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What is CNC machining?

Numerical control machining refers to a machining method for machining parts on a numerically controlled machine tool. The process specifications for numerically controlled machine tools and traditional machine tools are generally the same, but with significant changes. A digital machining method that controls the displacement of parts and tools. This is an effective way to solve a wide variety of parts, small lots, complex geometries, precision, efficiency and automated processing.

Basic interpretation

CNC machining means that the control system issues commands to allow the tool to perform various actions that meet its requirements. The shape and dimensions of the workpiece are expressed in the form of numerical values ​​and alphabets, and the processing required for the machining process is required. This refers to the process of machining parts with CNC machines.

CNC machines are computer-controlled machine tools, and the computers used to control machine tools, whether dedicated computers or general-purpose computers, are collectively referred to as numerical control systems. The movement and operation of CNC machine tools is controlled by auxiliary commands issued by the NC system. The instructions of the CNC system are compiled by the programmer according to the material of the workpiece, the processing requirements, the characteristics of the machine tool, and the instruction format (CNC language or symbol) specified by the system. The numerical control system sends operation or termination information to the servo device and other functional components according to program instructions for controlling various operations of the machine tool.

The machine will automatically stop when the part machining program is complete. No CNC machine tool of any kind can operate without input program instructions in the CNC system.

  • The controlled operation of a machine tool generally includes starting and stopping the machine tool.
  • Spindle start / stop, rotation direction and speed change.
  • Direction, speed, and mode of feed operation.
  • Tool selection, length and radius correction.
  • Tool replacement, coolant opening / closing, closing, etc.

Development background

CNC technology was born out of the needs of the aviation industry. In the late 1940s, a US helicopter company proposed it. In 1952, MIT in the United States developed a 3-axis CNC milling machine. In the mid-1950s, this type of CNC milling machine was used to process aircraft parts. In the 1960s, numerical control systems and programming work became more and more mature and perfect. CNC machine tools have been used in various industrial fields, but the aerospace industry has always been the largest user of CNC machine tools. Some large aviation factories are equipped with hundreds of CNC machine tools, including cutting machines. CNC machined parts include aircraft, rocket bulkheads, girders, skins, bulkheads, propellers, aircraft engine casings, shafts, discs, vane cavities, special chambers for liquid rocket engine combustors, and more. The early stages of numerical control machine tool development were based on continuous orbit CNC machine tools and continuous orbit control.

Continuous trajectory control, also known as contour control, requires the tool to move in a defined trajectory for the part. After that, we also actively developed point control CNC machine tools. Point control means that the tool moves from one point to another as long as it can reach the target accurately, regardless of the route it travels.

Operation process

CNC programming methods are manual programming and automatic programming. In manual programming, the entire contents of the program are manually written in the instruction format specified by the CNC system. Automatic programming is computer programming and can be divided into language and drawing-based automatic programming methods. However, no matter what automatic programming method you choose, you need the corresponding support hardware and software. You can see that the realization of numerical control machining programming is the key. But programming alone is not enough. In addition, CNC machining includes a series of preparations that need to be done before programming and aftermath processing after programming. In general, the NC machining process includes the following:

  • (1) Select and determine the parts and contents of CNC machining.
  • (2) Process analysis of numerical control of parts drawings.
  • (3) CNC process design.
  • (4) Mathematical processing of parts drawings;
  • (5) Creation of processing procedure.
  •  ⑹Programmed to generate a single control medium;
  • (7) Program verification and modification.
  • (8) Sample testing and on-site problem handling.

Stereotyped and archived digital processing technology files

A series of advanced numerical control processing techniques have also been developed and used in the aerospace industry to improve the degree of production automation, reduce programming time and reduce the cost of numerical control processing. This softly coupled computer numerical control system uses computer numerical control, ie, software stored in the computer to replace the controller in the CNC system using a small or microcomputer and perform computational and control functions. , The initial state of the numerical control system is gradually being replaced. Direct numerical control is suitable for short-term production of small batch aircraft by directly controlling multiple CNC machine tools using a computer. An ideal control system is an adaptive control system that can continuously change processing parameters. The system itself is complex and expensive, but it can improve processing efficiency and quality.

CNC Development In addition to improving hardware and CNC systems and machine tools, another important aspect is software development. Computer-aided programming (also called automatic programming) is a program written in the CNC language by a programmer. It is input to the computer for translation and the last punched tape or tape is automatically output by the computer. The widely used NC language is the APT language. It is roughly divided into a main and post-processing program. The former translates the program written by the programmer and calculates the toolpath. The latter edits the tool path as part of a machining program for CNC machine tools. CNC machining is to create a program on a computer in advance before machining a workpiece and input this program to a machine tool controlled by a computer program for instruction processing. Alternatively, instructions for direct processing can be written on the control panel of the computer program controller. The processing process is all automatic, including moving knives, changing knives, changing speed, changing direction, parking, and more. CNC machining is an advanced tool for modern mold making and machining. Of course, the numerical control processing method must correspond not only to the processing of mold parts but also to a wide range of applications.

Features of CNC Machining

Machined parts There are a wide range of problems in the CNC machining process. The following suggests key content that needs to be analyzed and reviewed in the context of programming potential and convenience.

In numerical control programming, the size and position of all points, lines, and faces are based on the starting point of the programming. Therefore, it is best to specify the coordinate size directly.

Geometric element conditions must be perfect and accurate

In programming, the programmer must have a complete grasp of the parameters of the geometric elements that make up the contour of the part and the relationships between the geometric elements. All geometric elements of the part contour are defined during automatic programming, so the coordinates of each node are calculated during manual programming. No programming can be done without any clearness. However, the parameters are often incomplete or ambiguous due to poor design considerations or neglect by the part designer. Such: arcs and straight lines, arcs and arcs touching, intersecting, or distant. Therefore, drawing reviews and analysis should carefully identify the problem and contact the designer in a timely manner.

Reliable positioning

In numerical control processing, processing operations are often concentrated, and it is very important to place the same reference. Therefore, it is often necessary to set up ancillary benchmarks. Or add some craft bumps to the blank.

Uniform geometry type or size

The shape and internal cavity of the part is preferably a uniform geometry or dimension, which can reduce the number of tool changes. You can also apply control programs and special programs to shorten the program length. The shape of the parts is as symmetrical as possible and it is easy to save programming time using the mirroring capabilities of CNC machine tools.

Basic principles of placement and installation

When machining parts with CNC machine tools, the basic principle of positioning and installation is the rational selection of positioning criteria and clamping programs. Please note the following points when selecting.

  • 1 aims to unify reference design, technology and programming calculations.
  • 2. Reduce the clamping time as much as possible after positioning clamping and treat all surfaces to be treated.
  • 3. Avoid manual adjustment processing plan of the machine to make full use of CNC machine tool.

Basic principle of selecting a jig

The features of CNC machining present two basic requirements for fixtures.

The first method is to ensure that the coordinate direction of the fixture and the coordinate direction of the machine tool are relatively fixed.

The second is to adjust the dimensional relationship between the part and the machine coordinate system. In addition, consider the following:

  • 1. If the amount of parts processed is not so large, please use combination fixtures, adjustable fixtures and other universal fixtures to shorten the preparation time and save the production cost. ..
  • 2. Consider the use of special equipment in batch production and strive for a simple structure.
  • 3. The loading and unloading of parts must be quick, convenient and reliable in order to reduce the downtime of machine tools.
  • 4. Make sure that the jig parts do not interfere with the machining of the parts on the surface of the parts. That is, the fixture must be open and its positioning and clamping mechanism components do not affect cutting tools (such as collisions) during the process.

Processing error

CNC processing error △ Numerical values ​​are due to errors such as programming error △, machine error △ machine, positioning error △ setting, knife error △ knife.

That is, △ numerical value + = f (△ programming + △ machine + △ set + △ knife)

inside that:

  • 1. Configuration of programming error Δ and rounding error due to approximation error δ.The approximation error δ is generated by the process of approximating a non-circular curve with straight or arc segments, as shown in Figure 1.43. Rounding error is the error generated by rounding the coordinate values ​​to the integer pulse equivalent during data processing. The pulse equivalent refers to the displacement of the corresponding axis of each unit pulse. A typical precision CNC machine tool has a typical pulse equivalent of 0.01 millimeters. More accurate numerical control The pulse equivalent of a machine tool is 0.005 mm or 0.001 mm.
  • 2. Machine tool error Δ is due to the error of the CNC system and the error of the feed system.
  • 3, Positioning error △ When the clamp is placed on the machine, the clamp is generated when the workpiece is placed on the fixture.
  • 4. Tool setting error Δ occurs when the relative position of the tool and the workpiece is determined.

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