Quick answer: Standard CNC milling produces a Ra 0.8–3.2 μm surface finish as-machined. Adding bead blasting costs +10–20%, anodizing +15–30%, and mirror polishing +50–150% on top of the base machining cost. For 80% of functional parts, the as-machined finish is sufficient. Surface finish is the third-largest cost driver in CNC after material and tolerance.
Surface Finish Methods: Ra Values, Cost & Applications
Finish Type
Ra Range (μm)
Appearance
Cost Multiplier
Best For
As-Machined
0.8–3.2
Visible tool marks
1.0x (baseline)
Functional parts, internal components
Bead Blast (Matte)
0.8–2.5
Uniform matte, hides marks
1.1–1.2x
Consumer products, cosmetic covers
Anodized (Type II)
0.8–2.0
Colored, corrosion-resistant
1.15–1.3x
Aluminum parts, branding colors
Anodized (Type III)
0.8–2.0
Dark gray, hard coat
1.3–1.5x
Wear surfaces, aerospace
Powder Coating
1.5–5.0
Thick colored layer
1.2–1.4x
Outdoor equipment, enclosures
Electropolishing
0.1–0.4
Bright, mirror-like
1.5–2.0x
Food/medical stainless parts
Passivation
No Ra change
No visual change
1.05–1.1x
Stainless corrosion protection
Mirror Polishing
0.05–0.1
Reflective mirror
1.5–2.5x
Decorative, optical surfaces
Black Oxide
No Ra change
Matte black
1.05–1.15x
Steel parts, mild corrosion resistance
Ra = Roughness Average in micrometers. Cost multipliers are additional to base machining cost. Actual costs vary by part size, geometry, and batch quantity.
Material-Specific Finish Compatibility
Material
Anodize
Powder Coat
Passivation
Electropolish
Bead Blast
Black Oxide
Aluminum 6061
✅ Best
✅
❌
❌
✅
❌
Stainless 304/316
❌
✅
✅ Best
✅ Best
✅
❌
Carbon Steel
❌
✅
❌
❌
✅
✅ Best
Titanium
✅
✅
✅
✅
✅
❌
Brass/Copper
❌
✅
❌
✅
✅
❌
Real Part Examples: Before and After Finish Cost
Part
Material
Qty
Base Machining Cost
As-Machined Total
Anodized Total
Bead Blast Total
Camera mount
Al 6061
50
$18/pc
$18/pc
$22/pc
$20/pc
Valve handle
SS 304
100
$15/pc
$15/pc
N/A
$17/pc
Drone bracket
Al 7075
20
$32/pc
$32/pc
$39/pc
$36/pc
When to Pay for Better Surface Finish
Anodize: Aluminum parts that are visible, need color coding, or require corrosion resistance
Bead blast: Parts where tool marks are unacceptable but tight tolerances must be maintained
Powder coat: Large enclosures, outdoor equipment, parts needing thick protective layer
Electropolish: Food contact surfaces, medical instruments, ultra-clean applications
Passivation: All stainless parts — it’s cheap ($0.50–2/part) and prevents rust
Skip it: Internal components, parts that will be painted later, parts assembled into hidden locations