With the continuous advancement of machining technology, CNC machining has undergone numerous changes. Many experts point out that CNC will become the mainstream machining method in the future. In the CNC machining process, tools are very important. Today, let’s walk through a comprehensive understanding of CNC tools.
What is CNC machine tool?
A tool is a device used for cutting processes in mechanical manufacturing. In a broad sense, cutting tools encompass both knives and grinding tools. The majority of tools are intended for machine use, although there are also hand tools. Since the tools used in mechanical manufacturing are primarily employed for cutting metal materials, the term “tool” is generally understood as metal cutting tools. Tools utilized for cutting wood are referred to as woodworking tools
The structure of various tools consists of the clamping part and the working part. In integral structure tools, both the clamping part and the working part are made on the tool body; for inserted-tooth structure tools, the working part (teeth or blades) is inserted into the tool body.
There are two types of clamping parts for tools: with holes and with shanks. Tools with holes are mounted on the main or sub spindles of the machine tool through internal holes, transmitting torsional torque via axial or end keys, such as cylindrical milling cutters, modular face milling cutters, and so on.
Tools with shanks usually come in three types: rectangular shank, cylindrical shank, and taper shank. Turning tools, planers, and the like generally have a rectangular shank; taper shank relies on the taper to withstand axial thrust and transmits torque through frictional force; cylindrical shank is generally suitable for smaller tools like twist drills and end mills, and torque is transmitted through the frictional force generated during clamping. The shank of many shank-type tools is made of low alloy steel, while the working part is made of high-speed steel and welded to the shank.
Types of CNC Machining Tools
CNC machining tools must adapt to the characteristics of CNC machine tools, which are characterized by high speed, high efficiency, and a high degree of automation. They generally include general-purpose tools, universal connecting tool holders, and a small number of specialized tool holders. Tool holders connect to the tools and are mounted on the machine tool’s spindle, so they have gradually become standardized and serialized. There are various methods for classifying CNC tools.
Based on tool structure, they can be divided into:
- Integral type;
- Inserted type, using welding or mechanical clamping connection, with mechanical clamping further divided into non-indexable and indexable types;
- Special types, such as composite tools, damping tools, and so on.
From a cutting process perspective, they can be divided into:
- CNC Turning tools, including outer circle, inner hole, thread, and cutting tools, among others;
- CNC Drilling tools, including drills, reamers, taps, etc.;
- CNC Milling tools.
CNC Turning tools
External Turning Tool
The external turning tool, also known as a turning insert, is used for CNC turning external cylindrical surfaces, end faces, hole bottoms, and more. Common types include straight edge external turning tools, V-edge external turning tools, and chamfered edge external turning tools. Straight edge external turning tools are suitable for workpieces with lower hardness, V-edge external turning tools are suitable for processing harder materials, and chamfered edge external turning tools are suitable for machining situations requiring high efficiency and good surface quality. When using external turning tools, it’s important to select the appropriate size and tool holder, and choose different types of inserts based on machining requirements.
Internal Turning Tool
The internal turning tool is used for turning the inner surface of holes and is divided into a blade-type and a lathe-type for cutting. Generally, a T-type clamping mechanism is used to secure the turning tool for machining deeper holes. Alternatively, a small-diameter shank clamping method is employed for shallower hole machining. The inserts for internal turning tools are typically categorized as ISO standard and non-ISO standard. Following the ISO standard for products can enhance their universality and interchangeability
The grooving tool is used for turning axial grooves on parts like slots and gears. Common materials for grooving tools include high-speed steel, cemented carbide, and ceramics. Depending on the groove shape and application, grooving tools are further categorized into various types, such as saw-tooth grooving tools, cylindrical-tooth grooving tools, orthogonal grooving tools, and arc-shaped grooving tools. When using grooving tools, it’s important to select the correct size and choose an appropriate tooth profile based on the grooving requirements.
Thread Turning Tool
The thread turning tool is used for turning internal and external threads and is divided into insert-type and groove-type. Cemented carbide is commonly used as the material for thread turning tools. The cutting angles, number of teeth, and tooth profiles of thread turning tools all affect the quality and efficiency of thread machining. When selecting thread turning tools, it is important to choose the appropriate type based on thread specifications and machining requirements.
The parting tool is used to cut through workpieces during turning operations and is commonly made from either cemented carbide or high-speed steel. Parting tools are typically mounted on the cutoff tool holder of machining centers to separate finished workpieces. They can also be directly installed on lathes to perform both turning and cutting operations. When purchasing, it’s important to choose the appropriate parting tool based on machining requirements and workpiece material.
CNC Drilling tool
Drilling tools used for drilling holes in solid materials or enlarging existing holes are collectively referred to as hole machining tools. Hole machining tools are widely used in machining operations.
Drill Bit: Primarily used for drilling holes in solid materials. Depending on the construction and purpose of the drill bit, it can be further categorized into twist drills, flat drills, center drills, and deep hole drills.
A twist drill is a tool that cuts round holes in a workpiece by rotating around its fixed axis. Its name comes from the helical shape of the chip flutes, resembling a spiral pattern similar to a “twisted” appearance.
The flat drill is the earliest type of drill used for hole machining. Due to its small point angle, difficulties in chip removal, poor guidance, and limited regrinding times, it has gradually been replaced by other types of drills in the past. However, the flat drill has a simple structure, easy manufacturing, low cost, small axial dimensions, and good rigidity. In recent years, it has regained attention.
The center drill is a type of drill used for machining round holes, suitable for processing harder materials such as metal, ceramics, glass, etc.
Deep hole drill
Deep hole drill is a type of drill specifically designed for machining deep holes, and it can be categorized into external chip removal and internal chip removal types. It is exclusively used for machining deep holes. In mechanical machining, holes with a ratio of depth to diameter greater than 6 are generally referred to as deep holes.
Tools for reworking existing holes, such as reaming drills, reamers, and boring tools.
A reaming drill is a tool used to enlarge the hole diameter and improve machining quality.
A reamer is a rotating cutting tool with one or more teeth used to remove a thin layer of metal from the surface of a previously machined hole. Reamers come with straight or spiral edges and are precision tools used for hole enlargement or hole finishing.
A boring tool is a type of hole machining tool, generally with a round shank, although larger workpieces may use a square shank, commonly seen in vertical lathes. It is most commonly used for internal hole machining, hole enlargement, contour replication, and similar applications.
A thread tap is a tool used for machining internal threads. It can be categorized based on its design into spiral flute taps, helical angle taps, straight flute taps, and pipe thread taps, among others. Depending on the application context, thread taps can be classified as hand taps or machine taps. Furthermore, they come in various specifications, including metric, Unified Thread Standard (UTS), and British Standard (Whitworth) thread taps. Thread taps are the most commonly used machining tools by operators in the manufacturing industry for thread tapping operations.
CNC Milling Tools
What are milling tools?
CNC Milling tools are a vital component of a milling machine for CNC milling process. Milling tools are rotating tools with one or more teeth used for milling operations. In the machining process, each tooth intermittently cuts away the remaining portion of the workpiece. Milling cutters are primarily used for milling flat surfaces, steps, grooves, contouring surfaces, and cutting workpieces.
Types of Milling Tools
Common CNC tools are classified into three types based on their shapes: end mills, ball nose mills, and bull nose mills. Each tool has a specific purpose.
Face Milling Cutter (Flat Mill)
In CNC machining centers, a face milling cutter, also known as a flat mill, is surrounded by primary cutting edges with secondary cutting edges at the bottom. Both the outer edge and the bottom of the face milling cutter have milling teeth to form cutting edges, allowing the vertical surfaces of the workpiece to be milled on a milling machine. The shape of the face milling cutter varies greatly, making it suitable for various machining tasks such as milling, grooving, and contouring surfaces. It can be considered the most widely used type of milling cutter.
When milling 2D shapes on a workpiece, very effective cutting distances and depths can be achieved because the parts in contact with the workpiece are the outer edge and the bottom. However, when cutting molds for 3D workpieces, the contact area with the workpiece is almost always a sharp point. Therefore, the tool-to-tool distance and cutting depth must be reduced, resulting in decreased machining efficiency. In other words, the face milling cutter is suitable for 2D-shaped workpieces but not suitable for 3D-shaped workpieces.
Ball End Mill (also known as R Mill)
A ball end mill, also referred to as an R mill, features a spherical bottom edge. Compared to a face milling cutter, the ball end mill is an essential tool for milling 3D workpieces. The bottom of the ball end mill lacks the sharp tip of the face milling cutter and has a blade with an R angle, making the tool more stable and less prone to breaking under pressure. In mold machining, the ball end mill is primarily used on 3D mold milling machines, especially for precision machining and corner cleaning. However, the contact area of the ball end mill with the workpiece is small, and it cannot extend distances. Therefore, it is not suitable for milling relatively flat areas on the workpiece.
Bull Nose Cutter
In CNC machining centers, a bull nose cutter, also known as a flat-bottomed R mill, is used for rough machining, flat surface finishing, and contour profiling. Compared to face milling cutters and ball end mills, the bull nose cutter combines the advantages of both, offering better work efficiency. The lateral spacing of the bull nose cutter’s blades can be larger than that of a ball end mill, providing similar advantages in precision machining. Therefore, bull nose cutters are suitable for both rough and precision machining.
Large workpieces with minimal surface variation, small areas with narrow grooves, and larger relatively flat regions are best treated with bull nose cutters. Secondary roughing methods can be employed to identify areas that require further machining. However, bull nose cutters may encounter blind spots on the tool body when dealing with certain concave areas, resulting in a “cusp” phenomenon.
CNC Machine Tool Materials
Carbon Tool Steel
Carbon tool steel is an alloy of iron and carbon with small amounts of other elements to enhance its properties. It is one of the least expensive materials for manufacturing cutting tools and is suitable for low-speed operations. These alloys often contain trace amounts of manganese, silicon, and copper. In some cases, chromium and vanadium are added to improve hardness and grain size. Tools made from carbon tool steel exhibit good wear resistance. Carbon tool steel tools are used for machining soft metals like aluminum, copper, and magnesium. They are limited to working at temperatures below 250°C. If the tool heats beyond this threshold, it will lose hardness, affecting milling operations.
High-Speed Steel (HSS)
High-speed steel is a high-alloy tool steel with added elements such as tungsten, molybdenum, chromium, and vanadium. HSS has high strength, toughness, and a certain level of hardness and wear resistance, making it suitable for various tool requirements. The manufacturing process of HSS tools is simple, and they can be sharpened easily, maintaining sharp cutting edges. Despite the emergence of various new tool materials, HSS tools still hold a significant share in metal cutting due to these advantages. They can be used to machine non-ferrous metals and high-temperature alloys. HSS is commonly used for machining iron risers, milling slots, and drilling oil holes in piston manufacturing due to its performance characteristics.
Cemented Carbide (Hard Alloy)
Cemented carbide is produced by powder metallurgy using hard metal carbides (such as WC, TiC, TaC, NbC) and a metallic binder (such as Co, Ni). Due to the presence of metal carbides, cemented carbides have high hardness, wear resistance, and heat resistance. The hardness of commonly used cemented carbides is 89-93 HRA, higher than that of HSS (83-86.6 HRA). They remain capable of cutting at temperatures of 800-1000°C. At 540°C, the hardness is 82-87 HRA, and it remains at 77-85 HRA at 760°C. Therefore, cemented carbide has much higher cutting performance than HSS, leading to significantly increased tool durability and 4-10 times higher cutting speeds.
Diamond is the hardest known mineral material with excellent thermal conductivity. Its wear rate when paired with various metals or non-metal materials in friction is only 1/50 to 1/800 of that of cemented carbide. It is the ideal material for making cutting tools.
Polycrystalline Cubic Boron Nitride (PCBN)
PCBN is formed by sintering CBN micro-powder with a small amount of binding phase (Co, Ni, or TiC, TiN, Al2O3) using a catalyst at high temperature and pressure. It has high hardness (second only to diamond), heat resistance (1300-1500°C), excellent chemical stability, much higher thermal stability (up to 1400°C) and thermal conductivity than diamond tools, and low friction coefficient, but its strength is lower.
The main advantages of ceramic cutting tool materials are their high hardness and wear resistance, with a room temperature hardness of 91-95 HRC. They exhibit high heat resistance, maintaining a hardness of 80 HRC at 1200°C. Additionally, their bending strength and toughness decrease minimally at high temperatures. Ceramics also possess excellent chemical stability, minimal affinity with metals, good high-temperature oxidation resistance, and little interaction with steel even at melting temperatures.
Properties of CNC Tool Materials
The hardness of tool materials must be higher than that of the workpiece material. Otherwise, the geometric shape of the tool edge cannot be maintained at high temperatures. This is a fundamental characteristic of tool materials.
|CNC Tool Material||Hardness (HV)|
|Polycrystalline Cubic Boron Nitride (PCBN)||4700-5000|
|Cemented Carbide (Hard Alloy)||1800-2000|
|High-Speed Steel (HSS)||850-1100|
|Carbon Tool Steel||600-700|
Adequate Strength and Toughness
The material in the cutting part of the tool must withstand significant cutting forces and impacts during machining. For example, when turning 45 steel with ap=4 mm and f=0.5 mm/r, the cutting force on the tool insert is about 4000 N. Therefore, tool materials must possess sufficient strength and toughness. The bending strength (measured in Pa) indicates the material’s strength, while impact toughness (measured in J/m2) represents its toughness, reflecting its resistance to brittle fracture and chipping.
High Wear and Heat Resistance
Wear resistance refers to a tool material’s ability to resist wear. Generally, the higher the hardness of the tool material, the better its wear resistance. Additionally, wear resistance is influenced by chemical composition, properties of hard particles, quantity, particle size, and distribution in the microstructure. The more carbides present and the finer and more evenly distributed they are, the higher the wear resistance. Wear resistance and heat resistance are closely related. Heat resistance is typically measured by the material’s ability to maintain high hardness at elevated temperatures, often called high-temperature hardness or red hardness. A higher high-temperature hardness indicates better heat resistance and stronger resistance to plastic deformation and wear at high temperatures. Tool materials with poor heat resistance experience significant hardness reduction at high temperatures, leading to rapid wear and even plastic deformation, resulting in loss of cutting ability.
Good Thermal Conductivity
Thermal conductivity of tool materials is measured in thermal conductivity [W/(m·K)]. Higher thermal conductivity indicates better heat conduction, allowing heat generated during cutting to dissipate more easily, thus lowering the temperature of the cutting area and reducing tool wear. Additionally, good thermal conductivity is important for intermittent cutting, especially when machining workpieces with poor thermal conductivity.
Good Processability and Economy
For ease of manufacturing, tool materials should exhibit good machinability, including forging, welding, machining, heat treatment, and grindability. Economy is one of the important criteria for evaluating and promoting the application of new tool materials. The selection of tool materials should consider the country’s resources to reduce costs.
Prevents mutual adhesion between workpiece and tool material molecules under high-temperature and high-pressure conditions.
Indicates that tool materials are less likely to undergo chemical reactions with the surrounding media at high temperatures.
Coatings for Cutting Tools
Titanium Nitride Coating
Abbreviated as TiN coating, which is a hard, thin film coating applied to the surface of tools to enhance their performance and durability. TiN coating is known for its golden color and is often used for cutting tools, drills, end mills, and other metalworking tools.
Titanium Carbonitride Coating
Titanium carbonitride coating, abbreviated as TiCN coating, is enriched with carbon to enhance tool hardness and achieve good surface lubrication. It is commonly used on high-speed cutting tools.
Diamond coatings improve the wear resistance of non-metallic material cutting tools. They are ideal for processing graphite, metal matrix composites (MMC), high-silicon aluminum alloys, and many other highly abrasive materials. Different coatings are used for hard milling, threading, and drilling operations, each serving a specific purpose. Additional coatings can also be applied between the tool surface and the tool substrate to further enhance tool lifespan.
General-Purpose PVD Coating
General-purpose PVD coatings enhance tool hardness and elevate oxidation temperature. They are suitable for high-speed steel cutting tools and forming tools, resulting in improved machining performance.
Chromium Nitride Coating
Chromium nitride coating, abbreviated as CrN coating, boasts excellent anti-bonding properties, making it an ideal choice for applications prone to chip buildup. This nearly invisible coating significantly enhances the performance of high-speed steel tools, cemented carbide tools, and forming tools.
Zirconium Nitride Coating
Zirconium nitride coating, abbreviated as ZrN coating, is an anti-adhesive coating that does not contain titanium or chromium. It is suitable for processing aluminum, copper, titanium, and their alloys, preventing chip buildup and adhesive wear.
Titanium Aluminum Nitride / Aluminum Titanium Nitride Coating
Titanium aluminum nitride aluminum titanium nitride (TiAlN/AlTiN) coatings extend high-temperature tool life by incorporating an aluminum oxide layer. These coatings are particularly suitable for dry or semi-dry cutting of cemented carbide tools. Depending on the ratio of aluminum to titanium in the coating, AlTiN coatings have higher hardness than TiAlN coatings. As a result, AlTiN coatings are another ideal choice for high-speed machining applications.
|Comparison of Various Coatings|
|Types of coating||Color||hardness (HV)||coefficient of friction||Oxidation temperature(℃）|
Characteristics of Tool Coatings
- The application of coating technology can significantly improve the surface hardness of tools without affecting their strength. Currently, the hardness has approached 100 GPa.
- In recent years, due to the rapid development of coating technology, the surface’s chemical stability and high-temperature oxidation resistance have been noticeably enhanced, enabling high-speed cutting processes.
- Lubricating thin films exhibit excellent solid-phase lubrication performance, effectively improving machining quality and also suitable for dry cutting processes.
- Coating technology is the final process for machining tools. It does not have any impact on the precision of the tools and can be applied multiple times.”
How to Choose the Right CNC Cutting Tools
The selection of cutting tools is carried out in the human-computer interaction state of CNC programming. The correct choice of tools and tool holders should be based on the processing capabilities of the machine tool, the material properties of the workpiece, the machining processes, the cutting quantities, and other relevant factors.
General principles for tool selection: Easy installation and adjustment, good rigidity, high durability, and accuracy. While meeting the processing requirements, choose shorter tool holders whenever possible to enhance tool rigidity. When selecting tools, ensure that the dimensions of the tools are compatible with the surface dimensions of the workpiece.
Future trend of Cutting Tools
Due to the increasing use of difficult-to-machine materials in parts operating under high temperature, high pressure, high speed, and in corrosive fluid media, the level of automation in machining and the demand for machining precision are on the rise. In order to adapt to this situation, the future development direction of cutting tools will involve the exploration and application of new tool materials. There will be further advancement in the gas-phase deposition coating technology for tools, depositing coatings with even higher hardness onto substrates with high toughness and strength. This will effectively address the contradiction between the hardness and strength of tool materials.
Furthermore, there will be continuous development in the structural design of indexable tools. Manufacturing precision of cutting tools will be enhanced to minimize variations in product quality and optimize tool performance.
Overall, the focus of cutting tool development will be on innovating materials, advancing coating technologies, enhancing tool structures, improving manufacturing precision, reducing product quality disparities, and achieving optimal tool utilization.