Do Parts Need a SPA Too? — How Surface Finishing Perfects Precision Machining
In the world of precision machining, surface treatment is far more than cosmetic enhancement — it’s a crucial process that directly affects a part’s performance, lifespan, and dimensional stability.
For CNC-machined components, if plating, anodizing, sandblasting, or coating processes are not properly aligned with the design and machining stages, even a visually flawless part may hide risks in assembly, tolerance control, or corrosion resistance.
✨ Why Is Surface Treatment So Important?
Every part’s surface acts as the first line of defense between the material and its environment.
Surface treatment improves:
Corrosion resistance (anti-rust, anti-oxidation)
Hardness and wear resistance
Electrical conductivity or insulation
Lubrication and surface aesthetics
However, different treatments can change the thickness, roughness, and dimensional tolerances of the surface.
If no machining allowance is reserved beforehand, the final dimensions may exceed specifications, leading to assembly jamming, excessive gaps, or even functional failure.
🧩 Common Surface Treatments and Their Characteristics
| Treatment Method | Thickness Change | Features & Applications | Effect on Tolerance |
|---|---|---|---|
| Plating | ~5–25 μm | Improves corrosion resistance, wear resistance, and conductivity (e.g., Ni, Cr, Zn, black nickel) | Slightly increases dimensions; size adjustment required |
| Anodizing | ~10–30 μm | Common for aluminum; enhances hardness and corrosion resistance, allows coloring | Outer layer hard but brittle; unsuitable for threaded fits |
| Electroless Plating | ~5–15 μm | Uniform coating; ideal for complex shapes | Even thickness but affects hole diameters and tolerances |
| Sandblasting | 0 μm (surface only) | Removes burrs, enhances adhesion, matte finish | Alters roughness; avoid treating mating surfaces |
| Coating / Painting | ~20–50 μm | Protective and aesthetic finish | Thick layer; threads and fits must be masked |
⚙️ The Critical Relationship Between Surface Treatment and Machining Tolerances
Although surface treatment thickness is measured in microns, for high-precision parts a few microns can decide whether a component fits or fails.
For example:
A nut plated with 15 μm may have its threads partially filled, making it impossible to screw in.
A precision shaft that is anodized but not re-ground afterward may exceed the tolerance range and seize inside the bearing.
Therefore, during design and process planning, it’s essential to account for coating thickness and adjust pre-treatment dimensions.
🧠 “Pre-Treatment Design” for Surface Coating Allowances
To guarantee consistent quality, Heng Chang implements three key design steps before processing:
Thickness Compensation
Adjust tolerance center values according to coating type (e.g., reduce outer diameters or enlarge inner holes before plating).Masking Design
Define masked areas on threaded, fitting, or grounding surfaces to prevent unwanted coating.Post-Machining
For high-precision components, perform secondary operations such as polishing or grinding after plating to restore target dimensions.
🔍 Practical Case Study
Heng Chang produced precision connector parts (material: SUS303) requiring black-nickel plating, 8–12 μm thick.
Without dimensional correction, the plated outer diameter would have exceeded the tolerance by 0.015 mm.
By presetting a –0.010 mm machining offset, the post-plating dimensions landed perfectly within the mid-tolerance range, ensuring smooth assembly and uniform appearance.
This kind of dimensional allowance design is an essential detail in true precision manufacturing.
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