How to Specify Critical Dimensions on a CNC Drawing
Contents
- Start With the Function
- Datums Before Dimensions
- Mark the Features That Actually Matter
- Choose the Inspection Level Deliberately
- Put the Process Information on the Drawing
- Always Send the 2D Drawing With the 3D Model
Callout conventions, inspection levels and the difference between engineering tolerances and manufacturing capability.
Start With the Function
Specify a tolerance because the function requires it, not because the number looks reassuring. Every tightened callout adds machining time, inspection time and cost. If a mounting hole only needs clearance, a general tolerance is enough; if a bore locates a bearing or a precision shaft, it needs a callout that reflects that job.
Datums Before Dimensions
Choose datum faces that match how the part is located in assembly, then dimension from those datums. This is what makes a drawing checkable: a parallelism or position callout referenced to a functional datum tells the machinist and the inspector the same story. Dimensions strung from an arbitrary edge are difficult to verify and easy to misinterpret.
Mark the Features That Actually Matter
Identify critical features explicitly — gripping faces, bearing bores, optical axes, mounting patterns, sealing surfaces. These are the features that drive the process plan and the inspection plan. On most automation parts, three to six features carry the function of the part; everything else is clearance geometry.
Choose the Inspection Level Deliberately
Standard final inspection is included with every order and covers visual checks plus key dimensions. A critical-dimension report records measured values for the features you identify. A full dimensional report records every drawing dimension. First article inspection approves a new process before the batch is released. Pick the level that matches the risk in the application and state it on the enquiry.
Put the Process Information on the Drawing
Include material grade and temper, surface finish and its specified thickness, thread insert type and depth, part marking, and any packaging requirement. Add a note for coating allowance on fits — anodizing grows the surface by roughly 0.01–0.03 mm per face depending on type, which matters on any H7 bore or precision gap.
Always Send the 2D Drawing With the 3D Model
A STEP file defines geometry, not tolerance. STEP alone cannot express which surfaces are datums, which bores are critical, or where the finish applies. A PDF drawing alongside the model lets us review manufacturability properly and quote the right process. If you only have a 2D drawing with complete dimensions and tolerances, that is enough to quote.
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