4.1 Form board and routing design

The Form Board (frequently called a Jig Board) is the template that transforms a theoretical 2D engineering drawing into a 3D wire harness. It is a highly precise calibration instrument. If the physical board geometry is incorrect, every harness built on it will be incorrect, complicating installation on the customer’s assembly line.

Designing the board and managing the physical routing requires an understanding of raw material behavior. Wires possess mass, stiffness, and bend limits. Furthermore, bundling materials (such as tie-wraps, tape, or lacing) exert compressive forces that can crush insulation over time. The primary objective is to guarantee the final harness installs smoothly into the chassis without forcing, stretching, or kinking.

Fixture design: the layout guideline

The form board acts as a “Third Hand” for the operator and the definitive “Go/No-Go Gauge” for all final harness dimensions.

Layout geometry and mechanical tolerance

A theoretical CAD drawing represents a wire as a zero-width line. Real wire bundles have a compounding physical diameter.

• The Centerline Collision Consideration: If routing nails are placed exactly on the theoretical CAD centerline, a thick bundle will increase outward, making the final harness too short for the required routing path.
• Mandatory Offset Design: All routing nails, pegs, and connector jigs must be offset by exactly half the calculated bundle diameter to ensure the true centerline of the built harness matches the engineering drawing.
• Dimensional Tolerance: The physical board must constrain the harness to the drawing tolerance (typically ± 10 mm for the overall harness length, and ± 5 mm for precise breakout positions).

Hard fixturing (rigid jigs vs. guarded nails)

• Proper Routing Nails: Used to define the physical routing path of the bundle.
  • The Requirement: Nails must be straight, rigid, and sleeved (covered with protective polymer tubing). Bare metal nails slice wire insulation during the tensioning of the pulling process.
  • Proper Head Height: All nails must be tall enough to contain the maximum bundle stack-up without the top wires spilling over during routing.
• Precision Connector Jigs: Connectors must not dangle loose. They must be held in fixed, machined mating jigs securely bolted to the board face.
  • The Function: The rigid jig holds the connector body still for pin insertion and mathematically sets the breakout length (the precise distance from the main trunk bundle to the connector).
  • Physical Keying Verification: Mating jigs must be designed to physically verify keying (preventing a “Key A” connector housing from being built as a “Key B”).

Visual aids and operator ergonomics

• The Full Scale Plot: The approved engineering drawing must be plotted at 1:1 scale and affixed directly to the board surface.
• Actionable Color Coding: Use distinct, color-coded zones painted on the board to dictate:
  • Red: No-Go zones / Keep-outs (e.g. actively representing a hot chassis bracket location).
  • Blue: Expected tie-wrap or lacing locations.
  • Yellow: Label and marking placement zones.
• Active Ergonomics: The board is often tilted (easel style) to reduce operator back strain. Hand tools (calibrated tie guns, tape dispensers) must be securely tethered or securely staged to prevent damage to the board surface.

Routing is the sequence of laying individual wires onto the board. This specific sequence dictates the internal nesting, outer diameter, and ultimate flexibility of the final assembled product.

The lay-up sequence

1. Heavy Gauge First: Large power cables (e.g. 10 AWG and up) are stiff and define the harness backbone. They must go down first, resting at the bottom of the bundle stack.
2. Delicate Signal Wires: Thin-gauge control wires are routed on top of or alongside the rigid power core.
3. Vulnerable Twisted Pairs: Data pairs must be laid flat and must not be crushed under the weight of heavier power wires.

Strict bend radius rules

Forcing a thick wire into a sharp 90-degree corner puts the conductor under stress and thins the outer insulation wall (dielectric degradation).

• The 3x / 5x Rule:
  • The Static Bend Limit: The minimum inner radius is mandated to be ≥ 3x the outer diameter (OD) of the wire bundle.
  • The Dynamic Bend (Flexing) Limit: For areas that will flex in the field, the radius must be increased to ≥ 5x to 10x the bundle OD.
• Intelligent Board Design: Routing nails positioned at corners must be spaced out to encourage this designed radius. Avoid a single nail to pivot 90 degrees; use a sweeping three-nail arc or a radiused block.

Service loops (maintenance loops)

Wires must not enter a rigid connector shell under physical tension.

• The DFM Guideline: Design a Service Loop (a small amount of S-curve slack) immediately before the wires enter the connector.
• The Purpose: This slack permits future field disconnect/reconnect cycles and emergency re-termination (cutting off a damaged contact) without rendering the entire harness too short.

Bundling mechanics: containment and the cold flow threat

Once routed, the loose wires must be secured into a unified bundle. The choice of restraint (Tie-Wrap vs. Lacing) dictates the long-term reliability of the harness.

Tie-wraps (cable ties): the “cold flow” risk

Nylon cable ties are cost-effective, but they apply constant compressive force to the insulation.

• The Cold Flow Phenomenon: Standard plastic insulation migrates away from persistent pressure over time. A tightened nylon cable tie will cut through soft insulation, causing a dead-short circuit inside the bundle.
• Tension Guns: Operators must never tighten ties by hand or with pliers. A calibrated Cable Tie Tension Gun equipped with an adjustable tension setting is required.
  • The Setting Rule: Set the gun to the minimum force required to hold the bundle round without crushing it. A tied bundle must still rotate slightly under the tie when securely twisted.
• Flush Cutting: The plastic tail of the tie wrap must be cut flush with the square head, leaving no protrusions.
  • The Consideration: A protruding tail cuts hands during installation and abrades adjacent wires inside the vibrating chassis.
• Regulated Spacing: Standard spacing is tying every 50 to 75 mm to organize the bundle without destroying its flexibility.

Lacing cord: the aerospace method

Heavy Lacing cord (thick waxed nylon or specialized polyester string) is mandated in high-reliability Class 3 aerospace, military, and severe-duty harnesses.

• The Advantages:
  • Zero Cold Flow Threat: The broader, flat surface area and lower applied tension prevent cutting into wire insulation over time.
  • Lowest Possible Profile: A lacing knot is smaller than a tie-wrap head (critical for tight wire ducts).
  • Extreme Temperature Stability: Standard nylon ties become brittle in extreme cold; aerospace lacing cord remains structurally stable.
• The Approved Technique: Requires manual continuous lacing (e.g. the locking Marlinespike hitch) or individual spot ties (e.g. the Surgeon’s Knot).
• The Guideline: Knots must be tight and locked. Loose lacing creates a Foreign Object Debris (FOD) tearing hazard.

Spot taping (industrial bundling)

Used for high-volume industrial harnesses.

• Heavy Friction Tape: Used for high-abrasion zones.
• Standard PVC Tape: Used for spot bundling. The Warning: Tape must be wrapped with a 50% overlap. Using too much tape makes the harness unnecessarily stiff; using too little allows it to unravel in the field.

A “Breakout” is where a single wire or a subgroup exits the main bundle to reach a connector. This is a common point of mechanical failure due to the Stiffness Transition.

The stress riser effect

The main trunk bundle is rigid (locked by wires + taping). The breakout wire is highly flexible. Engine or chassis vibration concentrates at this transition point where the rigid mass meets the flexible strand.

DFM breakout rules

1. The Smooth “Y” Transition: Avoid 90-degree right-angle breakouts. A sweeping “Y” shape flows the vibration stress safely and evenly.
2. Strain Relief Free Distance: The rigid bundle tie or tape must stop at least 25 mm (1 full inch) before hitting the hard connector body.
   • The Engineering Rationale: This “free length” absorbs vibration energy. If the rigid bundle touches the hard connector backshell, mechanical movement transfers directly to the crimp terminal inside, guaranteeing fatigue breakage.
3. Required Support at the Breakout Exit: Place a firm tie-wrap or locking lacing knot immediately before and immediately after the point the wire exits the main bundle. This locks the internal geometry so the breakout wire does not pull back freely into the bundle under tension.

Final Checkout: Form board and routing design

Focus Area	Engineering Guideline	Verification Action
Fixture Accuracy	The board geometry accounts for the bundle diameter offset, not just the theoretical centerline.	The built harness length must be measured against the drawing tolerance (typically ± 10 mm).
Nail Protection	All metal routing nails must be sleeved or radiused.	A visual check to confirm no bare metal nails touch wire insulation.
Bend Radius Control	The minimum inside bend radius of ≥3x bundle diameter is maintained at all corners.	The board layout flow must be audited; zero single-pin 90-degree sharp turns must be ensured.
Tie-Wrap Tension	Plastic ties are secure but can rotate slightly by force; insulation must not be crushed or dented.	A Calibrated Tie Gun is mandated; tension settings are verified at the start of every shift.
Flush Cutting	Zero sharp edges are permitted on any tie-wrap tails.	A tactile check (the inspector sweeps a finger over the tie head); reject if it is sharp or catches skin.
Breakout Strain Relief	A generous strain relief distance exists between the rigid bundle exit and the connector shell.	A visual check: Wires must not be under physical tension entering the rigid connector housing.
Connector Jigs	All connectors are firmly held in rigid, machined mating jigs during assembly.	Ensures a consistent breakout length and prevents bent-pin damage during insertion.