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ISO Turning Insert Designation Code Reference (CNMG, DNMG, VNMG)

An insert name such as CNMG 120408 looks cryptic until you split it by position. This reference decodes shape, clearance angle, hole type and the size digits so you can order the right insert or match a part number in the tool crib.

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Position in code	Letter	Meaning	Shop note
1 Shape	C	Rhombic, 80 deg point angle	Roughing and general turning, two usable corners per side
1 Shape	D	Rhombic, 55 deg point angle	Profiling and facing with good access
1 Shape	E	Rhombic, 75 deg point angle	Less common, used in some threading and profiling holders
1 Shape	F	Rhombic, 50 deg point angle	Rare, special profiling
1 Shape	H	Hexagonal, 120 deg	Six cutting corners, mostly drilling and grooving inserts
1 Shape	K	Parallelogram, 55 deg	Facing and chamfering holders
1 Shape	L	Rectangular, 90 deg	Special holders
1 Shape	M	Rhombic, 86 deg	Uncommon
1 Shape	O	Octagonal, 135 deg	Eight corners, chamfering
1 Shape	P	Pentagonal, 108 deg	Five corners, rarely used on lathes
1 Shape	R	Round	Strongest edge, light depth of cut, heavy feed capability
1 Shape	S	Square, 90 deg	Strong corner, up to eight corners on a double sided insert
1 Shape	T	Triangular, 60 deg	Common general purpose, up to six corners double sided
1 Shape	V	Rhombic, 35 deg	Fine profiling and copy turning, weak tip
1 Shape	W	Trigon, 80 deg	Three corners per side, good for multi corner economy
2 Clearance angle	A	3 deg	Used on some positive inserts for strength
2 Clearance angle	B	5 deg	Positive style, rarely seen
2 Clearance angle	C	7 deg	Positive style, light cuts
2 Clearance angle	D	15 deg	Positive style
2 Clearance angle	E	20 deg	Strongly positive
2 Clearance angle	F	25 deg	Strongly positive
2 Clearance angle	G	30 deg	Very positive, special
2 Clearance angle	N	0 deg	Negative insert, double sided, tilted in the holder to create clearance
2 Clearance angle	P	11 deg	Positive style, common for boring bars
5 Hole and breaker	N	No hole, flat, no chipbreaker	Clamped from above
5 Hole and breaker	R	No hole, chipbreaker on one side	Top clamp holders
5 Hole and breaker	F	No hole, chipbreaker on both sides	Top clamp holders
5 Hole and breaker	A	Cylindrical hole, no chipbreaker	Lever or pin clamp
5 Hole and breaker	M	Cylindrical hole, chipbreaker on one side	Most common CNMG, DNMG, VNMG, WNMG style
5 Hole and breaker	G	Cylindrical hole, chipbreaker on both sides	Double sided positive or negative inserts
5 Hole and breaker	W	Countersunk hole (40 to 60 deg), no chipbreaker	Screw clamp
5 Hole and breaker	T	Countersunk hole (40 to 60 deg), chipbreaker on one side	Screw clamp, common on small boring inserts
5 Hole and breaker	Q	Countersunk hole (40 to 60 deg) on both sides, no chipbreaker	Screw clamp
5 Hole and breaker	U	Countersunk hole (40 to 60 deg) on both sides, chipbreaker on both sides	Screw clamp
6 to 8 Size	12	Third group is insert size, for example 12 = 12.7 mm (1/2 in) inscribed circle on most shapes	In CNMG 120408 the 12 is the size, 04 thickness (4.76 mm), 08 nose radius (0.8 mm)
6 to 8 Size	04	Thickness code 04 = 4.76 mm (3/16 in); 03 = 3.18 mm; 06 = 6.35 mm	Pick the holder pocket to match
6 to 8 Size	08	Nose radius code in tenths of a mm: 04 = 0.4, 08 = 0.8, 12 = 1.2, 16 = 1.6	Match to depth of cut and finish requirement

The full ISO 1832 code has more positions (tolerance class, cutting edge and hand letters). Position numbers here follow the usual reading order of a CNMG 120408 style name. Always confirm against the tool maker catalogue before ordering.

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How to use this reference

Type a letter or a word such as rhombic or countersunk into the search box and the table filters live. Read an insert name left to right: the first letter is the shape, the second is the clearance angle, the third is the tolerance class, the fourth is the hole and chipbreaker style, then the size, thickness and nose radius digits. Export to Excel when you want a laminated crib card or to load descriptions into an item master.

Shape letter first: C, D, V, S, T, W, R are the ones you will meet most.
N means 0 deg clearance, so the insert must tilt in the holder.
Radius digits are tenths of a millimetre.

The standard behind the code

Indexable insert designations follow ISO 1832, with a separate American system (ANSI B212.4) that uses similar letters and inch based size digits. Metric inserts count the size as the inscribed circle in millimetres, while inch inserts count it in eighths of an inch. That is why a CNMG 120408 and a CNMG 432 are close to the same insert in two systems. Tolerance class letters change the allowed variation of the inscribed circle and the corner position, which matters for finish turning and for boring bars with tight shim control.

Worked example

Take CNMG 120408. C means an 80 degree rhombic insert, N means 0 degree clearance (a negative insert), M is the tolerance class, G means a hole with chipbreaker on both sides, 12 means a 12.7 mm inscribed circle, 04 means 4.76 mm thick and 08 means 0.8 mm nose radius. For heavy rough turning on steel this is a strong, economical choice. For a finishing pass on a small boring bar you would more likely look at a positive style such as a CCMT or VCMT.

Common mistakes

The usual errors are mixing inch and metric size digits, ordering a positive insert for a negative holder, and forgetting that nose radius digits are tenths of a millimetre, not hundredths of an inch. In an ERP item master, store the full ISO code plus the holder style it fits, so purchasing does not substitute an insert with the wrong clearance. Store the grade and chipbreaker name as separate attributes, since they are not part of the ISO code.

Frequently Asked Questions

What does CNMG 120408 mean?

C is an 80 degree rhombic shape, N is 0 degree clearance, M is the tolerance class, G is a cylindrical hole with chipbreaker on both sides, 12 is a 12.7 mm inscribed circle, 04 is 4.76 mm thick and 08 is a 0.8 mm nose radius.

What is the difference between CNMG and DNMG?

Only the shape letter changes. CNMG has an 80 degree point angle, giving a strong corner for roughing. DNMG has a 55 degree point angle, giving better access for profiling and facing, but a weaker corner.

Is a negative insert always better?

No. Negative inserts have more cutting corners and a strong edge, but they need more power and can rub on thin walls. Positive inserts cut more freely and suit finishing, small boring bars and long overhangs.

Do the letters include grade and coating?

No. Carbide grade, coating and chipbreaker name are added by the manufacturer after the ISO code. Two CNMG 120408 inserts can behave very differently depending on grade and breaker geometry.

If your tool crib items live in Epicor, SyteLine, SAP or Infor LN, Netray can help you automate duplicate item detection and insert descriptions from the ISO code.

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