Advanced surface finishes: the complete guide for CNC machining and 3D printing

The quality of a custom machined part is not measured only by its dimensional tolerances: surface finish determines tribological behaviour, corrosion resistance, aesthetic appearance and - in many applications - the functional performance of the part in service. A shaft with Ra 3.2 µm instead of Ra 0.8 µm is not simply less attractive: it has different friction coefficients, a greater tendency to wear and lower fatigue resistance. Yet surface finish is often the last thing considered during design in CNC machining processes.

Aluminium is the most versatile material from the standpoint of available surface finishes in aluminium CNC machining. Type II anodising - the standard - produces an aluminium oxide (Al₂O₃) layer of 5–25 µm, hard (300–400 HV), corrosion-resistant (ASTM B117 > 336 hours) and dyeable. Type III anodising - hard anodising - pushes the layer to 25–75 µm and hardness to 400–600 HV. For custom machined parts in aluminium with precision seats (H7 holes, bearing housings), plan post-anodising finishing operations on critical surfaces.

Steel has fewer options than aluminium but the available options are extremely effective for custom machined parts. Electrolytic zinc plating - 8–12 µm - is the standard treatment for carbon steel in moderately aggressive environments. Blackening (bluing) is purely aesthetic and protective in non-humid environments: Ra unchanged, uniform black appearance, zero impact on dimensions. For tribological applications - sliding guides, shafts with seals - ionic or gas nitriding brings surface hardness from 200–300 HV to 700–1,000 HV over a layer of 0.1–0.4 mm.

Each professional 3D printing technology has its own post-processing ecosystem. For FDM in ABS or ASA, shot blasting with fine grit (Al₂O₃, 80–120 mesh) evens out layer lines and prepares the surface for painting. For selective laser sintering in PA12 and MJF, dyeing penetrates 0.1–0.2 mm into the surface without altering dimensions and produces uniform, durable colours. For DMLS (metal 3D printing), the standard post-process is: support removal → shot peening (improves fatigue resistance by 10–30%) → fine-grit blasting → optional electrochemical polishing for functional surfaces.

Specifying Ra 0.8 µm across the entire part when only 3 surfaces require it can increase cost by 40–60% in precision machining. Correctly specifying differentiated Ra values is an act of respect towards the supplier and of genuine savings for your budget.

Surface finish is an integral part of the custom machined part design, not an afterthought. Specifying it correctly - with the type of finish, the required Ra value, the surfaces to which it applies and the reference standard - is how you obtain the right component at the right cost with no surprises at final inspection.