How to Read an ISO 1832 Insert Code

An indexable insert designation such as CNMG120408 is not a part number. It is a compact geometry specification written in a language that every manufacturer of turning tools speaks. Read it correctly and you know the insert's shape, its clearance angle, how it is toleranced, how it is held in the seat, its inscribed circle, its thickness, its nose radius and whether it has a left or right hand — all before you open a catalogue. The catalogue only tells you which brand-specific version of that geometry is in stock.

This guide walks through the eight positions of the ISO 1832 designation, position by position, with three worked examples drawn from the InsertBase reference database: CNMG120408, WNMG080404 and CCMT060204. Use the interactive decoder side by side for any code of your own.

The eight positions at a glance

An ISO 1832 designation is read left to right as a sequence of compact code groups. The standard layout is:

The two- or three-digit numeric blocks at the end are not arbitrary: 12 is the inscribed circle in tenths of a millimetre — so 12 means 12.7 mm because ISO 1832 stores the standardised circle values that the industry actually stocks. The same logic applies to thickness and nose radius.

Position 1 — Insert shape

The first letter tells you the outline of the insert as seen from above. The most common shapes are:

• C — rhombic with an 80° included corner angle, the workhorse for general turning because it offers both a strong cutting edge and reasonable accessibility.
• D — rhombic with a 55° angle, for finer finishing where access to shoulders matters.
• S — square (90° corner), used for facing and square shoulders.
• T — trigon (equilateral triangle with rounded corners), for heavy roughing where three cutting edges per side multiply tool life.
• V — rhombic with 35° included angle, for threading and very fine finishing.
• W — trigon with an 80° included point, a hexagonal-equivalent that combines the strength of C with three usable edges per side.
• R — round, for profiling and heavy roughing of harder materials where chipping is the failure mode.

Each shape has a family and an included angle that the InsertBase decoder reports verbatim. The corner angle is what determines edge strength: an 80° corner (C, W) is robust, a 55° corner (D) is more fragile but more accessible, a 35° corner (V) is the most fragile and is almost always fed at lighter depths.

Position 2 — Clearance angle

The second letter is the clearance angle between the insert's flank face and the workpiece surface. Six values dominate turning practice:

• N — 0° (neutral), the most common, used in holders that themselves supply the clearance.
• A — 3°, used in boring bars and some profiling holders.
• B — 5°.
• C — 7°, common in positive-rake finishing.
• P — 11°, for very positive light finishing.
• O — special, defined by the manufacturer.

If you put a C-clearance insert into a holder designed for N-clearance inserts, you change the actual rake and clearance the workpiece sees — a classic source of poor surface finish and unpredictable tool life. Read this letter before swapping inserts between holders.

Position 3 — Tolerance class and side count

The third letter reports two things together: how tightly the insert is toleranced on inscribed circle, thickness and nose radius, and whether it is single- or double-sided. The classes most often seen are:

• A, F, C — high-precision classes, normally single-sided, used for finishing and precision boring.
• M — moderate tolerance, can be single- or double-sided; the most common class for general turning.
• U — general-purpose, normally double-sided, for roughing.
• E, G, W — other classes, less common.

An M insert is double-sided when its chipbreaker pattern is identical top and bottom. The InsertBase decoder uses the side-count information together with the tolerance to decide whether a "double-sided" roughing insert can be flipped before being indexed — useful when roughing a long bar.

Position 4 — Fixing and chip-breaker type

The fourth letter describes how the insert is held and whether it has a chip-breaker formed into the rake face:

• N — no hole, no chip breaker (clamped only).
• A — with hole, no chip breaker.
• M — with hole and chip breaker (the most common in modern turning).
• G — with hole and chip breaker (a particular geometry family).
• F — no hole but with chip breaker (seldom seen on turning, more common on milling).

The presence of a hole means the insert is pinned or clamped through its body; the presence of a chip breaker means the rake face has a pressed groove that curls the chip so it evacuates cleanly. Both M and G are common on modern inserts and the choice between them is driven by the manufacturer's chip-breaker catalogue, not by ISO 1832.

Position 5 — Inscribed circle d1

The first numeric block is the inscribed circle: the diameter of the largest circle that fits inside the insert outline. For a round insert this is the insert diameter; for any other shape it is the diameter of the circle tangent to all straight edges. ISO 1832 stores these values in tenths of a millimetre and selects only the standardised ones the industry actually stocks. The most common values are:

• 06 — 6.35 mm (¼")
• 09 — 9.525 mm (3/8")
• 12 — 12.7 mm (½")
• 16 — 15.875 mm (5/8")
• 19 — 19.05 mm (3/4")
• 25 — 25.4 mm (1")

The inscribed circle determines how big the cutting edge is and, together with the thickness, how much depth of cut the insert can take. Bigger IC means more edges, more rigidity and higher metal-removal rates — and a more expensive insert.

Position 6 — Thickness s

The second numeric block is the insert thickness, also stored in tenths of a millimetre against standardised values. For T-prefix thicknesses (for example T3) the prefix marks a special thickness series — see the worked example for CNMG09T308 on InsertBase. Common metric values include:

• 03 — 3.18 mm
• T3 — 3.97 mm (T-series)
• 04 — 4.76 mm
• 06 — 6.35 mm
• 08 — 8.0 mm (varies by family)

Thickness sets how much clamping force the insert can survive and, together with IC, how much depth of cut it can take before fracturing. A thin insert on a heavy cut flexes and chips; a thick insert on a finishing pass wastes edge.

Position 7 — Nose radius rε

The third numeric block is the nose radius — the radius of the rounded tip where the two cutting edges meet. ISO 1832 stores these in tenths of a millimetre against standardised values:

• 02 — 0.2 mm
• 04 — 0.4 mm
• 08 — 0.8 mm
• 12 — 1.2 mm
• 16 — 1.6 mm
• 24 — 2.4 mm

The nose radius is the single most misunderstood parameter in turning. A smaller radius gives better surface finish at light cuts and reaches into corners; a larger radius is stronger, takes heavier cuts, but pushes the workpiece harder and is more prone to vibration if the setup is not rigid. As a rule of thumb, pick the largest nose radius that the workpiece geometry, the surface finish requirement and the spindle power will allow.

Position 8 — Hand (optional)

An eighth letter — R, L or N — describes whether the insert is right-hand, left-hand or neutral. Most modern neutral inserts are written without this letter; the hand is then determined by the holder. Inserts with a directional chipbreaker pattern (R or L) cannot be flipped or rotated without changing the cutting direction.

The InsertBase decoder accepts a trailing R, L or N and reports it in the hand position. A common confusion: CNMG120408R is not a different geometry from CNMG120408 — only the hand is added.

Worked example 1 — CNMG120408

One of the most stocked turning inserts in the world. Decoding position by position:

The full CNMG120408 model page on InsertBase adds the brand-specific layer: ordering codes, chipbreaker suffixes, carbide grades and source citations. Note how the geometry is identical for every brand — only the suffix changes.

Worked example 2 — WNMG080404

A trigon with 80° included point, used where three cutting edges per side multiply the number of usable edges per insert.

• W — trigon, 80° included point, the "sturdy alternative to C".
• N — 0° clearance, neutral.
• M — moderate tolerance, double-sided.
• G — with hole and chip breaker.
• 08 — d1 = 12.7 mm (the value stored by ISO 1832 for the 08 prefix family).
• 04 — s = 4.76 mm.
• 04 — rε = 0.4 mm nose radius, smaller than CNMG120408, suited to finer finishing and lighter cuts.

Because the corner angle is 80° on a trigon rather than on a rhombus, the same 0.4 mm nose radius sits on a sturdier corner. The trade is accessibility: WNMG cannot reach into a shoulder the way CNMG can. The WNMG080404 model page on InsertBase compares the two series directly.

Worked example 3 — CCMT060204

A positive-rake finishing insert, smaller and lighter than the previous two. The C-prefix at position 2 changes everything:

• C — rhombic 80° shape, the same as CNMG.
• C — 7° clearance. The insert itself carries clearance; the holder is a simpler flat pocket.
• M — moderate tolerance.
• T — with hole, no chip breaker (note: T at position 4 differs from the common M/G).
• 06 — d1 = 6.35 mm, a small insert.
• 02 — s = 2.38 mm, a thin insert.
• 04 — rε = 0.4 mm nose radius.

CCMT-class inserts are stocked for precision finishing on Swiss lathes and small boring bars where the cutting forces must stay low. The 7° clearance built into the insert makes the cutting edge sharper and the surface finish better — at the cost of edge strength. See the CCMT060204 model page for the catalogue variants.

Worked example 4 — T-series thickness

When position 6 starts with the letter T — for example CNMG09T308 — the insert uses a T-series thickness that ISO 1832 calls out separately. T3 means 3.97 mm, a value that fits certain thread-turning and boring profiles that the more common 3.18 mm (03) cannot. If you see T in position 6, look up the T-series table on the model page rather than reading the digit as a plain metric.

What the brand suffix changes — and what it does not

Everything after a dash — for example CNMG120408-PM 4325 — is brand-specific. The suffix tells you the chipbreaker geometry (PM, pre-machining) and the carbide grade (4325) that this manufacturer applies to the same CNMG120408 geometry. Crucially:

• The geometry (shape, IC, thickness, nose radius) is identical to the unsuffixed code. The suffix never alters ISO 1832 positions.
• The chipbreaker pattern pressed into the rake face differs and is not standardised — Sandvik's MR, PM, MM and PR are four different chipbreaker geometries on the same rhombic blank.
• The grade determines the substrate, the coating family (PVD or CVD), the ISO 513 application groups and the recommended cutting data.

So when you cross-reference two manufacturers against the same ISO 1832 code, geometry is interchangeable by definition. What is not interchangeable is the chipbreaker pattern (it changes chip flow and cutting forces) and the grade (it changes wear behaviour). Read the editorial policy for the FULL / PARTIAL / VERIFY framework InsertBase uses to express this honestly.

How to verify any code in seconds

Two practical workflows on InsertBase:

1. Interactive decode: type the code into the ISO 1832 decoder. The result shows every position, the decoded value and the standardised metric. Brand suffixes are flagged as "after-market" and routed to the catalogue pages.
2. Model page lookup: open the matching model page for the code. If the page is catalogued, it shows the geometry table plus every brand variant InsertBase has sourced. If the page is not yet catalogued, the geometry still decodes from the standard.

Both tools run from the same ISO 1832 code tables, so the values agree by construction. When a manufacturer catalogue gives a geometry that disagrees with the decoded value, the InsertBase editorial policy requires the conflict to be flagged on the model page — never silently corrected.

Common confusions and how to avoid them

• "CNMG and CNGG are the same shape" — yes, both are 80° rhombic. The difference is at position 3 (tolerance) and possibly position 4. Read the full code, not just the first letter.
• "A bigger nose radius always means a stronger insert" — only up to the limit your setup allows. Too large a radius loads the workpiece, induces chatter and shortens tool life.
• "I can swap CNMG and DNMG in the same holder" — never. The corner angle differs (80° vs 55°), so the seat geometry, the cutting edge direction and the accessibility all change.
• "The suffix after the dash changes the geometry" — no. CNMG120408-PM 4325 and CNMG120408-MM 1125 share geometry. Only chipbreaker pattern and grade differ.
• "T in position 6 is a typo" — no. T3 is a valid thickness series in ISO 1832.

Where to go next

• Try any code of your own in the ISO 1832 decoder.
• Read the companion guide on ISO 513 carbide grade selection to learn how to choose the suffix.
• Browse the full model index or filter by inscribed circle, thickness or nose radius.
• Use the cross-reference finder to locate a stocked equivalent for a code you already run.

Specifications on InsertBase are referenced from the ISO 1832 designation standard and from each manufacturer's officially published catalogue. Each cited value carries a source, a version and a date — see the editorial policy for the full methodology. Always verify the complete ordering code and cutting data against the current manufacturer catalogue before machining or purchasing decisions.