CNC milling is one of the core technologies of precision machining in modern manufacturing. Its principle is that under the control of a preset CNC program, the machine tool drives a high-speed rotating milling cutter to perform targeted cutting on the workpiece, and finally obtains the product shape that meets the design requirements by removing excess material.

The setting of machining parameters, as the “command center” of CNC milling, directly determines the rationality of the tool path and the machining quality. Understanding the characteristics of tools and the meaning of machining parameters is the basis for achieving efficient and precise machining. This article will take CNC milling machines as the object of explanation, from tool cognition to machining core, and then to key parameter analysis, to help you gradually master the basics of milling.

Cognition of CNC Milling Machine Tools

Tools are the direct execution components of CNC milling, and their size and type directly affect cutting efficiency, machining accuracy, and product surface quality. Differences in the structural design of different tools make them suitable for different machining scenarios and requirements.

Basic Classification Logic of Tools

In terms of cutting volume, there is a clear size difference between tools: large tools (such as flat-bottomed cutters with a diameter of more than 10mm) have a large single cutting volume, which is suitable for quickly removing excess material in the roughing stage; small tools (such as tapered ball cutters with a diameter of less than 2mm) have a small single cutting volume, which is more suitable for finishing or fine structure machining.

Divided by core shape, all CNC milling tools can be roughly classified into two basic types: flat-bottomed cutters and ball-end cutters. Most other tools are derivatives of these two types:

Combining flat-bottomed cutters with tapered cutters results in tapered flat-bottomed cutters;
Combining ball-end cutters with tapered cutters results in tapered ball-end cutters;
Combining flat-bottomed cutters with ball-end cutters results in bull-nose cutters.
The advantage of this derivative design is that it not only retains the core functions of the basic tools but also makes up for the limitations of a single tool. For example, tapered derivative tools can achieve fine machining through a small bottom diameter while ensuring the structural strength of the tool itself.

Common Tool Types and Applicable Scenarios

CNC milling machines have a rich variety of tools, and different types of tools have different focuses in cutting methods and applicable machining ranges. The following is a detailed explanation of the eight most commonly used tools in industrial production:

Tool Type Core Structural Features Main Applicable Machining Scenarios

Core of CNC Milling: Types and Basic Processes

After understanding the tools, you can enter the actual machining stage. The core of CNC milling is to plan the motion path and cutting parameters of the tool to ensure that the machining process is precise and efficient.

Three Core Types of Milling

According to different machining requirements, CNC milling can be divided into three basic types, covering most machining scenarios:

Single-line machining: Cutting for a single line, such as profile edges, groove lines, etc., where the tool moves along a preset single-line path;

Area machining: Milling for a large plane or area, where the tool covers the entire area through multi-path reciprocating cutting;

Curved surface machining: Machining for complex curved surfaces, where the tool needs to plan a 3D path that fits the curved surface shape to ensure the smoothness and accuracy of the curved surface.

 

Six-step Basic Process of Milling

The complete CNC milling process includes six steps, which can be flexibly adjusted according to the characteristics of the machine tool in actual operation:

Determine the machining object and scope (clarify the parts to be cut and size requirements);
Select suitable tools (choose tool type and size according to machining type and material hardness);
Plan the tool path (determine the motion path for single-line, area, or curved surface machining);
Set machining parameters (such as path spacing, axial layering, radius compensation, etc.);
Debug machine tool parameters (such as spindle speed, feed rate, which can be directly adjusted on the machine tool);
Start machining and verify (check the machining effect to avoid collisions or machining defects).

Analysis of Key Machining Parameters: The Core of Optimizing Tool Paths

Machining parameters are the key to adjusting machining quality and efficiency. Reasonable setting of these parameters can effectively avoid machining defects, protect tools, and improve product accuracy. The following is a detailed explanation of the eight core machining parameters and their application skills:

Relationship between Tool Cross-section and Groove Shape

Core rule: When cutting a workpiece with different types of tools, the cross-sectional shape of the formed groove is exactly the same as that of the tool.

Example: When milling a groove with a flat-bottomed cutter in a single line, the cross-section of the groove is rectangular; when milling a groove with a ball-end cutter in a single line, the cross-section of the groove is semicircular.
Application skill: Select tools according to the required groove shape. For example, choose a flat-bottomed cutter for rectangular grooves and a ball-end cutter for arc-shaped grooves.

Path Spacing

Path spacing refers to the distance between two adjacent tool paths during multiple or reciprocating milling (starting from the center of the tool axis), which is a key parameter affecting the surface quality of area machining:

Excessively large spacing

Spacing smaller than the tool diameter: The machined area can be covered by repeated cutting, resulting in a flat surface (especially suitable for plane machining with flat-bottomed cutters);

Application skill: For finishing, the path spacing is recommended to be set to 30%-50% of the tool diameter to balance machining efficiency and surface finish.

 

Axial Layering

When the total milling depth is large, to avoid tool chipping or wear due to excessive single cutting volume, the total depth needs to be divided into multiple cuts, which is called axial layering:

Example: If the total milling depth is 3mm and the single cutting depth is set to 1mm, it needs to be completed in 3 steps to gradually reach the target depth;
Application skill: The higher the material hardness, the smaller the single cutting depth should be (such as ≤0.5mm for cemented carbide materials), and it can be appropriately increased for soft materials (such as aluminum alloy).

Radius Compensation

During single-line engraving or profile cutting, the tool center point moves along the preset line by default. If radius compensation is not performed, the machining size will be smaller than the design size (the difference is the tool radius). The radius compensation function can solve this problem:

Without radius compensation: When machining a rectangle, the actual size = design size – 2×tool radius;
With radius compensation enabled: The tool automatically offsets a radius distance in the set direction to ensure that the machining size meets the requirements;

Right offset (corresponding to “outward offset” for profile cutting): Suitable for external profile machining to ensure the accuracy of external dimensions;

Left offset (corresponding to “inward offset” for profile cutting): Suitable for internal profile machining (such as grooves, apertures) to avoid undersized dimensions.

Tool Entry Angle

The tool entry angle is the inclination angle when the tool cuts into the workpiece, which serves to gradually increase the cutting volume from small to large, avoiding sudden exposure to large cutting forces:

Core advantage: Reduces tool impact, reduces the risk of chipping, and extends tool service life;
Application skill: When machining hard materials (such as steel, cemented carbide), the tool entry angle should be small (5°-15°); when machining soft materials (such as plastic, wood), the tool entry angle can be turned off and the tool can enter vertically directly.

Grooving

When milling a large area, the cutting volume of the first cut is the entire tool diameter (much larger than the cutting volume of the path spacing in subsequent tool paths), which easily leads to tool overload. The grooving function can decompose the load of the first cut:

Working principle: Divide the axial cutting depth of the first cut into multiple layers. For example, if the axial layering per layer (depth of cut) is 1mm and grooving is set to two layers, the first cut is divided into two cuts, each with a depth of 0.5mm. After two cuts, the milling depth reaches 1mm before normal milling starts;

Application skill: When machining materials with high hardness or using large-diameter tools for milling, it is recommended to enable the grooving function. The number of layers can be adjusted according to the tool diameter and material hardness (usually 2-3 layers).

Spindle Speed

Spindle speed refers to the speed at which the electric spindle drives the tool to rotate, in r/min (revolutions per minute), which directly affects cutting efficiency and tool wear:

Too low speed: Insufficient cutting force, easy to cause material adhesion to the tool, increasing surface roughness;
Too high speed: Accelerates tool wear, and even causes tool deformation due to high temperature;
Application skill: High speeds (3000-6000r/min) are required for hard materials (such as hardened steel), and can be appropriately reduced (1000-3000r/min) for soft materials (such as aluminum alloy). Refer to the tool manufacturer’s recommended parameters for specific values.

Feed Rate: Control of Tool Movement Speed

Feed rate refers to the speed at which the tool reference point moves relative to the workpiece along the path, in mm/min (millimeters per minute) or m/min (meters per minute), which balances machining efficiency and surface quality:

Too fast feed rate: Increases cutting force, easily causes burrs, tool marks on the workpiece surface, and even tool breakage;
Too slow feed rate: Low machining efficiency, and may cause surface burns due to long-term friction between the tool and the material;
Application skill: The feed rate should be slow (50-200mm/min) for finishing to ensure surface finish; it can be increased (200-500mm/min) for roughing to improve efficiency.