Numerical Control Technology and CNC Machine Tools
Numerical control technology, abbreviated as NC (Numerical Control), is a means of controlling mechanical movements and processing procedures with the aid of digital information. Currently, as modern numerical control commonly adopts computer control, it is also known as computerized numerical control (Computerized Numerical Control – CNC).
To achieve digital information control of mechanical movements and processing processes, corresponding hardware and software must be equipped. The sum of the hardware and software used to implement digital information control is called the numerical control system (Numerical Control System), and the core of the numerical control system is the numerical control device (Numerical Controller).
Machines controlled by numerical control technology are called CNC machine tools (NC machine tools). This is a typical mechatronic product that comprehensively integrates advanced technologies such as computer technology, automatic control technology, precision measurement technology, and machine tool design. It is the cornerstone of modern manufacturing technology. Controlling machine tools is the earliest and most widely applied field of numerical control technology. Therefore, the level of CNC machine tools largely represents the performance, level, and development trend of current numerical control technology.
There are various types of CNC machine tools, including drilling, milling, and boring machine tools, turning machine tools, grinding machine tools, electrical discharge machining machine tools, forging machine tools, laser processing machine tools, and other special-purpose CNC machine tools with specific uses. Any machine tool controlled by numerical control technology is classified as an NC machine tool.
Those CNC machine tools equipped with an automatic tool changer ATC (Automatic Tool Changer – ATC), except for CNC lathes with rotary tool holders, are defined as machining centers (Machine Center – MC). Through the automatic replacement of tools, workpieces can complete multiple processing procedures in a single clamping, achieving the concentration of processes and the combination of processes. This effectively shortens the auxiliary processing time and improves the working efficiency of the machine tool. Simultaneously, it reduces the number of workpiece installations and positioning, enhancing the processing accuracy. Machining centers are currently the type of CNC machine tools with the largest output and the widest application.
Based on CNC machine tools, by adding multi-worktable (pallet) automatic exchange devices (Auto Pallet Changer – APC) and other related devices, the resulting processing unit is called a flexible manufacturing cell (Flexible Manufacturing Cell – FMC). FMC not only realizes the concentration of processes and the combination of processes but also, with the automatic exchange of worktables (pallets) and relatively complete automatic monitoring and control functions, can perform unmanned processing for a certain period, thereby further improving the processing efficiency of the equipment. FMC is not only the basis of the flexible manufacturing system FMS (Flexible Manufacturing System) but can also be used as an independent automated processing equipment. Therefore, its development speed is quite rapid.
On the basis of FMC and machining centers, by adding logistics systems, industrial robots, and related equipment, and controlled and managed by a central control system in a centralized and unified manner, such a manufacturing system is called a flexible manufacturing system FMS (Flexible Manufacturing System). FMS can not only perform unmanned processing for long periods but also achieve the complete processing of various types of parts and component assembly, achieving the automation of the workshop manufacturing process. It is a highly automated advanced manufacturing system.
With the continuous progress of science and technology, in order to adapt to the changing situation of market demand, for modern manufacturing, it is not only necessary to promote the automation of the workshop manufacturing process but also to achieve comprehensive automation from market forecasting, production decision-making, product design, product manufacturing to product sales. The complete production and manufacturing system formed by integrating these requirements is called a computer-integrated manufacturing system (Computer Integrated Manufacturing System – CIMS). CIMS organically integrates a longer production and business activity, achieving more efficient and more flexible intelligent production, representing the highest stage of the development of today’s automated manufacturing technology. In CIMS, not only is the integration of production equipment, but more importantly, the technology integration and function integration characterized by information. The computer is the integration tool, the computer-aided automated unit technology is the basis of integration, and the exchange and sharing of information and data is the bridge of integration. The final product can be regarded as the material manifestation of information and data.
The Numerical Control System and Its Components
The Basic Components of the Numerical Control System
The numerical control system of a CNC machine tool is the core of all numerical control equipment. The main control object of the numerical control system is the displacement of the coordinate axes (including movement speed, direction, position, etc.), and its control information mainly comes from numerical control processing or motion control programs. Therefore, the most basic components of the numerical control system should include: the program input/output device, the numerical control device, and the servo drive.
The role of the input/output device is to input and output data such as numerical control processing or motion control programs, processing and control data, machine tool parameters, coordinate axis positions, and the status of detection switches. Keyboard and display are the most basic input/output devices necessary for any numerical control equipment. In addition, depending on the numerical control system, devices such as photoelectric readers, tape drives, or floppy disk drives can also be equipped. As a peripheral device, the computer is currently one of the commonly used input/output devices.
The numerical control device is the core component of the numerical control system. It consists of input/output interface circuits, controllers, arithmetic units, and memory. The role of the numerical control device is to compile, calculate, and process the data input by the input device through the internal logic circuit or control software, and output various types of information and instructions to control the various parts of the machine tool to perform specified actions.
Among these control information and instructions, the most basic ones are the feed speed, feed direction, and feed displacement instructions of the coordinate axes. They are generated after interpolation calculations, provided to the servo drive, amplified by the driver, and ultimately control the displacement of the coordinate axes. This directly determines the movement trajectory of the tool or coordinate axes.
In addition, depending on the system and equipment, for example, on a CNC machine tool, there may also be instructions such as the rotational speed, direction, start/stop of the spindle; tool selection and exchange instructions; start/stop instructions of cooling and lubrication devices; workpiece loosening and clamping instructions; indexing of the worktable and other auxiliary instructions. In the numerical control system, they are provided to the external auxiliary control device in the form of signals through the interface. The auxiliary control device performs the necessary compilation and logical operations on the above signals, amplifies them, and drives the corresponding actuators to drive the mechanical components, hydraulic, and pneumatic auxiliary devices of the machine tool to complete the actions specified by the instructions.
The servo drive usually consists of servo amplifiers (also known as drivers, servo units) and actuators. On CNC machine tools, AC servo motors are generally used as actuators at present; on advanced high-speed machining machine tools, linear motors have begun to be used. Additionally, on CNC machine tools produced before the 1980s, there were cases of using DC servo motors; for simple CNC machine tools, stepper motors were also used as actuators. The form of the servo amplifier depends on the actuator and must be used in conjunction with the drive motor.
The above are the most basic components of the numerical control system. With the continuous development of numerical control technology and the improvement of machine tool performance levels, the functional requirements for the system are also increasing. To meet the control requirements of different machine tools, ensure the integrity and uniformity of the numerical control system, and facilitate user use, commonly used advanced numerical control systems usually have an internal programmable controller as the auxiliary control device of the machine tool. In addition, on metal cutting machine tools, the spindle drive device can also become a component of the numerical control system; on closed-loop CNC machine tools, measurement and detection devices are also indispensable to the numerical control system. For advanced numerical control systems, sometimes even a computer is used as the human-machine interface of the system and for data management and input/output devices, thereby making the functions of the numerical control system more powerful and the performance more perfect.
In conclusion, the composition of the numerical control system depends on the performance of the control system and the specific control requirements of the equipment. There are significant differences in its configuration and composition. In addition to the three most basic components of the input/output device of the processing program, the numerical control device, and the servo drive, there may be more control devices. The dashed box part in Figure 1-1 represents the computer numerical control system.
The Concepts of NC, CNC, SV, and PLC
NC (CNC), SV, and PLC (PC, PMC) are very commonly used English abbreviations in numerical control equipment and have different meanings in different occasions in practical applications.
NC (CNC): NC and CNC are the common English abbreviations of Numerical Control and Computerized Numerical Control, respectively. Given that modern numerical control all adopts computer control, it can be considered that the meanings of NC and CNC are completely the same. In engineering applications, depending on the usage occasion, NC (CNC) usually has three different meanings: In a broad sense, it represents a control technology – numerical control technology; in a narrow sense, it represents an entity of a control system – the numerical control system; in addition, it can also represent a specific control device – the numerical control device.
SV: SV is the common English abbreviation of servo drive (Servo Drive, abbreviated as servo). According to the prescribed terms of the Japanese JIS standard, it is “a control mechanism that takes the position, direction, and state of an object as control quantities and tracks arbitrary changes in the target value.” In short, it is a control device that can automatically follow physical quantities such as the target position.
On CNC machine tools, the role of servo drive is mainly reflected in two aspects: First, it enables the coordinate axes to operate at the speed given by the numerical control device; second, it enables the coordinate axes to be positioned according to the position given by the numerical control device.
The control objects of servo drive are usually the displacement and speed of the coordinate axes of the machine tool; the actuator is a servo motor; the part that controls and amplifies the input command signal is often called a servo amplifier (also known as a driver, amplifier, servo unit, etc.), which is the core of the servo drive.
The servo drive can not only be used in conjunction with the numerical control device but also can be used alone as a position (speed) accompanying system. Therefore, it is also often called a servo system. On early numerical control systems, the position control part was generally integrated with CNC, and the servo drive only performed speed control. Therefore, the servo drive was often called a speed control unit.
PLC: PC is the English abbreviation of Programmable Controller. With the increasing popularity of personal computers, to avoid confusion with personal computers (also called PCs), programmable controllers are now generally called programmable logic controllers (Programmalbe Logic Controller – PLC) or programmable machine controllers (Programmable Machine Controller – PMC). Therefore, on CNC machine tools, PC, PLC, and PMC have exactly the same meaning.
PLC has the advantages of rapid response, reliable performance, convenient use, easy programming and debugging, and can directly drive some machine tool electrical appliances. Therefore, it is widely used as an auxiliary control device for numerical control equipment. Currently, most numerical control systems have an internal PLC for processing the auxiliary instructions of CNC machine tools, thereby greatly simplifying the auxiliary control device of the machine tool. In addition, in many occasions, through special functional modules such as the axis control module and positioning module of the PLC, the PLC can also be directly utilized to achieve point position control, linear control, and simple contour control, forming special CNC machine tools or CNC production lines.
The Composition and Processing Principle of CNC Machine Tools
The Basic Composition of CNC Machine Tools
CNC machine tools are the most typical numerical control equipment. To clarify the basic composition of CNC machine tools, it is first necessary to analyze the working process of CNC machine tools for processing parts. On CNC machine tools, to process parts, the following steps can be implemented:
According to the drawings and process plans of the parts to be processed, using the prescribed codes and program formats, write the movement trajectory of the tools, the processing process, process parameters, cutting parameters, etc. into the instruction form recognizable by the numerical control system, that is, write the processing program.
Input the written processing program into the numerical control device.
The numerical control device decodes and processes the input program (code) and sends corresponding control signals to the servo drive devices and auxiliary function control devices of each coordinate axis to control the movement of each component of the machine tool.
During the movement, the numerical control system needs to detect the position of the coordinate axes of the machine tool, the status of the travel switches, etc. at any time, and compare them with the requirements of the program to determine the next action until qualified parts are processed.
The operator can observe and inspect the processing conditions and working status of the machine tool at any time. If necessary, adjustments to the machine tool actions and processing programs are also required to ensure the safe and reliable operation of the machine tool.
It can be seen that as the basic composition of a CNC machine tool, it should include: input/output devices, numerical control devices, servo drives and feedback devices, auxiliary control devices, and the machine tool body.
The Composition of CNC Machine Tools
The numerical control system is used to achieve processing control of the machine tool host. Currently, most of the numerical control systems adopt computer numerical control (i.e., CNC). The input/output device, numerical control device, servo drive, and feedback device in the figure together constitute the machine tool numerical control system, and its role has been described above. The following briefly introduces other components.
Measurement feedback device: It is the detection link of a closed-loop (semi-closed-loop) CNC machine tool. Its role is to detect the speed and displacement of the actual displacement of the actuator (such as the tool holder) or the worktable through modern measurement elements such as pulse encoders, resolvers, induction synchronizers, gratings, magnetic scales, and laser measuring instruments, and feed them back to the servo drive device or the numerical control device, and compensate for the feed speed or the motion error of the actuator to achieve the purpose of improving the accuracy of the motion mechanism. The installation position of the detection device and the position where the detection signal is fed back depend on the structure of the numerical control system. Servo built-in pulse encoders, tachometers, and linear gratings are commonly used detection components.
Due to the fact that advanced servos all adopt digital servo drive technology (referred to as digital servo), a bus is usually used for connection between the servo drive and the numerical control device; in most cases, the feedback signal is connected to the servo drive and transmitted to the numerical control device through the bus. Only in a few occasions or when using analog servo drives (commonly known as analog servo), the feedback device needs to be directly connected to the numerical control device.
Auxiliary control mechanism and feed transmission mechanism: It is located between the numerical control device and the mechanical and hydraulic components of the machine tool. Its main role is to receive the spindle speed, direction, and start/stop instructions output by the numerical control device; tool selection and exchange instructions; start/stop instructions of cooling and lubrication devices; auxiliary instruction signals such as loosening and clamping of workpieces and machine tool components, indexing of the worktable, and the status signals of detection switches on the machine tool. After necessary compilation, logical judgment, and power amplification, the corresponding actuators are directly driven to drive the mechanical components, hydraulic, and pneumatic auxiliary devices of the machine tool to complete the actions specified by the instructions. It is usually composed of PLC and a strong current control circuit. The PLC can be integrated with the CNC in structure (built-in PLC) or relatively independent (external PLC).
The machine tool body, that is, the mechanical structure of the CNC machine tool, is also composed of main drive systems, feed drive systems, beds, worktables, auxiliary motion devices, hydraulic and pneumatic systems, lubrication systems, cooling devices, chip removal, protection systems, and other parts. However, to meet the requirements of numerical control and give full play to the performance of the machine tool, it has undergone significant changes in terms of overall layout, appearance design, transmission system structure, tool system, and operating performance. The mechanical components of the machine tool include the bed, box, column, guide rail, worktable, spindle, feed mechanism, tool exchange mechanism, etc.
The Principle of CNC Machining
On traditional metal cutting machine tools, when processing parts, the operator needs to continuously change parameters such as the movement trajectory and movement speed of the tool according to the requirements of the drawing, so that the tool performs cutting processing on the workpiece and finally processes qualified parts.
The processing of CNC machine tools essentially applies the “differential” principle. Its working principle and process can be briefly described as follows:
According to the tool trajectory required by the processing program, the numerical control device differentiates the trajectory along the corresponding coordinate axes of the machine tool with the minimum movement amount (pulse equivalent) (△X, △Y in Figure 1-2) and calculates the number of pulses that each coordinate axis needs to move.
Through the “interpolation” software or “interpolation” calculator of the numerical control device, the required trajectory is fitted with an equivalent polyline in units of “minimum movement unit” and the fitted polyline closest to the theoretical trajectory is found.
According to the trajectory of the fitted polyline, the numerical control device continuously allocates feed pulses to the corresponding coordinate axes and enables the coordinate axes of the machine tool to move according to the allocated pulses through servo drive.
It can be seen that: First, as long as the minimum movement amount (pulse equivalent) of the CNC machine tool is small enough, the fitted polyline used can be equivalently substituted for the theoretical curve. Second, as long as the pulse allocation method of the coordinate axes is changed, the shape of the fitted polyline can be changed, thereby achieving the purpose of changing the processing trajectory. Third, as long as the frequency of…