Last Updated: March 2026
Surface Finish Guide for CNC Machined Parts
The standard as-machined CNC surface finish is Ra 1.6µm — functional, with visible tool marks. Most parts from Rapid Manufacturing are delivered at this finish unless you specify otherwise. If your part requires corrosion protection, wear resistance, specific cosmetic appearance, or conductivity requirements, one of the post-processes below will meet your needs.
- • Corrosion protection on aluminium: Type II Anodise
- • Wear resistance on aluminium: Type III Hard Anodise
- • Colour + durability on steel: Powder Coat
- • Uniform coating on complex steel geometry: Electroless Nickel
- • Medical/food stainless: Passivation
- • Cosmetic uniform matte: Bead Blast
Surface Finish Options
As-Machined
The natural finish from the cutting tool. Visible tool marks. Suitable for most functional applications. Standard on all CNC machined parts unless otherwise specified.
Bead Blasted
Shot or glass bead blasting removes tool marks and creates a uniform matte finish. Does not improve dimensional accuracy. Commonly done before anodising for cosmetic aluminium parts.
Brushed Finish
Linear abrasive brushing creates a directional satin appearance. Common on stainless steel consumer products. Specify grit (e.g., 120 grit = finer than 60 grit).
Type II Anodising
Electrochemical process creating a 5–25µm aluminium oxide layer. Corrosion resistant, electrically insulating, can be dyed. Most common finish for aluminium consumer and industrial products.
Type III Hard Anodising
25–100µm thick coating at very high hardness (up to 70 HRC at surface). Significant wear resistance. Usually grey to dark grey/black. Used for high-wear sliding surfaces.
Powder Coating
Electrostatically applied polymer powder cured at ~180–200°C. Wide range of RAL colours, gloss levels, and textures. Excellent impact and abrasion resistance. Thicker than paint (~60–100µm).
Electroless Nickel
Uniform nickel-phosphorus deposit (~25µm typical) via chemical reduction. Deposits uniformly on all surfaces including bores. High hardness post-heat-treatment. Good corrosion resistance on steel.
Zinc Plating
Sacrificial zinc coating for corrosion protection on mild and carbon steels. Thinner than powder coat (~5–15µm). Often chromate conversion coated for additional protection.
Passivation
Acid bath treatment (nitric or citric) removes free iron and surface contaminants, improving the native chromium oxide passive layer. No visual change. No dimensional change.
Mirror Polish
Sequential abrasive polishing to optical finish. Labour intensive. Specified for sealing surfaces (O-ring grooves), optical reflectors, and high-end cosmetic applications.
Comparison Table
| Finish | Ra | Corrosion | Wear | Colour | Cost |
|---|---|---|---|---|---|
| As-machined | 1.6–3.2µm | — | — | — | $ |
| Bead blasted | 1.6–3.2µm | — | — | Matte grey | $ |
| Type II Anodise | 1.0–2.0µm | ★★★★ | ★★ | Clear/dyed | $$ |
| Type III Hard Anodise | 1.0–2.0µm | ★★★★ | ★★★★★ | Grey/black | $$$ |
| Powder coat | 2.0–5.0µm | ★★★★ | ★★★ | Any RAL | $$ |
| Electroless Nickel | 0.3–1.0µm | ★★★★ | ★★★★ | Silver | $$$ |
| Zinc plating | Sameµm | ★★★ | ★ | Silver/yellow | $$ |
| Passivation | Sameµm | ★★★★★ | — | — | $ |
How to Specify Surface Finish on Your Drawing
- →Use a general surface finish callout in the title block (e.g., "Ra 1.6µm unless otherwise specified")
- →Call out tighter or different finishes on specific surfaces using ISO 1302 symbols
- →For post-processes, add a general note: e.g., "Type II anodise, clear, per MIL-A-8625"
- →For powder coating, specify: RAL colour code, gloss level (matt/satin/gloss), and film thickness range
- →For anodised parts with tight tolerance bores, note "Machine after anodise" or provide pre-anodise dimensions with allowance
Need Help Choosing a Finish?
Rapid Manufacturing provides free DFM and surface finish recommendations with every quote. Upload your file and specify your functional requirements — we'll recommend the right finish at the right price.
Get a QuoteFrequently Asked Questions
What is the standard surface finish for CNC machined parts?
The typical as-machined surface finish for CNC milled parts is Ra 1.6µm (125 µin). This is the standard you will receive unless you specify otherwise. Turned parts are often slightly smoother at Ra 0.8–1.6µm depending on the operation. These finishes are suitable for most functional applications.
What is the difference between Type II and Type III anodising?
Type II (standard) anodising produces a coating 5–25µm thick. It provides good corrosion resistance, can be dyed in a range of colours, and provides moderate hardness. Type III (hard anodising) produces a thicker coating (25–100µm) with significantly higher hardness (up to 60–70 HRC at the surface). Hard anodising is used for wear-resistant applications — sliding surfaces, valves, and high-cycle mechanical parts. Type III is harder to dye and typically appears grey to black.
When should I use electroless nickel instead of anodising?
Anodising is specific to aluminium. Electroless nickel is used on steel, aluminium, and other metals when you need: (1) uniform coating on complex geometries including bores and internal features (electroless nickel deposits uniformly regardless of geometry), (2) very precise coating thickness control, (3) high hardness (as-deposited ~50 HRC, post-heat-treat ~68 HRC), (4) excellent corrosion resistance on steel. It is more expensive than anodising but suitable for a wider range of materials and geometries.
How do I specify surface finish on my CNC drawing?
Use Ra (roughness average) values per ISO 1302. Put a general surface finish symbol in the title block (e.g., Ra 1.6µm) and callout specific surfaces with different requirements. For post-processes, add a general note: e.g., "All surfaces: Type II anodise, clear, per MIL-A-8625" or "Powder coat: RAL 9005 Matt Black, 60–80µm".
Does anodising change the dimensions of a CNC machined part?
Yes. Anodising grows into and out of the surface. Approximately 50% of the coating thickness grows into the material, 50% grows out. Type II anodising (15µm typical) adds approximately 7.5µm per surface, so a bore machined to 10.000mm becomes approximately 9.985mm after Type II anodising. For tight tolerance bores and shafts, machine to account for the anodising buildup. Rapid Manufacturing's DFM review will flag dimensions that need anodising allowance.
What is passivation and when is it needed for stainless steel?
Passivation is a chemical treatment (typically nitric or citric acid bath) that removes free iron and other surface contaminants from stainless steel, forming a thicker, more consistent chromium oxide passive layer. It does not change the appearance or dimensions of the part. Passivation is recommended for: (1) medical and food-contact stainless parts, (2) parts exposed to marine or chloride environments, (3) any stainless part where contamination from machining (tool contact, steel fixtures) may have reduced corrosion resistance.