Tools used on a CNC lathe

The 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 highly demanding industries such as aerospace, automotive and medical. The potential of CNC machines is only realized through the correct selection and application of their most critical component: the cutting tool.

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

• Dimensional accuracy.

• Surface finish quality.

• Production cycle times.

• Profitability and competitiveness of the operation.

Types of CNC lathe tools

To understand the wide range of tools available, they need to be classified by function, construction and cutting direction.

Classification by machining operation

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

• Turning tools (cylindrical turning): To reduce the outside diameter of the part.

• Facing tools: To machine the front face of the part.

• Boring tools: To enlarge or finish internal diameters.

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

• Parting tools: To cut the finished part off the raw stock.

• Threading tools: To produce internal or external threads.

• Chamfering tools: To create a beveled edge.

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

Classification by construction

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

• Solid type (one-piece): 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, the whole tool must be replaced or reground.

• Brazed type: It consists of a tip of hard material (e.g. carbide) brazed onto a cheaper steel shank. It was a popular solution before the rise of indexable tools.

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

Classification by feed direction

The “handedness” of the tool determines the direction in which it can cut effectively.

• Right-hand tool: This is the most common type, designed to machine while feeding 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 has no preferred cutting direction and can machine in both directions, making it useful for profiling and contouring operations.

Most common materials for CNC cutting tools

A tool's performance 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 greater toughness than other materials, which allows them to withstand impacts and interrupted cuts. They are used in low-speed operations, on 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 cutting speeds far higher than HSS.

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

• Superhard materials (CBN and PCD):

• Cubic boron nitride (CBN): It is the second hardest known material and the choice for “hard turning” (machining steels harder than 45 HRC), cast irons and superalloys.

• Polycrystalline diamond (PCD): It is the hardest tool material available and offers unmatched wear resistance. It is not suitable for machining steels because of its chemical reactivity with iron. It is used mainly on non-ferrous materials (aluminum, copper) and highly abrasive materials (composites, reinforced plastics).

• Coatings (PVD and CVD): These are very thin ceramic layers 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 roughing 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

Selecting the optimal tool should follow a logical, systematic process to ensure that all critical variables are considered.

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

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

3. Selecting the insert geometry:

• Nose angle: A large angle (e.g. 80°) provides a strong cutting edge, ideal for roughing, while a small angle (e.g. 35°) offers better access for complex profiles.

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

• Nose radius (RE): A large radius allows higher feeds and gives a stronger edge, but can cause vibration. A small radius reduces vibration and allows better finishes, but limits the feed.

4. Selecting the grade and chipbreaker: Based on the material and the operation, the carbide grade and coating are selected (CVD for roughing steels, PVD for stainless steels and finishing). A chipbreaker matching the feed and depth of cut range is chosen to ensure proper chip control.

5. Selecting the tool holder: Choose a tool holder that offers maximum rigidity (the shortest possible overhang) and ensures clear 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.

Diagnosing and managing tool wear

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

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

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

• Built-up edge (BUE): Workpiece material welded to the cutting edge, common at low speeds. The solution is to increase the cutting speed or use a smoother coating (PVD type).

• Notching/chipping: Small fractures in the cutting edge, often due to a lack of toughness or vibration. It is solved by selecting a tougher grade or reducing the feed.

Applying coolant

Cutting fluid or coolant has several functions: lowering the temperature, lubricating, protecting against corrosion and helping to evacuate chips.

• Flood cooling: This is the traditional method, applying a large volume of coolant at low pressure.

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

Preventive machine maintenance

A poorly maintained machine can cause vibration or inaccuracies that lead to premature tool wear. A regular maintenance program including cleaning, checking lubricants, inspecting alignment and calibrating the axes is crucial to ensure a stable, precise platform so that tools perform correctly.