Reverse Engineering In-Depth: Complete Reconstruction and Manufacturing Process Optimization of High-Precision Ball Screws for Mori Seiki SL Series

Abstract

1. Failure Mode Analysis and Baseline Measurement

After 15–20 years of service, OEM ball screws (typically THK BNFN or NSK W series) in Mori Seiki SL machines exhibit the following dominant failure modes:

• Raceway Flaking/Peeling: Fatigue spalling caused by repeated alternating loads.
• Corrosive Wear: Coolant infiltration and lubrication breakdown leading to pitting and rust on raceways — the primary cause of rapidly increasing lead error and backlash.
• Nut Seizure: Loss of lubrication or preload degradation/overloading causing severe ball-raceway friction.

1.1 Establishing Digital Baseline Performance

Before disassembly, a comprehensive dynamic baseline must be recorded — this defines the target restoration specifications:

• Accuracy Testing: Renishaw QC20-W wireless ballbar for geometric and positioning accuracy. Typical 15–20-year-old SL series data over 800 mm travel: cumulative lead error 0.038–0.085 mm; axial backlash 0.045–0.120 mm.
• Assembly Documentation: Record original THK/NSK part markings, preload torque of bearing sets (usually paired angular-contact thrust bearings), and geometric relationship between nut housing and bearing supports.

2. High-Precision Geometric Measurement and Modeling

The core challenge of custom ball screw manufacturing lies in accurately reconstructing the Gothic-arch raceway geometry and bearing journal tolerances.

2.1 Structured-Light 3D Scanning and Point Cloud Processing

• Primary tool: Zeiss ATOS Triple Scan III blue-light structured-light scanner (volumetric accuracy ±0.008 mm) for full-field non-contact scanning.
• Supplementary measurement: Hexagon Absolute Arm RS6 laser scanner for hard-to-reach bearing journals and end features.

2.2 Critical Geometric Parameter Extraction

Parametric solid modeling is performed in Geomagic Design X. The most difficult aspects of reverse engineering involve deducing invisible key parameters:

• Gothic-Arch Raceway Geometry: SL series typically uses true Gothic-arch or ogival raceways. Raceway radius of curvature and contact angle (usually 45° or 50°) are the decisive factors for preload stiffness and load capacity — these must be precisely fitted from point-cloud cross-sections.
• Lead and Accuracy Grade: Common leads are 10 mm or 12 mm. Target accuracy set to C3-equivalent (lead error E ≤ 0.006 mm/300 mm) to restore sub-micron bidirectional repeatability.
• Bearing Journal Tolerances: Journal diameter and shoulder runout must be held within ±0.002 mm to ensure proper interference fit with bearing inner rings and maintain rotational concentricity.

3. Material, Heat Treatment, and Preload Design Optimization

3.1 Material and Heat Treatment Upgrade

Original THK/NSK screws typically use induction-hardened SCM415 or equivalent alloy steel.

• Material: 60CrMoV14-6 (DIN 1.7931) or domestic 9Cr18MoV.
• Heat Treatment: Dual-frequency induction hardening targeting surface hardness 60–63 HRC with effective case depth ≥ 1.2 mm.
• Cryogenic Treatment: Post-quench sub-zero treatment at –120 °C to ensure long-term dimensional stability under load and thermal cycling.

3.2 Preload and Stiffness Recalculation

Preload is the fundamental mechanism for eliminating backlash and maximizing system rigidity.

• Configuration: X-axis typically uses dual-nut fixed-fixed mounting.
• Calculation: In-house Python + ANSYS Mechanical scripts used to determine optimum preload based on maximum cutting force (SL-250 typical axial load ~8,500 N) and target stiffness (≥ 450 N/µm).
• Implementation: Shim-offset preload method using 0.03–0.05 mm precision shims to create controlled axial offset between the two nuts, achieving zero backlash and stiffness of 420–480 N/µm.

3.3 Finite Element Verification (FEA)

Full assembly simulations performed in ANSYS Mechanical:

• Static Analysis: 12 kN emergency-stop/crash load → maximum von Mises stress < 680 MPa (safety factor > 2.1).
• Modal Analysis: First natural frequency > 165 Hz (well above servo bandwidth).

4. Manufacturing and Ultra-Precision Process Route

High-precision ball screw manufacturing follows an extremely rigorous sequence:

1. Blank Preparation: Rough turning → stress-relief annealing.
2. Semi-Finish Thread Generation: Leistritz Polymat 100 high-speed whirlwind milling.
3. Precision Core Operations:
   • Induction hardening + cryogenic treatment for hardness and stability.
   • Thread grinding on Matrix 70 CNC thread grinder with continuous diamond-roll dressing.
   • Bearing journals centerless-ground or hard-turned to runout ≤ 0.002 mm relative to raceway.
4. Superfinishing: Raceways superfinished to Ra 0.08–0.12 µm.

5. Final Performance Validation and Engineering Benefits

5.1 Quality Control and Metrology

Every custom screw undergoes 100 % inspection:

• Full Dimensional CMM: Zeiss PRISMO with 500+ measurement points per lead on raceways.
• Accuracy Certification: Lead error ≤ 0.006 mm/300 mm (C3 equivalent).
• Profile Tolerance: Raceway form error ≤ ±0.002 mm.
• Dynamic Testing: 100 % dynamic balancing + vibration/noise testing.

5.2 Real-World Results (SL-250MC, 2002, X-axis 40 mm × 12 mm × 1050 mm)

Conclusion: Through systematic reverse engineering combined with state-of-the-art manufacturing, we do not merely “replace” discontinued parts — we reconstruct and upgrade the entire transmission system. Our custom ball screws consistently surpass late-1990s/early-2000s OEM performance in rigidity, accuracy, and service life.