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2.1 Wire Preparation: Single Conductor Processing

Wire preparation is the high-speed automated foundation of the entire harness manufacturing process. Errors introduced here — variable lengths, nicked conductor strands, or damaged insulation — are often impossible to detect after termination and result in latent reliability failures. This stage transforms raw material into precision components, requiring strict machine capability controls and visual standards that exceed simple dimensional checks.

2.1.1 Length Control and Machine Capability

Modern Cut-and-Strip machines are high-precision CNC tools. However, wire elasticity, spool tension, and drive roller slippage introduce variation. Relying on a single "First Article" measurement is insufficient; statistical control is mandatory.

Capabilities and Tolerances

  • Machine Capability (Cpk): The cutting process must demonstrate a Cpk ≥ 1.33. This ensures that 99.99% of wires fall within the tolerance window without constant operator adjustment.
  • Standard Tolerances: Unless specified otherwise, industry baseline length tolerances are:
    • Wire length < 1000 mm: ± 2 mm
    • Wire length > 1000 mm: ± 5 mm or 0.5% of total length.
  • Strip Length Tolerance: The length of the exposed conductor is critical for the crimp "brush" and "bellmouth." Typical tolerance is ± 0.5 mm.

Process Mandate: Production runs must use wire straighteners to remove the "memory" (curvature) from the spool. Kinked wire leads to inconsistent lengths and feed jams.

2.1.2 Stripping Quality: The Defect Atlas

Stripping is the mechanical removal of insulation without damaging the underlying conductor. The choice of blade (V-Blade vs. Die-Blade) and blade sharpness dictates quality.

Critical Stripping Defects (IPC/WHMA-A-620)

Defect Type

Mechanism

Acceptance Criteria (Class 3)

Nicked Strands

The blade cuts too deep, scoring or notching the copper strands.

Reduce Current/Fatigue Risk. Allowable nicks are strictly limited. Class 3 typically permits zero severed strands and minimal nicking (≤ 5% of strand diameter).

Cut/Missing Strands

Complete severance of one or more conductor strands.

Defect. Reduces the cross-sectional area and current carrying capacity.

Insulation Slug

A piece of waste insulation remains attached to the conductor.

Defect. Prevents proper insertion into the crimp barrel or solder cup.

Birdcaging

Strands flare out or separate, creating a "cage" shape.

Defect. Prevents insertion; strands may be bent back outside the terminal, causing shorts.

Insulation Damage

Blade marks, crushing, or burning on the remaining insulation.

Defect if the damage reduces insulation thickness by > 20% or exposes the conductor.

Setup Note: V-Blades are universal but risk nicking strands if the wire is not perfectly centered. Die-Blades (sized exactly to the conductor) are mandatory for high-reliability aerospace/medical stripping to guarantee concentricity.

2.1.3 End Preparation: Twisting and Pre-Tinning

Once stripped, the conductor strands act as individual loose wires. End preparation consolidates them for the next step.

A) Twisting

Twisting restores the lay of the strands that may have been disturbed during stripping.

  • Mandate: Twist must be in the same direction as the original manufacturer's lay.
  • Tightness: The twist must be tight enough to prevent splaying (stray strands) during insertion into a crimp barrel or PCB hole, but not so tight that it increases the effective diameter beyond the terminal's capacity.

B) Pre-Tinning (For Solder Terminations Only)

Pre-tinning involves dipping the twisted end into a solder pot to fuse the strands into a solid unit.

  • Application: Mandatory for wires intended for solder cups or PCB through-holes.
  • Prohibited for Crimping: Never pre-tin a wire that is intended for a crimp termination. Solder is a soft metal that "creeps" (cold flows) under compression, causing the crimp to loosen over time and fail.
  • Wicking Control: The critical quality metric is limiting solder wicking (capillary action) up the wire under the insulation.
    • The Defect: If solder wicks too far up, it creates a rigid "stress riser" point where the wire loses flexibility. Vibration will cause the wire to snap exactly at the end of the wicking.
    • Limit: Wicking must stop within 3 mm (≈ 0.125 inch) of the insulation end, or as defined by the spec. The wire must remain flexible immediately behind the termination.

Final Checklist: Wire Preparation Controls

Mandate

Criteria

Verification Action

Cpk Validation

Cut and strip machines must demonstrate Cpk ≥ 1.33.

Periodic capability studies using automated length measurement tools.

Strand Integrity

Zero cut strands allowed for Class 3. Nicks limited to <5% of strand diameter.

Visual inspection (10x magnification) of the first 5 pieces at setup.

Insulation Quality

Strip cut must be clean and square; no ragged edges or slugs remaining.

Visual check. Ensure strip length matches the terminal's required "brush" length.

Tinning Rule

Do not tin wires intended for crimping.

Process audit ensures tinning is only applied to soldered terminations.

Wicking Limit

Solder wicking under insulation must be minimized (typically < 3 mm).

Tactile check: The wire must remain flexible right up to the termination point.

Birdcage Prevention

Stripped ends must remain twisted and coherent.

Visual check to ensure no splayed strands that could cause shorts.


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