How Surface Finishes Can Affect Part Tolerances in CNC Machining
When designing CNC-machined parts, engineers often focus on tolerances, materials, and geometry, but overlook one factor that can subtly disrupt everything: surface finish.
It may seem cosmetic, but the finish you choose can significantly impact your part’s final dimensions, fit, and function.
Here’s what you need to know.
Why Surface Finish Isn’t Just About Aesthetics
Surface finishes like bead blasting, anodising, polishing, or brushing change the part’s surface texture. Some remove material, some add layers, and some alter surface geometry just enough to affect the tolerances you carefully specified.
Even a change of 0.01–0.05 mm may not seem significant. But when your part needs to fit into a tight assembly or align with precision components, it matters.
Common Finishes That Affect Dimensions
| Finish | Impact on Tolerances |
|---|---|
| Bead Blasting | Slight rounding of edges, removal of surface peaks |
| Anodizing (Type II/III) | Adds a layer (~10–25 μm), reduces internal clearances |
| Polishing | Removes minor surface material |
| Brushing | Minimal dimensional change, but affects flatness |
If your tolerance is ±0.05 mm and the finish adds 0.02 mm, you’ve already burned 40% of your margin.
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When to Be Extra Careful
- Press fits or interference fits: Added thickness from anodising or coatings can make assembly impossible without re-machining.
- Precision mating parts: Even slight deviations can cause misalignment or vibration.
- Optical or moving parts: Finishes that alter the surface geometry might introduce drag or reflection errors.
Get the Full Picture
Want to know when an as-machined finish is more than enough and when to pay for more?
How to Prevent Costly Mistakes
- Define tolerances with the finish in mind.
Always specify whether your tolerances apply before or after the finishing process.
- Communicate finish requirements early.
Don’t assume your supplier knows what you need, and don’t leave it open to interpretation.
- Use as-machined when possible.
For internal components or early prototypes, skipping the finish saves time, cost, and tolerance risk.
- Work with a single supplier.
Inconsistent finish interpretations across vendors lead to conflicting results. MakerVerse offers standardised options across technologies to keep results predictable.
