Computer Numerical Control (CNC) turning is a cornerstone of modern manufacturing. This process, in which a cutting tool removes material from a rotating workpiece to create cylindrical shapes, is crucial in demanding industries such as aerospace, automotive, and medical. The potential of CNC machines is only realized with the correct selection and application of their most critical component: the cutting tool .

The cutting tool is the link between the machine and the workpiece, the point where digital instructions become physical reality. An informed choice of tool material, geometry, and clamping system directly influences all performance metrics.

• Dimensional accuracy.

• Surface finish quality.

• Duration of production cycles.

• Profitability and competitiveness of the operation.

Types of CNC lathe tools

To understand the wide range of tools available, it is necessary to classify them according to their function, structure, and cutting direction.

Classification by machining operation

This classification groups tools according to the specific task for which they were designed. The main operations include:

• Turning tools (cylindering) : To reduce the outside diameter of the workpiece.

• Facing tools : For machining the front face of the workpiece.

• Boring tools : For enlarging or finishing internal diameters.

• Grooving tools : To create channels or grooves of a specific width.

• Cutting tools : For cutting the finished piece from the raw material.

• Threading tools : For creating internal or external threads.

• Chamfering tools : To create a sloping edge.

• Forming tools : With a specific profile to create complex geometries in a single pass.

Classification by structural design

The physical architecture of the tool defines its rigidity, cost, and modularity.

• One-piece (monoblock) type : The entire tool is made from a single piece of material (e.g., HSS or solid carbide). It offers excellent rigidity, but once the cutting edge wears down, the entire tool must be replaced or resharpened.

• Welding type (brazed) : This consists of a hard material tip (e.g., carbide) brazed to a less expensive steel shank. It was a popular solution before the rise of interchangeable tools.

• Tool holder type (index or indexable) : This is the dominant system in modern CNC machining. It consists of a reusable tool holder that secures a replaceable cutting insert. When the cutting edge wears down, the insert is quickly and economically rotated or replaced.

Classification by direction of advance

The "manual" nature of the tool determines the direction in which it can cut effectively.

• Right-Hand Tool : This is the most common type, designed to machine by advancing from the tailstock (right) towards the headstock (left).

• Left-Hand tool : Designed for the reverse operation, cutting from the headstock towards the tailstock.

• Neutral tool : It does not have a preferred cutting direction and can machine in both directions, making it useful for profiling and contouring operations.

Materials most commonly used in CNC cutting tools

The performance of a tool is intrinsically linked to the properties of its cutting edge material. The ideal material should have an optimal combination of hardness (wear resistance) and toughness (fracture resistance).

• High-speed steels (HSS) : These are complex steel alloys with superior toughness compared to other materials, allowing them to withstand impacts and interrupted cutting. They are used in low-speed operations, in less rigid machines, and for soft materials such as aluminum or low-carbon steels.

• Hard metals (cemented carbides) : These are the most versatile and widely used in CNC turning. They consist of hard tungsten carbide (WC) particles cemented in a cobalt (Co) matrix. They offer excellent hot hardness and wear resistance, allowing for cutting speeds much higher than those of HSS.

• Cermets and ceramics : Cermets offer greater wear resistance than carbides and produce exceptional surface finishes, making them preferred for high-speed finishing operations. Ceramics, even harder and more heat-resistant, allow for extremely high cutting speeds but are very brittle. They are used in machining gray cast iron and hardened steels.

• Superhard materials (CBN and PCD):

• Cubic boron nitride (CBN) : It is the second hardest known material and is the choice for "hard turning" (machining of steels with a hardness greater than 45 HRC), castings and superalloys.

• Polycrystalline diamond (PCD) : It is the hardest tool material available. It offers unparalleled wear resistance. It is not suitable for machining steels due to its chemical reactivity with iron. Its application is focused on non-ferrous materials (aluminum, copper) and highly abrasive materials (composites, reinforced plastics).

• Coatings (PVD and CVD) : These are very thin ceramic layers that are deposited on a substrate (usually carbide) to improve its surface properties without compromising the toughness of the core.

• CVD (Chemical Vapor Deposition) : Produces thick coatings ideal for high-speed grinding of steels and cast irons.

• PVD (Physical Vapor Deposition) : Produces thinner, smoother coatings, preferred for finishing operations and for machining materials that tend to stick to the cutting edge, such as stainless steels and superalloys.

How to choose the right tool for each operation

The selection of the optimal tool must follow a logical and systematic process to ensure that all critical variables are considered.

1. Part and material analysis : The starting point is always the part. The material (steels, stainless steels, cast irons, etc.) must be identified to select the appropriate grade (material and coating) of the insert. The geometry, tolerances, and surface finish requirements must also be analyzed.

2. Definition of the operation : The type of machining (cylindering, grooving, etc.) must be determined and a clear distinction must be made between roughing (prioritizes material removal) and finishing (prioritizes precision and surface quality).

3. Plate geometry selection:

• Tip angle : A large angle (e.g., 80°) provides a tough edge, ideal for roughing, while a small angle (e.g., 35°) offers greater accessibility for complex profiles.

• Insert size : It must be proportional to the depth of cut to ensure stability.

• Tip radius (RI) : A large radius allows for greater feed rates and produces a stronger cutting edge, but can cause vibration. A small radius reduces vibration and allows for better finishes, but limits feed rates.

4. Grade and chip breaker selection : Based on the material and operation, the carbide grade and coating are selected (CVD for roughing steels, PVD for stainless steels and finishing). A chip breaker is chosen that corresponds to the feed rate and depth of cut to ensure adequate chip control.

5. Tool holder selection : A tool holder should be chosen that offers maximum rigidity (the shortest possible overhang) and that guarantees free access to the machining area without risk of collision.

Tips for using CNC lathe tools correctly

Proper use and maintenance are crucial to optimize tool life and process efficiency.

Tool wear diagnosis and management

Wear is an inevitable process that provides valuable information about machining. Monitoring wear is key to preventing catastrophic failures. Common types of wear and their solutions are:

• Flank wear : Caused mainly by excessively high cutting speed. The solution is to reduce the speed or select a more wear-resistant grade.

• Cratering : Formation of a crater on the tool face, caused by high temperatures. The solution is to use a thicker coating (such as CVD) or reduce the speed.

• Filler edge (BUE) : Material from the workpiece welded to the cutting edge, common at low speeds. The solution is to increase the cutting speed or use a smoother coating (such as PVD).

• Nipping/chipping : Small fractures in the cutting edge, often due to lack of toughness or vibrations. This can be remedied by selecting a tougher grade or reducing the feed rate.

Refrigerant application

The cutting fluid or coolant has multiple functions: reducing temperature, lubricating, protecting against corrosion, and helping to evacuate chips.

• Flood cooling : This is the traditional method, which applies a large volume of refrigerant at low pressure.

• High-pressure coolant (HPC) : Directs jets of high-pressure coolant directly onto the cutting edge. It is extremely effective at breaking up and evacuating chips, allowing for significant increases in cutting parameters.

Preventive maintenance of the machine

A poorly maintained machine can induce vibrations or inaccuracies that cause premature tool wear. A regular maintenance program that includes cleaning, lubricant checks, alignment inspection, and spindle calibration is crucial to ensuring a stable and accurate platform for the tools to function correctly.