument.