Surface Finishing and Dimensional Requirements
For engineers and buyers whose parts are machined and then anodized, plated or otherwise finished.
A finish adds or removes material, so the drawing has to say which condition each dimension describes. This guide explains how to state both requirements together and how to check them.
How a Finish Changes Size
Most finishes change dimensions. A coating builds on every exposed surface, so a diameter or a slot width changes by twice the layer thickness: a shaft grows and a bore shrinks. Anodizing behaves differently, because the oxide grows partly into the base metal and partly outward, so the size change is smaller than the total coating thickness. Etching, pickling and blasting before or during finishing remove material instead.
Threads are the most sensitive feature. On a symmetric thread form, a layer on the flanks moves the pitch diameter by a multiple of the coating thickness; for common sixty-degree forms the change is about four times the layer thickness. A thread that gauges correctly after machining can fail the same gauge after plating, which is why threads need their own instruction.
Related: Materials and surface finishing
Common Finishes and Their Dimensional Effects
The table summarises how common finishes typically behave. Actual thickness and growth depend on the specification, class and the finisher's process, so the drawing should cite the specification and give a thickness range rather than rely on a general description of the finish or on a colour name alone.
| Finish | Typical dimensional behaviour | What to state on the drawing |
|---|---|---|
| Sulfuric acid anodizing on aluminium | Thin layer, partly penetrating; electrically insulating | Specification, type and class, colour, thickness range, masked areas |
| Hard anodizing | Thick layer, roughly half grown into the base metal; can raise roughness | Specification, thickness range, dimensions that apply after coating, masked areas |
| Chemical conversion coating | Very thin, usually negligible for size; stays conductive | Specification and class, and faces that rely on electrical contact |
| Electroless nickel | Even thickness, including inside bores and recesses | Specification, phosphorus range if relevant, thickness range, any heat treatment |
| Electroplated silver, gold or tin | Builds more on edges and outside corners than in recesses | Specification, underplate, thickness range at named surfaces |
| Passivation of stainless steel | No meaningful change in size | Specification and method |
| Bead or grit blasting | Removes or peens a thin layer and rounds sharp edges | Media type, finish reference and surfaces to protect |
Related: Specifying surface roughness
Thickness is a range
finish specifications allow a band of thickness, so the machined size has to leave room for both the thinnest and the thickest acceptable layer.
Before-Finish and After-Finish Dimensions
A drawing can state dimensions in one of two ways.
A bore shrinks and a shaft grows by twice the layer; thread flanks shift more — concept illustration
With after-finish dimensions, the size on the drawing is the finished size; the machinist works to a pre-finish size calculated from the expected layer, and the finisher's thickness range becomes part of the machining plan. With before-finish dimensions, the drawing gives the machined size and the finish is applied on top, so the finished size depends on the coating range and is not directly controlled.
For fits, threads, RF apertures and datum faces, designers usually specify the finished condition, because that is what assembles. A general note stating that dimensions apply after finish, with local exceptions where needed, keeps one rule for the whole part. Where a feature is measured at both stages, say which result is the acceptance value.
Related: Defining waveguide drawing requirements · Quality and inspection
Write the Finishing Requirement So It Can Be Checked
A finish called out only by name, such as black anodize or nickel plated, leaves the type, thickness and coverage to whoever orders the finishing. Each item below closes a gap that would otherwise appear as a question during quotation or as a disagreement when the parts arrive at incoming inspection.
Specification and class: the governing standard, type and class, and any sealing or post-treatment.
Thickness range: minimum and maximum, and where on the part it is measured.
Coverage and masking: surfaces that must stay bare or receive a different finish, shown on the drawing with boundaries.
Dimensional stage: a note saying whether dimensions apply before or after finish, with exceptions listed.
Functional faces: electrical contact, sealing or sliding surfaces with their own requirements.
Appearance: colour, uniformity and permitted marks, with a reference sample if appearance is accepted visually.
Related: Materials and surface finishing
Threads, Fits and Bare Contact Faces
Threads are usually handled in one of three ways: masked so they stay bare, machined with an allowance so the coated thread still gauges, or chased after finishing, which removes coating from the flanks.
Plugs and masking keep threads and contact faces bare while the rest of the part is finished.
External metric threads with a tolerance position such as g leave a small allowance for thin coatings, whereas position h leaves none. Internal threads are commonly plugged. State which method applies to each thread.
Anodic coatings are electrical insulators. A grounding point, an RF gasket face or a cover joint that must conduct needs either masking during anodizing or a conductive finish such as a conversion coating. Mark those faces with a boundary and a note, and check that the tolerance on the masking boundary suits the gasket or contact area.
Related: RF shielding covers and lids · Microwave cavity housings
Who Owns the Allowance When Finishing Is a Separate Step
Finishing is often done by a specialist after machining, arranged either by the machining supplier or by the buyer. If dimensions apply after finish, someone has to own the allowance: machining to the right pre-finish size, choosing a finisher whose thickness range fits, and checking the result. When the buyer sends parts to its own finisher, the machining supplier can only be accountable for the pre-finish condition, and the order should say so.
Rework needs the same clarity. Stripping a coating removes some base metal, and pre-treatment etches a little more each cycle, so a refinished part may no longer meet size. Agree in advance whether stripping and refinishing is allowed, how often, and how affected dimensions are rechecked before parts are shipped again.
Related: Quality and inspection
Two-stage inspection
for a critical feature, measuring after machining and again after finishing shows whether a problem came from the machined size or from the coating.
Records That Show the Finished Condition
The dimensional report and the finishing record answer different questions. The report should state the condition in which each feature was measured. The finishing record should identify the specification, the batch processed and the thickness checks performed. Agree which records you need before ordering, because preparing them is part of what is quoted.
specification, class, batch or lot, and the parts it covers.
the method, such as eddy current, magnetic induction or X-ray fluorescence, with locations and results.
adhesion, corrosion or seal-quality tests, as the finish specification requires.
the measurement stage and the features rechecked after finishing.
Related Pages
Continue with the closest related part family or guide.
Waveguide Drawing Requirements and Tolerances
Which waveguide features, datums, finish conditions and acceptance checks to define on the drawing before a machining quote.
Specifying Surface Roughness on Machined Parts
Choosing the roughness parameter, placing callouts on functional surfaces, stating the condition and checking the result on received parts.
What to Look for in an Inspection Report
What FAI, CMM and dimensional reports, conformance and material certificates prove, and which fields to check before accepting parts.
Questions About Finishes and Dimensions
If dimensions apply after plating, what does the machinist do?
The machinist calculates a pre-finish size from the specified thickness range, machines to that size and, where agreed, measures before the parts go to finishing. The finished parts are then accepted on the after-finish dimensions. This works only if the thickness range is stated and the finisher controls it, so give both on the drawing or in the order.
Do we have to mask every threaded hole?
Not necessarily. Some threads can be machined with an allowance and gauged after finishing; others are masked or chased afterwards. The choice depends on thread size and class, finish thickness and whether coating on the flanks is wanted for corrosion protection. State the method for each thread, or as a general note with listed exceptions.
Can anodized parts provide electrical grounding?
Not through the anodized surface, which insulates. Grounding and RF contact faces are commonly masked during anodizing or given a conductive conversion coating instead. Mark the faces and their boundary on the drawing, and agree how the bare or conductive area will be checked, for example by a resistance measurement if your design specifies one.
What happens if a finish has to be stripped and redone?
Stripping removes some base metal and can affect threads, edges and close fits, so a refinished part may not meet the drawing. Agree in advance whether rework is permitted, which dimensions are rechecked afterwards and how reworked parts are identified in the delivery records, so receiving inspection knows which parts were refinished.
Discuss Finishing and Dimensions Together
Send the drawing with the finish specification, the dimensions that apply after finishing and any faces that must stay bare.
Sales contact (pending)
Name, role and photo to be confirmed
Drawings are optional for an initial discussion.
Start with the information you have; file exchange can be arranged afterward.
Name to be confirmed
Sales, company name to be confirmed
- Say whether finishing should be arranged with the machining or handled by your own finisher.
- A short description is enough if the files are not ready.
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