Machinery's Handbook, 31st Edition
1352
CNC PROGRAMMING
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Y- Fig. 3. Incremental Motions. CNC Programming Process
CNC program (also called part program ) development starts with an engineering draw ing and material stock. Initial decisions include tasks to be done and goals to be achieved. The part program has to provide a safe and efficient method of machining a part, it should have a consistent structure (method of data entry), and be as portable as possible. Program portability allows the same program to run on machines that have similar characteristics. Typical logical process of program development can be summarized in a short, general list shown below. In actual use, specific steps will vary: Program Development List a) Evaluation of initial drawing and material. b) Sequence of machining operations. c) Tooling selection and arrangement of cutting tools. d) Part setup method. e) Technological data selection (speeds, feed rates, depths, etc.) f) Toolpath determination. g) Mathematical calculations. h) Program writing and preparation for transfer to CNC. i) Program testing. j) Program documentation. Word Address Format.— Writing any data to be processed by a computer requires a certain input format called a program. CNC program instructions are written in a special coded form obeying strict rules required by the programming language. These alpha- numerical instructions are called words . Where a single letter is followed by one or more digits—the letter must always be first. Special symbols, such as a decimal point or a nega tive sign may also be used, if required. Each letter, digit, or symbol represents one pro- gram character as well as one character in the control memory. A character is the basic unit of a a CNC program. CNC programs only use characters that are readily available on a standard computer keyboard, without any multi-key combinations. A word in CNC program consists of two or more characters and is a single instruction to the control sys- tem. It can be used individually or it can be grouped into a series of combined instructions (words), to be performed at the same time. A program word is the unit of instruction to the control system. Instructions performed simultaneously are called a block , which is al- ways represented by one line of the program. Individual instructions (blocks) are written in sequential order. Some typical words used in a block are: M08 D14 X7.875 H03 /M01 Y0 G01 S850 Z-2.5 N49 T0707 T05 F10.0
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