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INCREMENTAL POSITIONING DIMENSIONS—G91 MODE

In incremental mode, each programmed motion is independently measured from the previous point (Figure M-35).

For example, the drawing shows a four-hole pattern with each feature dimensioned relative to the other features. If the part were to be drilled starting from the setup point indicated, each tool positioning move would be an increment equal to the drawing dimensions. Positioning from the setup point to hole #1 would require a move in the X axis of 2.000 in. and a move in the Y axis of 1.000 in. When the tool arrives at hole
#1, the computer resets its counter to zero in both axes. The computer now assumes that this is a new starting point. The next incremental move to hole #2 requires a position move 2.000 in. farther along the X axis. The computer starts a new count at zero for the next 2.000 in. increment. Another way to think about incremental positioning and dimensions is to consider that the tool position is always at zero wherever it is.
Absolute Positioning Dimensions—G90 Mode In the absolute mode, each position location move is measured from a single start point called absolute zero (Figure M-36). A drawing may be dimensioned this way as well. An incrementally dimensioned drawing may be programmed in
absolute mode or have its dimensions converted if necessary.

The drawing in Figure M-36 is the same part as that in Figure M-35. The difference is that all the hole features are dimensioned from the absolute zero setup point. Hole #1 is at the absolute location X 2.000, Y 1.000. Hole #2 is located at X 6.000 and Y 1.000. Hole #3 is located at X 2.000 and Y 5.000. Hole #4 is located at X 6.000 and Y 5.000. When the absolute starting position is established by the program, all
moves will be measured from that point.
There are a few variations to be noted in the absolute mode. Depending on the location of the setup point, tool positioning may occur only in quadrant I. All axis motion values will be positive. No negative moves will be programmed, because all points in quadrant I are positive relative to absolute zero. If the setup point is the center of a circular hole pattern, then the tool will move into quadrants II, III, and IV, and it will be necessary to program negative axis values and also to maintain both axis locations relative to absolute zero. Keep in mind that in absolute G90 mode, you must tell the computer to maintain each absolute axis location programmed by continuing to place these words in the program.
If you do not enter both the absolute axial dimensions to maintain axis locations and leave them blank, the computer will assume that you want to program absolute zero in these blocks and will reposition the tool accordingly.
Now that you have an understanding of programmable axes, motion directions along and around these axes, and the methods of positioning dimensions, you can begin to develop the complete CNC program. This is the subject of the next unit.