Negative Turning Inserts

The series is built from double-sided, CVD-coated carbide inserts engineered for ISO P steel, from light finishing through severe interrupted heavy cutting. It pairs eight application-matched chipbreaker geometries - PF, PL, PM, PE, PR, PX, PY and PH - with three newly developed grades, GPT6220, GPT6225 and GPT6230, across the full ISO shape portfolio: CNMG, DNMG, SNMG, TNMG, WNMG, VNMG, KNUX and RNMG.

Because every insert is negative (0° clearance), both faces cut: a CNMG gives you four usable edges, a WNMG six, an SNMG eight - twice as many as a single-sided positive insert of the same corner size. On stable CNC lathes working carbon steel, alloy steel and cast steel, that is where the cost advantage comes from.

The chipbreaker decides whether a chip curls and breaks or wraps around the chuck as a bird's nest. Each geometry is matched to a narrow band of depth of cut and feed.

Read the three groups：

 PF and PL finish. PF is the first choice, with a three-stage platform that reaches depths and feeds conventional geometries cannot. PL adds a wave-shaped edge for profiling where the depth of cut fluctuates.

 PM and PE cover semi-finishing. PM is the general-purpose option - positive rake to bring resistance down, a double-antenna land to break the chip. PE trades chip-breaking for edge strength when the setup, not the chip, is the problem.

 PR, PX, PY and PH remove the bulk of the material. PR roughs with a flat, large edge-width land that survives the first scale-removing passes. PX reduces resistance through an inclination angle where machine power limits your depth of cut. PY holds edge strength on continuous heavy passes. PH, the first choice for heavy cutting, balances resistance against chip control with a wavy-line protrusion.

All three grades carry a newly developed multilayer CVD coating with exceptional adhesion, on substrates produced under the GEFORTECH process: the coating supplies wear resistance, the substrate supplies toughness.

GPT6220 is the wear-resistance grade - the highest cutting-speed capability of the three, and the longest tool life of the three on clean continuous cuts. GPT6225 is the balanced workhorse that covers most shop-floor work. GPT6230 is the toughness grade, for interrupted cuts, unbalanced parts and forged skins. Moving down the list trades speed for survival.

All three are also available across the Negative Q, Negative G and Positive U series, so one grade can be standardised across a mixed tool inventory.

Every P series insert follows ISO 1832, so the code on the box tells you exactly what you are holding. In CNMG120408: C 80° rhombic · N negative, 0° clearance · M tolerance class · G double-sided, centre hole, ground periphery · 12 12 mm inscribed circle · 04 4.76 mm thickness · 08 0.8 mm corner radius.

The last two digits of the ANSI code give the nose radius in 64ths of an inch: CNMG432 is a 2/64 inch radius, or 0.8 mm. Order by whichever code your shop uses.

Starting values for CVD-coated negative inserts in ISO P steel. For interrupted cuts, reduce Vc by 15–25% and move one step down in geometry. Confirm the final window against SW technical team.

In a documented production run, a WNMG080408-PM insert in grade GPT6220 roughed a 40Cr (AISI 5140 equivalent) half shaft at HB 200–340 - 75–219 m/min, 0.29 mm/rev, 1–2 mm depth of cut, dry. The PM geometry held chip control across the full depth-of-cut band while the GPT6220 CVD coating carried the surface speed on hardened alloy steel: a stable, predictable roughing operation on a part that is difficult to hold.

Choose the Negative P Series when the part is held rigidly, the machine has the power and mass, the cut is heavy or interrupted, or tool cost per edge drives the decision - the standard configuration for shafts, gears, hubs and flanges. Choose a positive geometry when the part is thin-walled, the boring bar overhangs far, the lathe is small or worn, or you are finishing and need the lowest cutting force.

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