How-To Measure Machine Backlash
This step-by-step procedure describes measuring backlash on a CNC Machine Axis using the commonly available Dial Indicator setup on a Magnetic Base.

CNC Machine Backlash Setup Backlash is measured at different locations for different reasons. However, only one location is required to explain how to measure backlash, so for the sake of explanation, this procedure uses the Axis level as shown on the image to the right.

1.    Select an indicator.  Use an indicator that has much more range of motion than the suspected amount of backlash.

2.    Move the Axis to the desired position for the backlash measurement.

3.    Setup the Indicator on the Axis Base. Setup the Indicator so that there is a gap between the Indicator probe and the Axis. The gap must be greater than any possible backlash amount.

4.    Move the Axis toward the Indicator to take up any backlash in that direction. The direction of the Axis move should not be reversed during this move to make sure that backlash is not reinserted into it. Continue moving the Axis into the Indicator until a preload is established greater than the expected amount of backlash then zero the Indicator.

5.    Move the Axis the desired distance away from the Indicator (the reverse direction from step 4). If there is no backlash the Indicator will show the same amount that the Axis moved. If the Indicator shows less than the distance the Axis moved then there is backlash (make sure the Indicator is still touching the Axis). The amount of backlash is the difference between the distance the Axis moved and the Indicated results.

Example:
The Axis was moved 0.010" (0.25mm).
The Indicator moved 0.008" (0.20mm).
0.010" - 0.008" = 0.002"
There is 0.002" or 0.05mm backlash in this example.

Note:  Once the backlash has been successfully measured in step 5, it can easily be measured in the reverse direction. Simply rezero the Indicator, then move the Axis the same distance in the reverse direction as step 5. This same process of rezeroing the Indicator then reversing the direction can be repeated as many times as desired.

I worked in the machine tool industry for 25 years, for simple measurement of backlash we always used the following protocol:

Move axis in positive direction to known position ie X110.000
Move axis in negative direction to X100.000
Set plunger or electronic indicatior to zero, ensure the indicator is "loaded"
Move axis in negative direction to X90.000
Move axis back in positive direction to X100.000
Difference between original zero and new reading equals backlash!
For more comprehensive assessment of axis backlash other "laser claibration" methods were also used.

Peter Groves 07/20/2012

For an in-depth perspective on backlash, referr the article, Axis Backlash.