1. Classification of CNC Milling Machines
Since the Industrial Revolution, the machine tool industry has experienced significant transformations. Most people are familiar with milling machines, lathes, and drill presses, commonly known as conventional machine tools. These devices rely on skilled operators to manually adjust the tool holder using hand wheels to position it for machining. Conventional machine tools require highly trained operators to produce high-quality parts under specific conditions. However, they tend to be less efficient and more costly compared to modern alternatives.
Numerical Control (NC) equipment has largely replaced traditional machinery in many industries. Unlike conventional tools, CNC machines operate automatically, eliminating manual intervention during the machining process. Before processing, all necessary information about the workpiece must be programmed, including the machining sequence, tool path, cutting direction, displacement, process parameters like spindle speed, feed rate, and cutting depth, as well as auxiliary actions such as tool changes, coolant control, and clamping. This data is written into a program using standard NC instructions, then input into the CNC control system, which guides the machine to perform the required operations. NC programming involves analyzing the part design, planning the machining process, creating mathematical models, writing and verifying the program, and can be done manually or automatically.
CNC milling machines are capable of drilling, boring, tapping, contour milling, face milling, pocket milling, and machining complex 3D surfaces. Machining centers and flexible manufacturing units have been developed based on CNC milling technology, primarily using milling as their main method.
CNC milling machines can be categorized into three types based on traditional classification methods:
(1) Vertical CNC Milling Machine
The vertical axis of this type of machine is perpendicular to the horizontal plane. It is the most common type of CNC milling machine, widely used in various applications. Most three-axis CNC vertical mills can perform three-axisè”动 (interlinked) machining.
(2) Horizontal CNC Milling Machine
The spindle axis of a horizontal CNC mill is parallel to the horizontal plane. To expand its capabilities, these machines often use a numerical-controlled rotary table or universal numerical-controlled table to achieve four- or five-axisè”动 machining. This allows for continuous contour machining on the side of the workpiece and multiple positions or surfaces to be machined in a single setup.
(3) Vertical and Horizontal Conversion Milling Machine
This type of machine allows the main spindle to switch between vertical and horizontal orientations, enabling both vertical and horizontal machining on the same machine.
2. Main Processing Objects of CNC Milling Machines
(1) Flat Parts
Flat parts are characterized by surfaces that can be parallel, perpendicular, or at a fixed angle relative to the horizontal plane. Most parts processed on CNC mills are flat, making them the simplest type of part in milling. Typically, two or three-axis linkage is sufficient for machining. The tool makes surface contact with the workpiece, and end mills or bull nose cutters can be used for both rough and finish machining.
(2) Surface Parts
Surface parts feature a space surface as the machined area. During machining, the cutter only contacts the workpiece at a point, and ball-end cutters are commonly used for finishing.
3. Coordinate System of CNC Milling Machines
To describe the location of points in space, a coordinate system is essential. CNC machines use a right-handed Cartesian coordinate system, where the positive directions of X, Y, and Z axes follow the right-hand rule, and the rotation axes A, B, and C follow the right-hand spiral rule. The Z-axis is typically aligned with the machine spindle, forming a three-dimensional coordinate system.
(1) Steps to Create a Machining Coordinate System
To machine a part on a CNC mill, the workpiece's position must first be determined. A coordinate system is established relative to the part, allowing the operator to create a machining coordinate system by pressing buttons on the control panel. Key steps include determining the coordinate system orientation, aligning the workpiece, and verifying the system’s accuracy.
(2) Elements for Establishing the Machining Coordinate System
Geometric elements like points, lines, and faces play a crucial role in setting up the coordinate system. The main factors involve locating features on the part and fixture, and aligning the coordinate axes accordingly. Common steps include selecting the coordinate plane, defining the axis direction, and setting the origin.
4. Zero Point of CNC Milling Machine
The position of the tool and the tool vector in the NC program depends on the machining coordinate system. Therefore, the exact position of the machining coordinate system must be determined before starting the machining process. On conventional machines, operators usually use the edge of the tool to determine the zero point. For CNC machines, the zero point is set via the control panel, and the machine’s reference point is fixed by the manufacturer.
5. CNC Milling Machine Offset
(1) Concept of Machine Offset
The distance between the machine zero and the work zero is called an offset. Each axis has its own offset value stored in the machine control unit. These values are used to guide the tool to the correct position during machining. Offsets can also be adjusted manually, allowing for fine-tuning of the workpiece dimensions.
(2) Setting and Adjusting the Machine Coordinate System
CNC machines can create multiple workpiece offsets, each corresponding to a different coordinate system. Commands like G54 to G59 allow the programmer to select the appropriate coordinate system. These offsets are stored in the machine control unit and remain after the machine returns to the reference point.
Table: Coordinate Offsets
Designation | X-offset | Y-offset | Z-offset
G54 | -30.221 | -65.864 | 0
G55 | -7.987 | -33.366 | -9.873
G56 | -15.765 | -7.832 | -35
G57 | -50.352 | -0.788 | -8.963
(3) Role of Workpiece Offset
Workpiece offsets allow for easy translation and rotation of the coordinate system without recalculating dimensions. These offsets are stored in the machine control unit and remain even after the machine returns to the reference point. When a G command is used, the CNC system recalls the selected coordinate system as the current one.
(4) Z-Axis Offset and Tool Length Offset
Z-axis offset is influenced by the tool mounted on the spindle. Tool length offset helps manage this by adjusting the Z-axis position based on the tool’s length. For example, if the program specifies a Z position of -100.0, and the Z offset is -12.5 with a tool length offset of 35.8, the final position would be -76.7. This ensures accurate tool positioning during machining.
6. Tool Parameter Preset
Tool presetting is used to measure and set parameters like tool length and diameter. Methods include test cutting, internal and external tool setting, and using a tool setting device. The latter is more accurate and efficient, especially when measuring tool geometry outside the machine.
(1) Tool Setting Method
The external tool setting method uses a measuring device to determine tool parameters without stopping the machine. The tool is placed in a tapered hole, and optical systems measure the tool’s length, radius, and diameter. These values are then entered into the machine control unit, ensuring precise machining.
(2) Composition of External Tool Setting Device
The tool setting device includes a positioning mechanism, a probe, and a measurement data processor. The positioning mechanism ensures accurate alignment, while the probe measures the tool’s dimensions. The data processor records and transfers the measurements to the machine control system.
(3) Precautions for Measuring Tool Parameters
Before use, the tool setting device should be calibrated with a standard mandrel. Static measurements may differ from actual machining results due to factors like tool rigidity, workpiece material, and coolant conditions. Operators should account for these differences by adding a small correction value, typically 0.01–0.05 mm, to ensure accuracy.
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