The CNC control unit is the control core of a CNC machine, usually consisting of a general-purpose or special microcomputer. The numerical control device is based on the input instructions for decoding, cnc machining, calculation and control to achieve its numerical control function.
Previously, the numerical control function of the numerical control equipment is to use the hardware structure of the special computer to achieve, so called hardware numerical control, referred to as NC. now mainly to the computer system control program to achieve part or all of the numerical control function, so called software numerical control or computer numerical control, referred to as CNC.
As the computer numerical control is a kind of numerical control system, plus now there is no hardware numerical control, so we now tube numerical control system collectively called CNC system.
- CNC is a recent development of an automatic control technology, in the field of machine tools refers to the use of digital signals on the machine tool movement and its control machining of a method. The definition of “machine tool” refers not only to metal cutting machine tools, but also includes other types of machine tools, such as wire-cutting machine tools, three-coordinate measuring machines, etc.
- The use of CNC technology control system called CNC system. Equipped with a CNC system controlled equipment called CNC equipment. Controlled equipment including machine tool industry, a variety of CNC machine tools and other industries, such as CNC flame cutting machine, EDM machine, CNC punching and shearing machine, CNC measuring machine, etc..
- Equipped with a CNC system called CNC machine tools, CNC machine tools contained in the CNC equipment. The fifth technical committee of the International Federation of Information Processing has defined CNC machines as follows: A CNC machine tool is a machine tool equipped with a program control system, which can logically process the program with the use of numbers, or other symbolic coding instructions specified.
- CNC control system is a control system, it can automatically read the input carrier on the pre-given digital values and instructions, and will be decoded, processing, and thus automatically control the machine tool to move and cnc machining parts.
The Components Of The CNC System
CNC machine tools are automatically controlled by the CNC system to process mechanical parts according to the given program. The CNC system is composed of user programs, input and output devices, computer numerical control devices (CNC devices), programmable logic controllers (PLC), spindle drives and feed drives, etc.
1. User program
It is a part processing program. According to the part drawing, the CNC machining program is prepared by manual programming or automatic programming and stored on an information carrier.
2. Input and output equipment
CNC system for automatic control of CNC equipment required for a variety of external control information and processing data, are through the input device into the CNC device memory. The input CNC device has parts processing programs, control parameters, compensation data, etc..
The main function of the output device is to display, print and output machining programs, control parameters, compensation parameters, etc.
3. CNC unit
The CNC unit consists of hardware and software.The hardware consists mainly of modules such as microprocessors, memory, position control, input/output interfaces, programmable controllers, graphic control, power supply, etc.
The software consists of management software and control software. The management software refers to the input and output of the part CNC Machining program, the display function of the system and the diagnostic function. The control software includes code machining, tool compensation, interpolation, position control and speed control.
4. Programmable controller (PLC)
The programmable controller for CNC equipment mainly completes the logical sequence control of the various actuators of the CNC equipment, i.e. instead of using relay control lines, the PLC program realises the auxiliary functions of the CNC equipment, the spindle speed function, the decoding and control of the tool function.
The PLC for CNC equipment has two types of built-in and independent type. Built-in PLC subordinate to the CNC device, PLC hardware circuit can be made with the CNC device other circuits on the same printed board, can also be made into an independent circuit board. Independent type PLC independent CNC device, itself has a complete hard and software functions, can be independent to complete the specified control tasks.
5. Servo System
The servo system includes two major parts: the drive part and the actuator. Servo system mainly refers to the spindle drive and feed drive of the control equipment, is the implementation part of the CNC system. Servo system is the role of a variety of instructions from the CNC device (pulse signal), into the movement of CNC equipment moving parts.
In the servo drive mechanism of CNC machine tools, commonly used drive components are power stepper motors, DC servo motors and AC servo motors, the latter two with induction synchronizers, encoders and other position detection components.
The main working process of CNC system
1. Preparation work before normal work
After the power is connected, the CNC device will check and diagnose the working status of the CNC system and the components of the CNC machine tool, and set the initial state.
2. Input of part control machining information
After the CNC system has normal working conditions, start inputting the part cnc machining program, tool length compensation values, tool radius compensation values and the coordinate value of the workpiece coordinate system origin relative to the machine origin.
3. CNC machining program decoding and pre-processing
After processing control information input, start processing operation, at this time the CNC device in the system control program, the CNC program for pre-processing, that is, for decoding and pre-calculation (tool complement calculation, coordinate transformation, etc.).
4. Interpolation calculation
After pre-processing the cnc machining information of a program segment, interpolation is carried out. Interpolation” means the densification of data points on a curve with a known start and end point. The task of interpolation is to calculate a number of intermediate points between the start and end points of a section of the part profile according to the feed rate required, and to issue a sequence of motion commands to each axis with a defined direction, size and speed.
5. Position control
The servo system of each axis uses the interpolation result as the command value for the position regulator of each axis and the displacement measured by the position detection element on the machine as the actual position value. The position regulator compares the two and, after adjustment, outputs the corresponding position and speed control signals to control the movement of the machine axes by the individual axis servo systems. Through the synthesis of the individual axis movements, the workpiece contour dimensions required by the CNC machining program are produced.
The characteristics of CNC machine tools
The advantages of CNC machine tools are more, mainly the following points:
- High machining accuracy and dimensional repeatability, good stability of machining quality
- Good flexibility, versatility
- Easy to achieve multi-functional, high complexity of the control
- High machining productivity
- Reduce labour intensity and improve working conditions
- High reliability and good economy
- Conducive to the development of advanced manufacturing systems
The shortcomings of CNC machine: large investment, repair and maintenance difficulties, as well as the technical level of technical personnel requirements are high.
The classification of CNC machine
A.According to the characteristics of the control system classification
1. Point control CNC machine
It is characterized by the process of moving the tool relative to the workpiece, not cutting process, no strict requirements for the positioning process of the trajectory, only to achieve from a coordinate point to another coordinate point of precise positioning.
Point control CNC machine tools are mainly: CNC coordinate boring machine, CNC drilling machine, CNC punching machine, etc.
2. Linear control CNC machine
Some CNC machine tools not only require the function of accurate positioning, but also require to move in a straight line from one point to another, and can control the speed of displacement. Because this type of CNC machine tools in the two points between the movement, to be cutting.
Straight line control CNC machine tools are mainly: CNC boring and milling machines, CNC lathes, machining centres, etc.. In general these CNC machine tools have two to three controllable axes, but at the same time control the number of axes only one
3. Contour control CNC machine tools
This type of CNC machine tool is characterized by the ability to control two or more axes at the same time, you can process the curve or surface of the part.
Contour control CNC machine tools are mainly CNC lathes, CNC milling machines, CNC grinding machines, machining centres, CNC electric machining machines, etc.
B.The Classification according to the control mode of servo system
1. Open-loop control system
This type of CNC machine tools without position detection feedback device. CNC device output command pulse by the drive circuit power amplification, drive stepper motor rotation, and then by the drive mechanism to drive the table movement, its control block diagram is shown below. Open-loop control of CNC machine tools work more stable, fast response, easy to debug, simple maintenance, but low control accuracy of these CNC machine tools are mostly economic.
2. Closed-loop control systems
This type of CNC machine tool with position detection feedback device. The position detection device is mounted on the machine table to detect the actual running position of the machine table and compare it with the commanded position of the CNC device, using the difference for control, the control block diagram is shown below.
Closed-loop control systems are characterised by high accuracy, speed, high technical requirements and high costs. The key is the stability of the system, which is prone to oscillation due to poor commissioning, so sufficient attention must be paid to stability during design.
Closed-loop control systems are mainly used in some high precision requirements of ultra-precision lathes, boring and milling machines, ultra-precision milling machines and precision machining centres, etc.
3. Semi-closed-loop control system
A semi-closed loop control system is a system in which the detection element is mounted on the motor or screw end to detect the angular displacement, the control block diagram of which is shown below. Since the closed-loop circuit does not include the screw, nut sub and table, a more stable control characteristic can be obtained. Its control accuracy is not as good as the closed-loop control system, but it is more convenient to debug, so it is widely used in small and medium-sized CNC machine tools.
C.According to the level of CNC device function classification
According to the CNC machine tool function level of CNC machine tools for classification, CNC machine tools can generally be divided into high, medium and low-grade three categories.
1. Resolution and feed rate
Resolution of 10 um, feed speed in 8 ~ 15 m / min for the low-grade; resolution of 1 pum, feed speed in 15 ~ 24 m / min for the mid-range; resolution of 0.1um, feed speed in 15 ~ 100 m / min for high-grade.
2. Servo feed type
The use of open-loop, stepper motor feeding system for the low grade; in the high grade is the use of semi-closed-loop or closed-loop DC servo system or AC servo system.
3. Linkage axis value
Low-grade CNC machine tools up to the number of linked axes for 2 ~ 3 axes, and in the high grade is 3 ~ 5 axes or more.
4. Communication function
Low-grade CNC machine tools are generally no communication function. Mid-range can have RS-232 or DNC (direct numerical control) interface. High-grade can also have MAP (manufacturing automation protocol) communication interface, with networking functions.
5. Display function
Low-grade CNC machine tools are generally only a simple digital tube display or simple CRT character display. The mid-range CNC machine tools have a more complete CRT display, not only character display, but also graphic display, human-machine dialogue function, self-diagnosis and other functions.
6. Built-in PLC
Low-grade CNC machine tools are generally no built-in PLC, in high-grade have built-in PLC. high-grade CNC machine tools with strong built-in PLC, with axis control of the expansion function.
7. Main CPU
Low-grade CNC generally using 8-bit CPU, mid-range and high-grade has been gradually by the 16-bit CPU to 32-bit CPU transition. Some new foreign numerical control system has been selected 64-bit CPU, and the choice of RISC central cnc manufacturing unit with a streamlined instruction set, in order to improve the system’s computing speed.
CNC technology and modern machinery manufacturing systems
A.The development trend of numerical control equipment and numerical control system
The main objectives of the development of CNC systems: reduce the price, improve reliability, broaden the function, improve operational comfort, improve integration, improve the flexibility and openness of the system.
| CNC system field | development trend |
| hardware | Open CNC system, PC integration technology, system modularization |
| system software | Open CNC system, standard operating system, real-time database |
| CNC core functions | 3D tool compensation, spline interpolation |
| programming function | 3D graphics simulation, WOP programming, online programming |
| server system | Using Modern Control Theory to Improve Following Accuracy |
| Monitoring and Diagnostics | Self-adaptive Technology , Intelligent Diagnosis Technology |
| communication | MAP, TCP/IP, field bus network, etc. |
| User Interface | (Tiana Data 1 God Academy user-designable interface |
B.CNC technology and computer integrated manufacturing system
To meet the requirements of modern production of multiple varieties, small and medium batches, product renewal cycle, the emergence of many advanced manufacturing methods and systems, in summary, mainly:
- Flexible manufacturing cell (FMC)
- Flexible manufacturing system (FMS)
- Computer Integrated Manufacturing System (CIMS)
Here, we mainly introduce the definition of FMC and FMS and their characteristics, etc.
(I) Flexible Manufacturing Cell (FMC)
1. The concept of FMC
Flexible Manufacturing Cell (FMC) consists of one or more CNC machines, automated logistics storage, transmission and exchange devices, and a cell control computer. The cell control computer coordinates and controls the operation of the equipment within the cell and communicates with the higher-level computer.
2. Composition of the FMC
In general, there are three subsystems in the FMC: information flow, logistics and processing. The information flow subsystem includes the FMC control system, monitoring system, control and management software; the logistics subsystem includes the loading, unloading, handling and storage of the workpiece and tool flows. The diagram below is a block diagram of the control structure of an FMC.
3. Features of FMC
- Under the control of the unit computer, different parts can be machined on different or the same machine tool;
- Under the control of the unit computer, flexible manufacturing systems can be formed and communicated;
- Automatic tool change and automatic transfer of workpieces during machine tool machining;
- Machining process monitoring can be achieved during the machining process of the machine tool;
(II) Flexible Manufacturing System (FMS)
1. Definition of FMS
Flexible Manufacturing System (FMS) is defined as: flexible manufacturing system is an automated manufacturing system composed of CNC processing equipment, material transportation and storage devices and computer control systems. It includes multiple flexible manufacturing units, which can be quickly adjusted according to the changes in the manufacturing task or production environment, and is suitable for multi-species, small batch production.
2. Basic composition of FMS
From the form of hardware, there are three components: multiple processing equipment, an automatic material storage and transportation system and a computer control system.
From the software content, the main components are: operation control software, quality control software and data management and communication software.
From a functional point of view there should be: the ability to automatically carry out the mass production of parts; change the software, it will be able to manufacture any parts of a certain family of parts; material transport and storage is done automatically; can solve the mixing ratio of parts under multi-machine conditions, and without additional costs.
3. Benefits of the FMS
Since the FMS is an automated system, there are significant benefits to be gained from the application of the FMS, mainly due to the following factors:
- High utilisation of major equipment.
- Reduction in major equipment costs.
- Reduction in direct labour costs.
- Reduction in work-in-process inventory and production lead times.
- Production is flexible and can adapt to changes in market demand.
- Ability to self-diagnose and maintain production.
- High product quality.
- Reduced floor space.
AC spindle servo motors and variable speed motors
A few basic concepts
- Calculation of spindle motor speed
- Calculation of the rated output torque and maximum output torque of the spindle motor.
- Calculation of the actual output power within the range of constant torque speed of the spindle motor.
- Calculation of the rated output torque and maximum output torque of the machine tool spindle. Calculation of the power required for the spindle and motor in the turning, milling and drilling mode
AC spindle servo motors and variable speed motors
● Basic concepts
1、The power load characteristics of the motor.
As a general drive load work rotary motors have the following three common power load characteristics.
- Continuous working system (S1): It means that the motor is allowed to work for a long time and without interruption under rated working conditions and load conditions.
- Short-time working system (S2): it means that the motor is allowed to run and work beyond the rated power within a short period of time, and its overload time is preferred to 10, 30 or 60 minutes, etc.
- Intermittent work system (S3): refers to the motor should work according to a certain on and off cycle, in order to ensure that the motor in the case of high current, overload does not cause the motor temperature is too high, CNC professional micro signal cncdar, worthy of attention breakdown of the absolute source and burned. In S3 working system under the work of the motor allowed each on, off work cycle for 10 minutes, for example FANUC AC spindle motor provisions in 50% ED rate (S3 working system), its cycle time cycle for 10 minutes (that is, ON: 5min, OFF: 5min).
An important feature of current AC spindle motors is that they allow operation in S2 or S3 operating systems, which are effectively a short operating system motor. Such as FANUC’s α11 type spindle motor rated continuous output power for 11kw. S2 working system under the 30-minute time allowed overload power for 15kw. because of CNC machine tools in the actual overload cutting time for each tool is difficult to go beyond 30 minutes, so many machine tool manufacturers in the production of a certain type of machine tool power parameters, commonly used spindle motor 30 minutes overload power as its sample on the main parameters of the spindle motor labeled.
Similarly, the design of CNC machine tools, designers also make good use of this feature of the spindle motor. In particular, it is necessary to point out that, at present, more than the standard type of ordinary frequency conversion motor, which can only work in S1 working system, does not allow the use of overload, CNC professional micro signal cncdar, worthy of attention so the designer must pay attention when choosing. But with the development of technology, the market has recently appeared a new type of inverter motor called “frequency spindle motor”, the constant power of the inflection point speed control frequency (wave) for 33.3Hz, not only significantly reduces the inflection point speed of the motor, improve the characteristics of the motor at low speeds, and allows 30 minutes of overload operation under the S2 working system, is a new type of motor with good price performance ratio.
The load characteristics of the motor and the inflection speed of the spindle motor nj.
Whether the AC spindle motor or variable speed motor, its inflection point speed above nj for stepless speed regulation, are basically a constant power speed regulation. That is, as the motor speed increases, its output power remains basically unchanged, while the output torque of the motor decreases with the increase in speed; its inflection point speed below nj for stepless speed regulation, CNC professional micro signal cncdar, worthy of attention are constant torque speed regulation, that is, as the motor speed decreases, its output torque is constant, while the output power of the motor decreases with the speed. Therefore, the machine tool spindle in the low speed section for rough machining and speed and fall into the motor’s constant torque section speed, the designer must carefully calibrate the actual output power of the motor at this time can meet the actual power required for cutting, otherwise there will be “bogging” phenomenon.
In our country, because the power plant using 50Hz frequency (circumferential) number of power generation, so the standard AC spindle motor (such as FANUC’s α series) and standard ordinary inverter motor, because more than 4 poles (4P) winding motor, the nj inflection point speed value should be 1500r/min. No matter what kind of speed motor, according to the load characteristics of the machine tool, the lower the nj inflection point speed value, and the highest motor The greater the ratio between the speed of the motor (constant power speed ratio), the better the force characteristics of the motor.
(a) Calculation of the spindle motor speed.
2 × F Formula: n = – × 60 P Where: n: motor speed F: control Hertz (wave) number P: the number of poles of the motor Note: Japan FANUC α series spindle motor parameters are as follows.
- Frequency range: α0.5~α6 type: 0~266.6Hz α8~α22 type: 0~200Hz α30~α40 type: 0~150Hz
- The number of poles (P): FANUC standard series spindle motor more than 4 to FANUC α12 type spindle motor as an example of calculation (0-200Hz): motor maximum output speed: 2 × 200 n = – × 60 = 6000r/min 4 motor constant power output speed (motor inflection point speed) 2 × 50 n = – × 60 = 1500r/min -×60=1500r/min 4 (Note: motor below 50Hz is constant torque characteristic output)
Spindle motor rated output torque and the maximum output torque calculation.
Where: N: spindle motor rated / 30 minutes overload power (KW) nj: spindle motor constant power inflection speed (r/min) FANUCα12 type spindle motor as an example of calculation (11/15kw) then: motor rated output torque (continuous output torque): CNC professional micro signal cncdar, worthy of attention motor maximum output torque (30 minutes overload):
Notes.
- According to the characteristics of CNC machine tools cutting operations, the calculation of spindle motor output torque are generally 30 minutes overload torque as a standard torque value.
- The torque value is kg.m when divided by 0.102 can be converted into Nm.
- Calculation of the output power of the spindle motor within the range of constant torque speed
Calculation of the purpose: when the spindle motor in the range of constant power speed for operation, with the decline in speed, the output power of the spindle motor also declined, at this time in the cutting parameters are not selected when the bogging phenomenon is very easy to appear, calculate the output power of the spindle motor under a specific speed, is to check the power consumed when cutting for the purpose. Formula: P (kw) = 1.0269 × n (r/min) × T (kg.m) / 1000 where: P (kw): the required motor power n (r/min): the actual speed of the motor (constant torque section) T (kg.m): the constant torque value of the motor Example: still FANUCα12 type spindle motor as an example (11/15kw)
- Output torque of the motor at 30 minutes overload.
- Find the output power of the motor at 1000 rpm (30 minutes overload)? P(kw)=1.0269×1000r/min×9.74(kg.m)/1000=10.002 kw
Calculation of the rated output torque and maximum output torque of the machine tool spindle.
Purpose of calculation.
The torque output on the motor is not equal to the torque output on the spindle, because the main drive chain in the machine tool often uses pulleys or variable gears, etc. for down and up speed, such as the use of down speed transmission, there is a down speed ratio i on the motor output torque amplification, which is often used on the mechanical design of a small power motor, CNC professional micro signal cncdar, worthy of attention through the large down speed ratio to dragging the reason for the large motion load. Formula: where: M: spindle output torque (kg.m) N: the main motor rated power or 30 minutes overload power (KW) Nj: the main motor constant power when the inflection point speed (r/min) i: drive chain speed reduction ratio η: mechanical transmission efficiency (pulley direct drive to take 0.96)
Note: According to the characteristics of CNC machine tool cutting operations, the calculation of the output torque of the spindle, generally according to the spindle 30 minutes overload when the maximum output torque to calculate.
Turning, milling, drilling spindle and motor power required under the way of calculation
Note: In Europe, the United States, Japan and other countries cutting manuals, information, in the cutting force and cutting power calculation, often using the concept of unit cutting force and unit cutting power. The so-called unit cutting force refers to “unit cutting area on the main cutting force”, with P, the unit is N/mm2; the so-called unit cutting power refers to “in the unit of time to remove a unit volume of metal consumed by the cutting power”, with Ps, the unit is KW / (mm3 / S).
The “calculation of the power required by the spindle and motor in turning, milling and drilling” described below is recommended in the manual of FANUC Japan. The values of MRt, MRm and MRd per kw of metal removed in the formula for calculating different cutting methods in turning, milling and drilling should be found in the corresponding cutting manuals or materials.
When CNC Cutting conditions
- Spindle speed: Ns (r/min)
- Workpiece diameter: Dt (mm)
- Feed rate: fr(mm/r)
- Depth of cut: t(mm) (cutting formula)
- Cutting speed: Vc = π × Dt × Ns (mm/min)
- feed rate: fm = fr × Ns (mm/min)
- Metal removal rate: Q=t×fr×Vc/1000(cm3/min) =t×fr×π×Dt×Ns/1000(cc/min) Q=π×Dt×t×fm/1000(cc/min)
- Machine tool spindle power required: Ps = Q/MRt(KW) where: MRt = metal removal rate per kw (cc/min/kw)
- Power required by the spindle drive motor: PM=1/η×Q/MRt where: η=efficiency of the spindle drive (%) b. When milling (cutting conditions)
- Spindle speed: Ns (r/min).
- Milling tool diameter: Dm(mm)
- Milling width: W(mm).
- Milling depth: t(mm)
- Number of milling cutter teeth: n (number of teeth).
- Feed: ft(mm/per tooth) (cutting formula)
- Cutting speed: Vc = π × Dm × Ns (mm/min)
- Feed rate: fm=ft×n×Ns(mm/min)
- Metal removal rate: Q=W×t×ft×n×Ns/1000(cm3/min) Q=W×t×fm/1000(cc/min)
- Machine tool spindle power required: Ps = Q/MRm (KW) where: MRm = metal removal rate per kw (cc/min/kw)
- Power required for the spindle drive motor: PM=1/η×Q/MRm(kw) where: η=efficiency of spindle drive (%) c. When drilling (cutting conditions)
- Spindle speed: Ns(r/min)
- Drill diameter: Dd(mm)
- Feed: fr(mm/r) (Cutting formula)
- Cutting speed: Vc=π×Dd×Ns(mm/min)
- feed rate: fm=fr×Ns(mm/min)
- Metal removal rate: Q=π/4×Dd2×fr×Ns/1000(cm3/min) Q=π/4×Dd2×fm/1000(cc/min)
- Power required for machine tool spindle: Ps=Q/MRd(KW) where: MRd = metal removal rate per kw
- Power required for spindle drive motor: PM=1/η×Q/MRd(kw) where: η=efficiency of spindle drive (%)
