Negative vs Positive Turning Inserts: Clearance Angle, Edge Strength and When Each Geometry Wins

Years ago I inherited a finishing job on a long, slender shaft — 42 mm bar, 300 mm out of the chuck, no steady rest. The previous setter had run it with CNMG120408, the same negative insert we used for everything steel, and the whole cell had accepted the chatter marks as weather. I swapped the holder for a screw-down one and put in a CCMT09T304 — same nose radius, same grade family, smaller insert. The chatter died on the first pass.

Nothing about the grade had changed. What changed was the second letter of the designation: N to C, 0° of clearance to 7°. That one letter moves more physics than most buyers realise, and it is the letter I see mis-specified most often on enquiry sheets. This guide is the decision order I now teach, demonstrated against the 341 models and 19,181 cited brand ordering codes in the InsertBase catalogue — every count below is something you can re-run against the linked pages.

What the second letter actually says

Position 2 of ISO 1832 is the clearance (relief) angle — the angle between the insert's flank and the workpiece as the insert sits flat in its pocket. The letters that matter in a turning catalogue:

An N insert has flanks at 90° to the top face — the thickest possible wedge of carbide behind the cutting edge. It becomes a "negative rake" cutting geometry only when the holder tilts it; the insert itself is simply square, and that squareness is what makes it reversible. A C or P insert has relief moulded or ground under the edge, so the wedge behind the edge is thinner and the insert can only cut on one face.

Two things follow immediately from that geometry, before any brand preference enters the room:

• Edge strength. The 90° wedge of a negative insert is the strongest section a carbide edge can have. Positive inserts trade that section away for a freer, lower-force cut.
• Edges per insert. A negative insert with chipbreakers on both faces is double-sided: a CNMG gives you four usable 80° corners, a WNMG six, where a single-sided positive CCMT gives you two. Per-insert edge economics favour negative by a factor of two — when the setup can carry it.

What the catalogue says the industry actually buys

Count the cited ordering codes rather than the models and the market's answer is lopsided: 13,142 of the 19,181 catalogue rows — 68% — sit on 0°-clearance negative models. The split by brand shows this is not one maker's bias:

Every major catalogue carries both families, and every one of them tilts negative — Sandvik by more than three to one. The most-catalogued designation in the entire library is CNMG120408 with 680 cited ordering codes; the most-catalogued positive, CCMT09T308, carries 317. Negative is the default geometry of production turning. Positive is the specialist you bring in when the default physically cannot do the job.

Where negative wins: anything that hits the edge hard

The thick wedge and the negative-holder seat make one thing possible that positives never match: surviving impact. Interrupted cuts, forged or cast skin, scale, out-of-round bar, high-feed roughing — anywhere the edge gets hammered rather than kissed. The 153 M-class moulded negative models in the library are the mass-market expression of that: economical, tough, double-sided, happy in a rigid machine at production feeds.

There is a subtler negative branch that buyers miss. Of the 193 negative models catalogued, 57 have no chipbreaker at all — the flat-top families: 48 with a plain cylindrical hole (the A-fixing CNMA, SNMA, DNMA line) and 9 clamped no-hole designs. With no breaker pressed into the face there is no stress-raising groove, and the edge presents a clean, strong land to the cut. That is the classic grey-iron geometry: cast iron makes short, crushing chips that do not need forming, and the work is pure abrasion. When I see a buyer spec a chipbreaker insert for continuous cast-iron work, I know they are paying for a groove the chips will never use.

And at the far end sit the single-sided negatives — 13 models with an M in position 4, like CNMM160612. One face, one breaker, thick section: heavy roughing where you would rather have one reinforced edge than two compromised ones.

Where positive wins: anywhere force is the enemy

Back to that slender shaft. A negative geometry drives the tool into the work and the work away from the tool; on a rigid lathe turning 80 mm bar you never notice. On a thin-walled part, a long overhang, a small collet lathe or a boring bar, those deflection forces are the whole story. The relieved flank of a positive insert cuts with markedly less radial and tangential pressure, and that is why the positive shelf exists.

Inside the positive family the catalogue draws a clean map. Of the 145 mainstream positive models (5°, 7° and 11° clearance), 73 carry G-class ground tolerance — against only 40 ground models among the 193 negatives. Positives are where the precision geometry lives. At the top of that line sit the E-class models, the tightest tolerance ISO 1832 names: all nine E-class inserts in the library are positive — the CCET, DCET and VBET finishing families, ground all round for jobs where the insert, not the machine, must hold the dimension.

Material pushes the same direction. Sticky, work-hardening alloys — the ISO 513 M and S groups — reward the sharper, freer-cutting positive edge, which is also why the PVD-coated sharp grades from the coating guide pair so naturally with positive geometry. Aluminium and other non-ferrous work goes further: polished, ground CCGT/DCGT edges exist almost purely for that group.

Then there is the 11° branch nobody should forget: 50 P-clearance models, 793 cited codes, and the largest share of them are the no-hole families — TPGN alone spans 12 models with 338 catalogue rows. Clamp-on, no fixing hole, 11° of relief: the geometry that ran the world's manual lathes and still runs in every simple toolholder that cannot take a screw-down insert. If you maintain older machines, this is the shelf your crib lives on.

The decision order I use

Five questions, in this order. The geometry answers itself by question four.

1. What does the holder allow? A negative pocket holder takes N-clearance inserts only; a screw-down positive holder takes its one inscribed-circle size. Check the holder before the catalogue — mixing families is not an option, the insert physically will not seat. If the machine only has one family of holders, your geometry decision is already made and you spend your energy on chipbreaker and grade instead.

2. Can the setup carry the force? Rigid machine, short overhang, solid workpiece: the negative default earns its double edges. Slender part, thin wall, boring bar, small lathe: go positive and do not apologise for the edge count — a deflected negative insert gives you zero usable edges, not eight.

3. What hits the edge? Interruptions, scale, cast skin, eccentric stock: negative, full stop, and look at the flat-top no-breaker family if the material is cast iron. Clean continuous bar: either family works, decide on the other questions.

4. What tolerance does the insert have to hold? Production finishing to tight size with minimal offset chasing: the ground G-class and E-class positives are built for exactly this. General roughing and semi-finishing: M-class negative is the economical default.

5. Only now: chipbreaker and grade. Geometry chooses the family; the chipbreaker chooses the feed band; the grade chooses the material. In that order. A perfect grade on the wrong clearance letter is still the wrong insert.

The short version

Negative is armour: 90° wedge, double-sided edges, 68% of every cited catalogue row, made for rigid machines and hard knocks. Positive is a scalpel: relieved flank, low force, ground precision, made for slender parts, small machines and sticky materials. Neither is "better" — they answer different constraints, and the second letter of the code tells you which constraint it answers.

If you take one action from this guide, make it this: the next time a job chatters or a finish drifts, read the second letter on the insert before you touch the speed and feed. And if you are speccing inserts for a new job and want the geometry decision checked against real catalogue rows, send the enquiry over — I answer these with the catalogue page attached.

Have a job where swapping clearance letters fixed what a grade change could not? The interesting failures are the ones I can verify against the catalogues — send them in and they may end up documented here, cited like everything else.