What are coating thickness and build-up?

Coating thickness is the thickness of a surface layer such as an anodic layer, electroless nickel, electroplated zinc or a powder coat, usually given in µm. Build-up is the change in a dimension caused by that layer. For a layer deposited onto the metal, the build-up per side is close to the coating thickness. An anodic layer is formed from the base metal itself and grows partly into the material and partly out of it; only the outward part changes the size. On a diameter, and on a width between two coated faces, the build-up counts twice: a shaft grows and a bore or slot narrows by twice the build-up per side. The distance between faces pointing the same way, or a centre distance, hardly changes with an even layer.

What do coating thickness and build-up depend on?

How thick and how even a layer turns out depends on the process, the material and the shape of the part. Electroless nickel is deposited without current and stays close to uniform in bores and internal corners, as long as the solution is refreshed there. Electroplated layers build up thicker on edges and corners and run thinner in bores and recesses. A powder coat is comparatively thick and covers sharp edges and internal corners less well than flat faces. An anodic layer also tends to run thinner in deep, narrow bores. The split between penetration and build-up depends on the process type, the alloy and the process conditions; for hard anodising a rule of thumb of roughly half outwards is often quoted. On a metric thread with a 60° thread angle, the pitch diameter changes by about four times the build-up on the flanks, twice as much as on a plain diameter.

What does the standard say about coating thickness and build-up?

A coating is usually specified through a process standard, for example ISO 7599 for decorative and protective anodising, ISO 10074 for hard anodising, ISO 4527 for electroless nickel-phosphorus or ISO 2081 for electroplated zinc on iron and steel. Many of these set a minimum thickness on the surfaces designated as significant; where the layer has a dimensional function, a nominal thickness with a tolerance is often agreed. For metric threads the ISO 965-1 limits generally apply before coating; the tolerance positions e, f, g and G leave room for a thin layer. Typical measurement methods are eddy current (ISO 2360) on aluminium, the magnetic method (ISO 2178) on steel and a microsection (ISO 1463). Which class applies, on which surfaces, and whether dimensions hold before or after coating should be stated on the drawing or in the order.

How do coating thickness and build-up work in practice?

Build-up calls for a decision before machining starts. You can adjust the size before coating, a shaft slightly smaller and a bore slightly larger, based on the build-up and spread your finisher quotes. That spread comes off the tolerance zone left for machining; on a tight fit it can be a large share. You can have a fit or a thread masked so no layer forms there. Or you have the layer applied thicker and machine the size afterwards: electroless nickel, hard chrome or a hard anodic layer is often ground, honed or lapped to size. Machining through a thin anodic layer leaves bare metal at that spot. Masking is usually hand work at the finisher; what it costs depends on the number of surfaces, the method and the batch size.

How do you see coating thickness and build-up in Blake?

In Blake the finishing sits with the part as a proposal from the drawing; you confirm or correct it. In the estimate, outsourced anodising, plating or coating is its own line from your subcontract library, and a missing price stays open. More on estimating.