1. Classification of CNC Milling Machines
Since the industrial revolution, the machine tool industry has undergone significant transformations. Most people are familiar with milling machines, lathes, and drill presses, which are commonly referred to as conventional machine tools. These devices require skilled operators to manually adjust the tool holder using handwheels to position it correctly for machining. Conventional equipment demands highly trained personnel and is generally inefficient and costly. In contrast, numerical control (NC) equipment has gradually replaced many traditional tools in various fields. CNC machines operate automatically, eliminating manual intervention during the machining process. All necessary information about the workpiece—such as the machining process, tool path, cutting direction, displacement amount, and parameters like spindle speed, feed rate, and depth of cut—must be programmed into a standard or specific NC program before being input into the machine’s control system. The programming process includes analyzing the part design, planning the machining sequence, and verifying the program. CNC programming can be either manual or automatic.
CNC milling machines are versatile and can perform drilling, boring, tapping, contour milling, face milling, pocket milling, and even complex 3D surface machining. Machining centers and flexible manufacturing units have evolved from CNC milling machines, primarily using milling as their main processing method.
CNC milling machines can be categorized into three main types based on traditional classification:
(1) Vertical CNC Milling Machine: The vertical axis is perpendicular to the horizontal plane. This type is the most common and widely used, with most three-axis CNC machines capable of three-axis linkage.
(2) Horizontal CNC Milling Machine: The spindle axis is parallel to the horizontal plane. To expand functionality, these machines often use a CNC rotary table for four- or five-axis linkage, allowing multi-angle machining without repositioning the workpiece.
(3) Vertical-Horizontal Convertible Milling Machine: This machine allows switching between vertical and horizontal machining modes, offering flexibility in operations.
2. Main Workpieces Processed by CNC Milling Machines
(1) Flat Parts: These parts have surfaces that are either parallel, perpendicular, or at a fixed angle relative to the horizontal plane. They are the simplest type of workpiece for CNC milling and typically require two- or three-axis linkage for machining. End mills or bull nose cutters can be used for both rough and finish machining.
(2) Surface Parts: These involve complex, spatially curved surfaces. The cutter only contacts the surface at a single point, and ball-end cutters are usually used for finishing.
3. Coordinate System of CNC Milling Machines
CNC machines use a right-handed Cartesian coordinate system. The X, Y, and Z axes follow the right-hand rule, while rotation axes A, B, and C follow the right-hand spiral rule. The Z-axis is typically aligned with the spindle, and the coordinate system is essential for accurate tool positioning.
(1) Steps to Create a Machining Coordinate System
To machine a part, the workpiece's position must first be defined. Operators can set up the coordinate system by aligning the part with reference points on the fixture or machine. The process involves determining the coordinate plane, aligning the axes, and setting the origin.
(2) Elements for Establishing the Machining Coordinate System
Key elements include defining the coordinate plane, aligning the axes, and setting the origin. These steps ensure the workpiece is accurately positioned relative to the machine tool.
4. Zero Point of CNC Milling Machine
The zero point is crucial for accurate machining. It serves as the reference point for all tool movements. CNC machines have a machine zero point, which is fixed and determined by the manufacturer. The workpiece zero point, or program origin, is set by the programmer and is used as the starting point for the NC program.
5. CNC Milling Machine Offset
(1) Concept of Machine Offset
Machine offset refers to the distance between the machine zero and the workpiece zero. These values are stored in the machine control unit and are used to adjust tool positions during machining. Adjustments can be made manually or automatically to improve accuracy.
(2) Setting the Machine Coordinate System and Offset
CNC machines allow multiple workpiece coordinate systems, such as G54 to G59. Each system defines a different origin, enabling efficient machining of multiple parts. Offsets are stored in registers and used to track tool movement accurately.
(3) Role of Workpiece Offset
Workpiece offsets simplify programming by allowing multiple coordinate systems to be used without recalculating every time. Once a G code is specified, the CNC system recalls the corresponding offset, ensuring accurate tool positioning.
(4) Z-Axis Offset and Tool Length Offset
Z-axis offset is influenced by the tool mounted on the spindle. Tool length offset compensates for this by adjusting the Z-axis position during machining. For example, if the program specifies a Z value of -100.0, and the offset is -12.5 with a tool length offset of 35.8, the actual position becomes -76.7. This ensures precise depth of cut and improves machining quality.
6. Tool Parameter Presetting
Tool presetting is used to measure and set parameters like tool length and diameter. Methods include test cutting, internal, and external tool setting. External tool setting tools offer high precision and efficiency, allowing operators to measure tool parameters without stopping the machine.
(1) Tool Setting Method
External tool setting tools use optical systems to measure tool dimensions. The tool is inserted into a taper hole, and the tool's length, radius, and diameter are measured. These values are then entered into the machine control system, ensuring accurate NC machining.
(2) Composition of an External Tool Setting Tool
These tools consist of a positioning mechanism, a probe, and a data processing device. The positioning mechanism ensures accurate alignment, the probe measures the tool, and the data processor stores and transmits the results to the machine.
(3) Precautions When Measuring Tool Parameters
Before use, the tool setting tool must be calibrated with a standard mandrel. Static measurements may differ from actual machining due to factors like tool rigidity and machine accuracy. A correction value is often applied to account for these differences. Additionally, the workpiece coordinate system must be carefully measured and adjusted to ensure consistent results during machining.
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