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, current edition 2002, 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. The class actually covers three dimensions, the inscribed circle, the thickness, and a third one called the cornerpoint that most people never hear about. The cornerpoint is the one that decides whether a fresh corner lands where the old one was, so it is the one that matters for indexing repeatability. M class holds all three to everyday tolerances, fine for general turning. G class is ground on the periphery, which tightens the inscribed circle and the cornerpoint to about plus or minus 0.001 inch, a real improvement, but it leaves the thickness at the same tolerance as M. And 0.001 inch is ten tenths, so if the job needs a fresh corner to repeat within a couple of tenths, G is not the class that does it. That is A or F territory, where the cornerpoint is held to about two tenths and the thickness is ground too. For most turning, M class is plenty.

The fourth letter describes the hole and the chipbreaker together. G means a cylindrical hole, straight walled with no countersink, and a chipbreaker groove on both faces. The straight hole is a real distinction. With no countersink there is no seat for a screw head, so a G insert is not held by a screw through the hole. It is retained by a lever lock, a pin, a wedge, or a top clamp. Screw down inserts carry a countersunk hole code instead, T and U for the 40 to 60 degree countersinks you see on CCMT and DCMT style inserts, and B, C, H, and J for the 70 to 90 degree family. That one letter tells you whether the insert will physically mount in the holder you are standing next to. Both faces is the other 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.

One warning before the integer rule bites you. When the fraction does not come out even, the inch system writes a decimal into the code. A CCMT 21.51 is a quarter inch IC, a thickness of one and a half sixteenths, which is 3/32, and a 1/64 radius. The 21.51 boxes in the crib stall everyone who only learned the whole number version. The metric system does the same dodge with a letter, using a T in the thickness field for the in between sizes, so T3 means 3.97 mm where 03 would mean 3.18 and 04 would mean 4.76.

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.

A second example that breaks the easy rules

Decode a DCGT 11T302 and three of the tidy rules above need their fine print. D is the 55 degree diamond. C is 7 degrees of clearance, a positive insert. Then comes G, and here it is a tolerance class, the ground periphery class from earlier, not the hole and chipbreaker meaning it carries in position four. The same letter shows up in different positions with completely unrelated meanings, so always count positions before you read letters. Fourth is T, a 40 to 60 degree countersunk hole with a chipbreaker on one face, which is the screw down single sided construction you would expect on a positive finishing insert. The 11 is the edge length code, about 11.6 mm of cutting edge on this shape. T3 is that in between thickness, 3.97 mm. And 02 is a 0.2 mm nose radius, a fine finishing point. Put together, that is the little positive screw down insert living in Swiss lathes and finish boring bars everywhere.

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. Standards referenced: ANSI B212.4-2002 and ISO 1832 (indexable insert designation).