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What is CNC Machining Technology
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What is CNC Machining Technology

With the development of science and technology, people's requirement for parts processing quality is becoming higher and higher. At the same time, product modification is frequent. In general mechanical processing, single and small batch products account for an increasing proportion. In order to ensure product quality, improve productivity and reduce costs, machine tools are required not only to have good versatility and flexibility, but also to have a high degree of automation in the process of processing. NC machining technology is a kind of high-efficiency and flexible automatic processing technology developed under this environment, which is controlled by digital information of NC machine tools. It is suitable for high-precision, complex parts and small and medium-sized batch production.

CNC machine tool is an efficient and flexible mechanical-electrical integration equipment which integrates computer technology, modern control technology, sensor detection technology, information processing technology, network communication technology, hydraulic pneumatic technology, optical and mechanical technology, and it is the basis of modern manufacturing technology.

1. Composition of NC Machine Tool and NC Machining Process

CNC machine tools refer to the machine tools that use digital information recorded in the media to control the machine tools through the CNC device, so that it can automatically implement the specified processing process.

1) Composition of CNC Machine Tools

NC machine tools automatically process workpieces according to pre-programmed processing procedures. CNC machine tools are usually composed of machine body, servo system, CNC device and control medium, as shown in Figure 2-61.

Figure 2-61 Composition of CNC Machine Tool

  1. Control medium. Control medium, also known as information carrier, is an intermediate medium which conveys the operation intention of human to CNC machine tools. It contains all the information necessary to process a part. Commonly used control media are perforated tape, magnetic tape, disk and other carriers that can store instruction information. For the short NC processing program, the NC device can be input directly through the keyboard on the NC operation panel.

  2. NC device. CNC device is the center of CNC machine tool. It receives the information from control medium, and converts it into pulse signal to control machine tool action.

  3. Servo system. Servo system is a device which connects the moving parts of machine tool with CNC system, including servo drive mechanism and movable parts of machine tool. It is the executive part of CNC system of machine tool. It transforms the impulse signal from CNC device into the movement of machine tool parts, so that the worktable or tool holder can be positioned accurately or moved strictly according to the expected trajectory, and the parts meeting the requirements of the drawings can be machined.

  4. Machine tool.  Machine tool is the mechanical part of NC machine tool. Besides the general parts such as main drive device, feed drive device, bed, worktable and auxiliary part, there are also special parts, such as tool storehouse, automatic tool changer and rotary worktable.

  5. NC Machine Parts Processing Process

    The NC machine tool parts processing process is that the operator first changes it into the content of manufacturing process according to the processing pattern. On the basis of familiarity with the processing technology, the processing program is compiled, and the human will is transformed into the acceptable information of CNC machine tools with the prescribed code and program format. Record information on control media (such as punched tape, tape or disk) to make it an instruction for controlling machine tools. After the input information is processed by the NC device, the command pulse is sent to the servo system of the coordinate axes of the machine tool to drive the corresponding moving parts of the machine tool, and other actions such as speed change, tool change and machine shutdown are controlled to automatically process the workpiece that meets the requirements of the drawings.

    Classification of 2 NC Machine Tools

    CNC machine tools can be classified according to process use, locus of motion, control mode and so on. This paper only introduces the classification according to process use. CNC machine tools are classified according to their processing characteristics or main processing procedures. They are mainly CNC lathes (including turning centers), CNC milling machines (including milling centers), CNC boring machines, CNC machining centers, CNC grinding machines (including grinding centers), CNC drilling machines (including drilling centers), CNC broaching machines, CNC planer, CNC cutting machine, CNC gear processing machine and CNC EDM machine (including EDM center, etc.). Figure 2-62 shows the appearance of CK50 NC lathe. Its processing range is the same as that of ordinary lathe. It mainly processes rotational parts such as shafts and discs. Figure 2-63 shows the appearance of XJK125 NC milling machine. It is especially suitable for the processing of complex parts. It can complete milling, drilling and boring.

    Figure 2-62   CK50 CNC lathe appearance chart

    Figure 2-63   XJK125 CNC Milling Machine Appearance Diagram

    Machining center (MC) is a kind of NC machine tool for multi-process machining of workpiece. It has a tool library and an automatic tool changer (ATC). After the workpiece is clamped once, under the control of the NC system, the machining center can automatically select and change the cutting tools according to different processes; automatically change the spindle speed, feed rate and the movement track of the tool relative to the workpiece and other auxiliary functions; sequentially complete the multi-process processing on several surfaces of the workpiece. Therefore, it reduces the time of workpiece clamping, measuring and machine adjustment, shortens the time of workpiece storage and handling, improves productivity and machine tool utilization, and it is an important development direction of CNC machine tools. Figure 2-64 shows the layout of JCS-018 machining center. Its shape is similar to that of vertical milling machine. There is a sliding seat 2 on the bed 1, and it moves horizontally (y axis direction). Workbench 3 moves longitudinally on the slide seat (x-axis direction). There are frame pillars 5 at the back of the machine tool. The spindle box 9 makes vertical lifting motion (z axis direction) on the column guide rail. The left and right parts of the column are the NC device 6, the left front part is equipped with the tool storehouse 7 and the automatic tool changing manipulator 8, and the left lower part is equipped with the lubrication device 4. There are 16 cutters in the tool library, which can complete all kinds of hole processing and milling. There are 16 cutters in the tool library, which can complete all kinds of hole processing and milling. The NC operation panel 10 is suspended in front of the operator's right for easy operation. The working state of the machine tool is displayed on the panel. Machining centers are usually divided into horizontal, vertical and universal machining centers according to the spatial position of the spindle during processing.

    Figure 2-64   JCS-018 Machining Center Layout

  6. machine tool body; 2-slide seat; 3-worktable; 4-lubrication device; 5-pillar; 6-NC device; 7-tool library; 8-tool changing manipulator; 9-spindle box; 10-operation panel

    3 Programming

    The so-called programming means that the process, process parameters, tool movement and direction and other auxiliary actions (tool changing, cooling, clamping, etc.) of parts are compiled into tables according to the sequence of movement and the instruction code and program format given by the NC machine tools used. This form is called "parts processing program sheet", or simply "procedure sheet". Then, all the contents of the program sheet are recorded on the control medium (such as perforated tape, tape, etc.), and then transmitted to the NC device to direct the NC machine tool processing. The whole process from the analysis of part drawings to the production of control media for NC machine tools is called programming.

    Generally speaking, the steps of NC programming are: process design→numerical calculation→preparation of parts processing program sheet→preparation of control medium or program input→program verification and trial cutting.

    1) Process design

    On the basis of comprehensive analysis of part drawings, the clamping and positioning method, processing route (such as tool setting point, tool changing point, feed route), tool and cutting parameters (such as feed speed, spindle speed, cutting width and cutting depth) are determined.

    2) numerical calculation

    According to the part drawing and the determined processing route, the tool trajectory is calculated.

    The general NC device has the functions of linear interpolation and circular interpolation. For processing simple parts consisting of arcs and straight lines, only the coordinate values of the intersection or tangent points (base points) of adjacent geometric elements on the contour of the part are calculated, and the starting and ending points of the straight line, the starting and ending points of the arc and the coordinate values of the center of the circle can be obtained. When the shape of parts is complex and the interpolation function of NC devices is inconsistent, more complex calculations are needed.

    3) Writing Procedure Sheet for Parts Processing

    According to the calculated coordinates of tool path, the determined cutting parameters and auxiliary actions, and the instruction codes and program formats prescribed by the NC system, a part processing program sheet is compiled.

    4) Preparing control medium or program input

    After the program sheet has been written, the operator or programmer must input the processing information into the NC device. According to the different input and output devices of the NC system, the program can be moved to some control medium first. Commonly used control media are U disk, disk, tape and so on.

  7. Program Checking and Trial Cutting

    The program must be checked and tested before it can be used formally. The method of checking is to input the contents of the control medium directly into the NC device to check whether the tool's trajectory is correct or not. On the CNC machine tool with CRT graphics display screen, the program checking and trial cutting can be carried out by simulating the cutting process of the workpiece; otherwise, the pen can replace the knife and coordinate paper can replace the workpiece, so that the machine tool can run and draw the processing trajectory.

    These methods can only check whether the tool's motion path is correct or not, and can not check the machining accuracy. Therefore, the trial cutting of parts should also be carried out. If it is found that the accuracy of the parts can not meet the requirements through trial cutting, the program sheet and the control medium should be modified, and the error compensation method should be adopted until the qualified parts are processed.

    4. Characteristics and Application of CNC Machine Tools

    1) Characteristics of CNC Machine Tools

    (1) High processing accuracy and stable processing quality. Because of the high manufacturing accuracy of CNC machine tool and the automatic processing according to the scheduled program, the artificial operation error is avoided, the consistency of the same batch of parts is good, and the product quality is stable.

  8. High productivity. Because many parts can be processed in one clamping process, many intermediate processes (such as marking) are omitted. Generally, only the first piece inspection is needed, which greatly shortens the production preparation time, so the productivity is high.

    (3) The high degree of automation reduces the intensity of labor and improves the working conditions. Except for the manual clamping of the blank, the whole processing process is automatically completed by the machine tool, which reduces the labor intensity of the operator and improves the working conditions.

    (4)It has strong adaptability and good economic benefit. When the processing object is changed, the NC program can be redesigned without redesigning tools, fixtures and moulds. This greatly shortens the product development cycle and provides a shortcut for the development of new products. At the same time, the processing accuracy is high, the quality is stable, the waste rate is reduced, the production cost is reduced, and the productivity is high, so good economic benefits can be obtained.

    (5) It is conducive to the modernization of production management. By using NC machine tools, the processing time of parts can be accurately calculated, and the management of inspection, fixture and semi-finished products can be effectively simplified, which is conducive to the modernization of production management. Because of the use of digital information, it is easy to form an integrated computer aided design and manufacturing system.

    However, the cost of CNC machine tools is high, the technology is complex and the maintenance is difficult, which requires higher quality of management and operators.

  9. Application of NC Machine Tool

    CNC machine tools are widely used, especially suitable for processing parts with the following characteristics: multi-variety, small batch production parts; parts with complex structure and high precision requirements; parts with frequent modifications. CNC machine tools can save a lot of tooling costs, reduce the overall cost. It is expensive, and it does not allow scrap of key parts; urgent parts with the shortest production cycle are needed.


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