Views: 12 Author: Site Editor Publish Time: 2024-10-14 Origin: Site

CNC machine tools are a general term for working machines that can complete the processing of metal blank parts into the required shape in terms of dimensional accuracy and geometric accuracy under digital control. CNC machine tools are usually composed of control systems, servo systems, detection systems, mechanical transmission systems and other auxiliary systems.

**I. Cartesian Coordinate System**

The** **coordinate system of CNC machine tools is a right-handed rectangular Cartesian coordinate system as specified by the International Organization for Standardization, as shown in the figure below.

1. Stretch out the thumb, index finger and middle finger of the right hand, and 90 ° to each other. Then the thumb represents the X coordinate, the index finger represents the Y coordinate, and the middle finger represents the Z coordinate.

2. The thumb points to the positive direction of the X coordinate, the index finger points to the positive direction of the Y coordinate, and the middle finger points to the positive direction of the Z coordinate.

3. The rotation coordinates around the X, Y, and Z coordinates are represented by A, B, and C respectively. According to the right-hand spiral rule, if the thumb points to the positive direction of any axis in the X, Y, and Z coordinates, then the rotation direction of the other four fingers represents the positive direction of the rotation coordinates A, B, and C.

**II. CNC Lathe Coordinate System**

Coordinate Axes and Their Motion Direction

1. Relative Motion of Machine Tools

On machine tools, we always assume that the workpiece is stationary and the tool is moving. In this way, programmers can determine the machining process of the machine tool based on the part drawing without considering the specific movement of the workpiece and tool on the machine.

2. The Provisions of Machine Tool Coordinate System

The X, Y, and Z coordinate axes in the standard machine tool coordinate system is determined by the right-handed Cartesian rectangular coordinate system.

On CNC machine tools, the movement of the machine tools is controlled by CNC devices. In order to determine the forming motion and auxiliary motion of the CNC machine tools, the displacement and direction of motion on the machine tools must first be determined. This requires the use of a coordinate system, which is called the machine tool coordinate system.

3. The Provisions of the Direction of Movement

The direction of increasing the distance between the tool and the workpiece is the positive direction of each coordinate axis, and the direction of the tool cutting into the workpiece is the negative direction.

4. Determination of Coordinate Axis Direction

(a) Z Coordinate

The movement direction of the Z coordinate is determined by the spindle that transmits the cutting power, i.e. the coordinate axis parallel to the spindle axis is the Z coordinate, and its positive direction is the direction in which the tool leaves the workpiece.

(b) X Coordinate

The X-coordinate is parallel to the clamping plane of the workpiece, generally in the horizontal plane. When determining the direction of the X-axis, there are two cases to consider:

1) If the workpiece rotates, the direction in which the tool leaves the workpiece is the positive direction of the X coordinate.

2) If the tool rotates, there are two situations: When the Z coordinate is horizontal, the observer looks toward the workpiece along the tool spindle, the +X movement direction points to the right; when the Z coordinate is vertical, the observer faces the tool spindle and looks at the column, the +X movement direction points to the right. The figure below shows the X coordinate of the CNC lathe.

CNC Horizontal Lathe Coordinate System

(c) Y Coordinate

After the positive direction of the X and Z coordinates is determined, the direction of the Y coordinate can be determined according to the right-hand rectangular coordinate system.

**III. Machine Coordinate System and Workpiece Coordinate System**

(A) The origin of the machine coordinate system is a fixed point set on the machine tool. It has been determined during the assembly and commissioning of the machine tool, and is the reference point for the CNC machine tool to perform processing movements.

1. The origin of the CNC lathe is on the CNC lathe. The origin is usually located at the intersection of the chuck end face and the spindle centerline. At the same time, by setting parameters, the machine origin can also be set at the positive limit position of the X and Z coordinates.

2. Machine reference point: the reference point is a fixed position point used for detecting and controlling machine movement.

The reference point is accurately adjusted by the machine manufacturer with limit switches on each feed axis, and the coordinates are entered into the CNC system. The coordinates of the reference point to the machine origin are therefore a known value.

Normally on a CNC milling machine, the machine origin and the reference point coincide; on a CNC lathe, the reference point is the limit point furthest from the machine origin. The figure below shows the reference point and the origin on a CNC lathe.

The origin and reference point of CNC machine tools

a) CNC Lathe b) CNC Milling Machine

(B) Workpiece coordinate system

The coordinate system set by the programmer during programming is also called the programming coordinate system.

The origin of the workpiece coordinate system is the workpiece origin, also called the workpiece zero point. Different from the machine coordinate system, the workpiece coordinate system is set manually.

**The general principles of selecting the origin of the workpiece coordinate system**

a. Try to choose the benchmark on the workpiece drawing to facilitate calculation, reduce errors and facilitate programming.

b. Try to choose the workpiece surface with high dimensional accuracy and low roughness value to improve the machining accuracy of the workpiece.

c. Easy for measurement and inspection.

d. For symmetrical workpieces, it is best to choose the symmetry center of the workpiece.

e. For general parts, select a corner of the workpiece outer contour.

f. The origin of the Z-axis direction is generally located on the surface of the workpiece.

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