G20, G21, G70, G71 (Length Units)

program g20 to use inches for length units program g21 to use millimeters it is usually a good idea to program either g20 or g21 near the beginning of a program before any motion occurs, and not to use either one anywhere else in the program it is the responsibility of the user to be sure all numbers are appropriate for use with the current length units g70 and g71 are added for cam software compatibility