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4.2 Mechanical CAD Package and Drawings

A perfect 3D model does not guarantee a perfect part. Manufacturing partners cannot inspect a physical part against a digital STEP file without explicit tolerances. The Mechanical CAD Package acts as the binding contract between Design and Factory. If a dimension or requirement is missing from the 2D drawing, the manufacturer is legally allowed to deliver whatever is easiest for them to produce.

The Deliverable Hierarchy

Define the "Master" source of truth immediately to prevent conflict between the 3D geometry and the 2D print.

Standard Rule: The 2D Drawing controls Tolerances, Material, Finish, and Critical Dimensions. The 3D Model controls basic geometry and tool paths.

The Golden Data Pack must contain:

  1. Native 3D Files (e.g., SLDPRT, PRT): Required for tool designers to analyze draft angles and electrode design.
  2. Neutral 3D Files (STEP AP214): Required for universal viewing and CAM systems. Use AP214 (not AP203) to preserve assembly hierarchy and colors.
  3. 2D Fabrication Drawings (PDF + DXF): The inspection master.
  4. Assembly Drawings: Exploded views showing build order.

2D Drawing Requirements: The "No-Interpretation" Rule

Drawings must be explicit. A drawing that relies on the machinist or molder "figuring it out" is a defect.

1. Materials and Finish

Do not use generic descriptors.

  • Bad: "Material: ABS", "Finish: Textured Black".
  • Good: "Material: ABS, LG Chem HF380", "Finish: MT-11010", "Color: Pantone Black 6C".
  • Logic: Different grades of ABS have different shrinkage rates (0.5% vs 0.8%). Using the wrong grade will make the part out of tolerance even if the tool is perfect.

2. Tolerances and Critical-to-Quality (CTQ) Dimensions

Every dimension needs a tolerance. Use a standard block (e.g., ISO 2768-m) for general features, but explicitly tolerance critical fits.

  • If feature mates with another part $\rightarrow$⭢ Then apply explicit tolerance (e.g., ± 0.05 mm) and mark as CTQ.
  • If feature is cosmetic only $\rightarrow$⭢ Then apply standard open tolerances.

3. Cosmetic Zones (A/B/C Classification)

Define visual acceptance criteria directly on the drawing to prevent subjective arguments during QC.

  • Zone A (Primary User Face): Zero scratches, specs, or sinks allowed.
  • Zone B (Side/Back): Minor defects allowed if < 0.5 mm² .
  • Zone C (Internal/Bottom): Flow lines, ejector pin marks, and flash allowed up to specified limit.

Pro-Tip: Always explicitly define "Ejector Pin Regret" zones. If you don't tell the toolmaker where to put the ejector pins, they will put them on your beautiful cosmetic surface because it is mechanically optimal.

The Virtual Fit Check (Interference Audit)

Before freezing the NPI gate, perform a mandatory assembly interference check. The most expensive ECOs occur because a connector doesn't align with a housing hole.

Validate the following:

  1. Static Interference: Run the CAD tool's "Interference Detection." Zero overlap allowed between solid bodies (exception: press-fit ribs).
  2. Connector Alignment: Verify USB/HDMI/Power connectors on the PCBA center exactly with the enclosure cutouts.
    • Rule: Cutout must be larger than the connector + the stack-up tolerance of the PCBA mounting holes + the connector float.
  3. Assembly Stack-up: Verify that screws are long enough to engage threads but short enough not to bottom out or pierce the other side.
  4. Component Headroom: Check the tallest component on the PCBA (usually electrolytic caps or inductors) against the enclosure ceiling and ribs.

Assembly Datums and Exploded Views

For the Assembly Instructions (MBOM context), provide drawings that guide the operator.

  • Exploded Views: Show every screw, washer, and adhesive liner in the order of operation.
  • Orientation Callouts: If a part is symmetrical but functionally directional (e.g., a fan or a gasket), add a physical feature (notch) or a clear label on the drawing: "INSTALL WITH NOTCH FACING UP".

Final Checklist

Check

Criterion

Critical Threshold / Requirement

Data Integrity

Native + STEP provided?

Must match exactly. STEP AP214 preferred.

Material

Specific Grade defined?

Manufacturer + Grade (e.g., Sabic C2950).

Cosmetics

Zones A/B/C defined?

Visual inspection map included in drawing.

Fit Check

Interference Report?

Pass. No collisions between PCB and Case.

Tolerances

CTQ dimensions marked?

Critical mating features have tight tolerances ( ± 0.05 – 0.1 mm).

Tooling

Ejector/Gate locations?

"Prohibited Zones" marked for gate/pin placement.

Assembly

Exploded View?

All parts visible; no "hidden" installation steps.

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