Backlash – unequal (machine error)
Plots
The plot has either unequal backlash steps, or, for machines with backlash compensation, backlash steps appearing in opposite directions on the same axis. In these plots unequal backlash is shown in the Y axis only.
Note: unequal backlash is not detected on one of the axis for a 220° test.
Diagnostic values
All backlash is quantified in the same way as described for positive backlash. Unequal backlash may be characterised by a large difference between the positive axis and negative axis values, or by both positive and negative backlash in the same axis.
For 220° diagnosis a backlash value is not displayed for the arc where data has not been captured. Unequal backlash will not be diagnosed in this case.
Cause
An excessive amount of windup in the ballscrew is causing a backlash effect. This depends on the position of the axis relative to the driven end of the ballscrew, which causes the unequal backlash type plot. Machines with backlash compensation may be adjusted to 'average' this difference, resulting in the opposing steps. The windup may be caused by a stiff or worn ballscrew, nut or guideway. This type of backlash is common in tests involving vertical movement, due to counterbalancing effects.
Effect
Inaccuracy in cut parts which change in magnitude depending on the position along the axis.
Action
Remove any backlash compensation which is applied to the machine. This will allow the extent of the problem to be assessed.
Check the ballscrew and guideways of the machine for signs of wear. If necessary maintain or renew these components.
If unequal backlash plots are the result of tests involving vertical machine movement, then the counterbalance is likely to be the source of the problem. Adjust the counterbalance system of the machine as required.