Machinery's Handbook, 31st Edition
CNC PROGRAMMING 1353 Within a word, there cannot be any spaces, but spaces are allowed in a block, between words only. The numeric value that immediately follows the address (letter) has to be entered in a format specific for that address. Using some of the words above, a meaningful block can be constructed: N49 G01 Z- 2.5 F10.0 M08 where: N49 = Block number (sequence number) G01 = Linear motion selected Z-2.5 = Z-axis motion only An early standard of Electronic Industry Association (EIA) RS-274 from the 1960’s defines various commands and functions for part programming. It was last updated in February 1980 to RS-274D but has never fully succeeded in its attempt to provide a uniform CNC programming environment for different control systems, mainly because of competitive forces in industry. However, many features of this standard have become common practice in programming and are used to this day. Program Development.— There are several methods of programming CNC equipment that will generate a working part program: a) Manual programming (also known as G-code programming) F10.0 = Feed rate specification M08 = Coolant ON function b) Computer based programming (also known as CAM programming) c) Shop-floor programming (also known as Conversational programming) d) Macro programming Manual (G-code) Programming requires the most effort, is time consuming, and prone to errors. The programmer writes every instruction separately, typically using a text editor on a computer to enter programming instructions. All calculations are done with the use of a pocket calculator. In spite of its disadvantages, manual programming has two major advantages—it offers complete human control over every detailed aspect of toolpath development and all related activities. Also, it provides important learning ground for part programming using computer methods. Originally, all manual programming was done all by hand and written on paper. Now, any computer can be used to write the program using a keyboard, several software based calculators can be used instead of hand-held devices, and simulation software can verify the toolpath graphically, on a computer screen. Computer Based Programming: Computer generated programs are based on graphical input, and typically require development of part geometry (point, lines, arcs, etc.). The final edited geometry serves as the toolpath geometry and can be assigned machining parameters. Specifying tool definition, spindle speeds and cutting feeds, and various cut ting conditions is part of the programming process. A good knowledge of manual pro gramming provides may necessary skills for computer based part programming. Computer based programming is sometimes called (incorrectly) “CAM programming”, because of the multitude of CNC software that uses the “cam” in the product title. It has nothing to do with a mechanical motion control device of the same name. Any CAD/CAM system that provides CNC capabilities also has to provide precise for matting of the final program output. This output is always based on a particular machine tool and control system combination. Software vendors offering program development software are very much aware of this need, yet face the extremely difficult task to cover every machine tool design with every control system available. When the toolpath is ini tially processed, it is generic—it has no connection to any particular control or machine specifications. The same toolpath has to be processed again (post processed) for a
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