Surface finish chart, roughness conversion and drawing callout symbols

Substrate first: anodizing only works on aluminum, while powder coats most metals. Then thickness — anodize adds roughly 10 µm per surface at Type II and about 25 µm at Type III, against 50 to 100 µm for powder, so powder is far more likely to affect fit. Anodize keeps edge definition and is integral to the surface, so it does not chip; powder covers surface defects and offers a wider durable colour range. Both are electrically insulating, so mask any ground path either way.

Most do. Powder coating is the largest common change at roughly 50 to 100 µm per surface, which is enough to bind a close-fitting bore or a thread. Type II anodize adds about half its coating thickness outward, so a 20 µm coating grows roughly 10 µm per surface; Type III at 50 µm grows about 25 µm. Plating adds its full thickness, typically 5 to 50 µm. Chemical conversion coating and stainless passivation are effectively dimensionally neutral. State whether a drawing dimension applies before or after finishing, and mask anything that must hold its machined size.

Name the surfaces, the acceptance basis and the roughness. Ra is the usual control: Ra 1.6 to 3.2 µm (63 to 125 µin) is normal as-machined, Ra 0.8 µm (32 µin) needs a finishing pass, and Ra 0.4 µm (16 µin) or better implies polishing or grinding. Add a colour reference rather than a colour name, say whether parts must match within a lot or across lots, and mark which faces are non-cosmetic so effort goes where it is visible.

The full range in this guide routes through the supplier network: anodizing in Type II at 5 to 25 µm and Type III hardcoat at 25 to 50 µm to MIL-A-8625, chem film to MIL-DTL-5541, powder coating at 50 to 100 µm per surface, wet paint, zinc plating to ASTM B633, electroless nickel to ASTM B733, black oxide to AMS 2485, passivation to ASTM A967, electropolishing to ASTM B912, bead blasting and mass finishing. Finishers holding ISO 9001, AS9100 and NADCAP accreditation are available where a programme requires it. State the specification, class, thickness, masked features and whether dimensions apply before or after coating.

It should separate function from appearance, name the base material, mark protected interfaces, and say whether dimensions apply after finishing. Color names, coating families and cosmetic notes are starting points; masking, rack marks, inspection and final-state size still have to be defined for the part.

Multiply Ra by about 1.11. The factor is exact only for a periodic profile, so treat a converted value as a reading aid rather than a substitute for the unit the drawing controls. If a supplier reports RMS and the drawing states Ra, say which one governs acceptance before parts are measured.

Ra 3.2 µm is 125 µin and corresponds to N8, the roughness a standard milled or turned surface commonly reaches without a dedicated finishing pass. It describes average roughness on the faces the callout covers; it does not state gloss, colour, direction of lay or edge condition, so add those separately when they matter.

They are the same measurement expressed two ways: an N grade is a shorthand label for a specific Ra value, so N7 is Ra 1.6 µm. Drawings from different regions favour one or the other, and mixing them in one package is a common source of quoting questions.

The substrate decides the shortlist before appearance does. Aluminum takes anodizing — Type II at 5 to 25 µm, Type III hardcoat at 25 to 50 µm — chemical conversion coating under 1 µm where the surface must stay conductive, powder and wet paint. Stainless takes passivation to ASTM A967 and electropolishing to ASTM B912, both of which remove material rather than add it, and it cannot be anodized. Carbon and alloy steels such as 4140 have no native protection and need a layer: zinc plating to ASTM B633 at 5 to 25 µm, black oxide, electroless nickel to ASTM B733 at 5 to 50 µm, powder or paint. Copper, brass and bronze are usually nickel- or tin-plated, or left to patina where appearance allows. Titanium takes color anodizing and passivation. Machined and molded plastics are limited to as-made texture, mechanical or vapor polishing, bead blasting and paint over a compatible primer.

Match the layer to the exposure rather than to a finish name. On aluminum indoors, chemical conversion coating under 1 µm is enough and keeps the surface conductive; for outdoor or handled parts, sealed Type II anodize or powder at 50 to 100 µm carries far more protection. On carbon and alloy steel, zinc plating at 5 to 25 µm with a passivate is the common baseline, while black oxide is closer to a mild inhibitor than a barrier and needs oil to perform at all. Stainless usually needs passivation rather than a coating, because the problem is free iron left on the surface by machining rather than the alloy. State the exposure, the expected service life and any salt-spray or cyclic test the part must pass; what the routed supplier can certify against that test is confirmed on the reviewed quote.

Ra is the arithmetic average deviation of the profile; Rz is built from peak-to-valley heights, so it reacts to isolated scratches and burrs that Ra averages away. That is why two surfaces can share an Ra and fail differently in service. There is no exact conversion between them, because the ratio depends on the process, the material, how uniform the profile is, the cutoff length and the measurement method. Published ratios run roughly 4x to 7x Ra, which is the band shown in the chart above. If a drawing controls a sealing or fatigue-critical face, specify Rz directly rather than converting from Ra.

ASME drawings use ASME B46.1-2019 for the parameters and ASME Y14.36 for symbol positions. ISO drawings use the ISO 21920 series published in December 2021: part 1 for indication on drawings, part 2 for terms and parameters, part 3 for specification operators. ISO 21920 replaced ISO 1302, ISO 4287, ISO 4288 and ISO 13565-2 and -3, all of which were withdrawn at the end of 2021. Older drawings still citing the withdrawn standards remain readable, but parameter values are not always numerically identical between the old and new profile definitions, so state which standard the drawing works to rather than leaving it to be inferred.