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OPTIONS
June 1999
CONTOURING A CCURACY
When milling a part it is desirable to machine it at the fastest feedrate possible and yet obtain the highest
accuracy. No machine can instantly accelerate or decelerate and this leads to positioning errors when axis
directions change. The higher the feedrate and the sharper the turn the more pronounced the effect. A high
feedrate selected for straight line milling will cause corner rounding at intersections when you actually want a
sharp corner. Similarly a high feed milling cut blending into a radius will cause the radius to be smaller than
programmed because of axis acceleration distance. To alleviate this the HAAS control uses a special function
called Contouring Accuracy (G187) so the operator can select the required accuracy.
When using this function the control will move up to the programmed feedrate in straight line moves and slow
down at intersections or radii to obtain the required accuracy you want. This accuracy is measured in true
three dimensional motion. It uses setting 85 to define a default value and G187 to program a new value directly
from your program. The amount of slow down depends on the accuracy specified and on how well one stroke
blends into the next. If two strokes blend into each other exactly (in one line), there is never any slow down.
Programming G187 is as follows:
G187 E0.01 (to set value)
G187 (to revert to setting 85 value)
The first line will set the required accuracy to 0.01 inches. G187 must be programmed on a line by itself. If
there is no E code, the accuracy reverts to setting 85. If MM programming is active, the units are millimeters.
The range of values possible are 0.0001 to 0.25 inches and 0.001 to 2.5 mm.
The most important thing to remember with Contouring Accuracy is that normal, well blended strokes should
not get a slowdown in feedrate. Only the sharp corners need this in order to achieve the programmed accu-
racy. However, if you set the accuracy to extremely small values (0.0005 and smaller) you may effect such a
slow feedrate that the machine appears to pause when you donîš’t think it should. Even well blended strokes
can have a slight error.
When roughing out a pocket the default setting (85) should be used for maximum speed then on the final clean
up passes you can specify a higher accuracy. A simple test part of milling a rectangle and varying the values
for Contouring Accuracy will demonstrate the principles outlined above.
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