Question

Under what conditions is incremental programming (G91) preferred over absolute programming (G90) for CNC machining?

Accepted Answer

Incremental programming (G91) is preferred over absolute programming (G90) for CNC machining under conditions where the tool movements are easily defined as distances relative to the current tool position, rather than absolute coordinates referenced to a fixed origin. G90 (absolute programming) defines positions in terms of a fixed coordinate system, while G91 (incremental programming) defines positions as distances from the current location. Incremental programming is particularly useful for repetitive operations that involve consistent step-over distances, such as creating patterns of holes or pockets, or machining multiple identical features spaced evenly apart. For example, drilling a series of equally spaced holes along a line can be simplified using incremental programming. The programmer only needs to define the distance between each hole relative to the previous hole, reducing the amount of code and potential for errors compared to calculating the absolute coordinates for each hole. Also, incremental programming is advantageous when writing subprograms or macros where the exact absolute location of the feature to be machined is not known until the subprogram is called; the subprogram can then operate based on relative movements from the current tool position. When small, repetitive changes are all that are required, using incremental mode also limits the opportunities for calculation errors in the absolute positions, simplifying the coding and debugging processes.

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Under what conditions is incremental programming (G91) preferred over absolute programming (G90) for CNC machining?