A buyer sends a drawing with a hard limit, “Ø10.00/9.99 mm, plate per spec,” and months later the parts arrive at 10.05 mm on a zinc deposit. Nothing was machined wrong. The problem is that plating allowance was never separated from the machining tolerance, so the deposit was applied on top of dimensions already cut to the top of the limit. The fix is not a tighter machine setting; it is deciding, before the first chip is cut, whether the drawing number refers to the part before plating or after plating, and then letting everyone in the chain measure at the same stage.
Why coating buildup changes fits, not just appearance
Electroplated and anodized coatings are not paint sitting on the surface in a fixed layer; they grow outward from the substrate. For electroplating, the deposit adds material over the part, so a coating applied to all surfaces increases external dimensions by roughly the coating thickness per plated face. The uniformity of that growth depends on the bath’s throwing power, which characterizes how evenly current, and therefore thickness, is distributed between areas close to the anode and areas far from it. A high Wagner number corresponds to more uniform deposition, and factors such as geometry, current density and bath chemistry all shift the outcome.
Anodizing behaves differently and is frequently mis-booked. Because the oxide grows partly into the aluminum and partly outward, the dimensional change is about half the oxide thickness on each surface. A page on anodizing notes the example of a 2 µm coating increasing the part dimension by about 1 µm per surface. That “half” rule is widely repeated for anodized aluminum, but it is an approximation tied to the specific alloy and process, and it should be confirmed for the actual finish called out rather than assumed. Coating thickness ranges themselves are process dependent: sulfuric-acid anodizing is commonly described as producing moderate coatings from about 1.8 µm to 25 µm (roughly 0.00007 in to 0.001 in), while hard anodizing is often quoted between 13 µm and 150 µm (about 0.0005 in to 0.006 in). Those ranges describe the process, not a guarantee for any one job.
The drawing convention that prevents double-counting
The single most useful decision is to state, on the drawing, whether the limits apply to the finished part (after plating, or “as plated”) or to the part before plating (often called the pre-plate or basis-metal condition). A drawing convention removes ambiguity; without it, the fabricator and the plater can each assume the other owns the allowance, and the error compounds silently.
If limits are specified after plating, the machinist has to hold size reduced by the expected buildup. For a coating specified on all external faces, the reduction is approximately two times the coating thickness on diameters, and one times the coating thickness on a single plated surface. If limits are specified before plating, then the plated part will finish larger by the same amount, and any dimension near the top of the limit may fail after coating. Many shops prefer to state the finished requirement and let the allowance be handled upstream, but that choice must be explicit, not left to interpretation. Whichever convention is used, it should appear in the notes and be repeated on the inspection report so that a 40 µm zinc deposit is never measured twice into the same stack.
Threads: pitch diameter is the sensitive dimension
Threaded features deserve separate attention because the functional fit is set largely by the pitch diameter, not the crest. Industry standards specify minimum and maximum limits for major diameter, minor diameter and pitch diameter for recognized thread sizes, and internal and external threads must satisfy those limits independently. If plating deposits a uniform thickness on a thread flank, it increases the pitch diameter of an external thread and decreases the effective clearance with a mating internal thread, which can cause interference or make hand assembly difficult.
This is why drawing and thread standards allow a plating allowance, or a deliberate difference, to be built into the pre-plate thread limits so that the plated thread still assembles. The practical consequence for a buyer or designer is to specify thread requirements in terms of the finished condition and to confirm that the pre-plate limits the shop uses already include the coating. A coating note that simply says “zinc plate, thickness as required” leaves the thread allowance undefined and invites a rejected gage. Where clearance is tight, consider whether the thread should be masked, whether a larger pre-plate allowance is appropriate, or whether the finish selection itself is the better lever.
Masking decides where the build actually lands
Not every surface should receive coating. Masking controls where the deposit is permitted, and by extension where dimensions must be compensated. Common masking decisions include keeping threads clean for gaging and assembly, protecting precision bores, sealing electrical contact surfaces that must remain conductive, and shielding sealing faces or datum features that must stay within a tighter band.
Masking is a design and process decision that the drawing should support. If a bore must remain unplated to preserve a press fit, say so. If a thread must be plated to a specified class, confirm that the allowance was planned rather than relying on the plater to guess. “Plate all over” is a legitimate instruction, but it is also a commitment that every dimension in the stack will move consistently, and it should be chosen knowingly. Where masking is impractical, an alternative is to specify the finished limits and accept that the pre-plate geometry is derived from them.
Gaging stage: measure the right part at the right time
Inspection arguments usually come down to the gaging stage. A thread plug gage that rejects a finished, plated part may pass the identical part in the pre-plate condition, and both readings can be correct. The part that matters is the one that will be accepted at the finished stage, so gaging hardware, GO and NO-GO limits and acceptance criteria must be referenced to the same stage as the drawing.
This is also where CMM or optical measurement of a plated surface needs care. A coating changes the measured surface, and coordinate measurement on an insulating anodic layer or a soft deposit can differ from what a contact method reads on the bare metal. Define whether dimensional inspection is performed before plating, after plating, or both, and state which one governs acceptance. If both are checked, the report should clearly label the basis-metal and finished readings rather than presenting a single ambiguous number.
Supplier questions that close the loop
The allowance is easy to lose in handoff because two processes and often two suppliers touch the same dimension. Before releasing a job whose fits matter, ask the following, and expect specific answers rather than general assurances:
- Does the drawing’s stated limits apply before plating or after plating, and is that written on the note block rather than implied?
- What coating thickness is the plater targeting, and is that a single value or a range with a stated standard or specification class?
- Which surfaces will be masked, and which dimensions are expected to move as a result?
- For threads, are the pre-plate limits already opened to include the plating allowance, and which thread standard class and gaging class applies?
- At what gaging stage will the part be accepted, and on which features is post-plate inspection required?
- How will the plater and machinist exchange the as-built pre-plate dimensions so both parties are working from the same numbers?
These questions are cheap to ask before production and expensive to discover after shipment. They also shift the conversation from opinion to evidence, because each answer can be tied to a drawing note, a specification class or an inspection record.
Planning the allowance without over-promising
The exact allowance is job-specific. Coating thickness varies with process, bath, geometry and how uniformly the deposit forms; standards such as ASTM test methods for coated products define thickness classes and measurement approaches, but they do not remove the need to plan the allowance for a particular part. Treat any single number as a planning input with stated conditions, not a universal constant, and verify it against the applicable specification before committing a hard-limit dimension.
Two habits pay off. First, keep the allowance visible: note the coating thickness, the convention and the masking plan on the drawing so the machinist, plater and inspector all read the same instruction. Second, inspect at the stage that matches the acceptance criteria, so a finished dimension is judged as a finished dimension. For related background, see how anodizing changes dimensions on anodizing and CNC dimensions, how to write thread requirements in thread callouts for CNC parts, and how finished features are verified in first article inspection for CNC parts, CNC machining quality inspection reports, and tight-tolerance work in precision CNC machining parts.
Sources: Wikipedia, “Electroplating”, “Anodizing”, “Screw thread” and “Unified Thread Standard” (accessed 2026-10-09). Range and allowance figures are process references with stated conditions, not guarantees; verify against the applicable, paid standard and specification for each job.
