Fundamental Control Principles of CNC Machining

The CNC Machining Process

After a CNC programmer completes a part program, the program can be input into the CNC unit through various methods including MDI input, input device input, and communication input. This program is stored in the part program storage area of the numerical control device. When looking for a reliable cnc milling machine for sale, understanding this fundamental process is essential for making an informed decision.

During machining, the operator can use menu commands to transfer the required part program to the machining buffer. When the CNC device receives the "cycle start" command from the machine tool control panel, it automatically processes the part program in the machining buffer (including motion trajectory processing, machine tool I/O processing, etc.), and outputs control commands to the corresponding execution components (servo units, drive devices, and PLC, etc.) to machine parts that meet the requirements of the drawings. Many manufacturers offering a cnc milling machine for sale provide comprehensive training on these operational procedures.

This automated process is what distinguishes CNC machining from conventional methods, providing unparalleled precision and repeatability. Whether you're a small workshop or a large manufacturing facility, investing in a quality cnc milling machine for sale can significantly enhance production capabilities.

1. Part Program Decoding

The part program is the original basis for CNC machining, containing contour information, process information, and auxiliary information of the part to be machined. However, the information expressed by these artificially specified codes cannot be recognized by the CNC device and must be translated and interpreted by a decoding program. When evaluating a cnc milling machine for sale, it's important to understand how its decoding system handles complex part programs.

The decoding process converts the alphanumeric G-codes and M-codes into a format that the CNC system can understand and execute. This translation is crucial because it bridges the gap between human-readable programming language and machine-executable commands. Modern cnc milling machine for sale options often feature advanced decoding systems that can handle complex programs with minimal processing time.

Effective decoding ensures that the machine interprets dimensions, feeds, speeds, and tool changes exactly as intended by the programmer. This precision is why manufacturers across industries seek out a reliable cnc milling machine for sale that offers robust decoding capabilities to maintain production accuracy.

3. Tool Radius Compensation Principles

3.1 Basic Concept of Tool Radius Compensation

The control point in a CNC machine during machining is the center of the tool, while the part program is usually prepared according to the part contour. In this case, the CNC system must convert the part contour into the tool center trajectory before machining can proceed. This process of converting the part contour into the tool center trajectory is called tool radius compensation. When searching for a cnc milling machine for sale, understanding its tool compensation capabilities is crucial for achieving precise results.

The CNC system allows programmers to program directly according to the tool center trajectory, in which case no tool radius compensation is needed. Tool radius compensation, as shown in Figure 3-4, means that when machining a workpiece with a tool of radius R, the tool center trajectory should be offset by a distance R from the programmed trajectory A, resulting in trajectory B. The tool center offset is called the tool radius compensation value. Therefore, the task of tool radius compensation is to obtain the tool center trajectory after adding the compensation value. Many manufacturers highlight their advanced compensation systems when marketing a cnc milling machine for sale.

3.2.3 Tool Length Compensation

Depending on the machining situation, sometimes not only tool radius compensation but also tool length compensation is required. This is particularly important when multiple tools are used in a single machining operation. When investing in a cnc milling machine for sale, ensuring it has robust length compensation capabilities can significantly improve production efficiency and accuracy.

Milling Cutter Length Compensation

The length compensation of a milling cutter is related to the control point. For zero-length tools that use the tip of a standard tool as the control point, generally no length compensation is needed. If non-standard tools with different lengths are used during machining, tool length compensation is required. The length compensation value is equal to the length difference between the tool used and the standard tool. Modern cnc milling machine for sale options typically offer easy-to-use interfaces for setting these compensation values.

When the intersection of the measurement reference plane of the tool length and the tool axis is used as the control point, the milling cutter length compensation always exists. Regardless of which tool is used, absolute length compensation of the tool must be performed to machine the correct part surface. In addition, after a milling cutter has been used for a period of time, its length becomes shorter due to wear, which also requires length compensation. This feature is particularly valuable in high-production environments, making it a key consideration when evaluating a cnc milling machine for sale.

Tool length compensation is implemented for the coordinate axis perpendicular to the main plane. For example, when programming in G17, the main plane is the OXY plane, so tool length compensation is implemented for the Z-axis. Tool length compensation is specified by G43 and G44 commands to indicate the offset direction, where G43 is the positive offset command and G44 is the negative offset command.

G43 and G44 are followed by the H command to indicate the offset number. The decoding program retrieves the corresponding offset value from the offset memory according to the offset number and stores it in the decoding buffer c_length for use by the tool length compensation program. During tool length compensation, the offset value is read from the decoding buffer c_length and compensation processing is performed. For G43, a plus sign is used, and for G44, a minus sign is used. The compensated coordinate value is then sent to the subsequent interpolation program. Tool length compensation is canceled with the G49 command or H00. When comparing different cnc milling machine for sale options, the ease of setting and adjusting these parameters is an important factor.

4. Acceleration and Deceleration Control

To ensure that the machine tool does not produce shocks, step losses, overtravel, or oscillations when starting, stopping, or when there is a sudden change in speed, the CNC system must control the feed speed command sent to the servo motor. When the machine starts and the feed speed increases significantly, the feed speed command value applied to the servo motor is controlled to increase gradually. When the machine stops or the feed speed decreases significantly, the feed speed command value applied to the servo motor is controlled to decrease gradually. This sophisticated control is a key feature to look for when considering a cnc milling machine for sale.

This software-implemented acceleration and deceleration control can be performed before interpolation, which is called pre-interpolation acceleration and deceleration control, or after interpolation, which is called post-interpolation acceleration and deceleration control. Each method has its advantages, and the choice often depends on the specific application requirements. High-end cnc milling machine for sale options typically offer configurable acceleration parameters to optimize performance for different materials and operations.

Pre-interpolation control adjusts the feedrate before the interpolation process, which is more efficient for complex contours. Post-interpolation control, on the other hand, adjusts the already calculated axis movements, providing more precise control over the actual motor movements. When selecting a cnc milling machine for sale, understanding which control method it uses can help match the machine to your specific machining needs.

5. Interpolation Principles

5.1 Interpolation Overview

After the part program undergoes interpolation preprocessing (decoding, tool compensation, speed planning), interpolation follows. Interpolation preprocessing prepares the tool center speed, trajectory shape information, and related parameters needed to describe the trajectory shape, such as: the start and end coordinates of a line and the feed rate; the start, end, and center coordinates of an arc, the feed rate, and the clockwise or counterclockwise arc interpolation command (G02/G03). This processing power is a critical factor when evaluating a cnc milling machine for sale.

The task of interpolation is to use a simple and fast algorithm to insert intermediate points of the contour trajectory between the start and end points of the given contour trajectory according to the curve equation and feed rate of the given contour trajectory, so as to achieve precise trajectory control, as shown in Figure 3-26. The quality of the interpolation algorithm directly affects the machining precision and surface quality, making it a key consideration when selecting a cnc milling machine for sale.

There are several types of interpolation algorithms used in CNC systems, including linear interpolation, circular interpolation, and parametric interpolation for complex curves. Linear interpolation is used for straight-line movements between two points, while circular interpolation handles arcs defined by start, end, and center points. Advanced cnc milling machine for sale models often include sophisticated interpolation algorithms for complex 3D contours.

The interpolation process runs in real-time, calculating each intermediate point at a rate that matches the machine's servo update frequency. This ensures smooth motion and precise positioning. When comparing different cnc milling machine for sale options, the interpolation accuracy and processing speed are important performance indicators that directly impact manufacturing capabilities.

6. Error Compensation Principles

In CNC machining, the input of instructions, decoding, calculation, and motor motion control are all controlled by the numerical control device, with no human error involved. However, there are some mechanical errors in the mechanical transmission mechanism. Compared with eliminating errors from the perspective of transmission mechanism manufacturing, error compensation by the numerical control device has the advantages of flexibility, convenience, easy implementation, and good economy. This section introduces the principles and methods of backlash compensation and pitch error compensation in mechanical transmission mechanisms. When investing in a cnc milling machine for sale, understanding its error compensation capabilities is essential for achieving the highest precision.

Backlash compensation addresses the play between mating components in the transmission system, such as between gears or leadscrews and nuts. The CNC system can be programmed to account for this backlash by slightly overshooting and then correcting, ensuring accurate positioning. Modern cnc milling machine for sale options often feature automatic backlash detection and compensation systems.

Pitch error compensation corrects for inconsistencies in the leadscrew's thread spacing, which can accumulate over long travels. This is typically achieved through a compensation table stored in the CNC system that adjusts the position command based on the axis position. When evaluating a cnc milling machine for sale, inquire about the resolution and range of its pitch error compensation system.

Advanced error compensation systems may also include thermal error compensation, which accounts for dimensional changes in machine components due to temperature variations. This is particularly important for high-precision applications or in environments where temperature control is challenging. Many premium cnc milling machine for sale models offer comprehensive thermal error compensation as a standard or optional feature.