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4.7 Tooling and Machine Capability

High-speed wire processing machines are the heartbeat of the harness shop, often producing thousands of leads per hour. However, speed without stability is a liability. A machine with worn rollers or dull blades will generate large volumes of non-conforming product before an operator notices. Therefore, machine capability is not assumed; it is proven through statistical qualification and sustained through aggressive preventive maintenance.

2.3.4.7.1 Machine Qualification: The Statistical Mandate

Before a machine is released for production, its ability to hold tolerance must be quantified. This is done using Process Capability (Cpk) studies on critical dimensions: Total Wire Length and Strip Length.

Qualification Protocol

    • Sample Run: A continuous run of 30 to 50 pieces is produced using a standard wire gauge (e.g., 18 AWG).
    • Measurement: Every piece is measured for Total Length and Strip Length using calibrated calipers or vision systems.
    • Calculation: The Cpk is calculated against the tolerance limits (e.g., ±1.0 mm).

      • Mandate: The machine must achieve a Cpk ≥ 1.33 (4σ) to be qualified. If Cpk < 1.33, the machine requires calibration, belt tensioning, or mechanical repair.

Statistical Process Control (SPC)

Once qualified, the machine is monitored using SPC control charts (X-Bar and R charts).

  • Drift Detection: SPC detects drive roller slippage (wire length drifting shorter) or blade mechanism play (strip length variance) before they create out-of-spec parts.
  • Control Limits: Limits are set based on the machine's proven capability, not the wide engineering tolerance.

2.3.4.7.2 Blade Maintenance: The Conductor's Enemy

Stripping blades are consumables. As they wear, they lose the ability to cleanly sever insulation and begin to tear or crush it. More critically, a dull blade requires more force, increasing the risk of scraping or nicking the conductor plating.

The Scraping Defect

A sharp blade cuts insulation cleanly. A dull blade compresses the insulation against the copper strands before cutting. This friction can scrape the tin or silver plating off the strands, exposing the base copper.

  • Class 3 Impact: Scraped plating reduces corrosion resistance and is a potential cause for rejection in high-reliability aerospace/defense harnesses.

Preventive Maintenance (PM) Schedule

Blade maintenance must be cycle-based, not time-based.

  • Cycle Count: PMs should be triggered by the machine's cut counter (e.g., inspect every 100,000 cycles).
  • Visual Inspection: Blades must be removed and inspected under 10x magnification.

    • Accept: Sharp, defined edge; coating intact (if TiN coated).

    • Reject: Chipping, rounded edges, or material buildup.

  • Changeover: Blades are replaced as matched sets. Never replace only one blade in a pair, as this causes misalignment and conductor damage.

2.3.4.7.3 Gripper and Roller Maintenance

The feed mechanism (rollers/belts) and the immobilization mechanism (grippers) determine length accuracy.

  • Roller Glazing: Urethane feed rollers collect dust and wax from wire insulation, becoming "glazed" (smooth/hard). This causes slippage. Mandate: Daily cleaning with Isopropyl Alcohol (IPA) to restore grip.
  • Gripper Pressure: If grippers are set too tight, they crush the insulation, altering the wire's dielectric properties (critical for Coax). If too loose, the wire slips during stripping, causing a short strip length.

Final Checklist: Tooling and Capability

Mandate

Criteria

Verification Action

Machine Qualification

Cut/Strip machines must demonstrate Cpk ≥ 1.33 on length tolerances.

Initial capability study performed and logged for each machine annually.

Blade Inspection

Blades inspected for wear/chipping based on cycle count (not just time).

PM Log requires "Pass/Fail" entry for blade edge condition under magnification.

Conductor Integrity

Zero scraping of conductor plating allowed.

First Article Inspection (FAI) includes microscopic check of the conductor surface after stripping.

Feed Maintenance

Feed rollers/belts cleaned daily to remove glazing.

Operator checklist confirms rollers are clean and provide consistent traction.

Process Drift

SPC Charts used to monitor strip length variation.

Operator stops machine if 7 consecutive points trend in one direction (indicating blade loosening or wear).


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