Surface finish is a critical part of the component manufacturing process. It can be for aesthetic purposes (e.g., if a component is visible on the final assembly), but surface finish is the final step in improving a part’s durability, resistance, and longevity. For example, electropolishing improves corrosion resistance while various plating processes protect against wear and tear.
Just as each surface finishing method imparts different properties, each method also has a unique impact on a part’s dimensions. For some techniques, like media tumbling or bead blasting, the impact is negligible. For techniques like electropolishing or powder coating, the impact is greater and can result in either material loss or gain.
Often, the change is minor; we’re talking millimeters after all. But in the world of precision components and complex assemblies, every micron matters.
When considering a surface finish (or finishes) for your parts, it’s important to understand how each method impacts dimensional accuracy in addition to the final aesthetic and functional properties.
The table below lists the most common types of surface finishes and compares the amount of thickness that gets added or removed to a completed part.
| Finish | Standard Thickness |
| Anodize | 5 - 150 µm |
| Powder Coat | 50 - 150 µm |
| Electroplating | 2 - 25 µm |
| Electroless Plating | 2.5 - 50 µm |
| Black Oxide | 0.5 - 1.0 µm |
| Media Blasting | -- |
| Passivation | -- |
| Chemical Conversion Coating | -- |
The rest of the guide will review the finishing methods that impact dimensional accuracy in more detail and provide design and machining tips to keep in mind when working with the respective finishing method.
Anodizing is a conversion coating process that adds an oxidized layer to aluminum surfaces, significantly impacting a part’s dimensions. During the conversion process, roughly 50% of the layer penetrates the substrate while the other 50% builds outward from the surface.
This layer ranges from 5 to 125 µm, depending on the anodizing type.
| Type | Thickness | Dimensional Change |
| Type II (decorative) | 5–25 µm | +5–25 µm |
| Type III (hardcoat) | 25–125 µm | +25–125 µm |
Design and Machining Tips for Anodizing
Related Read: Cost Comparison of the Most Common Surface Finishes 💲
Powder coating is a dry finishing process that can significantly impact tolerances and requires careful pre-treatment planning. A thermoplastic or thermoset powder is electrostatically applied to a metal surface and then heated to form a durable, uniform layer for both aesthetics and protection.
Unlike conversion coating processes, powder coating adds material to the part’s surface, and the material buildup can be substantial.
| Type | Thickness | Dimensional Change |
| Thin Film Powder Coat | 20–40 µm | +20–40 µm |
| Standard Powder Coat | 50–150 µm | +50–150 µm |
In the case of diameters, total buildup for powder coating can be 100–300 µm.
Powder coating is one of the most impactful processes on tolerances and can compromise many features, including threaded holes, fasteners, precision bores, shafts, sealing surfaces, and datum surfaces.
Design and Machining Tips for Powder Coating
Electroplating is by no means dimensionally neutral. It adds a thin layer of metal onto a part’s surface using an electric current, and this material buildup impacts dimensions. This largely depends on the type of metal that’s applied as a layer to the part.
| Type | Thickness | Dimensional Change |
| Nickel | 5–25 µm | +5–25 µm |
| Hard chrome | 2–25 µm | +2–25 µm |
| Zinc | 5–15 µm | +5–15 µm |
| Gold | 0.1–2.5 µm | +0.1–2.5 µm |
Design and Machining Tips for Electroplating
Electroless plating does impact dimensions on precision components, even more than electroplating, depending on the part’s complexity. If you’re working with precision parts and complex assemblies, you’ll need to make sure design, machining, and finishing are well-aligned.
Electroless plating chemically deposits a metal layer onto a substrate; no electricity, as the name implies. Because it's an autocatalytic process, the material gets uniformly distributed across all surfaces, even features like threads, internal bores, and other recessed features.
While electroless plating is great in terms of consistency, every feature grows, and there are dimensional impacts that have to be considered. It can be precision-friendly, but you have to plan for it.
| Type | Thickness | Dimensional Change |
| Electroless Nickel (Low-P) | 2.5–25 µm | +2.5–25 µm |
| Electroless Nickel (Mid-P) | 5–15 µm | +5–15 µm |
| Electroless Nickel (High-P) | 10–50 µm | +10–50 µm |
Design and Machining Tips for Electroless Plating
Black Oxide is extremely thin --- typically 0.5 to 1.0 µm --- and has a minimal, predictable impact on tolerances. Because it’s a conversion coating, meaning it doesn’t build up like plating or powder coating, it replaces a microscopic layer of the base metal. As a result, it doesn’t need to be compensated for in the machining process.
Despite its minimal impact on dimensions, be aware that black oxide can affect sealing performance in high-pressure or vacuum systems.
Design and Machining Tips for Black Oxide