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How do you specify coating and plating thickness on a drawing?
The short answer
Specify the process by a named standard, give a thickness range rather than one number, state how it is measured, list the surfaces that must stay bare, and say whether dimensions are met before or after finishing. A note that says copper plate 10 microns but names no standard, method or masking will be quoted differently by every shop.
Why a thickness note fails
Most surface treatment problems start on the drawing rather than in the tank, because a coating is a dimension that the drawing has not described. A plating bath does not deliver a uniform layer: current concentrates on edges and starves recesses, so a part specified only as zinc plate 10 microns can easily arrive at 20 microns on a corner and 4 microns inside a blind hole. Two consequences follow. First, a single nominal figure is meaningless without a range and a measurement method, because no process holds one value. Second, the coating changes the part, so a dimension that was correct before finishing may be outside tolerance after it. Anyone who has had a plated bore that no longer accepts a pin has met this problem. The fix is not a tighter process; it is a complete note.
The four blocks of a complete finish specification
A finish note does four jobs, and a note that skips any of them pushes the ambiguity downstream. It identifies the process by a recognised standard, never by a verbal description, so that the chemistry and the acceptance criteria are fixed by a published document rather than by a supplier's habit. It sets the thickness as a range with a stated measurement method, because too thin a coating fails to protect and too thick a coating introduces residual stress, changes dimensions and can impair adhesion. It lists the masking, naming threads, press fits, bearing bores, sealing faces and electrical contact points that must remain bare. And it defines the acceptance tests, whether that is a thickness check, an adhesion test or a salt-spray duration, and whether testing is on every part or on a sample from the lot.
Weak note against strong note
The difference between a note that works and one that does not is usually six lines. Naming the process as zinc plate leaves the thickness class, the passivation type and the standard open, so three shops will quote three different things; naming it as ASTM B633 FE/ZN 12 with a trivalent passivate fixes all three at once. Saying about 10 microns is not a specification, but a 12 micron minimum and a 25 micron maximum is one, and adding a measurement method such as five magnetic-induction readings per zone makes it verifiable. Masking that is not listed will not happen, and a bake requirement that is not written will not be performed, which for high-strength steel is a hydrogen embrittlement risk rather than a cosmetic one. Finally an acceptance line that says good finish cannot be inspected, while a defined salt-spray duration and a sample plan can.
| Element | What to write | Why it matters |
|---|---|---|
| Process | Named standard and grade, for example ASTM B633 FE/ZN 12 | Fixes chemistry and acceptance in one reference |
| Substrate | Alloy and heat treatment condition | Sets the pre-treatment and the bake decision |
| Thickness | A minimum and a maximum, never one number | Thin coatings fail, thick coatings stress and grow |
| Measurement | Method and reading count, for example ASTM E376 | Makes the result arguable with data rather than opinion |
| Masking | Named threads, bores, press fits and contact faces | Plating fills threads and blocks grounding |
| Post-treatment | Bake time and temperature where hydrogen risk exists | Protects high-strength steel from cracking |
| Acceptance | Tests, hours and sample plan | Turns a claim into a verifiable result |
Thickness bands by common coating
Because a range is the correct form of the specification, it helps to know where each coating normally sits. Electroplated zinc for corrosion protection commonly runs 5 to 25 microns depending on the class, with a trivalent passivate on top. Electroless nickel is usually specified between 10 and 50 microns, and the high end of that band is what carries the corrosion and wear performance. Type II anodising lands at 5 to 25 microns. E-coat is thinner, at 15 to 35 microns, because the film self-limits as it insulates the part. Powder coating is the thickest at 60 to 120 microns or more, which is why it is the coating most likely to interfere with a fit. The numbers are planning bands for standard lines; the coating on a specific part depends on the substrate, the bath and the racking, which is exactly why a measured result is more useful than a nominal figure.
Before or after finishing
The single most common cause of a plating-related rejection is a drawing that does not say whether a tolerance applies before or after the coating. The cleanest practice is to declare that critical dimensions apply after finishing, because that is the condition in which the part must function. A 10 micron layer adds 10 microns to a surface but 20 microns across a diameter, which is enough to close a bearing fit or lose a press fit. Where a dimension is genuinely sensitive, the machinist leaves a stock allowance and the final size is set after coating, or the surface is masked and left bare. If a design needs clearances tighter than about 50 micrometres, plating build-up cannot be ignored in the tolerance stack. State the rule once, and it applies to every hole, thread and face on the drawing.
Masking, baking and the limits of the note
Three items are routinely forgotten and three limits bound the method. The forgotten items are masking, post-treatment and testing. Masking keeps threads, bores, press fits and electrical contact faces bare, and it is hand labour, so a design that keeps the masked zones few and simple is cheaper to finish. Post-treatment matters most for high-strength steel, where a plating bake relieves hydrogen embrittlement and is a safety requirement rather than a finish choice. Testing defines what constitutes a pass. The limits are these: no coating process holds a uniform thickness across complex geometry; no coating is immune to the geometry effect, so internal corners always run thinner than external edges; and no drawing note can describe a result that the process cannot measure, which is why a coating thickness specification should always name the method. Every figure here is a planning range, and the evidence that settles a specific order is a thickness measurement and an acceptance test on the finished parts.
See surface finishing for the coating routes we run, CNC machining for the machining allowance that goes with a coating spec, and metal stamping for plated contacts and terminals where thickness control is critical.
Scope and sources. The seven elements of a complete surface treatment specification, the guide to naming a process by a recognised standard, the rule to specify thickness as a range with a measurement method such as ASTM E376, the masking examples for threads, press fits and grounding points, the bake at 190 to 204 degrees Celsius for a minimum of 23 hours within four hours of plating per AMS 2759/9, and the acceptance examples come from a tutorial on specifying surface treatments. The zinc plating callout structure, the note that the drawing should state whether dimensions apply before or after plating, the ASTM B633 and ISO 2081 references, the hydrogen embrittlement threshold above 1700 MPa or 46 HRC, and the failure modes in recesses and press fits come from a zinc plating guide for machined parts. The plating build-up arithmetic on a diameter, the rule that a 10 micron layer on each side of a shaft removes 20 microns of clearance, the guidance to state plating thickness as an after-plating requirement, and the note that the ASTM B456 classes define copper thickness come from a copper plating design note. The drawing callouts for nickel plating, the decorative-versus-precision cost trade and the tolerance stack-up impact come from a nickel plating design note. These are planning ranges; the coating on a specific part should be confirmed by a thickness measurement and an adhesion or salt-spray result.
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