Pre-plate dimension check

The Plating Tolerance Window — Does the Deposit Fit Inside Your Band?

A coating specified as a minimum on one surface lands on both walls of a bore and both flanks of a thread, so the diameter moves by twice the class minimum before any tolerance on the deposit itself is considered. This is the one calculation a machining supplier is better placed to run than a plating shop, because it decides the pre-plate dimension.

Coating class on the callout Thickness classes from ASTM B633-13 Table 1. The value is a minimum on a significant surface, so the real deposit sits at or above it.

Fit tolerance band available on the diameter, in µm Total band width, not the plus/minus. An ISO h7 fit at 20 mm is 21 µm; enter whatever your drawing actually allows.

Feature type Threads carry a second rule on top of the arithmetic: ASME B1.1 lets the allowance absorb the coating only if the maximum thickness is no more than one quarter of that allowance.

Exceeds the band

What the deposit does to the fit

Minimum deposit, one surface	12 µm
Diametral increase, two surfaces	24 µm
Share of the available band	114%
Band left for machining	none — the fit must be re-cut

Machining to the middle of the band and then plating to this class does not leave a tight fit — it leaves an interference. Move the pre-plate dimension, change the fit class, or mask the surface.

Send a Drawing for a Finish-Spec Review

Arithmetic on the class minimum only. Your plater’s deposit tolerance band sits on top of these figures, and thread roots and blind holes are exempt from the minimum unless the drawing designates them.

Sources: ASTM B633-13 Table 1 and Note 4 (the dimensional tolerance of most threaded articles does not permit much more than 8.0 µm of coating); ASME B1.1 Class 2A allowance rule. Zhenling Metal machines the component and coordinates the finishing operation; no plating line is operated in-house.

The Plating Tolerance Window — Does the Deposit Fit Inside Your Band?

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