Guides / Finishing

Ra 1.6 Is Free, Ra 0.4 Is Not

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

Surface finish is the requirement people most often specify by feel rather than by function, and it is one of the easiest places to spend money you did not need to spend.

Ra is the arithmetic average roughness, measured in micrometres. Lower is smoother. What matters is which values come off the machine for free and which ones need somebody to do more work.

What each band costs you

Ra	How it is achieved on aluminum	Cost impact
3.2 um	Roughing pass, no finish pass	Cheapest, visible tool marks
1.6 um	Normal finishing pass	Baseline, effectively free
0.8 um	Slower finish pass, sharp tooling, light cut	Modest, adds machine time
0.4 um	Dedicated finishing strategy, often a second operation	Significant
0.2 um and below	Lapping, polishing or grinding	Expensive, specialist

The step from 1.6 to 0.8 is small. The step from 0.8 to 0.4 is where the price starts moving, because the shop is no longer just slowing down, it is adding operations.

Where a fine finish genuinely earns its place

Sealing faces. An O-ring groove and the face it seals against need a controlled finish or the seal leaks. This is the most common legitimate reason to ask for 0.8 um or better, and it should be called out on that face alone.

Sliding and bearing surfaces. Anything that moves against another part. Usually paired with Type III hardcoat.

Optical and mounting datums. Faces that locate something precisely, where texture becomes positional error.

Fatigue critical parts. Cracks start at surface defects, so on a 2024 component under repeated load, finish is a structural requirement.

Vacuum and cleanliness. Rough surfaces trap contaminants and outgas.

Where it does not

Outside profiles, clearance holes, the backs of brackets, anything that gets painted, and anything that is going to be bead blasted anyway. On these, a finish callout adds cost and buys nothing.

Aluminum has its own quirk

Aluminum is soft and it smears. Push a dull tool through it and you get a built up edge, where material welds itself to the cutting tool and then tears off, leaving a torn rather than a cut surface. The measured Ra can look acceptable while the surface still looks bad.

This is why finish on aluminum depends as much on tooling and coolant as on feed rate, and why the softer 5000 series alloys are harder to finish well than 6061. If appearance matters on a 5052 part, say so, because getting there takes a deliberate effort a shop will not make unprompted.

Finish and anodizing interact

An anodized surface shows you whatever was underneath it. Clear anodize in particular hides nothing, so machining marks, a change in cutter direction and any chatter will all be visible on the finished part.

If a part is cosmetic, the usual answer is not a tighter Ra. It is a uniform texture: bead blast the whole visible surface before anodizing so everything reads the same, then the coating sits on one consistent finish. That is cheaper than chasing a fine machined finish and it looks better, because it removes the directional pattern rather than making it finer.

What to write

A general note plus specific exceptions:

SURFACE FINISH Ra 1.6 um UNLESS NOTED.
FACE 'A' AND O-RING GROOVE: Ra 0.8 um MAX.
BEAD BLAST ALL EXTERNAL SURFACES BEFORE ANODIZING.

That tells a shop exactly where to spend time and, more usefully, where not to.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Surface roughness, Ra and Rz
• ISO 1302, surface texture indication
• Built-up edge

Frequently asked questions

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