Types of turning inserts: complete selection guide

An indexable insert is a replaceable cutting tip made of sintered cemented carbide, cermet, ceramic or cubic boron nitride that is mechanically clamped to the lathe toolholder; when a cutting edge wears out, the insert is rotated (indexed) to a fresh edge or replaced, with no regrinding. Its identification is standardized: the ISO 1832 designation encodes, in a minimum of seven positions, the shape, clearance angle, tolerance, clamping and chipbreaker system, size, thickness and nose radius. Thus CNMG 120408 reads: shape C (80° rhombic), clearance N (0°), tolerance class M, fixing G (cylindrical hole with chipbreakers on both faces), size 12 (12.9 mm cutting edge on a 12.7 mm inscribed circle), thickness 04 (4.76 mm) and nose radius 08 (0.8 mm).

Selection verdict: for external roughing under stable conditions use negative 80° inserts (CNMG or WNMG); for finishing and profiling use 55° (DNMG) or 35° (VNMG); for internal turning and slender parts use positive inserts (CCMT, DCMT, VCMT, VBMT); and pick the carbide grade by the ISO 513 letter of the workpiece material: P for steel, M for stainless, K for cast iron, N for non-ferrous, S for superalloys and H for hardened materials.

ISO 1832 designation: how to read an insert position by position

ISO 1832 (functionally equivalent to the ANSI B212.4 system used in American catalogs) defines a code of seven mandatory positions, to which each manufacturer appends its own chipbreaker geometry and grade symbols. The table breaks down the CNMG 120408 example:

In the second position, the most frequent letters are N = 0° (negative), B = 5°, C = 7° and P = 11° (positive inserts). In the third, class M is the standard as-sintered tolerance, while class G identifies ground inserts with ±0.025 mm tolerance on the inscribed circle, required when the nose position must repeat between indexings without recalibrating the machine. In the fourth position, T (as in CCMT) means a partly countersunk screw hole with a chipbreaker on one face only. For thicknesses, code T3 equals 3.97 mm and 06 equals 6.35 mm; for nose radii, the number is the radius in tenths of a millimetre: 04 = 0.4 mm, 12 = 1.2 mm, 16 = 1.6 mm.

Insert types by shape: point angle and application

The point angle governs the central trade-off of insert selection: a larger angle backs the edge with more material, allowing higher feed and depth of cut, but generates more radial force and vibration tendency; a smaller angle reaches profiles and corners and reduces vibration, at the cost of a weaker tip that dissipates heat less effectively.

Point strength ranks from highest to lowest: R, S (90°), C and W (80°), T (60°), D (55°), V (35°). The trigon W offers three 80° points where the C rhombic offers two, lowering the cost per edge in roughing; in exchange it limits the toolholder lead-angle combinations.

Negative versus positive inserts

A negative insert (second letter N, 0° clearance) has both faces usable: a CNMG provides 4 edges, a TNMG or WNMG 6, and an SNMG 8. Since the insert itself has no clearance angle, the toolholder mounts it tilted; this raises cutting forces but leaves the edge backed by more carbide, making it suitable for roughing, interrupted cuts and high feeds on rigid lathes with well-clamped workpieces.

A positive insert (CCMT, TCMT, DCMT, VCMT, VBMT, with built-in clearance of 5°, 7° or 11°) cuts on one face only, so it yields half the edges; its advantage is a sharper cutting edge that reduces cutting forces and workpiece deflection. It is the correct choice for internal turning (the boring bar deflects less), for slender or thin-walled parts that a negative insert would push out of tolerance, and for low-power machines or small spindles. Practical rule: stable external work, negative; internal work or flexible part, positive.

Carbide grades and coatings (ISO 513)

ISO 513 classifies cutting materials according to the workpiece material they will machine, into six main groups identified by letter and color. Within each group, a number marks the position between wear resistance (low numbers, around 01 to 10: finishing at higher speed) and toughness (high numbers, around 30 to 50: roughing and interrupted cutting). The same ISO 1832 insert is produced in several grades: the geometric designation does not change; the substrate and coating do.

Beyond coated carbide, manufacturer catalogs assign other cutting materials to these same groups: cermet for finishing in steel, ceramics for cast iron and superalloys at high speed, cubic boron nitride (CBN) for group H and polycrystalline diamond (PCD) for group N.

CVD and PVD coatings

CVD coating (chemical vapour deposition, at temperatures near 1,000 °C) builds thick layers —typically between 5 and 20 µm according to manufacturers' technical guides— of titanium carbonitride, aluminium oxide and titanium nitride; it delivers the highest resistance to crater and flank wear and dominates steel and cast-iron turning at high speeds. PVD coating (physical vapour deposition, at about 400 to 600 °C) builds thin layers of 2 to 6 µm, for example TiAlN or AlCrN, that keep a sharp, tough edge: it is the option for stainless steels, superalloys, interrupted cuts, threading and parting, and for sharp-edged positive inserts.

How to choose the insert: material, operation and stability

1. Classify the workpiece material with its ISO 513 letter (P, M, K, N, S or H) and pre-select the grade the manufacturer recommends for that group.
2. Define the operation: roughing, medium or finishing. Roughing calls for a robust chipbreaker and nose radii of 0.8 to 1.6 mm; finishing calls for light-cutting geometries and radii of 0.2 to 0.4 mm.
3. Assess the stability of the machine-workpiece-clamping system: with good rigidity and external turning, negative insert; with long overhangs, internal work or thin walls, positive insert.
4. Choose the shape by the profile to be generated: 80° if only turning and facing; 55° or 35° for contour copying; round for radii and superalloys.
5. Size the insert, thickness and radius: the maximum permissible depth of cut is a fraction of the edge length that depends on the lead angle (check the toolholder table in the catalog), and the finishing feed should not exceed roughly half the nose radius; theoretical roughness grows with the square of the feed divided by eight times the nose radius (the classic kinematic turning formula).

Common errors when selecting inserts

• Roughing with 35° (V) shapes: the weakest point in the system chips under roughing feeds or depths; the 35° shape exists for profiling and finishing.
• Wrongly sized nose radius: a 0.4 mm radius in roughing overloads and breaks; a 1.6 mm radius on a slender part generates radial force and vibration.
• Using a P grade on stainless steel or superalloys: it produces built-up edge and notch wear; stainless calls for M grades and group S calls for tough PVD grades with a sharp edge.
• Machining aluminium with steel geometry: aluminium demands an N grade with a sharp, polished edge (or PCD); a steel chipbreaker smears and welds material.
• Mixing incompatible insert and toolholder: the second letter of the designation must match the toolholder seat; a CNMG does not mount in a CCMT toolholder.
• Finishing feed greater than the nose radius: theoretical roughness spikes and the tip works unsupported, with breakage risk.

Machining materials and quotes at TECTUL

TECTUL supplies the bars these inserts machine every day in Colombian workshops: 1045 steel bar (ISO group P, shafts and machine parts), 4140 steel bar (group P, quenched-and-tempered high-duty components) and SAE 660 bronze bar (group N, bushings and bearings). If your workshop also needs inserts, toolholders or other machining tools, TECTUL quotes them on request: write to us on WhatsApp with the full ISO designation (for example CNMG 120408 plus the material group to be machined) and we will reply with availability and lead time.