Surface Roughness Ra Chart: What Ra 0.4 to Ra 3.2 Means and What Each Finish Costs

Surface finish is the most over-specified number on mechanical drawings and the most under-measured. A callout of Ra 0.8 on a non-functional face can double the finishing time of a part for no benefit, while a missing finish note on a sealing face can leak in service. This chart translates the common Ra values into what they are for, what process produces them, and what they cost, based on what our lathes and mills hold in daily production.

CNC turning a steel shaft to a mirror-bright finish, fine feed and sharp tooling produce the low Ra value. — Finishing pass on a CNC lathe. Fine feed and a fresh edge are what a low Ra callout actually buys.

Ra values mapped to applications

Ra is the average absolute deviation of the surface profile, in micrometers. Lower is smoother. The table below maps the values that appear on most machined-part drawings to the functions they serve.

Ra value	Visual feel	Where it belongs	Typical process route
Ra 3.2	Visible tool marks, smooth to a fingernail	Non-critical faces, brackets, general machined surfaces	Milling or turning, standard feed
Ra 1.6	Tool marks just visible	Close-clearance fits, O-ring grooves with non-critical pressure, mounting faces	Finish pass, reduced feed
Ra 0.8	Reflective under light, no visible marks	Static sealing faces, bearing seats, shafts under rotating seals	Fine finish pass, sharp geometry
Ra 0.4	Mirror-like on flat faces	Dynamic seals under pressure, precision spools, medical implant contact surfaces	Fine turning or milling, often grinding
Ra 0.2 and below	True mirror	Gauge surfaces, optical seats, high-speed bearing journals	Grinding, lapping, or polishing

The cost gradient between rows is steep. Going from Ra 3.2 to Ra 1.6 on a milled face costs a lighter finish pass, a few percent of cycle time. Going from Ra 0.8 to Ra 0.4 can double finishing time and adds edge-prep discipline, because a worn or chipped edge prints its defect straight into the surface. Below Ra 0.4 the process usually leaves the machining hall entirely, into grinding or lapping.

What each process holds on production parts

These are the bands we hold in normal production on aluminum, steels, stainless, and titanium, measured on finished parts rather than test coupons.

Process	Production Ra band	Notes from the floor
CNC milling, standard pass	Ra 1.6-3.2	Step-over and feed dominate; tool condition shows up fast
CNC milling, finish pass	Ra 0.8-1.6	Light step-over, sharp ball nose on 5-axis for contoured faces
CNC turning, finish pass	Ra 0.4-0.8	Turning holds the lowest Ra of the cutting processes; nose radius and feed decide the number
Grinding	Ra 0.1-0.4	For hardened seats and faces below Ra 0.4
Anodizing or plating, after finish	adds 0.2-0.8 to the reading	Coatings grow roughness; quote the pre-coat value accordingly

Two floor details matter more than any chart. Coating comes last: hard anodize on an Ra 0.4 face can read Ra 1.0 after processing, so the drawing should state whether the finish applies before or after coating. And direction matters for seals: a shaft can measure Ra 0.4 and still weep if the lay of the marks runs like a spiral under a rotating lip seal. Specifying plunge-turned seal journals removes that risk entirely.

Surface roughness tester traversing a stylus across a machined steel face to record the Ra value. — Stylus-type roughness tester on a finished face. Every Ra claim on a report traces to a measurement like this.

How finish is actually measured

Ra claims on inspection reports come from stylus instruments that traverse the surface and average the profile, with the cutoff length set per the standard for the value being measured. Two practical notes for buyers. First, Ra and Rz are not interchangeable: Rz (mean peak-to-valley height) runs roughly four to seven times the Ra value on machined surfaces, and a drawing that mixes them without units invites disputes. Second, spot checks and full reports are different products; we measure finish on defined critical faces per the inspection plan, not on every face of the part.

Our 40+ precision inspection devices include the surface testers for this work alongside the CMMs that handle dimensional layout, and finish measurements ride along with the first-article report. The quality page lists the full equipment set and certificate scope.

Finished steel shafts and sealing caps with low-Ra turned surfaces laid out for final inspection. — Finished shafts and sealing caps. Turned seal journals keep the lay concentric, which matters as much as the Ra number.

Specifying finish without overpaying

Put tight Ra values only where a function needs them: sealing faces, bearing seats, sliding contacts, and anything a human body touches. Leave Ra 3.2 or a general note everywhere else. If your drawing carries a single finish note on every face, ask whether the sealing face really needs the same treatment as the mounting boss. Our process pages for turning and milling show the finish classes each route produces, and the tolerance standards reference pairs finish with dimensional bands.

Send the drawing through the quote form and the review will flag finish callouts that drive cost out of proportion to function, before the purchase order does. For corrosion or wear protection on top of the finish, the materials library covers how each alloy and coating combination behaves.

Frequently asked questions

Is a lower Ra always better?

No. Below the functional requirement, lower Ra adds cost and can even hurt: some sliding and adhesive joints need a little texture to hold lubricant. Specify the value the function needs and no tighter.

Why did my anodized part measure rougher than the drawing value?

Anodizing and plating grow surface texture. If the finish callout applies to the final coated state, the machining pass must start below it. State the measurement condition on the drawing and both sides measure the same thing.

What is the difference between Ra and Rz?

Ra averages the profile deviations; Rz averages the peak-to-valley heights over sampling lengths. On machined surfaces Rz is typically four to seven times Ra. Always compare like with like when reviewing reports.

Can you hold Ra 0.4 on a contoured face?

On 5-axis platforms with sharp ball-nose finishing passes, yes on most geometries, within the band shown above for the material. Very shallow open contours hold it best; deep pockets and sharp internal corners push the achievable value up.