Shenzhen Alu Rapid Prototype Precision Co., Ltd.
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Short answer: As-machined finish (Ra 1.6–3.2 µm) is included in the price. Everything beyond it — bead blasting, anodizing, powder coating, polishing — is a separate operation with its own cost. Specify finish only where it is functionally or cosmetically required.
This guide covers the finishes we apply most often, what each costs, and how to write it on a drawing.
Understanding Ra
Ra is the average surface roughness, measured in micrometres (µm). Lower is smoother:
| Ra value | Description | Typical process |
|---|---|---|
| Ra 3.2 µm | Clearly visible tool marks | Rough machining |
| Ra 1.6 µm | Standard machined finish | Normal CNC milling/turning |
| Ra 0.8 µm | Fine machined finish | Finishing pass, sharp tooling |
| Ra 0.4 µm | Very fine | Fine finishing, slow feeds |
| Ra 0.2 µm or better | Near-mirror | Polishing, lapping, grinding |
Note that roughness is not the same as flatness or dimensional accuracy — you can have a smooth surface on a part that is otherwise out of tolerance.
Common Finishes and Typical Cost
| Finish | Added cost | Best for |
|---|---|---|
| As-machined | Included | Internal, non-visible, functional parts |
| Bead blasting | +$5–$20 / part | Uniform matte look, removes tool marks |
| Anodizing, Type II | +$10–$40 / part | Color + corrosion resistance on aluminum |
| Hard anodizing, Type III | +$25–$80 / part | Wear resistance, thicker oxide layer |
| Powder coating | +$10–$35 / part | Thick, durable, wide color range |
| Polishing | +$15–$60 / part | Cosmetic, mirror finish |
| Electroplating | Quoted per part | Conductivity, wear, decorative (nickel, chrome, zinc) |
| Laser marking | +$1–$10 / part | Logos, part numbers, traceability |
Costs depend heavily on part size, batch size, and whether the finish is applied in-house or outsourced. Small parts finished in large batches cost far less per unit.
Anodizing: Type II vs Type III
Type II (standard anodizing) builds an oxide layer of roughly 5–25 µm. It accepts dye well, so it is the choice for colored, cosmetic aluminum parts. Type III (hard anodizing) builds 25–50 µm and is significantly harder and more wear resistant, but colors are more limited and parts grow slightly in dimension, which must be accounted for on tight-tolerance features.
Important: anodizing adds to the surface, so a bore that must fit a shaft should be masked or pre-sized accordingly. Discuss this before machining starts.
How to Specify Finish on a Drawing
State a default finish (for example "as-machined, Ra 1.6 µm unless noted")
Add specific callouts only on functional surfaces
Name the finish type exactly (e.g. "anodize Type II, black, per MIL-A-8625")
Specify which surfaces must be masked during finishing
Note any color standard or sample to match
See our surface finishing capabilities for the full in-house range.
FAQ
What is the standard surface finish for CNC machining?
Ra 1.6–3.2 µm, which is the as-machined finish included in normal pricing.
Does bead blasting remove tool marks?
It blends and softens them into a uniform matte texture, but deep marks can still show through.
Can stainless steel be anodized?
No. Anodizing is an aluminum process. Stainless is typically passivated, bead blasted, or polished instead.
Will anodizing change my part dimensions?
Yes. Type II adds roughly 5–25 µm and Type III roughly 25–50 µm, which matters on tight fits.
Is polishing worth it?
For visible, high-value parts yes; for internal or functional parts it is usually wasted cost.
Get a Quote With Finishing Included
Tell us the finish you need and we will quote machining and finishing together, with notes on any tolerance that must be adjusted for the coating.