GEOMETRIC REQUIREMENTS / CNC MACHINING INSIGHTS
A part can have the right nominal dimensions and still fail to assemble when key faces are not flat, parallel, or perpendicular in the way the design expects. These requirements should appear on the drawing only where they serve a clear function, and they should be tied to understandable datum context.
Start with function, then define the relationship
Ask what the feature must do. A mounting face may need to sit against another component. A bore may need to stay square to a sealing face. Two rails may need to remain parallel for a moving element. That assembly purpose guides whether the drawing needs a controlled geometric relationship beyond basic size dimensions.
The ASME Y14.5 overview is a primary source for the geometric-dimensioning language often used to communicate these relationships.
Choose datum surfaces that reflect assembly
Datums should be stable, identifiable features that represent how the part is located or inspected. A datum scheme is most helpful when it follows the real assembly sequence: for example, a mounting face as the primary reference, an adjacent locating edge as the secondary reference, and a bore or another feature as the third reference where appropriate.
| Requirement | Typical design question |
|---|---|
| Flatness | Does this face need to contact, seal, mount, or support another component evenly? |
| Parallelism | Must two surfaces or axes remain aligned for travel, clearance or assembly? |
| Perpendicularity | Must a bore, side wall, or locating feature be square to a primary reference? |
| Position or profile | Is the feature’s location more important than its standalone size? |
Do not apply controls without an assembly reason
Extra geometric callouts can make an order more difficult to interpret without improving the part. When a basic dimension is sufficient, keep the drawing simple. When a relationship is critical, identify the functional reason in the design review so the manufacturing team understands what must be preserved.
For a broader view of tolerance decisions, read when a CNC tolerance makes sense. For inspection planning, see what to include before production starts.
Show the requirement in the right view
Section views, enlarged details, and clear datum labels make geometric notes easier to review. Avoid placing a feature-control frame where it can be mistaken for a nearby feature. If the drawing has both machined and cosmetic surfaces, make the controlled surface unambiguous.
Agree on the inspection expectation early
Critical geometry should be discussed as part of the RFQ, together with the drawing revision and any requested report. The request can describe the feature that needs verification and the functional context; it does not need to invent a documentation format the project does not require. NIST maintains public calibration resources for measurement-traceability context.
Keep the drawing practical for manufacturing review
Clear, functional geometric requirements help prevent assumptions and focus inspection on the features that matter. ExactMach coordinates drawing-defined CNC milling, CNC turning, prototypes and custom parts around Shenzhen and Dongguan, with inspection and finishing support for international orders where required.