Turning inserts are one of the key elements of the CNC machining process. Their selection determines: process stability, tool durability, surface quality, and the ability to maintain the required tolerances. Manufacturers place information about the material application and recommended cutting parameters on the packaging of the inserts (usually packed in sets of several pieces). The correct interpretation of these markings is the foundation of proper technology.
In this article, we discuss inserts used on lathes: for rough turning, finishing, threading, grooving, and cutting. First, we would like to emphasize that it is always worth verifying the detailed catalog data with the tool manufacturers.
Table of contents
Material designations for inserts – groups P, M, K, N, S, H
Manufacturers of turning inserts classify materials according to a standardized system:
- P – steel,
- M – stainless steel,
- K – cast iron,
- N – non-ferrous metals (e.g. aluminum, brass, bronze,
- S – heat-resistant alloys and titanium,
- H – hard materials such as hardened steels.
Each insert grade has a specific carbide composition, coating type, and geometry adapted to a given group.
For example, inserts for stainless steel (M) are designed for ductile materials prone to build-up, while grades S and H must withstand high temperatures and intense abrasive wear.
One geometry, different materials – where are the limits?
In production practice, a single insert shape can be manufactured in different grades, intended for different materials. Compromise solutions are often used:
- Inserts (M), commonly referred to as “for stainless steel,” also work well with many steels (P) and cast irons (K).
- Inserts “typically for steel” (P) are not the right choice for S and H materials, but can be safely used for steel, aluminum, cast iron, brass, or bronze.
It is therefore crucial to make a distinction – the geometry (shape of the insert) may be the same, but the grade must be selected according to the material being machined and the machining conditions.
Inserts for rough turning
Rough turning requires inserts with high edge strength and the ability to work with higher feed rates. This group includes WNMG, CNMG, and DNMG, among others.
WNMG
Inserts used in MWLNL/R holders are most often available with corner radii of 0.4 / 0.8 / 1.2 mm.
In practice, a radius of 0.8 mm is most commonly used, less often 1.2 mm. A typical size is, for example, 0804. The tool is the basic solution for rough turning of small and medium-sized workpieces.
CNMG
DCLNL/R holders are designed for larger workpieces. They use inserts in sizes 12 and 16, with radii as for WNMG.
The design provides greater rigidity of the system at larger diameters and lengths.
DNMG
DNMG inserts (e.g., 1506) with radii of 0.4 and 0.8 mm are the classic solution for rough and semi-rough machining in a wide range of materials. They combine good strength with the ability to shape contours.
Inserts for finishing
Finishing requires inserts with smaller corner radii and stable, predictable performance in order to achieve the target dimensions and roughness.
DCMT
Inserts available in small and large versions, with radii of 0.2 / 0.4 / 0.8 mm (0.8 – in larger variants). Used for semi-finishing and finishing passes.
TNMG
Finishing inserts with radii of 0.2 / 0.4 / 0.8 mm. Used to “guide” the dimension within tolerance.
After each insert replacement, it is necessary to check the dimension (e.g., with a transameter) and, if necessary, correct the tool in the control system.
CCMT rhombic inserts
Available in small and large versions with radii of 0.2 / 0.4 / 0.8 mm.
Used for finishing cylindrical and end surfaces as well as contours, especially in areas with limited access.
VBMT
Inserts with a similar application to TNMG. Four-edged (0.4 / 0.8 mm) and single-sided (0.2 / 0.4 / 0.8 mm) versions are available. Selected where the geometry of the SVJ holder provides better access to the machined surface.
Thread cutting inserts
Thread cutting inserts are divided into solutions for internal threads (IR/IL) and external threads (ER/EL), in trapezoidal, metric, inch, and other profiles.
Inserts for grooves, channels, and cutting
Channel cutters
- Face – sizes 3, 4, 5 mm; for grooves on face surfaces.
- Internal – 1.04 / 1.0 / 1.2 mm; for channels in holes.
Cutters
Inserts with widths from 0.5 to 3.0 mm, mainly used on bar lathes for cutting off workpieces.
Practical application – a few rules
In technological practice, the following rules can be adopted:
Area of application Characteristics and selection rules
Rough turning Used as basic tools for high-volume machining. The most commonly selected radius is 0.8/1.2 mm. The selection of the insert grade depends on the material and the required performance.
Finishing turning Used to achieve the required dimensions and surface quality within the tolerance. After each insert replacement, it is necessary to check the dimensions and correct the settings if necessary.
Thread turning The insert is selected individually based on the thread profile and the requirements of the drawing. There are no universal solutions — each geometry requires the right tool.
Grooving and cutting The selection depends on the width and depth of the groove and the material of the workpiece.
The selection of a turning insert should be based on both the manufacturer’s catalog data and the experience of the technology and production team. The combination of these two sources of knowledge allows for a stable, repeatable CNC machining process and optimal use of tools.