CNC (computer numerical control)


January 17, 2017 Facebook Twitter LinkedIn Google+ CNC Machine



CNC (Computer Numerical Control)

Computer numerical control (Computerized numerical control, referred to as CNC) system is the use of computer-controlled processing functions to achieve numerical control system. The CNC system performs some or all of the numerical control functions according to the control program stored in the computer memory and is equipped with an interface circuit and a servo drive device for controlling the dedicated computer system of the automatic processing apparatus.

CNC 8065

Table of content

1. Development

2. Introduction

3. Basic structure

3.1 hardware structure

3.2 software structure

4. Basic classification

4.1 Motion trajectory

4.2 Processing technology

4.3 Servo system

4.4 Functional level

5. Selection factors

6. Five-axis CNC

6.1 Workpiece coordinate rotation

6.2 RTCP

6.3 Tool vector programming

6.4 Five-axis bevel machining

6.5 Five-Axis Interpolation

7. Workflow

8. Development

9. Common faults

10.Other references

11.List of CNC system manufacturers

1. Development

Computer numerical control is the use of a dedicated computer program can be stored in the implementation of some or all of the basic digital control functions of the NC system. Early NC system is composed of hardware circuit known as the hardware NC (Hard NC), after the 1970s, the hardware circuit components gradually replaced by a dedicated computer called the computer numerical control system, usually using a dedicated computer and equipped with interface circuits, Can realize the action of many numerical control equipment control. So now the CNC are generally CNC (computer numerical control), rarely use the concept of the NC.

NC (Numerical Control): on behalf of the old version, the first NC technology.

CNC (Computerized Numerical Control): computer numerical control technology – the new version of the NC preferred abbreviation.

NC may be CNC, but CNC is not referring to the old CNC technology.

2. Introduction

Numerical Control System, the early development and parallel evolution of the computer for the control of automated processing equipment, composed of tubes and relays and other hardware-based computing with a dedicated controller called hardware NC (Hard NC). After the 1970s, the separation of hardware and electronic components gradually integrated by a higher degree of computer processor instead, known as the computer numerical control system.

Computer numerical control (Computerized numerical control, referred to as CNC) system is the use of computer-controlled processing functions to achieve numerical control system. The CNC system performs some or all of the numerical control functions according to the control program stored in the computer memory and is equipped with an interface circuit and a servo drive device for controlling the dedicated computer system of the automatic processing apparatus.

CNC system from the NC program storage devices (from the early tape to the magnetic ring, to tape, disk to the computer common hard disk), computer control host (from a dedicated computer evolution to the PC architecture of the computer), programmable logic controller PLC), spindle drive and feed (servo) drive (including the detection device) and other components.

As a result of the gradual use of general-purpose computer, numerical control system has become a software-based color, and PLC instead of the traditional machine tool electrical control device, the system is more compact, its flexibility, versatility, reliability, easy to implement complex Of the NC function, use, maintenance is also convenient, and has a network connection and remote communication capabilities.

3. Basic structure

The world’s numerical control system a wide range of different forms, the composition of the structure has its own characteristics. These structural characteristics from the initial design of the basic requirements of the system and hardware and software engineering design ideas. For different manufacturers, based on historical development factors and their respective factors vary from place to place, in the design of thinking may also be different. For example, in the world in the 1990s, the United States Dynapath system uses a small plate structure, thermal deformation is small, easy to replace the board and flexible combination of the Japanese FANUC system tends to large board structure, reducing the inter-board connectors, it is conducive to system work Of the reliability. However, regardless of which system, their basic principles and composition is very similar. Generally the entire numerical control system is composed by three major parts, namely the control system, the servo system and the position survey system. The control system hardware is a special-purpose computer system with input and output function, which carries out interpolation operation according to the program of the workpiece and sends out the control instruction to the servo drive system. The measuring system detects the linear and rotary motion position and speed of the machine and feeds back to the control system and Servo drive system to modify the control instruction. The servo drive system compares the control commands from the control system with the feedback information of the measuring system and controls and controls the PWM current to drive the servomotor. The servo motor drives the machine to move according to the requirements. These three parts combine to form a complete closed-loop control of the CNC system.

The control system hardware is a special-purpose computer which has the interpersonal interaction function and has the input and output capability including the field bus interface. Servo drive system mainly includes servo drive and motor. Position measurement system is mainly used in long grating or circular grating incremental incremental encoder.

3.1 hardware structure

From the point of view of hardware structure, the numerical control system can be divided into two stages for six generations. The first stage is the stage of numerical logic control, which is characterized by having no CPU and relying on numerical logic to realize numerical calculation and logic Control, including the first generation is the tube numerical control system, the second generation is the transistor numerical control system, the third generation is the integrated circuit numerical control system; the second stage is the computer control stage, which is characterized by the direct introduction of computer control, relying on software to complete numerical control The main function, including the fourth generation is a small computer numerical control system, the fifth generation is the microcomputer numerical control system, the sixth generation is the PC numerical control system.

As the 90s of last century, the PC structure of the popularization of computer applications, PC architecture, computer CPU and peripheral storage, display, communication technology, high-speed progress, manufacturing costs significantly reduced, leading to PC architecture CNC system is becoming the mainstream of the numerical control system Architecture. PC NC system development, the formation of the “NC + PC” transitional structure, retaining the traditional NC hardware structure, only the PC as the HMI. Representative products include FANUC 160i, 180i, 310i, 840D and so on. There is a class of CNC functions will focus on the form of motion control card to achieve, by expanding the NC control board (such as DSP-based motion control card, etc.) to develop PC numerical control system. Typical representative of the United States DELTA TAU company PMAC multi-axis motion control card constructed PMAC-NC system. Another more revolutionary structure is the use of PC platform hardware and software resources, only to increase with the servo drive and I / O devices necessary for communication fieldbus interface, in order to achieve a very simple hardware architecture.

3.2 software structure

(1) Input the data processing program
It receives the input part program, the standard code that the processing instructions and data decoding, data processing, and stored in the required format. Some systems have to be compensated for, or interpolation and speed control for pre-calculation. Typically, the input data processing program includes input, decoding and data processing of three items.

(2) interpolation calculation program
CNC system based on the data provided in the workpiece processing program, such as the type of curve, the starting point, the end point, the speed of the middle of the output interpolation interpolation. The above calculation should not only strictly follow the given trajectory requirements but also meet the requirements of smooth motion acceleration and deceleration of the mechanical system. And a position command for forming a feed motion is sent to each coordinate axis based on the result of the calculation. This process is called interpolation. The calculated position command of the feed motion is controlled by the position loop, speed loop and current loop control in the CNC or in the servo system. The output current drives the motor to drive the workbench or the tool for corresponding movement and finish the processing tasks specified by the program.
CNC system is one side of the interpolation operations, while processing, is a typical real-time control.

(3) management procedures
Management program is responsible for data input, data processing, interpolation and other services for the processing of the various procedures for scheduling management. The management program also handles interrupts caused by panel commands, clock signals, fault signals, and so on. In the PC-based hardware architecture, the management process is usually supported by the real-time operating system to achieve.

(4) diagnostic procedures
Diagnostic function of the program is running in a timely manner to identify system failures, and indicate the type of failure. It is also possible to check the function of the main parts (CPU, memory, interface, switch, servo system, etc.) of the system before or after the fault and indicate the location of the fault.

4. Basic classification

4.1 Motion trajectory

(1) point position control numerical control system
The control tool moves from one machining point to the other one of the precise coordinate positions with respect to the workpiece and does not control the trajectory of the movement between the points and without making any machining during the movement. This type of system equipment, CNC drilling machine, CNC coordinate boring machine and CNC punch and so on.

tgFX - Build 791

(2) linear control CNC system
Not only to control the precise location of points and points, but also to control the two points between the tool movement trajectory is a straight line, and the tool can move in a given feed speed processing, the auxiliary function requirements than point control CNC system, as it may be required to have a spindle speed control, feed speed control and automatic tool exchange and other functions. This type of control equipment are mainly simple CNC lathes, CNC boring and milling machines,cnc router.

(3) contour control numerical control system
Such systems can be two or more coordinate direction of strict control, that is not only control the position of each coordinate travel, but also control the speed of each coordinate movement. The coordinates of the movement in accordance with the provisions of the proportional relationship with each other, precise coordination of continuous processing, to form the required line, slash or curve, surface. The use of such control equipment, CNC lathes, milling machines, machining centers, EDM and special processing machine tools.

4.2 Processing technology

(1)turning, milling CNC system
Aiming at the numerical control system of numerical control lathe control and the numerical control system for processing center. This kind of numerical control system belongs to the most common numerical control system. FANUC with T, M to distinguish between these two types of models. Siemens is in a unified NC on the configuration of different programming tools: Shopmill, shopturn to distinguish. The biggest difference between the two is that the turning system is capable of reflecting the distance of the tool point relative to the lathe axis at any time to express the radius of the currently machined workpiece, or by multiplying it by 2 as a diameter; the turning system has a fixed cycle of various turning threads; Turning system to support the spindle and the C axis of the switch to support the end of Cartesian coordinate system or rotary cylinder coordinate system programming, and CNC system to transform into polar coordinates to control; and for milling CNC system requires more complex curve, surface programming Processing capacity, including the five-axis and the slope of the processing. With the increasingly complex process of turning milling, CNC system requires both turning and milling functions.

(2) grinding CNC system
Special CNC system for grinding machine control. FANUC with G code distinction, Siemens must be configured. And the difference between the other CNC system is mainly to support the workpiece on-line meter access, measuring the size of the main instrument is in place, and inform the CNC system out of the grinding cycle. Grinding CNC system but also to support wheel dressing, and the modified wheel data as a tool data into the CNC system. In addition, grinding CNC system PLC also has a strong temperature monitoring and control circuit, also requires a vibration monitoring, ultrasonic grinding wheel access monitoring equipment, the ability to work together. For non-circular grinding, CNC systems and servo drives require a higher dynamic performance on the axes. Some non-circular processing (such as cam) due to be processed surface of high precision and high smoothness requirements, numerical control system on the curve smoothing technology also have special treatment.

 

 

(3) for special processing CNC system
Such systems often require special motion control processing and machining actuator control to accommodate special machining. For example, the parallel machine tool control needs to add the corresponding parallel structure decoupling algorithm to the conventional NC motion control algorithm; the wire cutting process needs to support the back path along the path; the punching and cutting machine control needs the C axis to keep the blanking head in the tangent of the motion path tangent; Gear processing requires the CNC system can achieve the law of the relationship between the gear ratio or the relationship between the electronic gear; laser processing to ensure that the distance between the laser head and the plate constant; EDM will control the discharge power; laser processing is required The numerical control system controls the laser energy.

4.3 Servo system

In accordance with the servo control system, you can CNC system is divided into the following categories:

(1) open-loop control numerical control system
Such numerical control system without detection devices, and no feedback circuit to stepper motor drive components. CNC device output of the feed command (mostly pulse interface) by the drive circuit for power amplification, control of the stepper motor stator winding thus energized / off the current pulse signal to drive the stepper motor rotation, and then by the machine drive The mechanism (gear box, lead screw, etc.) drives the table to move. This method is simple to control, the price is relatively low, from the 70’s, is widely used in economic CNC machine tools.

(2) semi-closed-loop control of the CNC system
The position detection element is mounted on the motor shaft end or the screw shaft end, and the actual working position of the machine table is calculated indirectly by the measurement of the angular displacement. Since the loop does not include screw, nut and machine tool The errors caused by these links can not be rectified by the loop. The control accuracy is not as good as that of the closed-loop control NC system, but the debugging is convenient and the cost is moderate, so the stable control characteristic can be obtained. , This approach is widely used.

(3) the whole closed-loop control numerical control system
The position detection device is installed on the machine table to detect the actual operating position (linear displacement) of the machine table and compare it to the commanded position (or displacement) calculated by the CNC device and adjust and control it with the difference. This type of control position control accuracy is high, but because it screw, nut vice and machine tool table on the link in the closed loop, resulting in the entire system connection stiffness variation, so debugging, the system is difficult to achieve high Gain, that is prone to oscillation.

4.4 Functional level

(1) economic numerical control system
Also known as simple numerical control system, usually using stepper motor or pulse train interface servo drive, does not have the position feedback or position feedback does not participate in position control; can only meet the general requirements of precision machining, can shape a straight line, , Arc and threaded parts, the use of microcomputer system for the single board computer or microcontroller system; usually does not have the user programmable PLC functions. Usually equipped with machine positioning accuracy of 0.02mm or more.

(2) universal type numerical control system
The numerical control system, which is between the simple type numerical control system and the high performance numerical control system, is characterized by the following four axes (including four axes), closed loop control (servo motor feedback information participation control), pitch error compensation and tool Management function, support user development PLC function.

(3) high-grade numerical control system
Generally refers to a multi-channel (two and above) numerical control equipment control, with dual-drive control, 5 axis and above interpolation linkage function, bevel processing, spline interpolation, bidirectional pitch error compensation, straightness and verticality Error compensation, tool management and tool length and radius compensation function, high static accuracy (resolution 0.001μm minimum resolution of 1nm) and high dynamic accuracy (servo error 0.01mm or less), high speed and complete PLC control functions NC system.

5. Selection factors

The applicability of numerical control system for CNC machine tool design selection is undoubtedly an important limiting factor. The following factors are important factors that must be considered in selecting a CNC system.

(1) drive capability
Different CNC supplier solutions in the servo power range and supporting the motor range is also different. First of all, should be able to match the motor type, power range to the initial screening. In particular, should pay attention to whether the NC machine tool program, including torque motor, linear motor, motor spindle synchronous spindle or synchronous spindle, the motor rated current demand and overload current demand, the maximum spindle speed requirements.

(2) full closed-loop demand and dual drive demand
CNC machine tools, especially large, heavy-duty CNC machine tools have a majority of closed-loop and dual drive demand. In the closed-loop control scheme, to choose between distance-coded grating, absolute value grating and ordinary incremental grating, the numerical control system should also support the corresponding feedback signal access
.
(3) five-axis control requirements
Five-axis machine tools need to be clear whether the five-axis linkage or only requires five-sided processing, the corresponding choice of CNC system functions are also different. For example, for five-sided box type processing, usually do not need RTCP, the choice is relatively large. While for five-axis function may involve CNC system supplier in the export license, after-sales service, technical support, etc. must also be seriously considered.

(4) production system requirements
CNC system support network to become a necessary condition for production system integration. For CNC machines to be incorporated into highly automated production systems, it is important to have a numerical control system with appropriate access solutions, including low-level PLC input / output points for direct access to advanced CNC system built-in OPC servers, The user opens the internal data of the numerical control system. In addition to the production system, automated online workpiece inspection and tool detection are also features that must be supported.

6. Five-axis CNC

With five-axis CNC machine tool can achieve a variety of attitude between the workpiece and the relative movement of the tool, on the one hand to maintain a better tool posture to avoid cutting tool center low cutting speed, but also to avoid the tool and workpiece, fixture Between the interference, to achieve a greater range of processing within a limited trip. Five-axis function is also an important indicator of the ability to measure the numerical control system.

Squad Selection WorldSkills Sao Paulo 2015

6.1 Workpiece coordinate rotation

For a five-axis machine with a turntable structure, the workpiece and the rotary table are consolidated, ie the workpiece coordinate system (WCS) is consolidated with the rotary table. When the table is rotated, the workpiece coordinate system (WCS) must be rotated accordingly. After that, X, Y and Z of the workpiece coordinate system are no longer consistent with the original machine coordinate system (XYZ). The five-axis interpolation algorithm needs to complete the rotation of the workpiece coordinate system automatically at any time to ensure the correct tool running trajectory.

Since the workpiece coordinate system rotates with the turntable, the CNC provides the user with the opportunity to select the machine coordinate system MCS or the workpiece coordinate system WCS in the manual operation mode. If the user has selected the manual operation under WCS and the WCS has been rotated, the manual operation will move in the direction of the axis after rotation. For example, if the C axis is rotated by 45 degrees from the initial 0 degrees, , The user to select the manual X-axis WCS, CNC machine tools will be XY axis linkage, XY plane to go 45-degree slash, the above-mentioned behavior for the workpiece side and manual positioning processing is very convenient, do not take into account how many degrees turn the table, The workpiece coordinate system shown in the drawings can be the direction of operation. In the automatic machining mode, all the G92, G54-G59, G52 are set under the WCS, will follow the WCS rotation and rotation.

It is important to note that if you are programming in the workpiece coordinate system, it is recommended that you use G53 to return to the MCS and then retract the tool in accordance with the MCS coordinate system. Otherwise, it is necessary to know the angle relationship between the current WCS and the MCS. For example, when the C-axis is 0 ° and 180 °, the WCS coordinate system is just opposite to the direction of the feed. When the feed start position C is 0 ° and the XY is the absolute value of the WCS, the retraction position is 180 ° C, To the starting point is to return to WCS absolute value of the negative.

For a machine with a swing-head structure, the five-axis CNC system only focuses on the coordinates of the control point (swing center) in the machine coordinate system MCS, while in the workpiece coordinate system WCS the five-axis NC system controls the tool point coordinates, as the picture shows. Combined with WCS rotation with the turntable, CNC control system so that the behavior of WCS always correctly reflect the relative position between the tool and the workpiece relationship, the user can feel comfortable with the workpiece drawing, consider the WCS under the workpiece programming can be, without regard to machine structure.

6.2 RTCP

In the five-axis machining, both the tool rotation and the turntable rotation cause the tool nose point to generate an additional motion of XYZ. The five-axis CNC system automatically compensates for the displacement between the workpiece and the tool nose point generated by these rotations and oscillations. This is called the RTCP (Rotation around the point of the tool) control function.

6.3 Tool vector programming

In the five-axis programming, it is recommended to use the attitude vector of the tool relative to the workpiece coordinate system (WCS) to express the attitude relationship between the workpiece and the tool. The result of this processing is that the user does not have to consider the specific type and structure of the five-axis machine tool. The same workpiece program can be processed on different types of five-axis machine tools. All coordinate processing related to the machine tool structure is done automatically by the five-axis CNC system.

6.4 Five-axis bevel machining

According to statistics, the world-wide, five-axis machine tool for real five-axis processing only 5%
73% for five-axis directional processing, such as V-type engine block, mold manufacturing, etc .; pentahedron processing accounted for 22% [1], such as machine tool box structure parts .

The concept of Frames is used in 840D to describe spatial slopes and coordinate systems.

The TNC530 uses the PLANE function to define the machining ramp.

Space angle A: The rotation angle SPA is revolved around the machine fixed X-axis; The space angle B: The rotation angle SPB is revolved around the machine fixed Y-axis; The space angle C: Rotation The angle SPC is revolved around the machine fixed Z-axis. In addition to the definition of space angle, TNC530 also supports projection angle, Euler angle, three points and other space slope definition.

The GNC 61 has a G92 coordinate system in the workpiece coordinate system WCS, which is responsible for the overall offset of the user-defined coordinate system on it, and can be used to express the datum of the fixture. In the G92 coordinate system, the user can define G54, G55, G56, G57, G58, G59 coordinate system, which can be used to express the coordinate system of multiple workpieces under the same fixture datum. GNC61 designed the program local coordinate system G52, the coordinate system is located under G54-G59, you can rotate the tilt. It is valid in the programmed machining program, and G52 is automatically cleared when a new program is loaded. GNC61 allows users to define G52 (space angle) directly in the program to specify a tilted coordinate system. In addition, GNC61 provides built-in functions for other tilted coordinate system definitions, including SG52_EULER, which specifies the G52 rotating coordinate system in Euler angles; SG52_2VEC, which defines the machining plane by using two vectors; SG52_3PT, To specify the G52 rotating coordinate system.

In addition to the definition of the slope, based on the five-axis CNC system also need to support automatic orientation of the tool to the attitude perpendicular to the slope. HEIDENHAIN TNC530 has three kinds of treatment MOVE, TRUN, STAY. In MOVE mode, when the RTCP is turned on, the automatic orientation of the tool is achieved. That is, the tool tip is not moved. In TRUN mode, the tool automatically orientates but does not turn on RTCP. The tool only wobbles and does not perform RTCP compensation movement. STAY Any movement is generated, but the corresponding required amount of motion is saved by the system variables.

6.5 Five-Axis Interpolation

Usually in the default state of the so-called five-axis CNC system using five-axis linear interpolation,
The ABC increment equals the linear increment for interpolation. Whether or not RTCP 5-axis linear interpolation is on is not directly bounded by the side edge of the tool, which may result in part sizes and shapes that do not meet the requirements. To this end, CNC manufacturers often also support other constraints side edge of the special five-axis interpolation.

(1) the plane vector interpolation

In the blanking mold, there are a large number of sidewall retaining surface requirements; aerospace thin-walled structural components also exist a large number of sidewalls tilt required cavity milling; welding parts welding groove also has milling inclined surface requirements. 840D to provide ORIVECT, and GNC61 G213 are the above functions. Normally this function starts RTCP automatically.

(2) Two-spline constrained interpolation
That is, to specify the spline curve of the tool nose point, and another spline curve to constrain the tool, the CNC system will complete the interpolation of the ruled surface with two spline curves. 840D provides ORICURVE, and GNC61 provide G6.3X are to achieve the above functions

(3) cone interpolation
Specifies that the tool vector travels along a specific conical surface.
The interpolation function is suitable for machining conical transition surfaces between conical and space bevels. 840D provides ORICONCW \ ORICONCCW \ ORICONIO \ ORICONTO that is to complete the above functions.

Space tool radius compensation
For 5-axis machining, RTCP functions as a tool length compensation. The five-axis tool radius compensation can not modify the five-axis machining program in the case of surface coordinates of the workpiece, adjust the various types of tools, can ensure the correct shape of the workpiece surface. FANUC in the most advanced 30i series CNC system and Siemens high-end 840D system supports the above functions.

Five-axis speed smoothing
In five-axis machining, the RTCP and various special five-axis algorithms, such as plane vector interpolation and double-spline constraint interpolation, can cause fluctuations in the linear velocity of the linear axes, which can sometimes cause Machine vibration, affecting the quality of surface processing parts, more than the allowable range of machine tools. For this five-axis CNC system need to smooth the shaft speed adjustment. FANUC the most advanced 30i series CNC system and Siemens high-end 840D system supports the above functions.

7. Workflow

(1) Input: part program and control parameters, compensation and other data input, can be used photoelectric reader, keyboard, disk, connect the computer DNC interface, network and other forms. CNC equipment in the input process is usually also complete the invalid code to delete, code validation and code conversion and so on.

(2) Decoding: Regardless of whether the system is working in MDI mode or memory mode, the part program is processed in a block unit. The parts contour information (such as start point, end point, line or arc) , Processing speed information (F code) and other auxiliary information (M, S, T code, etc.) in accordance with certain grammatical rules interpreted as the computer can identify the data form, and a certain data format stored in the specified memory dedicated unit. In the decoding process, but also to complete the grammar check on the program, if they find a syntax error immediately alarm.

(3) Tool compensation: Tool compensation includes tool length compensation and tool radius compensation. Usually the part program of the CNC device is programmed with the part contour path, which is used to convert the part contour path into the tool center trajectory. In a better CNC device, the tool compensation of the workpiece also includes the automatic transfer between blocks and over-cutting discrimination, which is the so-called C tool compensation.

(4) Feedrate processing: The programmed tool movement speed is the speed in the direction of the coordinates. Speed ​​processing first thing to do is based on the synthesis speed to calculate the speed of the coordinates of the motion. In some CNC devices, the minimum speed and maximum speed allowed by the machine, the automatic acceleration / deceleration of the software, etc. are also handled here.

(5) Interpolation: The task of interpolation is to “densify the data points” on a curve at a given start and end point. The interpolation program runs once per interpolation cycle, and during each interpolation cycle, calculates a small linear data segment based on the command feedrate. In general, after several cycles of interpolation, a block trajectory is processed by interpolation to complete the “data point densification” operation from the beginning to the end of the block.

(6) position control: Position control in the servo loop position loop, this part of the work can be achieved by software, can also be completed by the hardware. Its main task is in each sampling period, the theoretical position and the actual feedback position compared with the difference to control the servo motor. In the position control, it is usually necessary to complete the gain adjustment of the position loop, the pitch error compensation and the backlash compensation in each coordinate direction to improve the positioning accuracy of the machine tool.

(7) I / O processing: I / O processing mainly deal with CNC panel switch signal, machine electrical signal input, output and control (such as tool change, shift, cooling and so on.

(8) Display: CNC device display is mainly for the operator to provide convenient, usually used for part program display, parameter display, tool position display, machine status display, alarm display. Some CNC devices also have static and dynamic graphical displays of toolpaths.

(9) diagnosis: the system is not normal to check the situation, location, including online diagnosis and offline diagnosis.

8. Development

CNC system and related automation products mainly for supporting CNC machine tools. NC machine tool is a mechanical and electrical integration product formed by the infiltration of the new technology represented by the numerical control system to the traditional mechanical manufacturing industry. The machine tool equipped with the numerical control system greatly improves the precision, speed and efficiency of the parts processing. This NC machine tool is the national industrial modernization of the important material basis.

The concept of numerical control (“NC” or “NC”) is to convert the requirements of machined mechanical parts, such as shape, size, etc., into numerical data command signals to an electronic control device, which controls the drive of the machine tool Movement and processing of parts. In the traditional manual machining, these processes need to be manipulated by the machine to achieve, it is difficult to meet the complex parts of the processing requirements, especially for many varieties, small quantities of parts, processing efficiency is low, poor accuracy.

In 1952, Massachusetts Institute of Technology and Parsons to cooperate, invented the world’s first three-coordinate CNC milling machine. The control unit is made up of more than 2000 tubes, about one common laboratory size. Servo uses a small servo motor to change the hydraulic motor swashplate angle to control the speed of the hydraulic motor. The interpolation device uses pulse multiplier. The successful development of this NC machine tool marks the beginning of NC technology and mechanical manufacturing of a new, numerical control of the beginning of the era.

Software applications

In the 1970 Chicago exhibition, the first show by the minicomputer composed of CNC system. At about the same time, Intel invented the microprocessor. In 1974, the United States, Japan and other countries have developed a microprocessor as the core of CNC, sometimes called MNC. It uses the program in the computer memory to complete the requirements of the NC function. All or part of its control functions by the software, including decoding, tool compensation, speed processing, interpolation, position control. The semiconductor memory storage part processing program can replace the perforated parts tape program for processing, this program is easy to display, check, modify and edit, thus reducing the system hardware configuration, improve system reliability. The use of software control greatly increases the flexibility of the system, reducing the system manufacturing costs.

The Introduction of NC Standard

With the NC as an important equipment for automated mechanical processing, in the management and operation between the need for a unified terminology, technical requirements, symbols and graphics, which have a unified standard for worldwide technical exchanges and trade. NC technology development, the formation of a number of internationally accepted standards: the ISO International Organization for Standardization standards, IEC International Electrotechnical Commission standards and EIA American Electronics Industry Association standards. The earliest development of the standard NC machine tool axis and movement direction, NC machine code characters, NC machine block format, preparation functions and auxiliary functions, the size of NC tape, CNC terminology and so on. The establishment of these standards, the development of NC technology has played a role in regulating and promoting. ISO based on the needs of users and the next five years, information technology forecast, and in the brewing of the new standard “CNC controller data structure.” It focuses on AMT (Advanced Manufacturing Technology) on two main levels and the connections between them: the first level of CAM for the shop floor and its production machinery: the second level is the upper level, the data generation system , By the CAD, CAP, CAE and NC programming system and related database components.

The development of servo technology

Servo device is an important part of numerical control system. In the early 1950s, the feed drive of the world’s first NC machine was driven hydraulically. As the hydraulic system unit area is greater than the force generated by the electrical system (about 20: 1), inertia is small, fast response, so when a lot of NC system feed servo hydraulic system. The early 1970s, due to the oil crisis, coupled with the hydraulic pollution of the environment and the system bulky, low efficiency reasons, the United States GETTYS company developed a large DC servo motor, static torque and starting torque, good performance, FANUC quickly 1974 and introduced in the NC machine tools have been applied. Since then, the open-loop system is gradually replaced by a closed-loop system, hydraulic servo system is gradually replaced by electrical servo system.

The initial stage of the electro-servo technology is analog control. This control method has a large noise and drifts greatly. With the adoption of microprocessors, the introduction of digital control. It has the following advantages: ① no drift, good stability. ② based on numerical calculation, high precision. ③ through the parameters of the system settings, adjust to reduce. ④ easy to make ASIC circuit. For modern CNC system, the biggest breakthrough in servo technology can be summarized as: AC drive instead of DC drive, digital control instead of analog control, or software control instead of hardware control. The 20th century, 90 years, many companies have developed a linear motor, driven by all-digital servo, high rigidity, frequency response, and thus access to high-speed.

Automatic programming adoption

Programming methods are manual programming and automatic programming of two. According to statistical analysis, using manual programming, a part of the programming time and the ratio of machine processing, an average of about 30: 1. In order to improve efficiency, you must use a computer or programming machine instead of manual programming. Automatic programming requires an automated programming language, in which the APT language developed by the Massachusetts Institute of Technology is the most typical one of the NC language, which greatly improves the programming efficiency. Since the 1970s, the image NC programming technology effectively solved the geometric modeling, parts of the geometric shape display, interactive design, modification and tool path generation, pass simulation process simulation, and so on, thus promoting the CAD and CAM to Integration direction.

DNC concept from the “direct numerical control” to “distributed CNC” changes, its meaning has changed. “Distributed NC” indicates that a single computer can be used to control multiple CNC machines. In this way, mechanical processing from the stand-alone mode of automation to expand to flexible production lines and computer integrated manufacturing systems. From the communication function, you can increase the CNC system DNC interface, the formation of manufacturing communication network. Network is the most important feature of resource sharing, through the DNC function can form a network: ① parts of the program upload or download. ② read and write CNC data. ③PLC data transmission. ④ memory operation control. ⑤ system status acquisition and remote control.

The adoption of programmable controllers
In the 20th century, 70 years ago, NC controller and the machine strong sequence control relies mainly on relays. 1960s appeared semiconductor logic components, 1969 the United States DEC company developed the world’s first programmable logic controller PLC. PLC quickly shows the superiority: the design of graphics and relay circuit similar to the visual image, you can easily achieve the program display, editing, diagnosis, storage and transmission: PLC is not the kind of poor contact relay circuit, contact welding , Wear, coil burnout and other shortcomings. So soon in the NC machine tools have been applied. In the NC machine instruction execution time up to 0.085? S / step, the maximum number of steps to 32000 steps. Moreover, the use of PLC can also greatly reduce the system footprint, improve the system’s speed and reliability.

Sensor technology development
In addition to mechanical effects, the closed-loop system also depends largely on the sensors used, in particular the position and velocity sensors, such as the linear displacement and rotation that can be measured Angle of the linear induction synchronizer and circular induction synchronizer, linear and circular grating, magnetic tape, the use of reluctance sensors. These sensors by the optical, precision machinery, electronic components, the general resolution of 0.01 ~ 0.001mm, measurement accuracy of ± 0.02 ~ 0.002mm / m, machine table speed of 20m / min or less. With the continuous improvement of machine precision, the resolution of the sensor and precision also put forward higher requirements. So there is a “subdivision” circuit of high-resolution sensors, such as FANUC encoder developed by the breakdown can be done at a resolution of 10-7r. Using its high-precision CNC system for ultra-precision control and processing to create the conditions.

The Emergence of Open Technology

In 1987 the US Air Force published the famous “NGC (Next Generation Controller)” program, first proposed the concept of open architecture controller. One of the key elements of this initiative is the introduction of the Open Systems Architecture Standard Specification (SOSAS). The US Air Force defines an open architecture as allowing multiple manufacturers to sell interchangeable and interoperable modules in a competitive environment. Machine tool manufacturers can open the system platform to add a certain amount of hardware and software to form their own systems. Currently, there are basically two types of open systems in the market: ①CNC + PC board: Insert a PC board into a traditional CNC machine. The PC board mainly runs non-real-time control, and the CNC mainly runs real-time control based on coordinate axis motion. ② PC + motion control board: the motion control board into the PC’s standard slot for real-time control, and PC mainly for non-real-time control. In order to increase the openness, the mainstream numerical control system manufacturers often use the program ①, that is, without changing the basic structure of the original system on the basis of the addition of a PC board to provide keyboard allows users to PC and CNC linked together, greatly improving the man-machine interface Function. Typical examples are FANUC’s 150/160/180/210 system. Some manufacturers have also called this device fusion system (fusionsystem), because of its reliable, open interface, more and more welcomed by machine tool manufacturers, NC technology to become one of the trends.

9. Common faults

Position loop
This is the numerical control system sends out the control instruction, and compares with the position examination system feedback value, further completes the control work the key link. It has a high frequency of work, and connected with the peripherals, so prone to failure.

Common faults are: ① position control ring alarm: the measurement circuit may be open; measurement system damage, damage to the internal control unit. ② do not send commands on the movement, may be drifting too high, positive feedback, position control unit failure; measuring component damage. ③ measuring component failure, the general performance of non-feedback value; machine can not return to the base point; high-speed leakage pulse alarm may be due to raster or read the dirty; grating bad.

Servo drive system
Servo drive system and the power grid, mechanical systems and other related, but also in the work has been in the frequent start-up and operation of state, so this is more faulty part.

Power supply section
Power supply is the energy support part of the system to maintain normal operation, it is a direct result of failure or failure caused by the system shutdown or destroy the entire system.

10.Other references

cnc-milling-machine
cnc router
cnc-lathe-machine-buyers
3d-printer
cnc grinding machine

11.List of CNC system manufacturers

ManufacturerHQProducts Built
FANUC America CorporationUSCNC systems, motion control systems.
Heidenhain CorporationUSmachine controls and touch probes, length gauges and metrology solutions, digital readout systems and interface electronics.
NUM s.p.a.SwitzerlandCNC solutions.
Fagor AutomationSpainCNC control systems, feedback systems, digital readout systems.
Rockwell Automation Canada ControlsCanadaadvanced process control, condition monitoring&I/O, design and operations software, control systems, distributed control system (DCS).
Siemens Canada LtdCanadaautomation systems, customized automation.
Reviews