Best Practices for Creating Custom Cable Assembly Drawings

Key Takeaways (Executive Summary)

• The Golden Rule: A manufacturer builds to the Assembly Drawing, not the electrical schematic. The drawing must define physical reality (length, labels, protection).
• Essential Elements: Every print must have a BOM (Bill of Materials), a Pinout Table, and a Dimensional View.
• Don't Over-Constrain: Applying tight tolerances (+/- 1mm) to non-critical lengths increases cost. Use standard "loose" tolerances where possible.
• Orientation: If the connectors must face a specific direction (clocking), you must show a side view or cross-section.

The Blueprint of Your Product

We often receive an electrical schematic (a netlist) and are told, "Build this."

While a schematic tells us that Pin 1 connects to Pin 2, it doesn't tell us how. It doesn't mention the wire length, the type of heat shrink, the label placement, or the connector orientation.

To get a cable manufactured correctly, you need to bridge the gap between "Electrical Logic" and "Physical Reality." You need an Assembly Drawing.

Schematic vs. Assembly Drawing

• Schematic: A logical map. It shows how the electricity flows. It uses abstract symbols.
• Assembly Drawing: A physical map. It shows what the operator on the factory floor sees. It defines the "What, Where, and How Big."

Note: You need both. The schematic validates the circuit; the assembly drawing validates the build.

The 4 Pillars of a Perfect Cable Drawing

1. The Dimensional View (The "Picture")

This is the visual representation of the cable laid flat.

• Dimensions: Always dimension from the mating face of the connector to the mating face of the other connector.
• Breakouts: If it is a multi-leg harness, clearly dimension where the outer jacket stops and the individual legs begin.
• Labels: Draw the label position. (e.g., "Place label 50mm from Connector A").

2. The Bill of Materials (BOM)

A table listing every component.

• Item Number: Corresponds to bubbles on the drawing.
• Part Number: The Manufacturer P/N (e.g., Molex 39-01-2040). Do not just say "4-pin connector."
• Description: "4-Pos Rec Housing, Mini-Fit Jr."
• Quantity: Exact count per assembly.

3. The Pinout Table (The "Wire Run List")

This is the most critical part for electrical testing. It maps the connections.

4. The Notes Section

This captures the "invisible" requirements.

• Tolerances: "Unless otherwise specified, length tolerance is +/- 25mm."
• Standards: "Workmanship per IPC/WHMA-A-620 Class 2."
• Materials: "All materials must be RoHS compliant."

Common Mistakes to Avoid

1. The "Floating" Tolerance

• Mistake: Writing "Length: 300mm" without a tolerance.
• Reality: Cables are flexible. Is 295mm acceptable? Is 305mm?
• Fix: Standardize. Use +/- 1% of length or +/- 10mm (whichever is greater) for general tolerances.

2. Missing Connector Orientation (Clocking)

• Mistake: Drawing a cable with two right-angle connectors but not showing if they face the same way or opposite ways.
• Reality: Once we crimp the terminals, the wire has a "memory." If we twist it to fit, it creates stress.
• Fix: Add a "End View" showing the alignment of Connector A relative to Connector B.

3. Vague Materials

• Mistake: BOM says "Heat Shrink."
• Reality: There are 50 types of heat shrink. Polyolefin? Kynar? Adhesive-lined? 2:1 or 4:1 shrink ratio?
• Fix: Specify the type or at least the function (e.g., "Adhesive Lined Heat Shrink required").

Tools of the Trade: Do You Need CAD?

• Professional: SolidWorks Electrical, AutoCAD Electrical, or Zuken E3. These tools auto-generate the BOM and wire lengths based on 3D routing.
• Standard: Visio or PowerPoint. Believe it or not, 30% of the prints we receive are clear, well-labeled diagrams made in Visio. As long as the BOM and Pinout are clear, we can build from it.
• Napkin Sketch: Acceptable for a rough quote, but not for production. We will have to redraw it (for a fee) to create a control document.

Frequently Asked Questions (FAQ)

Q: Who owns the drawing? A: If you pay us an NRE (Non-Recurring Engineering) fee to create the drawing for you, you own it. We will provide the PDF and source files upon request.

Q: Do I need to draw every single wire in the bundle? A: No. Standard practice is to draw a single thick line representing the "Trunk" (main bundle) and individual lines fanning out at the ends for the connectors.

Q: What is "NTS"? A: Not To Scale. Cable drawings are rarely 1:1 scale because a 10-meter cable won't fit on a sheet of paper. Use break lines (zigzag lines) to indicate the cable is longer than shown.

About the Author

Michael Wang

Senior Technical Engineer

As the technical lead at TeleWire, Michael bridges the critical gap between complex engineering requirements and precision manufacturing. With deep expertise in Design for Manufacturing (DFM) and signal integrity, he oversees the technical validation of custom interconnect solutions for mission-critical automotive, industrial, and medical applications.

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