One of the most expensive habits in mechanical design is applying tight tolerances everywhere. A CNC machine may be capable of excellent accuracy, but capability is not the same as economical production.
Specify Function, Not Precision for Its Own Sake
Start with what the feature must accomplish. A locating hole may need a controlled fit, while a non-critical cover dimension may only need a broader tolerance. Every tight tolerance should have a functional reason.
General Tolerance vs Specific Tolerance
A drawing can use a general tolerance for ordinary dimensions and apply tighter limits only to critical features. This makes the drawing easier to read and helps manufacturing focus inspection effort where it matters.
Example
| Feature | Possible design approach | Reason |
|---|---|---|
| Decorative overall length | General drawing tolerance | No critical fit |
| Bearing seat | Specified fit/tolerance | Functional location |
| Mounting slot | Controlled width and position | Assembly alignment |
| Threaded hole | Standard thread callout | Compatibility with fastener |
Hole Tolerances Need Special Attention
Hole diameter, position, perpendicularity, depth, thread specification, and surface condition can all affect assembly. If a hole is functionally located, its positional tolerance may matter more than an unnecessarily narrow size tolerance.
Surface Finish and Tolerance Are Different
A dimension controls size or location. Surface roughness controls texture at the specified scale. Do not use a tighter dimensional tolerance to solve a surface-finish problem, or vice versa.
Inspection Must Be Possible
Before specifying a tight limit, consider how the supplier will measure it. A feature may require a calibrated micrometer, bore gauge, CMM, optical system, or functional gauge. The inspection method should be realistic for production volume and feature geometry.
Five Questions Before Tightening a Tolerance
- Does the function actually require it?
- Is the tolerance based on a stack-up?
- Can the process hold it consistently?
- How will it be inspected?
- What happens if the tolerance is relaxed?
Conclusion
Good CNC drawings do not contain the smallest possible tolerances. They contain the tolerances needed for function, communicated clearly enough for manufacturing and inspection. This balance is one of the simplest ways a designer can reduce cost without reducing performance.