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Backlash is any non-movement that occurs during axis reversals. Say, for instance, the X axis is commanded to move 1 inch in the positive direction. Immediately after this X movement, the X axis is commanded to move 1 inch in the negative direction. If any backlash exists in the X axis, then it will not immediately start moving in the negative direction, and the motion departure will not be precisely 1 inch.

Backlash in the X or Y axis of machining centers is most obvious when milling full circles. If any backlash exists, there will be a nasty witness mark on each quadrant line (X+, X-, Y+ and/or Y- side of the circle). As you probably know, backlash is caused by normal usage, and checking for backlash (easily done with a dial indicator) should be part of your preventive maintenance program.

There is a feature called backlash compensation that electronically compensates for backlash. However, this feature (commonly set by parameters) simply causes the control to add the amount of backlash to each axis reversal. While this improves positioning accuracy, it does nothing to minimize the excessive vibration a machine will experience when performing powerful machining operations if the axis drive systems are sloppy. The best way to correct axes (at least with box way construction) is to adjust the machine's gibs to eliminate the backlash all together.

When you must machine holes in precise X, Y locations (when finish boring, for example), backlash can cause positioning errors in hole locations if the motions required to machine the holes cause a reversal in axis direction. Consider, for example the series of X, Y motions to machine the holes in the workpiece shown in Figure 1.

As the boring bar moves from hole No. 1 to hole No. 2, it's moving in the X+ and Y directions. As it moves to hole No. 3, the Y axis experiences a reversal in direction. Any backlash in the Y axis will tend to shrink this Y-axis movement. Hole No. 3 will be out of position by the backlash amount. In similar fashion, the X axis is moving in the positive direction from holes No. 1 to No. 2 to No. 3. But during the X motion to hole No. 4, the X axis reverses direction. If there is any backlash in the X axis, the X distance between holes No. 1 and No. 3 will be different than the X distance between holes No. 4 and No. 5. Note that if it's properly adjusted, backlash compensation will eliminate this potential problem.

One long-term way to minimize errors in positioning movements (especially for critical holes machined by boring bars), is to force the boring bar to approach the hole from the same direction in X and Y for every hole. For the most part, the direction from which the hole is approached is not too important. An exception might be a large horizontal machining center. Since the headstock moves to form the Y axis, and since it might be quite heavy if not properly counterbalanced, it would be wise to always approach from the negative Y direction. While the appropriate approach motion for each hole can be programmed long hand using simple motion commands, many controls have a special feature called single direction positioning that makes programming for this application very simple.

Fanuc and Fanuc-compatible machining center controls use a G60 word to specify the single direction positioning mode. Note that G60 is modal and must be canceled (with G64 with Fanuc controls) once you're done machining holes. The distance and direction for the approach is set by parameters (not programmable for most controls). For the machines we've seen, the amount of approach is commonly about 0.1 inch and the direction for the approach is from the negative direction in X and Y.

Once installed, single direction positioning will alter all movements in X and Y. First, the boring bar will be sent to a position about 0.1 (on the negative side) from the coordinates in X, Y. Then the boring bar will be brought to the programmed coordinates. Even if there is some backlash in the X or Y axis, since all movements will be done in this fashion, the distances between the holes will be perfect.