Surface Finish Guide

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

Q: How tight should I specify surface finish?

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.

Q: What are SPI finish grades?

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.

Q: What is the difference between bead blast and sand blast?

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.

Q: Does surface finish affect corrosion resistance?

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.

Q: How is Ra measured?

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.

Ra values, finish callouts, SPI mold grades, and when to specify each — written for design engineers by practicing manufacturers. Reference this before your next drawing release.

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01 · Ra reference

Ra values & what they feel like.

A rule of thumb: lower Ra = smoother, more expensive. Specify the highest Ra (roughest) that still satisfies function.

Ra (µm)	Ra (µin)	Description	Typical process	Use case
12.5 µm	500 µin	Rough	Flame cutting, rough turning	Non-critical hidden surfaces, weldments
6.3 µm	250 µin	Coarse	Rough milling, heavy feed turning	Non-visible structural, brackets
3.2 µm	125 µin	Medium	Standard machining, as-machined	Default as-machined for most CNC parts
1.6 µm	63 µin	Fine	Finish milling, precision turning	Mating parts, general cosmetic
0.8 µm	32 µin	Very fine	Finish turning, grinding, fine bead blast	Sealing surfaces, hydraulic bores
0.4 µm	16 µin	Ground	Cylindrical grinding, precision	Bearing seats, precision shafts
0.2 µm	8 µin	Polished	Lapping, honing, fine grinding	Optical surfaces, precision optics
0.1 µm	4 µin	Mirror	Lapping, polishing	Optical instruments, medical implants
0.05 µm	2 µin	Super mirror	SPI-A1 mirror polish	Optical-grade, injection mold cavities

02 · SPI finish grades

SPI mold finishes.

Standard mold-finish grades defined by the Society of the Plastics Industry. Most commonly specified on injection-molded part drawings.

A · Polished

SPI-A1 Mirror polish, Ra 0.05 µm

SPI-A2 Diamond polished, Ra 0.1 µm

SPI-A3 Fine diamond, Ra 0.15 µm

Optical clarity, cosmetic A-grade enclosures, medical devices

B · Paper

SPI-B1 Fine paper polish

SPI-B2 Medium paper polish

SPI-B3 Coarse paper polish

Cosmetic consumer products, automotive interior

C · Stone

SPI-C1 Fine stone polish

SPI-C2 Medium stone polish

SPI-C3 Coarse stone polish

Matte satin industrial parts, visible non-cosmetic

D · Dry Blast

SPI-D1 Fine dry blast

SPI-D2 Medium dry blast

SPI-D3 Coarse dry blast

Textured industrial enclosures, most consumer (hides scuffs)

03 · Drawing callouts

How to specify on your drawing.

√

Basic surface finish symbol

The classic check-mark symbol indicates any surface requiring controlled finish. Typically paired with Ra value above the symbol.

√Ra 1.6

Specific Ra value

Specify Ra value directly above the finish symbol. Value in µm by default in metric drawings, or specify "µin" for imperial.

√Ra 3.2 / Ra 0.8

Multiple values

Two values indicate upper and lower limits. Useful when finish can be "at least X but not smoother than Y" (for paint adhesion, for example).

FAQ

Surface finish questions.

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.