Introduction: What is Backlash?

In a ball screw assembly, "Backlash" is the amount of axial play (movement) between the screw and the nut when the direction of rotation is reversed. If your motor turns, but the table doesn't move for the first few microns, you have backlash.

While TOCO offers Zero-Backlash (Preloaded) nuts, all mechanical systems will eventually develop some play due to natural wear. Knowing how to measure this accurately allows you to compensate in your CNC software or decide when it’s time for a replacement.

1. Required Tools for Measurement

To measure backlash accurately, you cannot rely on the naked eye. You will need:

• Magnetic Base: To hold your indicator firmly to the machine frame.

• Dial Indicator: Preferably a "Digital Micrometer" style with a resolution of at least 0.001mm (1 micron).

• Manual Pulse Generator (MPG): Or a way to move your machine in precise, small increments (the "Jog" function).

2. The "Push-Pull" Test (Static Measurement)

This is the simplest way to check for mechanical looseness in the nut or the support bearings.

1. Lock the Screw: Disable the motor so the screw cannot rotate.

2. Position the Indicator: Place the tip of the dial indicator against the side of the moving table (carriage). Set the dial to zero.

3. Apply Manual Force: Firmly push the table in one direction, then pull it in the other.

4. Read the Dial: The total movement shown on the needle is your mechanical backlash.

   • Note: If this value is high, it could be the ball nut OR a loose locknut on the support bearing.

3. The "Reversal" Test (Dynamic Measurement)

This test identifies the "Lost Motion" that occurs during actual machine operation.

1. Move to a Starting Point: Jog the machine in a positive direction (e.g., +X) for a few centimeters to take up any slack.

2. Zero the Indicator: Place the indicator against the table and zero it.

3. The Micro-Move: Program the machine to move exactly +0.100mm. Verify the dial shows exactly 0.100mm.

4. The Reversal: Program the machine to move exactly -0.100mm (back to the starting point).

5. Calculate the Difference: If the dial indicator reads 0.015mm instead of 0.000mm, your backlash is 15 microns.

4. Diagnosing the Result

Where is the play coming from?

• Constant Backlash: If the error is the same at both ends of the screw, the problem is likely in the Support Bearings (BK/BF units) or a loose coupling between the motor and the screw.

• Variable Backlash: If the play is worse in the middle of the screw than at the ends, the Screw Raceways are worn in the most frequently used area. The screw must be replaced.

5. Can Backlash be Fixed?

• Software Compensation: Most modern CNC controllers (Mach3, LinuxCNC, Fanuc) allow you to enter a "Backlash Compensation" value. The software will "over-rotate" the motor to make up for the gap.

• Mechanical Re-balling: In some cases, a TOCO nut can be restored by installing slightly larger (oversized) ball bearings to restore the preload.