What Each Ra Number Costs on a Stainless Part

September 21, 2026

Surface finish is the second most common cause of a stainless part being rejected, after tolerance. It is also the requirement most often written as a word rather than a number.

What Ra actually is

Ra is the arithmetic average deviation of the surface profile from its mean line, measured over a defined length. It is a single number that summarises a complicated profile, and that is both its usefulness and its limitation.

Units cause half the confusion. Ra 0.8 micrometre is the same as 32 microinch. Drawings from North America often use microinch, drawings from Europe and Asia use micrometre, and a drawing that mixes them causes real errors. Write the unit.

Two other symbols appear on stainless drawings:

• Rz, the average of peak to valley heights. It is more sensitive to isolated defects than Ra, which is why cleanability specifications sometimes use it.
• Rmax, the single largest peak to valley height in the sample. The strictest of the three.

The practical bands

Ra	Microinch	How it is produced	Where it fits
3.2	125	Normal milling or turning	General machined surfaces, non critical
1.6	63	Good finishing pass	The common default, most industrial parts
0.8	32	Careful finishing pass, or bead blast and polish	Food contact per 3-A, seal faces, sliding fits
0.4	16	Grinding, fine turning, polishing	Pharma contact, high pressure seals
0.2 and below	8 and below	Electropolishing, lapping, superfinishing	Vacuum, high purity, some medical

Each step down that table is roughly a doubling of effort, and the last two often mean a second supplier and extra days. It also means material comes off, which matters on a tight tolerance feature.

Why stainless makes this harder than steel

Austenitic stainless does not shear cleanly. It is ductile and it tears, so a worn edge smears metal across the surface instead of removing it. The result is a surface that measures acceptably on Ra and looks wrong, or that looks fine and fails a closer inspection.

The fix is sharp tooling, rigid setups and enough feed to cut under the hardened layer rather than rub on top of it. That is the same list as in work hardening, because it is the same problem.

Two consequences for a buyer:

1. Ra 0.8 micrometre by machining alone is achievable but not free. It needs a dedicated finishing pass with a fresh tool. Asking for it on every surface multiplies that cost by the number of surfaces.
2. Ra 0.4 micrometre by machining alone is optimistic on most geometry. Expect a secondary process.

Ra is not cleanability

This is the trap in food, dairy and pharmaceutical work.

A machined surface at Ra 0.6 micrometre can still contain a torn manganese sulphide inclusion, a folded lip of metal from a dull tool or a single deep mark from a chip that got dragged. None of those move the average much. All of them hold product and bacteria.

Electropolishing addresses this directly. It removes more metal from the peaks than the valleys, so it lowers Ra and rounds the profile rather than simply flattening it. It also opens nothing up, unlike mechanical polishing which can smear metal over a defect and hide it.

So for a cleanable surface, specify both:

Product contact surfaces: Ra 0.8 micrometre maximum, electropolished per ASTM B912.

Not:

Polished finish, food grade.

The food contact article goes into what 3-A and NSF expect.

Specify by surface, not by part

The single biggest saving available on a stainless finish specification is to stop applying it everywhere.

A typical valve body has one or two sealing faces, one bore that matters and a dozen surfaces nobody will ever touch. A blanket Ra 0.8 micrometre note in the title block applies the expensive requirement to all of them.

Mark the critical surfaces on the drawing with their own symbol and leave the rest at the general note. On a part with many faces this routinely changes the quote by a large margin, and it costs nothing in function.

Where the finish is decided

• Seal and gasket faces. Ask the seal manufacturer for their band. Many elastomeric seals want Ra 0.4 to 0.8 micrometre. Too smooth is a real failure mode.
• Sliding fits. A smoother surface spreads contact load and reduces the chance of galling. See Nitronic 60 and galling.
• Before plating or coating. The coating follows the surface underneath. A rough substrate gives a rough coating.
• Before passivation. A rougher surface has more area and more embedded iron. It passivates less predictably. See citric against nitric.
• Fatigue critical parts. Surface finish is a fatigue variable. Tool marks are stress raisers.
• Cosmetic parts. Say which face is visible, because a bead blast finish on a visible surface is usually cheaper than a polished one and looks deliberate.

What to put on the drawing

1. A general note for everything, usually Ra 3.2 or 1.6 micrometre
2. Specific symbols on the surfaces that matter, with the unit stated
3. The process where the process is the requirement, for example electropolished per ASTM B912
4. The measurement method if it is not obvious, and whether a report is required
5. A note on visible surfaces if appearance matters

That is enough for three shops to quote the same thing. The full RFQ checklist covers the rest of the drawing.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• Choosing a Passivation Method Under ASTM A967
• Weld Then Machine, or Machine Then Weld?