Axis Drift | Machine Drift

Machine tool axis drift (and what to do about it)

Gluzman, Dave

Machine tools equipped with rotary encoders can experience position drifting when they reverse their axial direction of movement. Precision milling operations require a linear accuracy of 0.0003" (0.0076 mm) over 15" (380 mm) of axial travel, and drift can make attainment of that accuracy impossible.

Machines that exhibit this phenomenon are normally equipped with a rotary encoder, a ballscrew with a ballnut-leadscrew coupling, a pair of matched thrust bearings that support the screw, and a CNC control that can compensate for backlash and pitch error.

During tests conducted to find out why axial drift occurs, each axis traveled a certain number of steps in each direction. At every stop point, a laser interferometry data acquisition/analysis system automatically measured the difference between the actual position and the target position. That difference equals the axis error. To detect backlash, after the last stop the axis did an overrun (a short "forward-reverse" motion), and the system read data again. On some machines we tested, the error curve shifted with each run and never settled down. We call this phenomenon axis drifting.

Drift severity varied from one machine to another, ranging from barely measurable to 0.0001" (0.0025 mm) per run. Machines went through a warm-up period before the tests to eliminate any temperature-- induced effects.

Normally, with the axis reversing direction at each end of the travel, any noncompensated backlash should appear on an error plot as a shift in the curves for forward and reverse moves for one run. A difference in backlash at the ends of travel, though, results not only in shifted forward-- reverse curves, but in drifting as well. Putting it another way, axis drift occurs because backlash at the right-hand reversing point differs from backlash at the left-hand reversing point.

With unequal backlashes, compensating backlash at one end leaves the other end not completely compensated. If both end backlashes were absolutely equal, a change in the direction of movement would not cause drift, even without backlash compensation.

It's well known that backlash occurs because of loose or worn ballnut-leadscrew couplings; loose, worn or non-- similar axis thrust bearings; improper bearing preload; or improperly adjusted gibs. Experience suggests thrust bearings are the major contributors to drift. Thrust bearing replacement and proper pre-load minimize drift. Because some machines we tested were nearly drift-free, we believe that proper installation and adjustment procedures, and replacement of faulty components, could cut drifting to a negligible level.

Keep in mind that mills equipped with linear scales, as opposed to rotary encoders, don't experience this drift. Even if a mechanical backlash occurs, the linear scale eliminates its effect. It's also important to differentiate between drift caused by warming and that resulting from backlash.

Dave Gluzman

Equipment Reliability Engineer Sulzer Carbomedics, Inc. Austin, TX

Copyright Society of Manufacturing Engineers Nov 1998
Provided by ProQuest Information and Learning Company. All rights Reserved