Mirror or standard – the truth about surface specifications.

After tolerancing and material selection, surface finish is the third major lever in turned parts costing. Specifying Ra 0.4 across the board often means paying 20–30 % more – with no functional benefit. This article shows when which surface finish makes sense.

In Germany, the surface finish of a turned part is specified in Ra values (arithmetic mean roughness), per DIN EN ISO 4287 and DIN EN ISO 4288. Lower values = smoother surface. Typical values for CNC turned parts: Ra 1.6 (standard, easily achievable), Ra 0.8 (elevated), Ra 0.4 (high, often already special machining), Ra 0.2 or below (grinding territory).

On a modern CNC turning automatic, Ra 1.6 is reproducibly achievable with standard tooling and standard feeds. Machine time is standard. Ra 0.8 requires slightly reduced feeds or finer tooling – machine time roughly 10–20 % longer. Ra 0.4 demands special tooling (wiper geometry, inserts with fine cutting edge preparation) and further reduced feeds – machine time 30–50 % longer than at Ra 1.6.

Below Ra 0.4 things get interesting. Ra 0.2 is still possible in the turning process with the right tooling and a very fine feed strategy, but variation grows and the unit price rises considerably. Ra 0.1 and below is usually more economical with downstream grinding or polishing. At Marquart, our turning processes achieve Ra 0.4 reproducibly; for tighter values we work within the my-precision group with our sister company Haas Präzisionstechnik (precision grinding).

Functional justifications for tight surface finishes: sealing surfaces (hydraulics, seal seats – usually Ra 0.4 required), sliding surfaces (bearing seats, shafts under bearings – Ra 0.4 to Ra 0.8), aesthetic requirements (visible furniture/design components – Ra 0.8 or finer). For all other surfaces, standard Ra 1.6 or the general specification per DIN ISO 1302 is sufficient.

A common cost driver: the blanket specification 'Ra 0.4 on all surfaces'. In practice, a turned part usually needs only 1–3 functional surfaces at Ra 0.4 – the remaining outer surfaces are fine with standard values. We regularly work out this differentiation on the drawing together with our regular customers.

Material influence: different materials behave differently in turning. Stainless steel 1.4305 achieves Ra 0.4 with moderate effort. Titanium Grade 5, on the other hand, is considerably more demanding – Ra 0.4 here often means doubling machine time. When changing materials, revisit the surface requirements as well.

Ra describes only the arithmetic mean roughness and suppresses individual outliers. For sealing and sliding surfaces, Rz – the mean roughness depth from five individual measuring lengths – or Rmax as the largest single roughness depth is therefore often more meaningful, because a single deep groove endangers the sealing function while barely changing the Ra value. Even closer to function are the Abbott parameters from the material ratio curve: Rk characterizes the load-bearing core roughness depth, Rpk the reduced peak height (running-in wear) and Rvk the valley depth for lubricant retention. We specify these parameters deliberately where a plain Ra figure would be too coarse.

Measurement is done tactilely using the stylus method per DIN EN ISO 21920 (successor to the withdrawn DIN EN ISO 4287 and 4288). The key point is the separation of roughness and waviness via the cut-off wavelength λc: for Ra 0.4 µm typically 0.8 mm, for coarser surfaces 2.5 mm. From this follow the sampling length and evaluation length of typically five cut-off lengths. The same surface yields different numerical values depending on the chosen filter. So that measured values remain comparable, cut-off and measuring length belong on every drawing and in every measurement report.

The roughness achievable in turning is not a matter of chance but follows the kinematics. The theoretical roughness can be estimated via Rth ≈ f² / (8 · rε), where f is the feed per revolution and rε the corner radius of the cutting edge. A smaller feed and a larger corner radius reduce the roughness depth quadratically and inversely proportionally, respectively. This is exactly where the trade-off lies: halving the feed for finish machining approximately doubles the cutting time. We therefore choose feed and tool geometry so that the required finish is achieved reliably with acceptable machine time.

We check feasibility, suggest optimizations and get back to you within 48 hours with a first assessment.