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A machined part can leave the CNC center perfectly in tolerance and still fail inspection after finishing. The reason is simple: most coating and surface treatment processes either add material to the surface or convert the surface itself, and both change the final dimensions of the part. On a hole, a shaft, or a threaded interface, a few microns of buildup can be the difference between a smooth assembly and a rejected batch.
This guide explains how the most common finishing processes affect dimensional accuracy, how much growth to expect from each one, and what engineers and buyers can do to keep finished parts within tolerance. It is written from the perspective of a precision cnc machining factory that handles machining and finishing under one roof.
Every surface treatment affects dimensions through one of two mechanisms:
Knowing which mechanism applies to your finish is the first step in predicting the final size of the part.
The table below summarizes typical layer thicknesses and the approximate growth per coated surface. For diameters, remember that growth appears on both sides, so the total change on a shaft or hole is roughly double the per-surface value.
| Process | Typical Thickness | Approx. Growth per Surface |
|---|---|---|
| Anodizing Type II (decorative) | 5–25 µm | About half the layer: 2.5–12.5 µm |
| Anodizing Type III (hardcoat) | 25–125 µm | About half the layer: 12–62 µm |
| Nickel / chrome / zinc electroplating | 2–25 µm | Full layer: 2–25 µm |
| Electroless nickel plating | 2.5–50 µm | Full layer, very uniform on all features |
| Powder coating | 50–150 µm | Full layer: 50–150 µm |
| Black oxide / blackening | 0.5–1 µm | Negligible for most tolerances |
| Passivation / phosphating | — | No meaningful change |
Anodizing converts the aluminum surface into a hard oxide layer. Roughly half of that layer penetrates into the base material while the other half builds outward, so a 25 µm anodic layer adds only about 12 µm per surface. Decorative Type II anodize is thin enough to ignore on loose tolerances, but Type III hardcoat can reach 125 µm and will easily consume a ±0.01 mm tolerance band if it is not planned for. Threads, press-fit bores, and sealing faces are the features most often affected.
Plating is fully additive: the entire metal layer sits on top of the machined surface. Nickel typically adds 5–25 µm, hard chrome 2–25 µm, and zinc 5–15 µm. Electroless nickel deserves special attention because it deposits chemically and therefore coats every surface uniformly, including threads, internal bores, and deep recesses that a machinist might forget to compensate for. That uniformity is excellent for consistency, but it also means every feature grows at the same time.
Powder coating is the heaviest finish on the list, adding 50–150 µm per surface. On a diameter that means 100–300 µm of total growth, which will close up threaded holes, jam fasteners, and ruin sliding fits. Powder-coated parts should either be designed with generous clearance or have their functional features masked before spraying.
Black oxide is a conversion coating only 0.5–1 µm thick, and passivation removes free iron without adding measurable material, so neither requires dimensional compensation in most cases. Nitriding and PVD/CVD coatings sit between the two extremes: they are thin, but on ultra-precision fits even a few microns of case growth or film thickness should be confirmed with the finisher before machining.
Consider a turned aluminum shaft with a journal specified at 25.000 mm ±0.01 mm, finished with 50 µm of hardcoat anodize. The anodic layer grows about 25 µm outward per surface, which adds roughly 50 µm to the diameter. A shaft machined to 25.000 mm would measure about 25.050 mm after anodizing, which is 50 µm above nominal and five times the permitted deviation.
There are two standard fixes. The first is compensation: machine the journal to approximately 24.950 mm so the anodized diameter lands on 25.000 mm. The second is masking: keep the anodize off the journal entirely. Either approach works, but it must be agreed before the parts reach the finishing line, not after inspection rejects them.
Dimensional problems caused by finishing usually come from a gap between the machine shop and the coating vendor. ANOK removes that gap by offering coating and surface treatment as part of a one-stop machining service, covering anodizing, electroplating, powder coating, passivation, blackening, nitriding, phosphating, and PVD/CVD. On aluminum surface treatment, dimensional variance is controlled to within 5 µm.
Because machining and finishing are planned together, compensation values, masking requirements, and inspection points are defined at the DFM stage. With machining tolerances down to ±0.002 mm and ISO 9001:2015 certified quality control, ANOK delivers finished parts that meet the drawing on the first inspection, not after rework.
Coating and surface treatment always change part dimensions; the only question is whether the change was planned. Additive finishes such as plating and powder coating add their full thickness to every surface, while conversion finishes such as anodizing add roughly half. Once you know the expected growth, you can compensate during machining, mask critical features, or correct dimensions after treatment.
If your project involves tight tolerances and finished surfaces, send your drawings to ANOK. Our engineers will review the finishing requirements together with the machining plan and quote parts that arrive in tolerance, coating included.