CNC machine tool coordinate system
1. Coordinate System
The machine tool's coordinate system includes all actual axes of the machine. It serves as the foundation for defining positions and movements within the machine.
The reference point, tool change position, and pallet exchange location are all determined based on the machine coordinate system. This system consists of a defined coordinate framework, an origin point, and movement directions that guide the machine’s operation.
According to ISO standards, the coordinate system used in CNC machines is a Cartesian system following the right-hand rule. This ensures consistency across different types of CNC equipment.
Figure 1 Right-handed Cartesian Coordinate System
2. Coordinate Axes and Naming Principles
Regardless of the machine structure, the movement in a CNC machine is defined as the motion of the tool relative to the workpiece. This approach ensures consistent programming regardless of whether the workpiece or the tool is moving.
It is assumed that the workpiece is stationary, and the tool moves accordingly. The programming is based on the tool's path, which simplifies the process of setting up the machine for various operations.
The direction of movement of the machine parts is related to the machine type. According to JB3051-82, increasing the distance between the workpiece and the tool is considered the positive direction of movement.
a) CNC Lathe b) Vertical CNC Boring and Milling Machine c) Horizontal CNC Boring and Milling Machine
Figure 2 Axes of the CNC Machine and Their Direction of Motion
(1) Definition of Z-axis Movement
The Z-axis is parallel to the spindle of the machine. Its positive direction is from the table toward the tool holder, meaning the tool moves away from the table.
(2) Definition of X-axis Movement
1) For rotating workpieces (like lathes and grinders), the X-axis is radial and parallel to the horizontal slide. The positive X-direction is outward from the center of rotation.
2) For rotating tools (such as milling and drilling machines):
Vertical: X-axis is horizontal, parallel to the clamping surface axis. Positive direction is to the right when viewed from the main axis column.
Horizontal: X-axis is horizontal, parallel to the workpiece clamping plane axis. Positive direction is to the right when viewed from the main axis side.
(3) Definition of Y-axis Movement
The positive direction of the Y-axis is determined using the right-hand rule, based on the X and Z axes.
(4) Definition of Rotation Axis Movement
The positive directions of the A, B, and C axes follow the right-hand thread direction along the X, Y, and Z axes respectively.
5. Workpiece Coordinate System
The workpiece coordinate system is used to define the geometry of the part. NC programs should reference this coordinate system to ensure accurate machining.
Fig. 3 Workpiece Coordinate System Fig. 4 Lathe Coordinate System
Figure 5 Vertical Milling Coordinate System Figure 6 Horizontal Milling Machine Coordinate System
3. Coordinate Origin
In CNC machines, multiple reference points can be defined. These points are essential for positioning and programming:
(1) Machine Origin (M): The starting point for the machine tool's coordinate system. It is fixed and set by the manufacturer.
(2) Machine Tool Reference Point (R): A physical position set by the travel switch. It has a fixed relationship with the machine origin and is typically used for automatic tool changes.
Fig. 7 Machine Coordinate System
(3) Program Origin (W): Also known as the Part Origin. It is the reference point defined by the programmer during NC programming.
(4) Mounting Origin: Also called the Fixture Origin, it may coincide with the workpiece origin depending on setup.
Figure 8 CNC Lathe Figure 9 CNC Boring and Milling Machine
4. Absolute Coordinates and Relative Coordinates
Absolute coordinates represent the position of the tool as a distance from the origin. This method is commonly used in G90 programming commands.
Figure 10 Absolute Coordinate Representation
Figure 11 Relative Coordinate Representation
Relative coordinates express the position as an increment from the previous location. This is often used in G91 programming. Some systems use X/Y/Z for absolute and U/V/W for relative coordinates. Both can be combined in a single program for more flexibility.
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