"Ra 0.4" on a drawing does not mean that the surface has to look shiny; it means that the arithmetic mean deviation obtained with a specific measuring method must not exceed 0.4 µm. Not knowing this difference leads to unnecessary grinding and polishing operations or, conversely, to functional surfaces that fall short. This article is the surface-side counterpart of our tolerance selection article.
Basic Parameters
- Ra (arithmetic mean roughness): the average of the absolute deviations of the profile from the mean line. The most common parameter; it describes general surface quality well, but can hide a single deep scratch.
- Rz (mean maximum height): the average over five sections of the difference between the highest peak and the deepest valley in each sampling length. It reacts strongly to individual deep marks; on sealing and bearing surfaces it is more meaningful than Ra. Roughly Rz ≈ 4–7 × Ra (depending on the machining method; the conversion is not exact).
- Rmax / Rt: the largest peak-to-valley difference over the whole evaluation length; the strictest parameter.
- Rq, Rsk, Rku, Rmr: additional parameters used in special applications (bearings, sealing, coating adhesion).
Ra and Rz cannot be converted into each other on the same surface; whichever is requested must be measured. Rz is more common on German-origin drawings, Ra on English and Turkish drawings.
The Old N Grades
N grades (old version of ISO 1302) are still seen on old drawings and in some standards:
| N grade | Ra (µm) | Typical method |
|---|---|---|
| N12 | 50 | Rough casting, flame cutting |
| N10 | 12.5 | Rough turning/milling |
| N9 | 6.3 | Normal turning/milling |
| N8 | 3.2 | Good turning/milling (CNC standard) |
| N7 | 1.6 | Fine turning/milling, reaming |
| N6 | 0.8 | Very fine turning, grinding, Swiss-type turning |
| N5 | 0.4 | Grinding, honing, precision turning (brass/aluminium) |
| N4 | 0.2 | Fine grinding, honing, lapping |
| N3 | 0.1 | Lapping, polishing |
Achievable Values in CNC Turning and Milling
| Operation | Routine Ra (no extra cost) | Achievable Ra (extra pass/care) | Beyond that |
|---|---|---|---|
| CNC turning, steel | 1.6–3.2 | 0.8 | Grinding (0.2–0.4) |
| CNC turning, aluminium/brass | 0.8–1.6 | 0.4 | Polishing, diamond turning |
| Swiss-type turning, brass/free-cutting steel | 0.4–0.8 | 0.2–0.4 | Roller burnishing, grinding |
| CNC milling, flat surface (face milling) | 1.6–3.2 | 0.8 | Grinding, lapping |
| CNC milling, side wall | 1.6–3.2 | 0.8–1.6 | Grinding |
| Hole: drilled | 3.2–6.3 | — | Reaming (0.8–1.6), honing (0.2–0.4) |
| Hole: boring | 0.8–1.6 | 0.4 | Honing |
| Threading | 1.6–3.2 | — | Thread rolling |
Values are typical; tooling, cutting parameters, material and machine condition affect them. On difficult-to-machine materials such as stainless steel and titanium, routine values are one step coarser.
Cost Impact
Going from Ra 3.2 to Ra 1.6 is usually achieved by changing parameters and has very little effect on cost. Ra 0.8 is achieved on the lathe with a finishing pass and tool selection (5–15% extra). Ra 0.4 and below almost always means a second operation on steel (grinding, honing, polishing) and a separate machine, set-up and measurement; this can multiply the cost for the surface concerned by 2–3. That is why the most expensive mistake is to write "all surfaces Ra 0.8" or "Ra 0.4" in the title block: non-functional surfaces are also refined unnecessarily. The right approach: general surface Ra 3.2 (or 1.6), finer values only on functional surfaces. In our DFM article we cover this principle together with the other cost items.
Which Value for Which Surface?
- Free surfaces, not visible: Ra 3.2–6.3
- Mounting and contact surfaces: Ra 1.6–3.2
- Sliding and bearing surfaces (bushing, shaft): Ra 0.4–0.8; specifying Rz is more meaningful
- O-ring and seal surfaces: Ra 0.4–0.8 (dynamic: 0.2–0.4; check the seal manufacturer's recommendation); no directional marks
- Hydraulic cylinder bore: Ra 0.1–0.4, honed
- Surface before coating: coating does not hide roughness, it usually increases it; decorative plating requires an Ra 0.4–0.8 base
- Food and pharmaceutical equipment contact surface: Ra 0.8 (usually the upper limit in hygienic design), below 0.4 with electropolishing
- Medical implants: Ra 0.05–0.5 depending on the application, special standards apply
How Is It Measured?
Surface roughness is measured with a contact profilometer (a diamond-tipped stylus is drawn across the surface) or optical methods. The result depends on the measuring parameters: cut-off wavelength (λc, typically 0.8 mm), sampling length and measuring direction (perpendicular to the machining marks). If these parameters are not stated on the drawing, the defaults of ISO 21920 (formerly ISO 4288) apply. Comparison plates (by eye/fingernail) are only for rough checks and do not go into the inspection report. For tightly specified surface requirements the parameter and instrument must be stated in the inspection report; for reading inspection reports see our quality assurance article.
Writing the Surface Symbol Correctly on the Drawing
- State the parameter explicitly: "Ra 0.8" or "Rz 6.3"; just "0.8" is ambiguous.
- Upper limit or range? The default is an upper limit (measured value ≤ specified). If a lower limit is required (on some sealing and adhesion surfaces), indicate it with "L".
- Write the general surface requirement in the title block or the corner of the drawing with exceptions in brackets: for example general Ra 3.2, with special surfaces (Ra 0.8, Ra 1.6) in brackets.
- If the lay (parallel, perpendicular, crossed) is functional, indicate it in the symbol; directional marks on sealing surfaces cause leakage.
- Note whether it applies before or after coating.
- Write the Ra value instead of the old N grade; N grades no longer appear in current ISO standards.
Frequently Asked Questions
Are Ra 0.8 and Rz 6.3 the same thing?
They may correspond approximately to a similar surface quality, but they are different parameters; one cannot be converted exactly into the other. Whichever is written on the drawing is measured.
Is Ra 0.4 possible on a CNC lathe without grinding?
On brass and aluminium with suitable tooling and parameters, yes; on steel it is difficult to achieve consistently, and grinding or roller burnishing is required. On Swiss-type lathes Ra 0.4 is considered routine on small diameters.
Can we say a part is higher quality if its surface roughness is lower?
No; some functions such as coating adhesion and oil retention require a certain roughness. The right value is the value the function requires.
Does surface roughness go into the inspection report?
Special surface requirements stated on the drawing, yes; the general surface value is usually verified by visual/comparison checks. In FAI and PPAP all specified surfaces are measured.
Conclusion
Surface roughness guarantees the function when specified correctly and silently multiplies the cost when specified incorrectly. Ra 3.2 (or 1.6) for the general surface, finer values only on functional surfaces and Rz where needed protects both the part and the budget. To see the cost impact of the surface requirements on your drawings, send your files via our quote form; recommendations come with the quote. For our surface-treatment and finishing capabilities see our CNC milling and CNC turning pages.
