What is CNC Machining Technology?

1. Basic concepts of CNC technology

CNC technology refers to the use of digital instructions to achieve the operation of one or more mechanical equipment control technology, CNC technology is usually involved in the control of mechanical quantities such as position, angle, speed, and the flow of mechanical energy related to the switch quantity. CNC machine tools is the use of CNC technology machine tools, is a set of machinery manufacturing, computers, automatic control, electric motors and drag, power electronics, sensors, machine electrical and PLC, gas and hydraulic and network communications technology as one of the automated metal cutting equipment, is a typical mechanical and electrical products. Machine tool numerical control technology includes the machine tool body, numerical control system and peripheral technology, as shown in the below picture.

CNC control device is the core of CNC technology, is a special computer for CNC machine control, the operation of CNC machine tools to undertake the data input, processing and calculation, control signal input and output, as well as network communication.

2. Types of CNC machines

a. CNC lathe

CNC lathe is mainly used to process various rotary surfaces of shaft, sleeve and disk parts, such as internal and external cylindrical surface, tapered surface, shaped rotary surface and thread, etc. According to the configuration of the spindle, there are horizontal CNC lathes (spindle axis in horizontal position) and vertical CNC lathe (spindle axis in vertical position); according to the different types of processing, there are several types such as common type, universal type and multifunctional type. The CNC system of common type CNC lathe is simple in function and can complete the conventional turning processing, the bed structure of lathe adopts horizontal bed, the spindle speed adopts manual mechanical gear shift or automatic gear shift with electrical stepless speed control, the automatic tool changer adopts four-sided electric tool holder, and the spindle chuck adopts manual chuck or hydraulic chuck. The CNC system of general CNC lathe is similar to that of common type, the b e d adopts inclined bed structure, the spindle adopts electrical stepless speed control, the automatic tool changer adopts hydraulic or electric turret tool holder of 6, 8 and 12 stations, the main Axis chuck using hydraulic chuck.

 

The multifunctional CNC lathe is developed on the popular CNC lathe, and its CNC system is more powerful, the lathe spindle has directional control and feed interpolation control, and the automatic tool changer is a turret tool holder equipped with a power head. In addition to conventional turning processing, the multifunctional CNC lathe can also perform special processing such as milling groove on cylindrical surface or end face, milling spiral groove on cylindrical surface, end face drilling and tapping thread.

b. CNC Milling Machines and CNC Machining Centers

CNC milling machines have the ability to machine complex planes and curved contours in addition to the technological capabilities of ordinary milling machines. CNC milling machine has vertical CNC milling machine, horizontal CNC milling machine and gantry milling machine and other types. Vertical CNC milling machine spindle axis perpendicular to the horizontal, suitable for the processing of discs, covers and molds and other parts.

Horizontal CNC milling and boring machine spindle axis parallel to the horizontal, in order to expand the processing range, by increasing the CNC rotary table so that the workpiece can be a clamping to complete the processing of all sides, especially suitable for processing complex boxes, pumps, valves and shells and other parts.

The CNC gantry milling machine is a milling machine with a door frame and a long horizontal bed, which is suitable for milling large parts.

 

CNC Machining center is developed on the basis of CNC milling machine, with tool magazine and automatic tool changer multifunctional CNC machine tools, which milling, boring, drilling, tapping and thread cutting and other functions in one machine tool. Workpiece in the machining center after a clamping, through automatic tool selection and tool change, in turn to complete multiple processes of processing, greatly improving productivity. CNC Machining centers are vertical machining centers like the picture below.

Horizontal machining centers (like the picture below) and gantry machining centers and other types, equipped with tool magazines are bucket type, disc type and chain type and other forms.

c.  CNC Grinder

A CNC grinder is a machine tool that uses abrasive tools to grind the surface of a workpiece. Most CNC grinders use high-speed rotating grinding wheels to perform grinding operations. CNC grinders are available as CNC surface grinders, CNC cylindrical grinders, CNC internal grinders, and CNC profile grinders.

 

 

d. CNC EDM and WEDM

The CNC EDM is a machine tool that uses two electrodes of different polarities to produce an electrical discharge in an insulating liquid to remove the material, and is particularly suitable for the machining of complex shapes and difficult materials.

EDM wire cutting process is referred to as “wire cutting”. It uses electrode wire (molybdenum wire, tungsten-molybdenum wire, etc.) as the tool electrode. On the one hand, under the action of the pulse power supply, a spark discharge is formed between the tool electrode and the workpiece, which generates a large amount of heat instantaneously, causing the surface of the workpiece to melt or even vaporize; on the other hand, the electrode wire moves along a predetermined trajectory, thus completing the cutting of the part contour, CNC EDM and WEDM machines are widely used in the field of mold processing.

e. CNC turret punch machines and CNC bending machines

CNC turret punch machines are used for the processing of various types of sheet metal parts, through a simple combination of punching die can automatically complete a variety of complex hole patterns and shallow stretch forming processing, but also with a small punching die to punch large round holes, square holes, waist-shaped holes and various shapes of holes in a stepwise manner.

CNC bending machine is a kind of processing equipment for bending and forming sheet metal by means of concave and convex die.

3. Machining characteristics of CNC machines

Compared with ordinary machine tools, CNC machine tools process parts automatically according to the machining program, taking CNC lathes as an example, the process of machining parts is shown below.

The programmer prepares the machining process (machine selection, tool selection, fixture selection, toolpath planning and numerical calculation, and cutting amount determination) and writes the machining program based on the part drawing. The operator enters the machining program into the CNC and executes it after clamping the workpiece, adjusting the tool and setting the tool. CNC system for processing data after the program, the output control signal control machine parts action (such as spindle start, stop and spindle speed, the tool relative to the workpiece feed trajectory, automatic tool change, cutting fluid on, off, etc.), for automatic cutting processing. The basic features of CNC machine tool processing are.

• The process is carried out without excessive human intervention, minimizing human influence on machining accuracy.
• The CNC machine tool itself has high mechanical and control accuracy, and each step of the machining process is performed in strict accordance with the machining program to ensure the machining accuracy of the part and the consistency of the accuracy maintenance.
• CNC machine tools with a high degree of automation, such as automatic tool change, automatic tooling, etc., effectively reducing the non-cutting time, high machining efficiency.

4. Basic control of CNC machines

Although there are various types of CNC machines, t h e mechanical drive mechanism, electrical control and basic CNC commands of the machines are similar. Take the vertical machining center shown in Figure 0- 3d as an example, its mechanical structure and electrical control cabinet as shown below.

The mechanical movements of a CNC machine are controlled electrically and driven, including the control of main movements, feed movements and auxiliary functions, as shown in Figure 0-11.

a. Control of the main movement

The mechanical drive of main motion includes spindle components and drive, such as belt drive, gear drive, etc.; the electrical drive of spindle includes spindle motor and spindle driver, etc. The CNC system processes the data of the input M03, M04, M05 and S commands and outputs control signals to the spindle drive, which realizes the basic control of the spindle motor such as start, stop and speed control. The basic functions of spindle control include spindle forward and reverse rotation and spindle speed control. The drive of the spindle motor is realized by the spindle drive. The basic requirement for spindle control is to keep the spindle speed stable. For this purpose, an encoder is installed on the spindle motor or spindle for speed detection and feedback. Special functions of the spindle control include spindle quasi-stop, positioning control, spindle gear oscillation control during gear changes, thread cutting, rigid tapping and synchronization control. In order to ensure the accuracy of the position control, the spindle is equipped with an encoder to detect and feedback the spindle position and speed. With the development of high-speed CNC machine tools, some CNC machine tool spindle using the electric spindle technology. The electromechanical technology involved in spindle control includes spindle construction and spindle mechanical drives, spindle motors and drives, as well as spindle speed and position detection.

b. Control of feed motion

Feed control includes point control and trajectory control (also known as contour control), as shown in below.

• Point control The machining center drilling, reaming, reaming, tapping and boring are all point control. In point control, the tool does not cut during the rapid movement from the current position to the next position, and the movement path can be set artificially, butthe position accuracy between points must be guaranteed.
• Trajectory control CNC milling machines, machining centers, CNC lathes and CNC grinding machines for contouring are trajectory controlled. In trajectory control, the tool cuts the part according to the specified path to ensure both the positional accuracy and the stability of the tool motion speed. The mechanical drive of the feed motion includes the ball screw nut and guide rail of each motion axis; the electrical drive of the feed includes the servo motor and servo driver. The CNC system processes the input G01, G02, G03, G00 commands, coordinate values and feed speed F commands, and then outputs control signals to the servo drive, which outputs power to the servo motor to drive the ball screw, so that the table can realize the feed motion, and finally make the tool move relative to the workpiece according to the trajectory and speed specified in the machining program to realize the cutting process. The core of the CNC system control of the feed motion is interpolation and position control. The control goal of the feed motion is position control and speed control, and to ensure the accuracy of the position control, feed speed stability and the rapidity of the command response. To achieve this goal, the CNC machine tool feed motion control has position detection and speed detection, such as grating and encoder. All the measures taken in the servo system, such as mechanical transmission for preload, reverse clearance adjustment and compensation, screw pitch accumulation error compensation, as well as the use of high-precision position detection device and servo adjustment optimization, etc., are to ensure the stability of the feed motion and position accuracy. With the development of high-speed CNC machine tools, some CNC machine tools using a linear motor drive, eliminating the ball screw, shorten the feed mechanical drive chain, improve the mechanical drive stiffness and accuracy, as well as the rapid response of the servo system. The electromechanical technology involved in feed motion control includes feed mechanical drive mechanisms, servo motors and drives, position and speed detection devices, servo system performance and adjustment, etc.

c. Control of auxiliary functions

The basic auxiliary functions of a CNC machine include cutting fluid control and lubrication control. Tool holders and tool changes in CNC lathes, hydraulic tailstock and hydraulic chucks, and tool selection and tool changes in machining centers are the most distinctive auxiliary functions. The auxiliary functions are performed by the PLC and peripheral control circuits of the CNC system.

5. Trends in CNC technology

a. Speeding up

Speed, accuracy and efficiency are the key indicators of mechanical cutting. High-speed cutting processing can not only increase productivity, but also improve machining quality. The high speed of CNC machine tools is demonstrated by.

• The CNC system is capable of high-speed processing of machining programs consisting of small program segments to calculate the rotation angle and speed of the servo motor, and requires the servo motor to respond quickly.
• Increased cutting and feed speeds, reduced auxiliary time. The machine uses an electric spindle with a maximum speed of 100000r/min or more; maximum feed rate of 200m/min at a resolution of 1μm using a linear motor. Advanced machining centers have tool change times of less than 1s.

b. High precision

Advanced CNC machine tool accuracy in addition to the conventional static geometric accuracy, including machine tool motion accuracy, t herm al deformation, as well as the detection of vibration and compensation. The high accuracy of CNC machine tools is demonstrated by.

• The CNC system adopts high-speed interpolation technology to improve the resolution, and adopts high-resolution position detection device to improve the position detection accuracy; in the position servo control, feed-forward control and non-linear control methods are used to improve the position control accuracy.
• The use of error compensation technology. CNC system through the control software to artificially generate a new error to offset the current original error, in order to reduce the machining error, improve the machining accuracy of the parts of the purpose. In addition to backlash compensation, screw pitch accumulation error compensation and tool error compensation and other technologies, but also on the machine’s thermal deformation for error compensation. Research results show that the comprehensive compensation can reduce 60% to 80% of the machining error.

c. High reliability

The key technical indicator of the reliability of a CNC system is the mean time without failure. The high reliability of a CNC system is demonstrated by.

• Improve the reliability of the CNC system hardware, and make the hardware function software, to adapt to the requirements of various control functions.
• The CNC system carries out fault diagnosis and alarm for system hardware, software and various external devices by running various diagnostic procedures such as start-up diagnosis, online diagnosis and offline diagnosis, and uses the alarms to eliminate faults in time.
• Fault-tolerant technology, “redundant” design of critical components for self-recovery from failure.
• Various testing and monitoring techniques are used to automatically protect against overtravel, knife damage, interference, power failure, and other accidents.

d. Multi-axis linkage and functional compounding

5-axis linkage machining center or CNC milling machine has become the hot spot of CNC machining. Ball milling cutter using 5-axis linkage control when processing free-form surfaces, ball milling cutter in the cutting process can always maintain a reasonable cutting speed, thereby significantly improving the quality of the machined surface, and significantly improve the processing efficiency. With the development of CNC technology, the number of control axes of the CNC system is also increasing, some up to 16 axes, while the number of linked axes has reached 6 axes. Composite machine tool is a machine tool to achieve from the blank to the finished product of a variety of elements of processing, such as by the milling and drilling composite machining center, by the turning and milling composite turning center, as well as by the milling, boring, drilling and turning composite composite machining center. The use of composite machine tools for processing, reducing the number of workpiece clamping and positioning errors, changing and adjusting the auxiliary time of the tool, improve the accuracy of parts processing, shorten the product manufacturing cycle, improve productivity and enterprise market responsiveness, compared to the traditional processing processes and dispersion of production methods have obvious advantages.

e. Application of industrial robots in CNC machining lines

In the field of automotive parts (such as shafts, discs, covers, etc.) processing, there is a large number of parts, complex parts and processing stations, the characteristics of multiple CNC machine tools need to work together. In the traditional machining process, the loading and unloading, handling of workpieces need to be completed manually, there is a problem of low productivity. The integration of CNC machine tools and industrial robots, CNC machining production line, with a high degree of automation, short production cycle, high flexibility characteristics. In the CNC machining production line, industrial robots are mainly used for the handling and loading and unloading of parts between CNC machine tools, and there are truss type, guide rail mobile type and fixed rotary type.

f. Intellectualize

The significance of intelligent CNC systems is that the visualization of the human-machine interface significantly improves work efficiency and ensures more comfortable and safe work. Intelligence is demonstrated by.

• Adaptive control technology for machining process. By detecting the cutting force, power, current and voltage of the spindle and servo motor during the machining process, the machine identifies the tool force, wear and stability of the machine, and adjusts the spindle speed and feed rate in real time according to these conditions, so that the machine is in the best operating condition to improve the machining accuracy, reduce the surface roughness of the machined surface, and improve the safety of the machine operation.
• Process expert system. Expert experience, the general and special laws of cutting and processing into the computer, supported by a database of processing parameters, the establishment of an expert system with artificial intelligence, providing optimized cutting parameters, so that the machine is always in the optimal and most economical cutting state, thereby improving the efficiency of programming and processing technology, shortening the production lead time.
• Intelligent spindle and servo drive technology. The spindle and servo system parameters are automatically optimized for optimal operation.
• Intelligent fault diagnosis technology. It is able to record all kinds of information of the system completely, reason about the fault through the expert knowledge base, find out the cause of the fault and give a solution to the problem. At the same time, the computer continuously updates the expert knowledge base through deep learning of fault diagnosis.

g. Networking

CNC system networking refers to the CNC system and other external control system or the upper computer for network communication. CNC system networking, so that CNC machine tools with twoway, high-speed networking communication function, to ensure that the flow of information between the various departments in the workshop unimpeded, both to achieve resource sharing, but also to achieve remote monitoring of CNC machine tools, control and management, etc., can also achieve digital services of CNC machine tools, such as remote diagnosis and maintenance of CNC machine tool failure.

h. Environmentally friendly

Machine tools not only consume energy during the cutting process, but also produce solid, liquid and gaseous waste, causing direct or indirect pollution t o the working environment and the natural environment. Greening includes.

• New structures and materials are used to reduce power losses in the moving parts, while maintaining rigidity and light weight.
• Dry cutting, no cutting fluid is used to reduce pollution emissions.
• The use of advanced tools, the selection of reasonable tool geometry and cutting parameters, effectively improve the cutting efficiency, reduce the power required in the cutting process, extend the tool life, so as to achieve the purpose of obtaining greater output with less resource consumption.