In CNC machine tool machining, the selection of CNC cutting tools and the determination of cutting parameters are important parts of the CNC machining process. They not only affect the machining efficiency of CNC machine tools, but also directly influence the machining quality. The selection of cutting tools and the determination of cutting parameters in CNC machining are completed under human-computer interaction.

This requires programmers to master the basic principles of tool selection and cutting parameter determination. By discussing issues such as tool selection and cutting parameter determination that must be addressed in CNC programming, this paper puts forward several principles and suggestions, and discusses the issues that should be paid attention to.

Characteristics of Commonly Used Tools in CNC Machining

CNC machining tools must adapt to the characteristics of CNC machine tools, such as high speed, high efficiency and high degree of automation. Commonly used tools include general-purpose tools, general-purpose connecting tool holders and a small number of special tool holders, and a standardized and serialized system has been formed at present.

According to the structural form of tools, CNC machining tools can be divided into welded integral tools, assembled tools and combined tools.

According to the tool material, CNC machining tools are further divided into high-speed steel tools, cemented carbide tools, diamond tools, and tools made of other materials (such as cubic boron nitride tools, ceramic tools, etc.).

To meet the requirements of CNC machine tools for tools, such as durability, stability, easy adjustment and replaceability, indexable tools with mechanical clamping have been widely used in recent years, accounting for 30% to 40% of CNC tools in terms of usage quantity.

CNC machine tools mainly have the following characteristics:

① Good rigidity and high precision;

② Small vibration and thermal deformation, good interchangeability, and convenient for quick tool change;

③ Long service life;

④ Stable and reliable cutting performance, and easy size adjustment (which can reduce tool change adjustment time);

⑤ Reliable chip breaking or chip curling (to facilitate chip removal); ⑥ Serialization and standardization, which is conducive to programming and tool management.

 

Basic Properties of CNC Machine Tool Materials

CNC machining tool materials should possess the following basic properties: high hardness, sufficient strength and toughness, high wear resistance and heat resistance, good thermal conductivity, high chemical stability, as well as good processability and economy. Currently, the materials used for CNC machine tools mainly include the following types.

High Speed Steel (HSS)

High speed steel is a type of tool steel containing a relatively high proportion of alloying elements such as tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V). It has good mechanical properties and processability, and can withstand large cutting forces and impact loads.

Cemented Carbide CNC Machine Tools

Cemented carbide used for cutting wood materials is mainly cobalt-rich cemented carbide, whose main components are tungsten carbide (WC) and metallic cobalt (Co). It has high hardness and good wear resistance, and is often used for processing brittle materials and non-metallic materials. Common grades include YG6, YG8, YG15, and YG6X (the number in the grade represents the percentage content of Co; the higher the Co content, the better the toughness).

 طلاء

Coated tools are made by depositing a thin layer of wear-resistant refractory metal compound on the tool substrate. Common coating materials mainly include titanium nitride (TiN), titanium carbide (TiC), and aluminum oxide (Al₂O₃).

Polycrystalline Diamond (PCD)

Tools made of this material have high hardness and good wear resistance, but low strength and high brittleness. They also have a strong affinity with iron, so they are usually used for high-speed precision machining of non-metallic materials.

 

Common Tool Structural Forms and Characteristics

In CNC milling machines and machining centers, all types of tools are mounted on the tool magazine respectively, and tool selection and tool change are carried out at any time in the sequence specified by the machining program.

Therefore, standard tool holders must be used to enable tools for processes such as milling, drilling, and scraping to be quickly and accurately installed on the machine spindle or tool magazine. Currently, machining centers in China generally adopt the TSG tool system, whose tool holders are divided into two types: straight shanks (three specifications) and taper shanks (four specifications).

In economical CNC machining, since tool regrinding, measurement and replacement are mostly done manually, which takes a long time of auxiliary work, it is necessary to reasonably arrange the tool arrangement sequence and minimize the number of tools as much as possible. After a tool is clamped, it should complete all machining operations that it can perform.

Tools for rough machining and finish machining should be used separately; even if the tools are of the same size and specification, milling should be carried out first, followed by drilling. The finish machining of the outer contour of 3D curved surfaces should be done first, and then the finish machining of 2D contours. The automatic tool change function of CNC machine tools should be utilized as much as possible to improve machining efficiency.

 

Selection of CNC Machine Toolس

When selecting tools (inserts), key factors to consider primarily include the type and properties of the workpiece material, the cutting process to be adopted, the geometric shape of the workpiece, and quality requirements.

When choosing a tool, first, the tool size must match the size of the machined surface of the workpiece. For processing the peripheral outer contour of plate-type parts, small-diameter cylindrical end mills or corn mills with indexable cemented carbide inserts are often used.

When milling large flat surfaces, indexable cemented carbide insert face mills should be selected; for processing grooves, high-speed steel or cemented carbide inserted-tooth shank end mills are suitable; for processing rough workpiece surfaces or rough machining of holes, corn mills with indexable cemented carbide inserts can be used; for processing the edge profiles of plate parts, small-diameter integral inserted-tooth end mills should be chosen; for processing through square grooves, disc mills are applicable; for processing 3D solid profiles, ball nose end mills or spoon-shaped end mills are selected. The flat surface milling tools and their machining diagram are shown in Figure 1. 

Figure 1: Flat Surface Milling Tools and Machining Diagram

During curved surface engraving and machining, since the cutting speed at the tip of a ball nose tool is zero, the cutting stepover is generally set very dense to ensure machining accuracy. Therefore, ball nose tools are often used for the finish machining of curved surfaces. Flat-end tools outperform ball nose tools in both surface machining quality and cutting efficiency.

Thus, on the premise of ensuring no over-cutting, flat-end tools should be preferred for both rough machining and finish machining of curved surfaces. In addition, the tool life and accuracy are closely related to the tool price. However, it is important to note that in most cases, although choosing high-quality tools increases tool costs, the resulting improvement in machining quality and efficiency will instead significantly reduce the overall machining cost.

(1) Milling large flat surfaces:To improve production efficiency and reduce the surface roughness of machined parts, indexable insert disc mills are generally used.

(2) Processing profiled parts:To ensure that the cutting edge of the tool is tangent to the machined contour at the cutting point and avoid interference between the cutting edge and the workpiece contour, ball nose end mills are generally used.

Two-flute end mills are used for rough machining, and four-flute end mills are used for semi-finish machining and finish machining. The profiled surface milling tools and their machining diagram are shown in Figure 2.

Figure 2: Profiled Surface Milling Tools and Machining Diagram

(3) Milling small flat surfaces or step surfaces:General-purpose end mills are generally used for milling small flat surfaces or step surfaces. The small flat surface milling tools and their machining diagram are shown in Figure 7.

Figure 3: Small Flat Surface Milling Tools and Machining Diagram

(4) Milling grooves:To ensure the dimensional accuracy of the groove, two-flute tenon groove end mills are generally used. The grooving tools and their machining diagram are shown in Figure 4.

Figure 4: Grooving Tools and Machining Diagram

(5) Processing holes:Hole-making tools such as drills and shank end mills can be used. The hole-making tools and their machining diagram are shown in Figure 9.

 

 

Selection of Main Technical Parameters for Milling Cutters

Selection of milling cutter angle parameters

The marked angle parameters of a milling cutter include: the rake angle γ, which represents the inclination of the rake face relative to the base surface and mainly affects chip deformation

The clearance angle α, which indicates the inclination of the flank face relative to the cutting plane and primarily influences the friction between the tool flank and the workpiece; the wedge angle β, which is the angle between the rake face and the flank face, reflecting the sharpness and strength of the tool’s cutting part; with the relationship γ + β + α = 90°.

For combined cemented carbide formed assembly milling cutters, the recommended wedge angle β is 30°~40° (30° for soft materials and 40° for hard materials); the clearance angle α is generally 10°~20°; the recommended rake angle γ is 25°~35° when machining soft materials and 10°~25° when machining hard materials.

In actual cutting processes, the angles of the milling cutter change due to factors such as cutting movement, cutting edge installation height, and tool wear.

Selection of milling cutter cutting parameters

Cutting parameters mainly refer to the cutting layer thickness, tool axis rotational speed, and feed rate. The processing parameters determined according to the final machining accuracy and surface roughness requirements of the product should be written the processing program, and the corresponding cutting thickness, rotational speed, feed rate, and number of tool teeth should be determined based on the final required surface roughness and the corresponding feed per tooth of the tool.

When milling or drilling wooden materials, the cutting speed should generally be controlled between 30~70 m/s. CNC machine tools use milling cutters with relatively small diameters and high spindle speeds, typically between 3000~24000 r/min (50~400 r/s). For example, if the cutting speed is 30 m/s and the spindle speed is 24000 r/min, the cutting circle diameter of the milling cutter is approximately 24 mm.

In practice, the diameter of the installed milling cutter is often smaller than this value (e.g., D=6 mm), resulting in a cutting speed much lower than the controlled value. Since the spindle outputs constant power or torque with high power, attention must be paid to the rigidity, strength, and durability of the tool to ensure production efficiency and machining accuracy.

The feed rate of the workpiece depends on the requirements for the surface quality of the cutting process. The surface roughness of the cut workpiece largely depends on the feed per tooth of the milling cutter during cutting.

Excessively large feed per tooth leads to overly rough machined surfaces, while excessively small feed per tooth causes burning on the machined surface. Therefore, the feed per tooth of the milling cutter must be appropriate. Recommended values for milling cutter cutting speed and average chip thickness are shown in Table 1.

Table 1 Recommended Milling Cutter Cutting Speeds and Average Chip Thicknesses

Cutting Material	Cutting Speed / (m·s⁻¹)	Average Chip Thickness / mm
Coniferous wood	60~90	0.2~0.8
Broad-leaved wood	60~80	0.2~0.8
Modified wood	50~85	0.2~0.8
Laminated veneer lumber	70~100	0.3~0.6
Solid wood panel	50~90	0.2~0.8
Blockboard	60~90	0.2~0.8
Plywood	50~80	0.3~0.6
Medium and high-density fiberboard	60~100	0.2~0.8
Particleboard	60~80	0.35~0.8

For wood products, the commonly used feed per tooth is 0.3~1.5 mm, depending on the milling object and surface quality requirements. For CNC milling of wood products (which is a finishing process), the commonly used feed per tooth is 0.3~0.8 mm.

The cutting width is proportional to the tool diameter and inversely proportional to the cutting depth. In economical CNC machining, the general range for the cutting width L is L = (0.6~0.9)d.

ملخص

Programmers must be familiar with tool selection methods and principles for determining cutting parameters to ensure machining quality and efficiency of parts, give full play to the advantages of CNC machine tools, and improve the production efficiency and economic benefits of enterprises.