What "Ra 3.2" actually looks like. A practical reference.

Ra is the arithmetic mean roughness — the average deviation of the surface profile from its mean line. Lower Ra means smoother surface. Standard as-machined is Ra 3.2 µm (rough), fine-machined Ra 1.6 µm, precision-machined Ra 0.8 µm, mirror polished Ra < 0.1 µm. Ra is measured in micrometers (µm) or microinches (µin) where 1 µm ≈ 40 µin.

Match finish to function. For structural parts hidden from view, Ra 3.2 µm (as-machined) is fine. For cosmetic parts visible to users, Ra 0.8 µm or bead-blasted uniform matte. For mating sealing surfaces, Ra 0.8 µm. For hydraulic cylinder bores, Ra 0.4 µm ground. For optical surfaces, Ra 0.05 µm polished. Specifying tighter than needed adds cost.

Society of Plastics Industry (SPI) mold finish grades apply primarily to injection-molded parts. SPI-A1: mirror polish (Ra 0.05 µm). SPI-A2: diamond polish. SPI-A3: fine diamond. SPI-B1/B2/B3: paper polished. SPI-C1/C2/C3: stone polished. SPI-D1/D2/D3: dry blast (most common for most enclosures). Specify the grade appropriate for your cosmetic requirement.

Bead blast uses glass beads — produces uniform matte finish without cutting surface material. Sand blast uses abrasive grit (aluminum oxide, garnet, silicon carbide) — produces coarser texture and cuts material (used for paint stripping, heat-scale removal, or heavy surface preparation). For cosmetic applications, specify bead blast. For paint prep, sand blast.

Yes. Rougher surfaces have more surface area for corrosion attack and trap moisture/contaminants. For stainless steel, smoother finishes (electropolish to Ra 0.4 µm) significantly improve corrosion resistance by reducing attack sites. For painted or coated steel, roughened surfaces (sand blasted) provide better paint adhesion — trade-off between corrosion resistance and coating adhesion.

Surface roughness is measured with a profilometer — a stylus traces across the surface, recording micrometer-level height variations. We maintain Mitutoyo surface roughness testers in our QA lab. Calibrated measurement reports provided on request; typical cost for a single Ra verification is $10–30 per part.