4.2 Mechanical CAD Package and Drawings

A perfect 3D model does not guarantee a perfect physical part. Manufacturing partners cannot functionally inspect a physical piece of plastic or metal against a digital STEP file without explicit 2D tolerances. The Mechanical CAD Package is the legally binding contract between the Design team and the Factory. If a dimension or requirement is missing from the 2D drawing, the manufacturer is legally permitted to deliver the cheapest physical interpretation.

The Deliverable Hierarchy

The “master” source of truth must be defined immediately to prevent arguments between the 3D geometry and the 2D print.

Standard Rule: The 2D Drawing controls the Tolerances, Material, Finish, and Critical Dimensions. The 3D Model controls only the basic geometry and CNC 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 for injection molding.
2. Neutral 3D Files (STEP AP214): Required for universal viewing and CAM systems. AP214 (not AP203) must be used to preserve the assembly hierarchy and color coding.
3. 2D Fabrication Drawings (PDF + DXF): The absolute inspection master.
4. Assembly Drawings: Exploded views showing the exact build order.

2D Drawing Requirements: The “No-Interpretation” Rule

Drawings must be mathematically explicit. A drawing that relies on the machinist or molder “figuring it out” or “using common sense” constitutes an incomplete specification.

1. Materials and Finish

Using generic material descriptors is prohibited.

• 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 physical shrinkage rates during cooling (e.g., 0.5% vs 0.8%). Approving the wrong resin grade will yield a part that is out of tolerance, even if the steel tool is cut perfectly.

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

Every dimension needs a tolerance. A standard block (e.g., ISO 2768-m) must be used for general features, but the critical mating fits must be explicitly toleranced.

• For features that mate with another part, an explicit tight tolerance (e.g., ± 0.05 mm) must be applied and marked as CTQ.
• For cosmetic or free-space features, standard open tolerances should be applied to save machining cost.

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

Visual acceptance criteria must be defined directly on the drawing to prevent subjective arguments during Quality Control (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/Hidden): Flow lines, ejector pin marks, and flash allowed up to a specified mechanical limit.

Best Practice

“Ejector Pin Regret” zones must always be explicitly defined. If the toolmaker is not explicitly told where to put the ejector pins, they may place them on primary cosmetic surfaces because it is mechanically optimal for pushing the plastic out of the mold.

The Virtual Fit Check (Interference Audit)

Before freezing the NPI gate, an assembly interference check must be performed. Expensive ECOs (Engineering Change Orders) often result from a connector failing to align with a housing hole, which then necessitates expensive tool modifications.

The Validation Protocol:

1. Static Interference: The CAD tool’s “Interference Detection” must be executed. Zero overlap is permitted between solid bodies (exception: deliberately designed press-fit ribs).
2. Connector Alignment: USB, HDMI, and Power connectors on the PCBA must be verified to center exactly within the enclosure cutouts.
   • Rule: The cutout must be larger than the connector + the stack-up tolerance of the PCBA mounting holes + the connector’s own manufacturing float.
3. Assembly Stack-up: All screws must be verified to be long enough to properly engage the threads, but short enough that they do not bottom out or pierce the other side of the plastic.
4. Component Headroom: The tallest component on the PCBA (usually electrolytic capacitors or large inductors) must be checked against the enclosure ceiling and internal mechanical ribs.

Assembly Datums and Exploded Views

For the Assembly Instructions (MBOM context), drawings must be provided that guide the operator linearly and logically.

• Exploded Views: Every screw, washer, and adhesive liner must be shown in the exact sequential order of operation.
• Orientation Callouts: If a part is symmetrical but functionally directional (e.g., a cooling fan or a sealing gasket), a physical “poka-yoke” feature (like a notch) must be added or a prominent label must be placed on the drawing: “INSTALL WITH NOTCH FACING UP”.

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Recap: Mechanical CAD Package and Drawings

Parameter	Requirement	Value / Criterion	Action	Document
Material & Finish	Specify exact grade and code.	e.g., “ABS, LG Chem HF380”, “Pantone Black 6C”.	Mandatory on drawing.	2D Drawing
Critical Dimensions	Apply explicit tolerance to mating features.	e.g., ±0.05 mm, marked as CTQ.	Mandatory on drawing.	2D Drawing
Cosmetic Zones	Define visual acceptance per zone.	A: Zero defects. B: Defects <0.5 mm². C: Ejector marks/flash allowed.	Mandatory on drawing.	2D Drawing
Interference Check	Validate assembly fit.	Zero overlap between solid bodies (press-fit ribs excepted).	Perform before NPI gate freeze.	3D Model
Connector Alignment	Verify housing cutout clearance.	Cutout > (Connector + PCBA stack-up tolerance + connector float).	Perform before NPI gate freeze.	3D Model / 2D Drawing