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Measuring Axis Backlash

Introduction

There are as many ways of measuring backlash as there are CNC technicians. Each method will give you a different answer. This is my way, which is reasonably representative of practical cutting speeds and conditions.

Procedure

Set up a 1" travel indicator bearing against the axis. Take some time to make sure it is straight and square.

Jog the axis until the indicator is a few turns into its travel, then zero the indicator. Don't worry about getting it exact: you don't really care what number the indicator stops at; you are only looking for differences.

Create and load a program similar to this:

  g1 g91 f8
  x.05
  x-.05
  g4 p1
  x-.05
  x.05
  g4 p1
  x.05
  x-.05
  g4 p1
  x-.05
  x.05
  g4 p1
  x.05
  x-.05
  g4 p1
  x-.05
  x.05
  g4 p1
  ; ... and so on ...

For your convenience, pre-written CNC files are available here:

Mill X Axis	xlash.cnc	LashProgs.zip
Mill Y Axis	ylash.cnc
Mill Z Axis	zlash.cnc
Lathe X Axis	ulash.cnc
Lathe Z Axis	wlash.cnc

Run the program. It will move the axis 0.050" out and back, pause for a moment, then move 0.050" out and back in the other direction, and pause again.

Look at the difference in the indicator position between the two moves. Remember again that you don't care whether the needle points to zero or not; you only care what the difference is between the position after moving plus-then-minus and the position after moving minus-then-plus.

If the indicator comes up short, then you need to increase your backlash compensation on that axis. If the indicator overshoots, then you need to decrease your backlash compensation on that axis.

Where do I find the backlash compensation amounts?

In recent software versions (anything since 2003 or so):

1. Press F7/Utility
2. Press F6/User Maint
3. Press F2/Lash
4. Edit the desired backlash compensation value
5. Press F10/Save
6. Press ESC as needed to return to the main screen

In older software versions (pre-2003):

1. Press F1/Setup
2. Press F3/Config
3. Enter the configuration password (usually "137")
4. Press F2/Machine
5. Press F2/Motor
6. Arrow over to the Lash Comp column
7. Edit the desired backlash compensation value
8. Press F10/Save
9. Press ESC as needed to return to the main screen

What if the needle overshoots, but the backlash compensation is already zero?

This usually means that there is some backlash, but that the axis is tending to "coast" into the backlash range after the servo motor comes to a stop.

You can reduce this by making the stop more gradual. Either decrease the Deadstart value on the Machine Configuration -> Jog Parameters screen, or increase the Accel Time value on the PID Configuration -> PID Parameters screen.

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Copyright © 2021 Marc Leonard
Last updated 18-Apr-2021 MBL