CNMG120408: How to Choose a Chipbreaker and Grade from 680 Catalogue Rows

A purchasing agent once forwarded me an enquiry that read, in full: "CNMG120408, 500 pcs, best price." I sent back one question — which suffix? — and got silence, then a phone call. On their screen, CNMG120408 was one line item. On the five catalogues I check, it is 680 distinct ordering codes: 77 different chipbreaker suffixes and 173 different grades parked behind the same ten characters. Choosing "CNMG120408" is not choosing an insert. It is choosing a shelf.

This guide walks that shelf the way I do when a real job is attached to the question. Every count below is pulled from the cited catalogue rows on the CNMG120408 model page — you can re-run every number against the linked sources.

First, what the ten characters have already decided

Before any suffix enters the discussion, ISO 1832 has fixed the geometry. Run it through the decoder and CNMG120408 resolves to:

• C — 80° rhombic, the workhorse shape with two strong 80° corners per face
• N — 0° clearance: a negative insert, square flanks, double-sided
• M — medium (as-sintered) tolerance class, the economical moulded standard
• G — cylindrical fixing hole, chipbreakers pressed on both faces
• 12 — 12.7 mm (1/2") inscribed circle, the most common turning size in the library
• 04 — 4.76 mm (3/16") thick
• 08 — 0.8 mm (1/32") nose radius, the general-purpose corner (see the nose radius guide for why R0.8 is the default and when R0.4 or R1.2 beat it)

US buyers will recognise the same insert as CNMG432 — the ANSI alias the catalogues print alongside. Same pocket, same insert; only the naming convention differs. All 680 rows share every dimension above. What separates them is everything after the geometry: the suffix that forms the chip and the grade that survives the material.

The shelf, measured

Here is how the 680 cited rows divide across the five catalogued brands:

Read the last two columns together and you can see each brand's philosophy. Kennametal offers the widest grade matrix — 24 suffixes crossing 90 grades. ISCAR goes the opposite way: all 18 of its rows carry the same GN geometry, and the entire choice is pushed into the grade column, 18 grades deep. Neither is wrong. They are two answers to the same question — where should the decision live, in the breaker or in the carbide?

The next-most-catalogued designations in the library — CNMG120412 at 546 rows and WNMG080408 at 529 — show the same pattern at slightly smaller scale. What follows for CNMG120408 applies to them, and to any heavily catalogued code.

Step one: pick the chipbreaker by feed band

The suffix between the geometry and the grade is the chipbreaker, and chipbreakers sort by one axis before any other: how much feed and depth of cut the land and groove are built to survive. Finishing breakers have narrow, sharp lands that curl a thin chip at low feed; roughing breakers have wide, reinforced lands that protect the edge when the cut gets heavy. The chipbreaker codes guide decodes the naming families in detail — here is what the CNMG120408 shelf actually stocks, top suffixes by cited row count:

The letter patterns are brand dialects, but the axis underneath is universal. Sandvik is the one catalogue in the library that publishes its intended operation directly on each row, and its 163 rows split 57 finishing / 78 medium / 28 roughing — a useful reality check on how the industry actually deploys this insert: mostly in the middle band, semi-finishing and general turning, with finishing next and true roughing third.

Two suffix situations deserve a flag before you order:

• Wiper breakers. Sandvik's WF, WM, WMX and Korloy's LW rows are wiper geometries — a flattened secondary land at the nose that irons the surface, letting you hold finish at higher feed. If the drawing's surface requirement is why you are reaching for a smaller nose radius, check the wiper rows first; the nose radius guide covers the trade.
• No suffix at all. 35 of the 680 rows carry no chipbreaker suffix — 32 from Kennametal, 3 from Mitsubishi. These are the plain double-sided forms. Do not assume that makes them cast-iron specials: the catalogued material groups on those rows span P, M, S and N. Treat the plain form as what it is — the same geometry without a pressed groove — and read the grade column instead.

One honest limitation, because it matters for how you use this site: a suffix like MP means medium-duty in Korloy's dialect and appears in Kennametal's list too, but identical letters are not certified-identical geometries. Suffixes are brand property. The only safe transfer between brands runs through measured equivalents — which is what the cross-reference finder and its 191 catalogued mappings are for.

Step two: pick the grade by material, then coating

The breaker is chosen by the cut; the grade is chosen by the workpiece. Across the 680 rows, the catalogued ISO 513 material-group coverage looks like this (a grade certified for several groups counts once per group):

Two conclusions fall out of that table. First, CNMG120408 in an M-class moulded body is overwhelmingly a steel, stainless and cast-iron insert — that is where 173 grades concentrate. Second, the thin ends are informative: only five rows cover N-group aluminium (a double-sided moulded edge is the wrong tool for aluminium; the polished, ground positive inserts exist for that), and no catalogued row claims H-group hardened work — a 0.8 mm nose on a 4.76 mm moulded insert is not where the catalogues put hard turning. If your job sits in N or H, the answer is not a cleverer suffix on this code; it is a different geometry family.

Coating follows material, and here the library is deliberately uneven. Where the catalogue states the coating, the split on this model runs Sandvik 116 CVD / 42 PVD (5 rows with coating unrecorded); Korloy 70 CVD / 39 PVD (18 unrecorded); ISCAR 11 CVD / 4 PVD / 3 uncoated — CVD dominant for the steel-and-iron mainstream, PVD for the sharper, heat-sensitive work, exactly the pattern the CVD vs PVD guide explains. Mitsubishi lists 2 PVD and 3 uncoated rows with 19 pending verification, and all 348 Kennametal rows carry coating data still flagged as pending against the paper catalogue. I would rather show you a gap than a guess — the editorial policy explains the citation rule, and the grade names guide shows how to read what each brand's grade string does tell you in the meantime.

For orientation, the single most-catalogued grade on this model at each brand: Kennametal KCP10B (13 rows), Sandvik 4415 (15 rows, CVD TiCN+Al₂O₃+TiN, rated K/P), Korloy NC3120 (11 rows, CVD, steel medium duty), Mitsubishi MC6115 (3 rows), and across ISCAR's 18 single-row grades the uncoated IC28 sits at the aluminium end. None of these is "the answer" — they are the centres of gravity, the variants each brand stocks deepest, which is the closest thing a catalogue has to a recommendation.

The decision order, compressed

When the enquiry says CNMG120408 and nothing else, this is the sequence:

1. Confirm the geometry is even the question. Rigid setup, steel or iron, general turning to semi-finishing — proceed. Slender part or sticky alloy at low speed — the negative vs positive guide first; the answer may be CCMT, not a better CNMG.

2. Fix the feed band, fix the suffix family. Finishing, medium or roughing is a decision your cutting data has already made; the suffix just has to match it. Use the brand tables above as the map into each catalogue's finishing-to-roughing ladder, and Sandvik's 57/78/28 split as the sanity check on where most work actually lands.

3. Fix the material, fix the grade. P, M or K group first — 211, 163 and 138 rows deep respectively on this model — then CVD versus PVD within it. If you are crossing brands, never transfer the suffix; transfer through the cross-reference finder or not at all.

That is the whole method. Geometry, then breaker, then grade — each step throws out most of the shelf, which is exactly what a shelf of 680 is for.

The short version

CNMG120408 is not a product; it is a 680-row decision space that happens to share one pocket. The ten characters buy you the geometry — 80° rhombic, negative, double-sided, half-inch IC, R0.8 nose. The suffix buys you the feed band. The grade buys you the material. Choose them in that order and the choice takes minutes; choose them out of order and you are comparing 680 rows by price, which is how a 500-piece order of the wrong insert happens.

If you are speccing this code for a real job and want the breaker-and-grade combination checked against the catalogue rows before you commit, send the enquiry over — I answer with the cited catalogue page attached, not a guess.

Have a CNMG120408 variant that earned its place on your shelf — or one that failed in a way the catalogue didn't predict? The verifiable cases are the ones worth documenting. Send them in.