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# 11.2 G81 Cycle

The G81 cycle is intended for drilling.

0. Preliminary motion, as described above.

1. Move the Z-axis only at the current feed rate to the Z position.

2. Retract the Z-axis at traverse rate to clear Z. This cycle was used in the description of G80 above but is explained in detail here.

Example 2 - Absolute Position G81

Suppose the current position is (1, 2, 3) and the following line of NC code is interpreted.

G90 G81 G98 X4 Y5 Z1.5 R2.8

This calls for absolute distance mode (G90) and OLD_Z retract mode (G98) and calls for the G81 drilling cycle to be performed once. The X value and X position are 4. The Y value and Y position are 5. The Z value and Z position are 1.5. The R value and clear Z are 2.8. OLD_Z is 3.

The following moves take place.

1. a traverse parallel to the XY plane to (4,5,3)

2. a traverse parallel to the Z-axis to (4,5,2.8).

3. a feed parallel to the Z-axis to (4,5,1.5)

4. a traverse parallel to the Z-axis to (4,5,3)

Example 2 - Absolute Position G81

Suppose the current position is (1, 2, 3) and the following line of NC code is interpreted.

G91 G81 G98 X4 Y5 Z-0.6 R1.8 L3

This calls for incremental distance mode (G91) and OLD_Z retract mode (G98). It also calls for the G81 drilling cycle to be repeated three times. The X value is 4, the Y value is 5, the Z value is -0.6 and the R value is 1.8. The initial X position is 5 (=1+4), the initial Y position is 7 (=2+5), the clear Z position is 4.8 (=1.8+3), and the Z position is 4.2 (=4.8-0.6). OLD_Z is 3.

The first preliminary move is a traverse along the Z axis to (1,2,4.8), since OLD_Z < clear Z.

The first repeat consists of 3 moves.

1. a traverse parallel to the XY-plane to (5,7,4.8)

2. a feed parallel to the Z-axis to (5,7, 4.2)

3. a traverse parallel to the Z-axis to (5,7,4.8) The second repeat consists of 3 moves. The X position is reset to 9 (=5+4) and the Y position to 12 (=7+5).

1. a traverse parallel to the XY-plane to (9,12,4.8)

2. a feed parallel to the Z-axis to (9,12, 4.2)

3. a traverse parallel to the Z-axis to (9,12,4.8) The third repeat consists of 3 moves. The X position is reset to 13 (=9+4) and the Y position to 17 (=12+5).

1. a traverse parallel to the XY-plane to (13,17,4.8)

2. a feed parallel to the Z-axis to (13,17, 4.2)

3. a traverse parallel to the Z-axis to (13,17,4.8)

Example 3 - Relative Position G81

Now suppose that you execute the first g81 block of code but from (0, 0, 0) rather than from (1, 2, 3).

G90 G81 G98 X4 Y5 Z1.5 R2.8 Since OLD_Z is below the R value, it adds nothing for the motion but since the initial value of Z is less than the value specified in R, there will be an initial Z move during the preliminary moves.

Example 4 - Absolute G81 R > Z

This is a plot of the path of motion for the second g81 block of code.

G91 G81 G98 X4 Y5 Z-0.6 R1.8 L3

Since this plot starts with (0, 0, 0), the interpreter adds the initial Z 0 and R 1.8 and rapids to that location. After that initial z move, the repeat feature works the same as it did in example 3 with the final z depth being 0.6 below the R value.

Example 5 - Relative position R > Z

Next: 11.3 G82 Cycle Up: 11. Canned Cycles Previous: 11.1 G80 Contents Index