Basic knowledge of CNC MACHINING

Basic knowledge of CNC MACHINING.

1. CNC machine tool and CNC system
CNC means computer digital control.
1. CNC machine tool
⑴. For metal cutting
Hole processing, tapping, boring, milling, turning, thread cutting, plane cutting, contour processing, surface grinding, cylindrical grinding
Cutting, internal grinding, etc.
(2) Wire electrode cutting machine.
(3) Punch, step punching, stamping, metal forming, bending and other machine tools.
(4) Industrial robots.
(5) Injection molding machine.
(6) Detection and measuring machine.
(7) Woodworking machinery.
(8) Special material processing machinery: such as processing stone, glass, radioactive mineral aggregate, etc.
(9) Special processing machinery Laser processing machine, gas cutting machine, welding machine, drawing machine, printing machine, etc.
With the development of electronic technology, computer technology and IT technology, at present, these machine tools and processing equipment can be controlled by numerical computer with numerical data, which is called CNC control. Need more learning materials. Group 373600976 can help you
2. CNC system
CNC system means computer numerical control system.
Basic configuration of CNC system
CNC control of machine tools is a multidisciplinary integrated control technology.
A CNC system includes: ⑴. CNC control unit (numerical controller part). (2) Servo drive unit and feed servo motor. (3) Spindle drive unit and spindle motor. (4) PMC (PLC) controller. (5) Input/output (I/O) unit of control signal of strong current cabinet (including tool magazine) of machine tool. (6) Position measurement and feedback unit of the machine tool (usually included in the servo drive unit). (7) External shaft (mechanical) control unit. Such as: drive shaft of tool magazine, exchange table, loading and unloading manipulator, etc. (8) Information input/output equipment. Such as computers, disk drives, memory cards, keyboards, special information devices, etc. (9) Network. Such as Ethernet, HSSB (high-speed data transmission port), RS-232C port, etc. and the local area network of the processing site.
The hardware of CNC unit (controller part) is actually a special microcomputer. It is the core designed and produced by CNC equipment manufacturers for the control of machine tools. The following figures show the basic hardware modules; Basic control function module and an actual controller hardware.
2. Motion coordinate and feed axis of machine tool
A machine tool has several moving axes to perform cutting feed during machining, so it is called feed axis. After the machine tool is started, the mechanical coordinate system (rectangular coordinate system) of the machine tool is established based on the zero point of the machine tool. Each axis corresponds to one of the corresponding coordinates. Having linear motion or rotary motion. The international standard ISO specifies the direction and name of the coordinate axis. See the figure below.
According to the regulations, define each coordinate axis according to the right hand rule of the rectangular coordinate system, and the positive direction of the Z axis is generally the direction of the machine tool spindle. 10. Y and Z are defined as linear motion axes; U, V, W are linear motion axes parallel to X, Y, Z respectively; A. B and C are rotary motion axes, which respectively move around X, Y and Z, and their positive directions conform to the right-hand spiral rule.
During CNC control, program commands X, Y, Z, U, V, W, A, B, C and other commands are used to control the coordinate axis, and numerical commands are used to command the distance of its movement, the direction of movement of positive and negative commands, and the speed of movement of F commands. For example:
G01 X120 Y-300 F1000;
The meaning is G01: X axis and Y axis move in harmony to process a straight line; X120, Y-300: axis X moves 120mm; Y-axis travel - 300mm; F: The feed speed is 1000mm/min.
3. CNC interpolation and output of position control command
1. Interpolation calculation of track motion -- Interpolator
CNC controls the coordinate movement of the machine tool. In the control principle, this is the position control system. What needs to be controlled is: the linkage of several axes, and the calculation of motion path (machining contour). The most important thing is to ensure the motion accuracy and positioning accuracy (dynamic contour geometric accuracy and static position geometric accuracy); Movement of each axis (mm); Moving speed (mm/min); Moving direction; Starting/braking process (acceleration/deceleration); The resolution of the move.
Modern CNC system is a pure electrical control system. The movement of the feed axis is carried out by the servo motor. Usually, a feed shaft is driven by a servo motor. The motor is powered by a servo amplifier. The work of the servo amplifier is controlled by the distributed output signal of the CNC interpolator.
CNC controls the feed axis of the machine tool by executing the previously prepared processing program instructions. The program command is the machining tool motion path compiled according to the part contour (as shown in the figure above). The program is compiled by segments according to the part contour. A program segment processes the contour of a shape. Different program instructions (part contour shape elements) are used for different contour shapes. For example, G01 -- linear motion command;
G02 - Clockwise arc movement command; G03 - Anticlockwise motion arc command; G32 (G33) --- thread processing
However, in a section of processing instructions, only the end point of this section is programmed. For example, the following program segment needs to process an arc on the X-Y plane, and only the coordinate value X100 of the end point is instructed in the program; Y-200:
G90 G17 G02 X100. Y-200. R50. F500;
The starting point of this paragraph has been written in the previous paragraph, which is the end point of the previous paragraph. Therefore, when machining this segment, as shown in the figure above, the NC controller, i.e. the computer processor, only knows the starting point and ending point coordinate values of this segment. The coordinates of other points on the tool path in the segment must be calculated by the processor. The processor calculates according to the contour command (G02) of this segment and the coordinates of the starting point and the ending point, that is, it must calculate the contour of the workpiece to be machined, and calculate the positions of the intermediate points where the tool moves along the X axis and Y axis simultaneously during the execution of this segment of command.
The composite motion of X axis and Y axis forms the workpiece contour path for tool processing.
In addition, the program must command the movement speed (processing speed), such as F500 (mm/min). In position calculation, the tool moving direction speed of the corresponding point shall be calculated according to the contour position. In this example, the corresponding velocities of points along the X axis and points along the Y axis are calculated respectively.
The mechanism for realizing the above operations is called the interpolator. Need more learning materials. Group 373600976 can help you
Each operation of the interpolator is called an interpolation cycle, generally 8ms; The interpolator for calculating complex profiles uses a high-speed CPU, and the interpolation cycle can be shortened to 2ms at present. A program segment is divided into multiple interpolation cycles, depending on the shape and size of the contour.
The instruction to execute the above program segment is to interpolate clockwise arcs. It is an interpolation subprogram based on the arc calculation formula. The judgment condition during calculation is: continuously execute the feed of the tool along the X and Y axes, and judge whether it reaches the end point and exceeds the tolerance by one pulse equivalent of each feed. The calculation direction is clockwise, and the feed equivalent is 1 μ M/pulse, the speed is 500mm/min.
CNC system control software includes multiple interpolation subprograms. Each geometric element of the workpiece shape corresponds to a geometric movement of the tool. Therefore, CNC is required to have corresponding interpolation subprograms. This is the G code in the CNC system control software to control the coordinate axis movement. For example, G01, G02, G03, G32, G33, G05, G08. There are also some subprograms that control the tool movement according to the requirements of the processing technology. The more G codes, the stronger the CNC function. Use these G codes to program the processing of parts.
CNC system control software is compiled in assembly language. Different types of machine tools use different CNC systems. Of course, the control software of these systems is completely different.
The hardware of the interpolator is the main CPU of CNC. Of course, there are interpolators with pure hardware.
2. Distribution output of interpolation pulse
After the interpolation operation, the distance (amount of movement) of each coordinate axis in the same time cycle (interpolation cycle) for the workpiece shape required for processing is calculated, which is expressed by the number of pulses. For example, the X axis feeds 25 pulses in this interpolation cycle; The Y-axis feeds 50 pulses, which are sent to the corresponding coordinate axis as the position movement command of the corresponding axis. The pulse sequence has positive and negative signs, and the command corresponds to the direction of motion of the axis; The pulse sequence is output at a certain frequency to command the motion speed of the axis. This device is called pulse distributor. In order to prevent the impact of processing movement and improve the processing accuracy and finish, the feed speed is accelerated/decelerated before the pulse is distributed to each feed axis. As shown in the figure below, CNC can realize two kinds of acceleration/deceleration control: acceleration/deceleration before interpolation and acceleration/deceleration after interpolation.
Linear or exponential acceleration/deceleration methods are usually used after interpolation: the speed change of exponential acceleration/deceleration is relatively smooth, so the impact is small, but the lag of speed command is large. On the contrary, the speed of linear acceleration and deceleration changes rapidly. If the time constant is set too small, it will cause impact and vibration of the machine tool. However, the machined part contour may be close to the programmed contour.
Linear acceleration and deceleration method is used before interpolation, which can reduce the shape error of machining. In addition, in order to improve the machining accuracy and speed, CNC software such as pre reading/pre-processing multiple program segments, fine acceleration and deceleration, etc. are also developed.
3. Offset and compensation of machining tools
The position pulse of the above interpolation is calculated according to the program compiled by the workpiece contour, that is, the running track of the tool center point is the workpiece contour. Considering that the tool has radius and different lengths, the tool center cannot travel according to this path during actual machining. The actual radius and length of the tool must be included according to the actual tool used. The center path of the actual tool is calculated by CNC, and the tool movement is controlled according to this path. This function is called "tool offset and compensation".
⑴. Tool radius offset, compensation is as shown in the figure below. The actual tool center track and the CNC machining program track prepared according to the part contour size offset by a tool radius. During programming, use the G command (G41, G42) to tell the CNC interpolator to perform the offset calculation of the tool radius. The interpolator calculates the center track of the tool according to the actual tool radius to control the tool travel. That is to say, the number of pulses for each feed axis output by the above pulse distributor is the number of feed pulses for the tool center track after the interpolated part contour is offset by a tool radius. The compensation pulse of each axis is sent to the corresponding feed axis.
The radius value of the actual tool must be entered into the tool compensation memory before machining. The tool compensation memory can store the geometric dimensions (radius values) of multiple tools at the same time. The tool used in processing is specified by the program with the tool number, such as T102. According to the tool number instructed in the program, the CNC interpolator finds the actual tool radius value and performs the calculation.
G41 is left tool compensation: when looking along the direction of tool travel, compensation is added on the left side of the workpiece; G42: Add compensation on the right side of the workpiece along the direction of tool travel.
Before processing, use the length of a tool as the reference, measure the positive and negative differences between the tool length of each tool used in actual processing and the reference tool length, and input the difference into the tool compensation memory according to the tool number as the above tool radius value. When the machining program is compiled, the tool number is programmed. At the beginning of machining, set the tool with the tip of the reference tool. When CNC executes the processing program, find out the difference of tool length according to the tool number instructed in the program, and make compensation according to the symbol of the difference of tool length. The figure above shows the cutter length compensation of the milling machine, only in the Z direction. For the lathe, there are X and Z directions. As shown in the figure below.
In CNC system of milling machine, G43 and G44 are used to command tool length compensation. G43 is positive tool compensation, that is, the tool compensation value is added to the end point coordinate value of the program command. G44 is negative tool compensation, that is, the tool compensation value is subtracted from the end point coordinate value of the program command. Need more learning materials. Group 373600976 can help you
Machining complex shaped parts (such as molds) requires the use of multi coordinate interpolators with multiple axes moving at the same time. Of course, multi coordinate (multi-dimensional) tool compensation must also be used.
4. Motion error compensation
1. Clearance compensation (loss of momentum compensation) when the moving shaft is in reverse direction
When the machine tool workbench moves in the opposite direction from a certain direction, the pulse will be lost due to the gap between the ball screw and the nut or the deformation of the lead screw, which is called the loss of momentum.
Print a table on the machine tool to measure the reverse movement clearance of each axis, and set the compensation amount ------ compensation pulse number (1) according to the measured clearance value μ M/pulse). In this way, when the worktable is in reverse direction and before executing the movement of the CNC program command, the CNC will output the compensation pulse to the servo amplifier of the corresponding axis through the pulse distributor at the rate set by the CNC in advance to compensate for the loss of momentum.
The reverse clearance value is related to the moving speed of the worktable. By setting the relevant parameters, the system can compensate the clearance under G00 (rapid movement) and feed speed (F) respectively.
2. Pitch error compensation
The pitch of the ball screw used in the machine tool has errors. CNC can compensate the measured pitch error of the ball screw of each feed shaft. The pitch error of ball screw is usually measured by laser interferometer. The measured reference point is the zero point of the machine tool. Set a compensation point every certain distance, which is set with parameters. Of course, each axis can be set arbitrarily. For example, if the stroke length of X axis is 50 mm, one point can be added; if the stroke of Z axis is short or the movement accuracy is required to be high, 20 mm, one point can be added.
The compensation value is determined according to the actual measured ball screw error, and its value (the number of compensation pulses) is set into the CNC pitch error compensation memory according to the compensation point number (calculated from the reference point, namely the zero point of the machine tool), as shown in the following figure. In general, the equivalent of a compensation pulse is μ m. The compensation value can be positive or negative. When the feed axis is moving, CNC detects the moving distance in real time, and outputs compensation values to each axis at the corresponding compensation points of each axis according to these preset parameter values, so that the corresponding axis moves more or less than the corresponding number of screw compensation pulses based on the CNC interpolation output pulse.
Recently, the CNC system has developed the bidirectional pitch error compensation function according to the moving direction of the worktable. The moving accuracy of the feed shaft is further improved.