CNC descended from NC (Numerical Control), also known as Tape Numerical Control, to distinguish that earlier generation from CNC. Tape NC did not have computing capablilty in the control, and some units were only able to position to an X-Y coordinate, and then drill a hole there. Other units could travel in only one axis at a time, or do linear interpolation only. The most sophisticated ones could also do circular interpolation. Cutter radius compensation, helical interpolation, blending of line segments and program lookahead were generally too hard to implement in raw hardware, and only became available when computers became small and inexpensive enough to connect directly to the machine tool.
CNC systems generally (but not always) had CRT screens, which could display more useful messages to the operator, were able to show some 'peephole' view of the program blocks to be executed next, and had a number of fields to show which modal functions were in force at the moment.
Most CNC systems use the IEEE standard RS-274D as their input language, providing a great flexibility of mixing CAD, CAM, CNC and other systems together, regardless of manufacturer, to get exactly what you need done, without paying for unneeded features. Note that a substantial number of systems only support proprietary command languages, Heidenhain being one of the major ones. While these proprietary systems are often very powerful, in that very complex tasks can be described in very few lines, their lack of standardization is a serious drawback. Of course, the large commercial shop with several machining centers from different makers immediately sees the benefit of being able to take a partially completer workpiece off, say, an Okuma machine, and finish it on a Cincinnatti machine with just a few keystrokes, when a breakdown stops the first machine. But, even the small shop with just one machine can get a benefit from being able to mix and match CAD, CAM and CNC systems that 'all talk one language'.
The above paragraph is too simplistic, of course, in that, due to imprecision in the RS-274D standard, every control builder has their own dialect of RS-274D, and so a CAM program needs to have some details filled in before it can write a program acceptable to a specific machine.
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