CNC machine classification: as an industry insider, you have to know

March 12, 2022

By siedery

We have been providing CNC machining services to small and medium-sized businesses around the world for over a decade. We hope that this article will be of some help to those of you who are new to the industry and welcome more companies from around the world to work with us.

Classification by use

  • Metal cutting CNC machine
    Metal cutting CNC machine includes CNC lathe, CNC milling machine, CNC drilling machine, CNC boring machine, CNC grinder, CNC gear shaper, CNC boring and milling machine, CNC cam grinder, CNC cutter grinder, CNC curved surface grinder, etc. Grinding center and machining center (MC) are CNC machine tools with force library and automatic tool change device, such as machining center CNC grinder, etc.
  • Metal forming CNC machine
    Metal forming CNC machine includes CNC bending machine, CNC pipe bending machine, CNC hydraulic forming machine, and CNC press.
  • CNC special processing machine
    CNC special machining machine includes CNC wire cutting machine, CNC EDM machine, CNC electric pulse machine, CNC laser machining machine, etc.
  • Other types of CNC machine
    Such as water jet cutting machine, shoe sample cutting machine, engraving machine, CNC CMM, etc.

Classification by movement mode

  • Point control CNC machine
    As shown in Figure 3-1, the feature of point control NC machine is that the CNC device only controls the precise coordinate position of the tool or worktable from one machining position to another, and then carries out fixed-point machining. In the process of moving and positioning, the trajectory is not strictly controlled and no cutting is carried out. The CNC system of the machine only needs to control the coordinate value of the end of the stroke, regardless of the motion trajectory, so the motion between several coordinate axes does not need any connection. In order to reduce the movement time of the moving part as much as possible and improve the positioning accuracy, the moving part first moves quickly, slows down when it approaches the end coordinate, and accurately moves to the end positioning.This kind of CNC machine tools mainly include CNC coordinate boring machine, CNC drilling machine, CNC punch, CNC spot welding machine and CNC pipe bending machine. The corresponding numerical control device is called point numerical control device, and the control system of point numerical control device is relatively simple.
  • The linear control NC machine
    shown in Figure 3-2. The characteristic of the linear control NC machine tool is that the moving parts of the machine tool should not only realize the accurate movement and positioning from one position to another, but also realize the linear cutting movement in the direction parallel to the coordinate axis at a given feed speed. Although linear control CNC machine tool expands the process range of point-to-point control CNC machine tool, its application is still greatly limited.

    This kind of CNC machine mainly include simple CNC lathe, NC boring and milling machine and CNC grinder. The corresponding CNC device is called linear CNC device.
Figure 3-2 linear control CNC machine
Figure 3-2 linear control CNC machine
  • Contour control CNC machine

    contour control CNC machine tool is also called continuous control CNC machine or trajectory control CNC machine. This type of machine can be used for two or more than two coordinate axes shall be strictly controlled at the same time, that is, not only the stroke position of each coordinate but also the speed and displacement of each coordinate in the whole machining process shall be controlled; In other words, the tool path should be controlled to process the workpiece into a certain contour shape. The movement of each coordinate shall cooperate with each other according to the specified proportional relationship, accurately coordinate and continuously process, and approach the specified straight line, oblique line or angle , Contour curve and surface with the minimum error. Figure 3-3 shows the working principle of coordinate contour control CNC machine.
    Figure 3-3 contour control CNC machine
a) NC turning b) NC Milling

    Figure 3-3 contour control CNC machine
    a) NC turning b) NC Milling a) NC turnin

    Commonly used CNC lathes, CNC milling machines and CNC grinding machines are typical contour CNC machine tools. They can replace all types of profiling processing, improve machining accuracy and productivity, and shorten production preparation time. CNC flame cutting machine, EDM machine and CNC plotter also adopt contour control system. The control structure of the contour control system is more complex than that of the point position linear control system. In the machining process, interpolation operation needs to be carried out continuously, and then the corresponding speed and displacement control is carried out.

    The number of control coordinate axes can be divided into 2-axis control, 2.5-Axis control, 3-axis control, 4-axis control, 5-axis control, etc., as shown in Figure 3-4 and figure 3-6.
 Fig. 3-4 2.5 spatial surface of axis machining
  Fig. 3-4 2.5 spatial surface of axis machining
Fig. 3-5 2.5 machining of axis linkage NC milling machine
Fig. 3-5 2.5 machining of axis linkage NC milling machine
Fig. 3-6 3 NC machining of axis linkage
Fig. 3-6 3 NC machining of axis linkage

Classification according to control principle

  • Open-loop control CNC machine
    The feed servo drive of this kind of machine is open-loop, that is, there is no detection feedback device. Generally, its driving motor is a stepping motor. The main feature of the stepping motor is that the motor rotates one step every time the control circuit changes the command pulse signal Angle, and the motor itself has self-locking ability. The block diagram of its control system is shown in Figure 3-7.
    Figure 3-7
    Figure 3-7

    The feed command signal output by the NC system controls the driving circuit through the pulse distributor. It controls the coordinate displacement by the number of transformation pulses, the displacement speed by the frequency of transformation pulses, and the direction of displacement by the distribution order of transformation pulses. Therefore, the biggest feature of this control mode is convenient control, simple structure and low price. The command signal flow sent by the NC system is one-way, so there is no stability problem of the control system. However, the displacement accuracy is not high because the error of mechanical transmission is not corrected by feedback. Early CNC machine used this control mode, but the failure rate is relatively high. At present, due to the improvement of driving circuit, it has still been widely used, especially in China, this control mode is mostly used in the NC transformation of general economic CNC system and old equipment. In addition, this control mode can be configured with single chip microcomputer or single board computer as numerical control device, so as to reduce the price of the whole system.
  • Closed-loop control CNC machine
    As shown in Figure 3-8, the closed-loop control system is directly equipped with a linear position detection device (such as grating ruler) on the moving parts of the machine, which is installed on the saddle of the machine, that is, it directly detects the linear displacement of the machine coordinates. The detected actual displacement value is fed back to the comparator of the NC device, compared with the input original command displacement value, and the compared difference is used to control the moving part for supplementary displacement, and the repair is not stopped until the difference is eliminated positive movement to achieve accurate positioning.
    Figure 3-8

    Figure 3-8

    Through feedback, the transmission error in the whole mechanical transmission chain from motor to machine saddle can be eliminated, so as to obtain high static positioning accuracy of machine. However, in the whole control loop, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the whole mechanical transmission chain is very large compared with the electrical response time, which brings great difficulties to the stability correction of the whole closed-loop system, and the design and adjustment of the system are quite complex. Therefore, this full closed-loop control mode is mainly used for ultra precision lathes, boring and milling machines, ultra precision milling machines, CNC precision grinding machines and precision machining centers with high precision requirements.
  • Semi closed-loop control CNC machine
    This system is a derivative of the closed-loop system, as shown in Figure 3-9. The difference between it and closed-loop system is that its position feedback adopts angle detection element (encoder, etc.) and is directly installed at the rotating part of the transmission chain (such as servo motor or ball screw end). What it detects is not the actual displacement of the workbench, but the rotation angle of the rotating shaft related to the displacement, which is then fed back to the comparator of the NC device, compared with the input original command displacement value, and controlled with the compared difference to make the moving parts supplement the displacement until the difference is eliminated. Its accuracy is slightly worse than that of the closed-loop system.
    Figure 3-9

    Figure 3-9

    Since most mechanical transmission links are not included in the system closed loop, more stable control characteristics can be obtained. However, the mechanical transmission errors such as lead screw can not be corrected at any time through feedback, and the software setting compensation method can be used to appropriately improve its accuracy. This system has the advantages of simple structure, easy adjustment and low price of detection components, so it is a widely used NC system at present.

Classification by CNC system type

  • Economical CNC system (also known as simple CNC system)
    This grade of CNC machine can only meet the processing of general accuracy requirements, and can process straight lines, oblique lines, arcs and threaded parts with simple shape. The microcomputer system adopted is single board computer or single chip microcomputer system, which has the functions of digital display and CRT character display. The feed of the machine tool is driven by stepping motor, and the number of controlled axes and linkage axes are 3 axes or less.
  • Universal CNC system (commonly referred to as full-function CNC system)
    This kind of CNC system has many functions. In addition to the functions of the general CNC system, it also has certain graphic display functions and user oriented macro program functions. The microcomputer system adopted is a 16 bit or 32-bit microprocessor with RS-232C communication interface. The feed of the machine tool is driven by AC or DC servo. The general system can realize the linkage control of 4 axes or below.
  • High-grade CNC system
    The microcomputer system used is a 32-bit microprocessor system. The feed of the machine tool is mostly driven by AC servo. In addition to the function of general NC system, it should be able to realize the linkage control of at least 5 axes or more. It has three-dimensional animation graphics function and pleasant graphical user interface. At the same time, it also has rich tool management function, wide speed regulating spindle system, multi-functional intelligent monitoring system and user oriented macro program function. It also has a strong intelligent diagnosis and intelligent process database, which can realize the automatic setting of processing conditions, networking and communication with computer.
  • PC based Open CNC system
    The development of NC system with general microcomputer technology can get strong hardware and software support. The technology of these software and hardware is open. At this time, the general microcomputer not only has its own functions, but also has all the functions of full-function NC system.

Interpolation Theory

  • Basic concept of interpolation

    The core problem of machine tool digital control is how to control the movement of tool or workpiece. For the motion trajectory of plane curve, two motion coordinates are required to coordinate the motion, while for spatial curve or three-dimensional surface, more than three motion coordinates are required to produce coordinated motion in order to get out of its trajectory. During NC machining, as long as the information is sent to the NC device according to the regulations, it can be controlled. The input information can be obtained by direct calculation. For example, for the trajectory motion of y = f (x), the y value can be given incrementally according to the accuracy requirements, and then the x value can be calculated according to the functional formula. As long as the range of X is determined, the approximate trajectory can be obtained, and the accurate trajectory can be obtained by correctly controlling the speed ratio of X and y. However, in this direct calculation method, the higher the order of the curve, the more complex the calculation is, and the more difficult it is to control the speed ratio. In addition, there are some curves and surfaces represented by discrete data, and the surfaces (list curves and surfaces) are difficult to calculate. Therefore, NC machining does not use this direct calculation method as the input of control information.

    Various workpieces for contour machining on machine tools are generally composed of some simple and basic geometric elements (straight line, arc, etc.). If the machining object is composed of other conics and high-order curves, a small straight line or arc can be used for fitting (parabola or high-order curve fitting is required in some cases), which can meet the accuracy requirements. This fitting method is “interpolation”. In essence, it completes the “data encryption” of “filling the gap” according to the limited information, that is, the NC device generates the basic line type (straight line, arc, etc.) according to a certain method according to the limited data during programming, and completes the fitting of the required contour trajectory on this basis.

    It can be seen that the CNC system calculates a series of machining points of the tool according to the limited information of the contour line of the part, and completes the so-called data “densification” work. Interpolation has two meanings: one is to use small line segments to approximate to produce basic line types (such as lines, arcs, etc.); The other is to fit other contour curves with basic line type.

    Whether ordinary CNC (hardware CNC) system or computer CNC (CNC, MNC) system, there must be a part to complete the “interpolation” function. The device that can complete the interpolation is called interpolator. The interpolator in NC system is composed of digital circuit, which is called hardware interpolation; In CNC system, the interpolator function is realized by software, which is called software interpolation.
  • Interpolation method

    In the numerical control system, the commonly used interpolation methods include point by point interpolation, digital integration, time division and so on. The interpolation process of point by point comparison method, which is the most used method in NC system, and the interpolation operation method of straight line and arc are briefly introduced as follows.

    Point by point comparison method is also called algebraic operation method and drunk step method. The basic principle of this method is that in the process of controlling machining, the computer can calculate and judge the machining error point by point, compare it with the specified motion trajectory, and determine the moving direction of the next step from the comparison results. The point by point comparison method can be used for both linear interpolation and circular interpolation. The characteristic of this algorithm is that the operation is intuitive, the interpolation error is less than one pulse equivalent, the output pulse is uniform, and the speed change of the output pulse is small and easy to adjust. Therefore, it is widely used in two coordinate linkage NC machine tools.

    The interpolation principle of point by point comparison method can be summarized as “point by point comparison and step by step approximation”. The interpolation process of point by point comparison method is divided into four steps:
  1. Deviation discrimination. Judge the relative position between the current position of the tool and the ideal line segment according to the deviation value, so as to determine the direction of the next step.
  2. Coordinate feed. According to the discrimination result, move the tool one step in the X or Y direction
  3. Deviation calculation. When the tool moves to the new position, the deviation from the ideal line segment is calculated to determine the direction of the next step.
  4. Endpoint discrimination. Judge whether the tool reaches the end point. If it does not reach the end point, continue interpolation; If the end point is reached, the interpolation ends.

    (1) Linear interpolation

    As shown in Figure 3-10, it is the case of linear interpolation using the interpolation principle of point by point comparison method. In a certain program, the machine tool needs to process an OA straight line with an included angle of a with the x-axis. When machining on the NC machine tool, the motion path of the tool does not strictly follow the OA straight line, but the step broken line step by step. The maximum deviation between the broken line and the straight line does not exceed the allowable range of machining accuracy. Therefore, these broken lines can be approximately regarded as OA straight lines.
    Figure 3-10 linear interpolation
    Figure 3-10 linear interpolation

    We stipulate that when the machining point is above or on the OA straight line, the deviation value FN of the point is ≥ 0; If it is below the OA straight line, the deviation value FN is less than 0. The logic function of the NC device of the machine can automatically judge the walking step according to the deviation value. When FN ≥ 0, feed one step in the + X direction; When FN < 0, feed one step in the + Y direction. Each step will be automatically compared, and the tool will walk while judging. The tool will approach the OA straight line with the broken line 0-1-2-3-4 -…. – A in turn. In this way, feed point by point from point O until point a. This function of distributing pulses along a smooth straight line is called linear interpolation, and the device to realize this interpolation operation is called linear interpolator.

    (2) Circular interpolation

    As shown in Fig. 3-11, the point by point comparison interpolation principle is applied to arc interpolation. In a certain program, the machine tool needs to process the AB arc with radius R. when machining on the NC machine tool, the motion path of the tool also follows the step broken line step by step. The maximum deviation between the broken line and the arc does not exceed the allowable range of machining accuracy. Therefore, these broken lines can be approximately regarded as AB arc. We stipulate that when the machining point is outside or on the AB arc, the deviation value (the ratio of the distance from the point to the origin o to the radius R) FN ≥ 0; if the point is inside the arc AB, the deviation value FN < 0. During machining, when FN ≥ 0, feed one step in the – X direction; when FN < 0, feed one step in the + Y direction. The tool approaches the arc AB along the broken line a-1-2-3-4 -…. – B in turn, and feed point by point from point 4 until point B. This method distributes pulse work along the arc It can be called arc interpolation, and the device to realize this interpolation operation is called arc interpolator.
    Figure 3-11 circular interpolation

    Figure 3-11 circular interpolation

    (3) Quadrant processing of point by point comparison method
    • Separate treatment
      There are 4 groups of calculation formulas for linear interpolation of 4 quadrants; For counterclockwise arc interpolation of 4 quadrants and clockwise arc interpolation of 4 quadrants, there are 8 groups of calculation formulas, as shown in Figure 3-12.
      Fig. 3-12 four quadrant feed directions of linear interpolation and circular interpolation

      Fig. 3-12 four quadrant feed directions of linear interpolation and circular interpolation

      Interpolation is real-time, which directly affects the motion of the tool. The speed and accuracy of interpolation operation are important indexes of NC device. Interpolation principle is also called trajectory control principle.
    • Coordinate transformation method
      The deviation function of the first quadrant inverse circle interpolation is used to calculate the deviation of the third quadrant inverse circle interpolation and the second and fourth quadrant forward circle interpolation, and the deviation function of the first quadrant forward circle interpolation is used to calculate the deviation of the third quadrant forward circle interpolation and the second and fourth quadrant inverse circle interpolation.

4 comments on “CNC machine classification: as an industry insider, you have to know”

  1. A very detailed article on the classification of CNC machines from a different perspective! Thanks to Siedery.

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