CNC Machining Parts Post Processing: What Each Finish Actually Does

• 1
  What CNC Machining Parts Post Processing Really Changes
• 2
  Surface Texture: Ra Numbers and What They Hide
• 3
  Anodizing, Plating and Coating: Where Each One Belongs
• 4
  Deburring and Edge Condition: The Step That Prevents Failures
• 5
  How Post Processing Interacts With Tolerance and Fit
• 6
  Materials and Finishes: What Pairs Well and What Does Not
• 7
  Choosing a Finish by Function

Machining leaves a surface that is geometrically correct but physically unfinished. Cutting tools leave a feed mark pattern, a cold-worked layer a few micrometres deep, and burrs at every edge where the tool exited. Post processing removes, adds or rearranges material on that outer layer. Nothing about the bulk metal changes.

That last point matters more than most people expect. Anodizing does not make 6061 stronger. Polishing does not fix a bore that is out of tolerance. Post processing controls three things only: surface texture, surface chemistry and edge condition. If your problem is stiffness, flatness or a press fit, no finish will solve it.

So the right question is never "what is the best finish." It is "what is the failure mode I am protecting against." Corrosion, galling, wear, electrical contact, a sealing face, a grip surface, a cosmetic panel. Each maps to a small number of treatments, and the rest are decoration.

A single part often needs two or three treatments in sequence. Bead blasting before anodizing, for example, or tumbling before black oxide. Sequence errors show up as blotchy colour, poor adhesion or a burr that reappears after plating.

Ra is an average roughness value. It tells you the mean deviation from the center line, but not the shape of the profile. Two surfaces can both read Ra 1.6 μm and behave differently: one with fine uniform scratch, one with deep periodic grooves from a worn insert. Sealing and wear depend on the profile, not just the average.

As-machined aluminum typically lands at Ra 1.6–3.2 μm with a sharp cutter and a stable setup. That is fine for brackets, housings and most internal parts. When a surface is a bearing seat, an O-ring groove or a sliding interface, we aim for Ra 0.8–1.6 μm, which usually means a finishing pass with a smaller stepover or a wiper insert.

Below that, Ra 0.2–0.8 μm needs either a dedicated finishing strategy or an abrasive process. On our 5-axis centers we can hold ±0.005 mm and a fine finish in one setup on curved geometry, which avoids the mismatch you get when a surface is re-fixtured for polishing.

Be careful with polishing as a fix. It rounds edges and can push a burr inward instead of removing it. If a part has a critical edge, deburr deliberately before any cosmetic polish.

• 1
  Ra 1.6–3.2 μmAs-machined. General fit, no sealing.
• 2
  Ra 0.8–1.6 μmBearing seats, O-ring grooves, sliding faces.
• 3
  Ra 0.2–0.8 μmOptical, vacuum and tight sealing surfaces.

Anodizing converts the aluminum surface itself into an oxide layer. Type II clear or coloured runs a few micrometres to about 25 μm and is mostly cosmetic plus mild corrosion protection. Hardcoat anodizing goes thicker and harder, and it is the one to choose for sliding aluminum parts that would otherwise gall.

Hardcoat costs you something. It builds on edges and in tight features, so a Ø6 H7 bore will shrink. Threads get tight, and masking becomes necessary. Tell us which dimensions are functional before anodizing, not after.

Electroless nickel goes on steel, stainless, copper alloys and aluminum. Unlike electroplating it covers uniformly, which is why it is used on complex geometry and internal channels. The trade-off is a roughly 25 μm per side build that has to be allowed for in the drawing.

Zinc plating is the cheap corrosion workhorse for steel brackets and fasteners. Silver and gold plating are about contact resistance, not appearance. Black oxide is thin, dimensional and used on tooling, but it offers almost no corrosion protection without an oil film.

A burr is a stress riser. On a fatigue-loaded part, a 0.1 mm burr at a hole exit can start a crack that no surface finish will stop. On a hydraulic manifold, a loose burr becomes a contaminant that jams a spool. Deburring is not cosmetic work.

Hand deburring with a carbide scraper or abrasive cord handles small batches and complex internal edges. Tumbling, either vibratory or barrel, handles volume and gives a consistent radius but reaches only where the media reaches. Thermal deburring and abrasive flow deburring reach every internal path and are worth the extra cost on manifold-type parts.

Specify the edge condition you want. A 0.2 mm radius on all external edges is a clear instruction. "Deburr all edges" is not, because two shops will deliver two different edges.

One caution with tumbling: it work-hardens and rounds sharp corners. If a part locates on a sharp edge for inspection or assembly, mask it or skip the tumble.

Every coating that adds material changes dimensions. Plating and anodizing build outward; polishing and etching remove material. The drawing has to say whether the stated tolerance applies before or after the finish. Most quality disputes we see trace back to this omission.

The practical rule: dimensions that mate with something else must be specified as post-finish. For a plating build of 25 μm per side, a Ø20.00 +0.02/0 part will be out of tolerance after plating unless the pre-plate dimension is reduced. We adjust the CAM program accordingly when the drawing says so.

Tight features are the second issue. Anodizing and plating both build up in slots, small holes and thread roots faster than on flat surfaces. A hardcoat on a 1 mm slot can close it by a measurable amount. Masking solves it, but masking is manual work and adds cost.

When a part combines a hard functional bore with a cosmetic outer surface, split the treatment. Mask the bore, finish the outside. It is cheaper than finishing everything and then re-machining the bore.

Aluminum anodizes well. 6061, 6063, 6082 and 7075 all take Type II and hardcoat. 7075 shows more colour variation because of its copper content, so if colour match matters across a batch, 6061 is the safer base.

Stainless steel does not anodize. It gets bead blasted, brushed, polished or passivated. Passivation after machining is standard practice for 303, 304, 316 and 17-4PH to remove free iron from the surface and restore corrosion resistance.

Titanium and Inconel are usually left as-machined or bead blasted. Anodizing titanium is possible for colour coding but it is a different process from aluminum anodizing and should not be assumed.

Plastics behave differently again. POM and PEEK machine to a good finish and need little more than a deburr. ABS, PC and PMMA get vapour polished or painted when appearance matters. Carbon fibre is normally left with a machined edge and a clear coat.