In production we treat sheet metal surface finish standards as non-negotiable process controls. The drawing callout drives everything: Ra value, coating type, thickness range, and visual grade. Once the part leaves the press or laser, the finishing line is locked to those numbers. CAM and process engineers do not improvise; they match the specified standard to the available line capability and reject any deviation that cannot be held inside the stated tolerance band.
Where process capability forces the Ra limit we actually hold
Ra requirements show up as a hard limit because the upstream cutting and forming steps already set the starting roughness. Laser-cut edges typically land between Ra 3.2–6.3 µm before any secondary work. Punch and blanked edges sit higher. When the drawing asks for Ra 1.6 or better on functional surfaces, we have to add vibratory finishing, brushing, or light grinding. Those operations remove material and change dimensional stack-up, so the allowance is built into the blank size at the programming stage. If the drawing is silent on which faces need the tighter Ra, we default to the functional contact faces only and leave the rest at the as-cut condition. That decision is made during DFM review, not on the floor.
Powder coating, anodizing and galvanizing each bring their own process window. Powder lines run to AAMA 2603/2604 or ISO 12944 film-thickness bands; we target the middle of the specified range because edge build-up and Faraday-cage effects push thickness higher on complex geometries. Anodizing follows MIL-A-8625 or ISO 7599 thickness and seal quality; Type II clear is the most common, but Type III hard-coat changes both thickness and final dimension. Hot-dip galvanizing is controlled to ASTM A123 or ISO 1461 coating mass; the zinc layer grows thicker on thinner gauge, so we adjust immersion time and quench accordingly.
What escapes the line when the finish callout is ignored or vague
When the drawing only says "powder coat" without color, gloss, thickness or appearance grade, the part comes back with orange peel, edge pull-away or wrong shade. The assembly line then rejects it for fit or cosmetics. Anodizing without a clear Type and class callout produces uneven color on different alloy batches and can leave the part outside the final dimension tolerance because the oxide layer grows outward. Galvanizing without coating-mass limits leaves excess zinc on threads and holes that must be chased later, adding secondary operations and delay. In every case the scrap or rework cost lands on the fabricator; the shipment date moves.
How the finishing line actually locks the standards in place
We start with the drawing note. Preferred format is a clear callout such as "Powder coat RAL 9005, 60–90 µm, AAMA 2604, appearance grade A on visible faces." For anodizing we expect "MIL-A-8625 Type II Class 1, 10–25 µm." Galvanizing needs "ASTM A123, coating grade 75." Ra is written next to the surface symbol or as a note: "Ra 1.6 max on sealing faces." Once the callout is confirmed, the process sheet is generated with the exact pretreatment, coating parameters and inspection plan.
Powder coating thickness is checked with a magnetic or eddy-current gauge at five locations per part; we reject if any reading falls outside the band. Appearance is judged under controlled lighting against a physical master or SPI grade sample. Anodizing thickness is verified by eddy-current or metallographic section; dye and seal quality are checked by the standard boil or acid-dissolution test. Galvanizing is sampled for coating mass by the strip-and-weigh method or magnetic gauge, and the surface is inspected for bare spots, dross and runs. Ra is measured with a stylus profilometer on the critical faces; we take three traces and average them. All data stay with the traveler so the part can be released or quarantined before it leaves the plant.
When the factory will accept a relaxed or partial finish
We allow exceptions only when the drawing explicitly states them. Non-visible internal surfaces can drop to a lower appearance grade or even bare metal if the note says so. Functional Ra can be limited to sealing or sliding faces while the rest of the part stays at as-fabricated roughness. For low-volume prototypes we sometimes accept a commercial-grade powder or a thinner anodize layer when the customer signs a deviation, but the same thickness and adhesion tests still apply. Structural galvanizing can use a lower coating mass on thick sections if the drawing permits, provided corrosion life remains acceptable. In every case the relaxation is written on the traveler and confirmed before the finish process starts; silent assumptions are not used.
The practical rule is simple: the tighter the surface-finish requirement, the more upstream process control and secondary operations are needed. Clear, complete callouts on the drawing keep the finishing line efficient and the scrap rate low. Vague notes force us to stop and ask, which is exactly what the standards are meant to prevent.
