CNC Machining Surface Finish: Ra Values and How to Specify Them

Surface finish is the most over-specified number on machining drawings — and the most under-specified. Buyers write "smooth" when they mean Ra 1.6, or demand Ra 0.4 on faces nobody will ever see, and pay for it on every part. This guide explains what the numbers mean, what ordinary machining delivers for free, and how to call out finish only where it earns its place.

What Ra Actually Measures

Ra is the average height of the peaks and valleys on a surface, measured in micrometres (µm) or microinches (µin). It describes roughness — the fine texture left by the cutting tool — not waviness, not flatness and not defects like scratches. Two surfaces with identical Ra can look and behave completely differently, which is why the number alone never tells the whole story.

The standard symbol on drawings is the check-mark with a number: Ra 3.2 means an arithmetic average roughness of 3.2 µm maximum. If no finish is specified at all, the drawing is asking for the natural result of the process — which is often exactly right.

What Ordinary Machining Delivers

Condition	Typical Ra	What it looks like	Typical use
Rough milled / turned	3.2–6.3 µm	Visible tool marks	Non-critical faces, early prototypes
Standard finished milled	1.6–3.2 µm	Fine tool marks, uniform	Most machined faces — the free default
Fine finished pass	0.8–1.6 µm	Smooth to the eye, marks visible under light	Sealing faces, sliding surfaces
Fine turned	0.8–1.6 µm	Even, satin cylindrical surface	Shaft seats, bearing journals
Precision ground	0.2–0.8 µm	No directional marks, dull sheen	Bearing fits, gauge surfaces
Lapped / polished	0.05–0.2 µm	Mirror	Optical, sealing and decorative work

The practical takeaway: a well-set-up milling or turning operation lands around Ra 1.6 without any special effort. Asking for better than that is a paid operation — slower feeds, extra passes, or a secondary process like grinding.

How Finish Requirements Affect Price

Each step beyond the natural finish costs time in a predictable ladder:

1. Ra 1.6 from the cut — usually free on good equipment with fresh tooling.
2. Ra 0.8 from the cut — a dedicated finishing pass with light depth and reduced feed; expect a modest adder on critical faces only.
3. Ra 0.4 and below — the territory of grinding, honing or lapping, with a separate setup and inspection. Justified on bearing and sealing surfaces; expensive everywhere else.

Tight finish plus tight tolerance compounds the cost, because both are achieved the same way: slower, lighter, more controlled passes with cold, stable tooling.

How to Specify Finish on a Drawing

1. Default to the process. Leave faces unspecified unless the function demands a number. Most faces of most parts are fine at the natural machined finish.
2. Call the number only where it matters. Bearing seats, sealing faces, mating guides and optical or cosmetic surfaces earn a callout; the rest do not.
3. Mark the extent. A finish symbol with no scope is ambiguous. Note the faces it applies to, or annotate each one.
4. Use Ra, not words. "Smooth", "polished" and "fine finish" mean different things to every shop. A number is contract-grade.
5. Think direction. For a sealing face, the lay (direction of tool marks) can matter as much as the height. Cross-hatched or random patterns seal better than circular ones.

Finish After Machining vs Finish From Machining

Surface treatments change the numbers you will measure. Anodising slightly roughens and adds thickness; bead blasting homogenises tool marks into a matte texture; polishing removes them entirely. If a drawing specifies both a fine machined Ra and a heavy texture treatment, the second operation will override the first — decide which one the function actually needs. Our surface treatments page covers the common options and what each does to dimensions.

Frequently Asked Questions

What Ra should I put on a general machined face?

None at all, unless there is a functional reason. Unspecified faces come out at the process's natural finish — around Ra 1.6–3.2 — which suits the vast majority of applications.

Is a lower Ra always better?

No. Extremely smooth surfaces can resist paint adhesion and can even increase friction in some sliding applications. Match the finish to the function, not to an instinct that smaller is better.

How is Ra measured?

With a stylus profilometer that traces a short line across the surface and averages the peaks and valleys. For critical parts we can measure and report actual values — see our article on quality control and inspection.

Can you hold Ra 0.4 on a milled face?

On small, accessible faces, sometimes, with a slow finishing pass. On large or deeply pocketed faces it is a grinding job, and the price reflects that.

Related Reading

• Surface Treatments
• Anodizing vs Powder Coating: Which Finish to Choose?
• CNC Machining Metal Materials
• CNC Machining Cost: 7 Factors That Drive Your Per-Part Price

Unsure which faces on your drawing need a finish callout? Send it over — we will mark up where a number matters and where it only adds cost.