After the machine stops, the work isn't done. The surface finish of a CNC-machined part determines not just how it looks, but how it functions — friction, sealing, fatigue life, and assembly tolerance all depend on finish quality.
This guide covers the 6 most common CNC machining surface finishes, from budget-friendly as-machined to high-cost mirror polishing. Each finish includes Ra values (roughness average), typical applications, cost impact, and real-world selection advice.
| Finish | Ra (μm) | Cost Factor | Visual Appearance | Best For |
|---|---|---|---|---|
| As-machined | 1.6–3.2 | 1.0x (baseline) | Visible tool marks | Hidden surfaces, non-cosmetic |
| Bead blasted | 0.8–1.6 | 1.1–1.3x | Matte, uniform | Cosmetic covers, consumer goods |
| Brushed | 0.4–1.0 | 1.2–1.5x | Linear grain | Decorative panels, heat sinks |
| Medium polish | 0.4–1.0 | 1.3–1.8x | Semi-reflective | Medical devices, food contact |
| High polish | 0.1–0.4 | 1.8–2.8x | Reflective | Pistons, sealing surfaces |
| Mirror polish | <0.1 | 2.5–5.0x | Full reflection | Optics, mold cavities |
Ra: 1.6–3.2 μm · Cost: baseline
The default finish from the machine tool. Visible tool marks from the end mill or lathe. This is the finish you get if you don't specify anything else.
How it's achieved:
When to use: ✅ Internal features not visible in the assembly ✅ Structural parts where cost matters ✅ Parts that will be painted, anodized, or plated (the coating covers tool marks) ✅ Prototypes
When to avoid: ❌ Cosmetic surfaces that will be touched by users ❌ Sealing surfaces (needs Ra < 0.8 μm for O-ring seals) ❌ Parts in sliding contact (accelerated wear with rough finish)
Cost tip: As-machined is the fastest and cheapest. If your part doesn't need to look pretty, don't pay for polishing — the machine marks are part of the aesthetic.
Ra: 0.8–1.6 μm · Cost: 1.1–1.3x
Bead blasting propels fine glass beads at the part surface, creating a uniform matte texture. This is the most common cosmetic finish for consumer-facing CNC parts.
How it's achieved:
When to use: ✅ Consumer product enclosures ✅ Handheld devices (nice tactile feel) ✅ Parts that need an OEM look ✅ Hiding minor scratches and imperfections
Material considerations:
Warning: Bead blasting adds a slight surface texture that can trap contaminants. Not suitable for food or medical contact surfaces without additional passivation.
Ra: 0.4–1.0 μm · Cost: 1.2–1.5x
A linear brush pattern applied with abrasive belts or pads. Produces a consistent directional grain similar to brushed stainless steel appliances.
How it's achieved:
When to use: ✅ Architectural panels, face plates ✅ Heat sinks (increases surface area slightly) ✅ Consumer electronics housings ✅ Parts that need a "premium" industrial look
Material best for:
Limitation: Brushed finishes are directional. If the part rotates in use, the grain direction will be obvious. Bead blasting is better for isotropic appearance.
Ra: 0.4–1.0 μm · Cost: 1.3–1.8x
Semi-reflective polish typically achieved with 320–400 grit abrasive. Removes visible tool marks and creates a smooth, showroom-ready surface.
How it's achieved:
When to use: ✅ Medical device components ✅ Food processing equipment ✅ Parts that will be clear anodized or PVD coated ✅ Customer-facing prototypes
Material considerations:
Ra: 0.1–0.4 μm · Cost: 1.8–2.8x
High polish requires 600–1000 grit abrasives followed by buffing compounds. Achieves a reflective surface that you can see your face in — but not quite mirror.
How it's achieved:
When to use: ✅ Pistons and cylinder bores (needs low friction) ✅ Sealing surfaces ✅ Display parts in showrooms ✅ Electrode contacts
When to avoid: ❌ Parts that will be painted (paint doesn't stick well to high-polish) ❌ Parts with deep cavities (polishing tools can't reach) ❌ Soft aluminum (5052, 1100) — smears instead of polishing
Ra: < 0.1 μm · Cost: 2.5–5.0x
The highest grade of polish. Mirror finish requires diamond paste (3–6 μm) and multiple buffing stages. Achieved on suitable materials only.
How it's achieved:
When to use: ✅ Injection mold cavities (creates SPI A-1 finish in plastics) ✅ Optical components and reflectors ✅ Medical implants (requires Ra < 0.05 μm) ✅ High-end decorative hardware
Materials that can achieve mirror finish: | Metal | Mirror possible? | Effort | Notes | |---|---|---|---| | 6061-T6 Aluminum | ✅ | High | Best of common Al alloys | | 7075 Aluminum | ✅ | Medium | Higher hardness = cleaner result | | Stainless 304 | ✅ | High | Work-hardening slows process | | Stainless 316 | ✅ | Very high | Slower than 304 | | Tool Steel (P20, H13) | ✅ | Medium | Good for mold cavities | | Brass | ✅ | Low | Polish-friendly, bright result | | Titanium Grade 5 | ⚠️ | Very high | Tough, but beautiful when done | | Copper | ⚠️ | Low | Soft — easy to scratch after polish |
| Finish | Cost Multiplier | When to Use |
|---|---|---|
| As-machined | 1.0x | Always the default |
| Bead blast | 1.1–1.3x | Most consumer products |
| Brushed | 1.2–1.5x | Premium industrial look |
| Medium polish | 1.3–1.8x | Medical, food, cosmetic |
| High polish | 1.8–2.8x | Functional sealing/sliding |
| Mirror polish | 2.5–5.0x | Molds, optics, implants |
Real-world tip: A common mistake is specifying "mirror polish" for the entire part when only one surface needs it. Specify finishes per surface — make the visible faces polished and leave hidden faces as-machined. This saves 40–60% in finishing cost.
Not all finishes work with all post-processing:
| Post-process | As-machined | Bead blast | Polish |
|---|---|---|---|
| Clear anodize | Visible tool marks, poor aesthetic | ✅ Excellent | ✅ Excellent |
| Black oxide | ✅ Good | ✅ Better | ❌ Too smooth, poor adhesion |
| PVD coating | ✅ Good | ⚠️ Medium | ✅ Excellent adhesion |
| Powder coat | ⚠️ Good but visible texture | ✅ Hides undercoat well | ❌ Poor adhesion |
| Passivation | ✅ No effect | ⚠️ May change color | ✅ No effect |
Is the surface visible?
├── No → As-machined (cost lowest)
└── Yes →
Is it a consumer product?
├── Yes → Bead blasted (best cost/value)
└── No →
Is it medical or food?
├── Yes → Medium polish (cleanable, Ra < 0.8)
└── No →
Functional or cosmetic?
├── Functional (sealing/sliding) → High or mirror polish
└── Cosmetic → Bead blast or brushedQ: Can I bead blast and anodize? Yes — this is actually the most common combination for aluminum CNC parts. Bead blast before anodizing produces a uniform, professional matte finish.
Q: Does polishing remove material? Yes, typically 0.1–0.2 mm per surface for high polish, up to 0.3 mm for mirror. Design your part dimensions with this in mind.
Q: Which finish is best for aluminum? For cosmetic parts: bead blast + clear anodize. For functional: medium polish. For sliding surfaces: high polish.
Q: What Ra value do I need for an O-ring groove? For static O-ring seals (NBR/Viton): Ra < 1.6 μm. For dynamic (piston/rod): Ra < 0.4 μm. For PTFE seals: Ra < 0.2 μm.
Q: Why is mirror polish so expensive? It's labor-intensive (2–4 hours per small part), requires skilled workers, and consumables (diamond paste, buffing wheels) cost significantly more than abrasives.
This guide reflects standard industry practice for CNC machining finishes. Actual results depend on material, machine condition, and operator skill. Always request a finish sample before full production.
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