I. What are the functions of a CNC milling machine?
Point-to-point control function: The point-to-point control of CNC milling machines is mainly used for hole machining of workpieces, such as center drilling positioning, drilling, reaming, countersinking, boring, and other hole machining operations. Continuous control function: Through linear interpolation, circular interpolation, or complex curve interpolation movements of the CNC milling machine, the planes and curved surfaces of the workpiece are milled.
1. Linear interpolation
One of the most basic functions required to complete CNC milling is linear interpolation, which can be divided into planar linear interpolation, spatial linear interpolation, and approximate linear interpolation.
2. Circular interpolation
One of the most basic functions required to complete CNC milling is planar circular interpolation and approximate circular interpolation.
3. Fixed cycle
Fixed cycles refer to fixed subroutines created by the system, which adapt to different machining requirements through various parameters. They are mainly used to perform typical machining actions that need to be repeated multiple times, such as machining various holes, internal and external threads, grooves, etc. Using fixed cycles can effectively simplify program development.
4. Tool compensation
It generally includes functions such as tool radius compensation, tool length compensation, and tool spatial position compensation.
(1) Tool radius compensation - planar contour machining
If the tool radius compensation function is programmed directly according to the workpiece contour, the actual contour will be larger than the tool radius value when machining the inner contour, and smaller than the tool radius value when machining the outer contour. Using tool radius compensation, the CNC system automatically calculates the tool center trajectory, causing the tool center to deviate from the workpiece contour by one tool radius value, thus machining a contour that meets the drawing requirements. By changing the tool radius compensation amount, the tool wear and machining errors can also be compensated for, enabling both roughing and finishing of the workpiece.
(2) Tool length compensation - setting tool length
The tool length compensation function changes the amount of compensation for the tool length, which can compensate for the length deviation after tool change, and can also change the plane position of the cutting process to control the axial positioning accuracy of the tool.
(3) Tool space position compensation - surface machining
II. Classification of CNC Milling Machines
(I) Classification by Process Application
1. Ordinary CNC machine tools
Ordinary CNC machine tools generally refer to automated machine tools that achieve digital control at one stage of the machining process, such as CNC milling machines, CNC lathes, CNC drilling machines, CNC grinding machines, and CNC gear machining machines. Ordinary CNC machine tools are not yet fully automated; tool changes and part clamping still require manual operation.
2. Machining Center
A machining center is a CNC machine tool equipped with a tool magazine and an automatic tool changer. It combines the functions of a CNC milling machine, a CNC boring machine, and a CNC drilling machine. After a part is clamped once, most of its machining surfaces can be milled.
(II) Classification of Exercise Modes
1. Point-to-point control CNC machine tools: The CNC system only controls the precise position of the tool from one point to another, without controlling the motion trajectory. The movements between the coordinate axes are independent, and no workpiece is machined during the movement. This type of CNC machine tool mainly includes CNC drilling machines, CNC coordinate boring machines, and CNC punching machines.
2. Linear Control CNC Machine Tools: In addition to controlling the precise position between control points, the CNC system must also ensure that the movement trajectory between two points is a straight line, and the movement speed must also be controlled; this is also called point-to-point linear control. This type of CNC machine tool mainly includes relatively simple CNC lathes, CNC milling machines, and CNC grinding machines. CNC machine tools solely for linear control are now rare.
3. Contour Control CNC Machine Tools: The characteristic of contour control is its ability to simultaneously and continuously control the displacement and velocity of two or more motion coordinates. It not only controls the starting and ending coordinates of the machine tool's moving parts, but also the velocity, direction, and displacement of every point throughout the entire machining process; it is also known as a continuous control CNC machine tool. This type of CNC machine tool mainly includes CNC lathes, CNC milling machines, CNC wire cutting machines, and machining centers.
(III) Classification of Control Methods
1. Open-loop control CNC machine tool
These types of machine tools do not have position detection feedback devices and typically use stepper motors as actuators. The input data is processed by the CNC system, which issues pulse commands to make the stepper motor rotate through a step angle. This is then converted into linear movement of the worktable through a mechanical transmission mechanism. The speed and displacement of the moving parts are determined by the frequency and number of input pulses.
2. Semi-closed-loop control CNC machine tools
Detection elements (such as inductive synchronizers or photoelectric encoders) are installed at the ends of the motor or lead screw. The displacement of the moving parts is indirectly detected by detecting their rotation angle, and then fed back to the CNC system. Since most of the mechanical transmission links are not included in the closed-loop circuit of the system, relatively stable control characteristics can be obtained. Although its control accuracy is not as high as that of closed-loop control CNC machine tools, it is easier to debug and is therefore widely used.
3. Closed-loop control CNC machine tools
These CNC machine tools are equipped with a position detection feedback device, which uses a linear displacement detection element that is directly installed on the moving parts of the machine tool. The measurement results are directly fed back to the CNC device. Through feedback, transmission errors in the entire mechanical transmission chain from the motor to the moving parts of the machine tool can be eliminated, ultimately achieving precise positioning.
