【正文】 solves some of the problems that exist in coordinating a direct numerical control system. There is another type of distributive numerical control that is a spinoff of the system previously explained. In this system, the NC program is transferred in its entirety from a host puter directly to the machine’s controller. Alternately, the program can be transferred from a mainframe host puter to a personal puter (PC) on the shop floor where it will be stored until it is needed. The program will then be transferred from the PC to the machine controller.CNCIn 1970, a new termputerized control (CNC)was introduced to the NC vocabulary. The development of a CNC system was made economically possible by electronic breakthroughs which result in lower coasts for miniputers.The physical ponents of CNC units are the same regardless of the type of machine tool being controlled. On soft wire units, it is not the MCU but the executive program, or load tape, which is loaded into the CNC puter’s memory by the control manufacturer. It is this executive program that makes the control unit “think” like a turning center or a machining center. Therefore, if there is a needed to change the functions of a softwire or CNC unit, the executive program can be changed to suit. Usually it is the manufacturer, and not the user, who makes the change.There have been great of developments in MCUs with the introduction of largescale ICs and microprocessors. Many features not available a few years ago give much greater flexibility and productivity to NC systems. As developments in control units continue new features which were not found on the pre1970 hardwired control units.1. CathodeRay Tube (CRT)The CRT is like a TV screen which serves a number of purposes:a. It shows the exact position of the machine table and/or the cutting tool at every position while a part is being machinedb. The entire program for a part can be display on the screen for editing or revision purposes.c. The screen assists in work setups, and some models receive messages from sensors which indicate machine or control problems.2. Absolute and Incremental ProgrammingBy using the proper address code G90 or G91, most CNC units will automatically program in that particular mode. Some of the newer CNC units are capable or handling mixed data in a given block or data. 3. Inch or MetricMost CNC units are capable of working in inch or metric dimensions. Either a switch on the control unit or a specific code on the part program will determine the measurement system used when machining a part.4. EIA or ASC II CodeMany of the newer CNC units will read either the Electronic Industries Association (EIA) or the American Standard Code for Information Interchange (ASC II) standard code. The control unit identifies each one by the parity check: odd for the EIA code and even for the ASC II Code.5. Manual Data InputMost CNC units provide some method of making changes to the part program, if necessary. This may be necessary because changes were made to the part specifications, to correct an error, or to change the machining sequences of the part.6. Program EditingVery few part programs are free from error from the start, and the flaws generally show up on the shop floor. Program editing is a feature which allows the part program to be corrected or changed right at the control unit.7. InterpolationWhile early models of control units were capable of only linear, circular or parabolic interpolation, the newer models include helical and cubic interpolation. 8. Point—to—Point and Continuous—Path PositionAll MCUs are capable of point—to—point or continuous—path positioning or any bination of each.9. Cutter Diameter and Length CompensationOn newer MCUs, it is possible to manually enter the diameter and/or the length of a cutter which may vary from the specification of the part program. The control unit automatically calculates what adjust for the difference.10. Program StorageThe newer CNC units generally provide largecapacity puting and data storage. This allows information about a part program to be entered and stored for future use. Therefore, whenever the part program is can be recalled from memory rather than reread from the tape. This not only protects the quality of the tape, but the information is recalled much faster.11Canned or Fixed CyclesStorage capacity is generally provided in the MCU for any cycle. This is used or repeated in a program. When the program is being written and a previous cycle is required. The control until will recognize that code and recall from memory all the information required to perform that cycle again.12. Subroutines and MacrosA parametric subroutine, sometimes called a “program with program,” is used to store frequently used data sequences which can be recalled from memory as required by a code in the main part program. An example of a subroutine could be a drilling cycle in which a series of 3/8 inchdiameter holes 1 in. deep must be drilled in a number of locations on a work piece.A macro is a group of instructions or data which are permanently stored and can be recalled as a group to solve recurring problems such as bolthole circle locations, drilling and tapping cycles, and other frequently used routines. An example of a macro would be the XY locations of various holes on any vided, the MCU makes all the calculations for hole locations and caused the machine tool slides to move into the proper position for each hole.13. Axis InversionAxis inversion is the ability of the MCU to reverse plus and minus values along and axis or program zero to produce an accurate lefthand program. This ability applies to all four quadrants and reduces the time that would be required to program each part.14. DigitizingThe digitizing feature allows a part program to be made directly from an existing part. The original part is traced while the CNC unit records all ma