THE CNCPROS STANDARD FOR ALIGNING YOUR BALLSCREW AFTER REPLACEMENT

Ballscrew Alignment: The CNCPros International Standard

Precision isn't just about the parts you install—it's about how you align them. Aligning a precision ground ball screw is like tuning the tracks for a high-speed train; if the rails aren't perfectly parallel and level, the train will struggle, vibrate, or eventually derail. In a CNC machine, the ball screw is the "muscle" that translates rotational motion into precise linear movement. If it is misaligned, you introduce stress, friction, and binding, which leads to premature wear of the bearings and the ball nut. 

At CNCPros International, we’ve spent over 25 years serving as the industry’s technical authority for Fadal maintenance. Brian Denny few all over the world fixing Fadal's CNC Machining Centers only when nobody else could.  Our promise was if we flew to your location and could not fix the machine, the trip was free.  Never did we not get paid.  We don't just provide the hardware; we provide the expert methodology to ensure your machine holds sub-micron tolerance.

1.  Preparation and Inspection

Precision ground ball screws have extremely tight tolerances; even a small speck of dirt or metal chip can cause the ball nut to bind or become misaligned.

• Cleaning: Thoroughly clean all mounting surfaces, bearing housings, and the screw shaft itself.
• Surface Check: Inspect all surfaces for nicks or debris that could prevent flush mounting.
• Pre-installation Check: Before locking everything down, confirm that the screw spins freely by hand.  If it feels "gritty" or binds at any point, do not proceed.  Re-examine the cleanliness of the tracks and the bearing seals.

2.  General Alignment Principles

The goal is to ensure the axis of the ball screw is perfectly parallel to the axis of the guideways (linear rails).

• The "Floating" Method: Leave the bearing housing bolts slightly loose. By moving the machine carriage along its travel, the carriage naturally positions the ball nut housing, allowing the assembly to "float" into its most relaxed, stress-free position before you fully tighten the fasteners.
• Parallelism Check: Use a dial test indicator (DTI) to verify that the screw is parallel to the linear rails in both the horizontal and vertical planes.
• Using Shims: If you detect misalignment, use precision shim stock to adjust the height of the bearing blocks or the nut mounting bracket. Never "force" a screw into place by overtightening mounting bolts, as this will put the shaft under radial load.

3. Bearing and Preload Setup

The way you secure the ends of the screw is critical for precision.

• Fixed-Supported Configuration: The "fixed" end handles both axial and radial loads (often using angular contact bearings), while the "supported" end allows for thermal expansion.
• Adjusting Preload: If it's too loose, you’ll have backlash; if it's too tight, you will feel significant resistance when turning the screw by hand, leading to heat buildup and failure. Use an indicator to monitor the assembly as you tighten the lock nuts.

4. Electronic Verification and Alignment

Once mechanically set, use the CNCPros electronic protocol to ensure the axis is perfectly parallel.

• Step A: Install the ballscrew and align it from end to end.
• Step B: With a clamp-on ammeter, clamp to the brown wire connected to the axis amplifier. If using an AC amplifier, attach your ammeter to one of the output wires R, S, or T.
• Step C: Jog the axis to the + limit.
• Step D: Enter one of the following commands into MDI, adjusting values to reflect total travel:

X-Axis: G1  F25.   G91  X-_.  M49

Y-Axis: G1 F25. G91 Y-_. M49

Z-Axis: G1 F25. G91 Z-_. M49

• Step E: Monitor the ammeter display while traversing; readings should not vary more than 2 amps for MM screws and 1 amp for inch ballscrews.
• Step F: Watch for variations at either end when being approached; if higher than normal values are observed, check and adjust alignment.

5. Final Calibration

• The "Acid Test": If the screw binds at one end of the stroke but not the other, it indicates the bearing housing is not perfectly aligned with the linear rail axis; re-shim until rotation force remains constant.
• Completion: If the ballscrew is realigned, adjust the backlash again and retest.

Struggling to get your axis alignment within spec? Reach out to our technical team at 208-888-9236. We’ll help you verify your alignment and ensure your machine is running smoother than it ever has.