In mechanical engineering drawings that follow SI practice, what is the primary base unit used for dimensions on parts and assemblies?

Introduction / Context:
Dimensioning conventions affect manufacturing accuracy, tolerance schemes, and interoperability. Mechanical industries using SI overwhelmingly dimension parts in millimeters to avoid decimal clutter and to match machining resolutions.

Given Data / Assumptions:

• Industry domain: mechanical design and manufacturing.
• Standards: SI units and common drafting practices (e.g., ISO, ASME metric drawings).
• Typical part sizes: from small components to medium assemblies.

Concept / Approach:
The millimeter offers fine granularity without frequent decimals. Using centimeters introduces needless decimal conversion; meters fit site or civil scales, not detailed parts; kilometers are for mapping, not fabrication.

Step-by-Step Solution:

Verification / Alternative check:
Look at CNC, CAM, and metrology equipment: metric programs and gauges reference millimeters for setup and measurement.

Why Other Options Are Wrong:

• Centimeter: rarely used in engineering drawings; ambiguous and imprecise for machining.
• Meter/Kilometer: suited to building/site scales, not component detailing.

Common Pitfalls:
Mixing inches and millimeters on a single drawing; always state units clearly in the title block to prevent costly errors.

Final Answer:
Millimeter