Surface Finish Callout: Ra, Rz and How to Specify per Surface
The most expensive four words in machining are "Ra 0.8 all over". They appear on brackets that will never touch anything smoother than a hand, and they add a finishing pass to every face. On a seal bore the same words cost nothing, because that bore needed the pass anyway.
A surface finish callout specifies function, not quality. Getting it right means knowing which surfaces do work and which merely exist.
Ra is an average, and averages hide the peaks
Ra is the arithmetic mean deviation of the roughness profile over a sampling length. Because it is a mean, a surface with one deep scratch in a smooth field has the same Ra as a uniform sawtooth. For a bearing seat those surfaces behave identically; for a seal or a fatigue-critical fillet they behave nothing alike.
Rz is the peak-to-valley height within the sampling length. It is sensitive to the single worst peak — exactly the feature that breaches a seal or starts a crack.
In turning, Rz runs about 4 to 6 × Ra, so Ra 0.8 µm means Rz 3.5 to 5 µm. If your seal supplier asks for Rz 2.5 µm maximum, that is Ra 0.4 to 0.5 µm in turning terms, and a surface called out at Ra 0.8 will fail the seal test while remaining "in specification".
Where Ra stops meaning anything
Four applications need Rz, or a profile parameter, rather than Ra:
Sealing surfaces. An O-ring or a lip seal bears on the peaks. A plateau surface with a few deep valleys has a low Ra and leaks, because the ring cannot bridge the valleys. Specify Rz max, and for dynamic seals also a material ratio (Rmr) so the bearing area is defined.
Optical and contact surfaces. Light scatter and contact resistance depend on the height distribution, not the mean. A mirror at Ra 0.01 µm and a ground surface at Ra 0.4 µm are not interchangeable.
Coating adhesion. Anodize and zinc plating need a mechanical key. Below about Ra 0.4 µm, adhesion falls off and the coating becomes prone to blistering, so a polishing pass before anodizing is often worse than no pass at all.
Fatigue surfaces. A steel fillet at Ra 0.4 µm carries notably higher alternating stress than the same fillet at Ra 3.2 µm, because roughness is a population of small notches. On a fatigue-loaded part, the finish belongs next to the radius, not in a general note.
See also: Chamfer vs Fillet: Which Edge Treatment Belongs on the Drawing and Datum Scheme: How the Datum You Choose Changes the Price of the Part.
Calling it out surface by surface
The working structure is one general note plus exceptions:
- A title-block note: "Surface roughness Ra 3.2 µm unless otherwise specified."
- Specific callouts on the ten surfaces that matter: seal bores, bearing seats, sliding fits, optical faces, datum faces used for location.
- A note on direction: "measured across the lay" for anything functional.
- A note on what must not be polished: "no polishing on sealing faces" where adhesion or plateau problems apply.
One global value is either expensive or inadequate. On a typical housing, five surfaces need Ra 1.6 and forty are fine at Ra 6.3; a single Ra 1.6 note costs a finishing pass on all forty-five.
Symbols matter too. The check mark with a number is universally read; older texture symbols with material-ratio boxes are interpreted differently by different shops. Write "Ra 1.6 µm", not "1.6".
Measurement direction and cutoff
Measured along the tool marks, a turned surface reads low, because the stylus rides in a groove and never climbs a peak. Measured across the lay, it reads the true profile — often twice the value. That is why "measured across the lay" belongs in the note.
Cutoff matters as much. Standard practice uses λc = 0.8 mm for Ra up to 2 µm and 2.5 mm for coarser surfaces, with at least five sampling lengths. A shop reading a Ra 3.2 surface at 0.8 mm cutoff reports a different number than one at 2.5 mm, and both claim conformance.
On a bore under Ø20 mm, a skid-type probe often cannot traverse far enough, so ask for a replica instead.
What tightening the callout costs
The table is the cost multiplier we apply to a finishing operation relative to a standard Ra 6.3 milled or turned surface, on a part of moderate size.
| Ra | Rz typical | Process that gets there | Cost factor | Typical use |
|---|---|---|---|---|
| 12.5 µm | 50 µm | Rough face mill, saw cut | 0.8× | Non-functional clearance |
| 6.3 µm | 25 to 30 µm | Standard milling or turning | 1.0× | General surfaces, covers |
| 3.2 µm | 13 to 18 µm | Finish turning, finish milling | 1.15× | General fits, non-sealing faces |
| 1.6 µm | 6 to 9 µm | Fine turning, wiper insert | 1.3× | Static O-ring grooves, bearing seats |
| 0.8 µm | 3.5 to 5 µm | Fine boring, cylindrical grinding | 1.5× | Bearing seats, seal bores |
| 0.4 µm | 1.5 to 2.5 µm | Honing, fine grinding | 2.0× | Dynamic seals, hydraulic rods |
| 0.2 µm | 0.8 to 1.2 µm | Grinding plus lapping | 2.6× | Precision spools, gage pins |
| 0.05 µm | 0.2 to 0.3 µm | Lapping, polishing | 4.0× | Optics, reference surfaces |
Tightening from 3.2 to 1.6 costs about 13 percent of the finishing time, usually worth it. Tightening from 1.6 to 0.4 roughly doubles it and often needs a different machine — which is why Ra 0.4 on a milled face is a red flag.
Geometry matters as much as process. A Ø20 bore is fine-bored to Ra 0.8 in one pass; a 200 × 150 mm face at the same value needs a fly cutter, a wide radius and a slow feed, which can triple the cycle time on that face. Ask whether Ra 1.6 plus flatness would meet the function.
Getting the numbers to hold
The finish a shop produces is set at the programming stage. In turning, theoretical Ra is roughly f²/(32·r), where f is feed per revolution and r is the insert nose radius. A 0.1 mm/rev feed with a 0.8 mm nose radius gives about Ra 0.4 µm; double the feed to 0.2 mm/rev and it becomes roughly Ra 1.6 µm. That is the whole trade: feed rate against finish, with the nose radius as the lever.
In milling, stepover and runout dominate. A 12 mm face mill with 0.05 mm runout on one insert leaves a mark no feed adjustment removes. On a 5-axis machining center cutting a contour, stepover must shrink as curvature rises, or the finish varies across the part.
If a surface is functional, tell us what it does. A seal bore, a sliding fit and a cosmetic cover all take different finishes even when the drawing says the same Ra, and the difference is entirely in the process choice. Send the drawing with the functional surfaces marked and we will quote the finish that actually works.
Frequently asked questions
Is Ra the same as Rz?
No. Ra is the mean deviation of the profile, Rz is the peak-to-valley height. Turning typically gives Rz at four to six times Ra, so an Ra 0.8 surface can be Rz 5 µm and fail a seal specified at Rz 2.5 µm.
What does Ra 0.8 µm feel like?
Fine but not mirror-like. A fingernail will not catch, a reflection will be diffuse, and it looks like a good machined finish. Ra 0.4 is noticeably shinier, and Ra 0.2 starts to reflect shapes.
Can milling hold Ra 0.8 µm over a large face?
Only with a fly cutter, a wide nose radius, low feed and a rigid setup, and even then it is slow. Above about 100 mm wide, Ra 1.6 µm is the practical limit before cost climbs steeply.
Should I specify Ra on every surface?
No. A general Ra 3.2 note plus explicit callouts on the five to ten functional surfaces is cheaper and clearer. Applying a tight value everywhere raises cost on surfaces that will never be inspected.
Related reading
Send a drawing for a quote and DFM review.
STEP, IGES, X_T, PDF or DWG up to 50 MB. Tell us quantity and finish, and the quote comes back with tolerance and process notes, not just a price.