Surface Finish in CNC Machining: What Ra, Rz, and Your Drawing Callouts Actually Mean

A 32 Ra callout on every surface of a machined part doubles cycle time. Most of the time, the part needs that finish on exactly two faces. The rest can be “as machined” — and the buyer never knows they were paying for finish they couldn’t see.

Surface finish — also called surface roughness — is one of the most over-specified, least-understood items on a typical engineering drawing. Here’s what the numbers actually mean, how they’re measured, and how to specify finish without burning your budget on faces that don’t need it.

What Ra Actually Is

Ra — roughness average — is the arithmetic mean of the absolute deviations of a surface’s profile from its mean line, measured over a defined sampling length. In English: if you ran a stylus across the surface and recorded the up-and-down, Ra is the average of how far the stylus moved from a flat reference. Lower number means smoother surface.

Ra is expressed in microinches (µin) under the imperial system or micrometers (µm) under metric. Common conversions:

32 µin Ra = 0.8 µm Ra
63 µin Ra = 1.6 µm Ra
125 µin Ra = 3.2 µm Ra
250 µin Ra = 6.3 µm Ra

Ra Is Not the Whole Story — Meet Rz, Rmax, and Rt

Ra averages everything. Two very different surfaces can have the same Ra value — one with shallow, frequent peaks, another with rare, deep gouges. That’s where the other measures come in:

Rz — the average of the five highest peaks and five lowest valleys within the sample length. Rz catches outliers that Ra hides.
Rmax (or Rt) — the single greatest peak-to-valley distance in the sample. Used when a single bad spot would fail the part (sealing surfaces, fatigue-critical parts).
Rq (RMS) — root-mean-square roughness. Older drawings (pre-1996) often use RMS instead of Ra. As a rough conversion, RMS ≈ 1.11 × Ra.

For most commercial work, Ra is the only callout you need. For sealing surfaces, bearing seats, or fatigue-critical components, Rz or Rmax should be specified as well.

What Finish Each Machining Process Produces

Knowing the natural finish of a process helps you specify what’s actually achievable without secondary operations.

Process	Typical Ra (µin)	Typical Ra (µm)
Rough milling	125–250	3.2–6.3
Finish milling	32–125	0.8–3.2
Turning	32–125	0.8–3.2
Drilling	63–250	1.6–6.3
Reaming	32–63	0.8–1.6
Grinding	8–32	0.2–0.8
Lapping / polishing	1–8	0.025–0.2

What Finish You Actually Need

Real-world finish requirements, drawn from common engineering practice:

250 Ra (6.3 µm) — Rough as-machined surfaces, internal faces, anything cosmetic that won’t be seen.
125 Ra (3.2 µm) — General machined surfaces. The default for most non-critical machined faces.
63 Ra (1.6 µm) — Mating surfaces with light loading. Press-fit bores, sliding fits with light contact.
32 Ra (0.8 µm) — Bearing seats, gear teeth, surfaces that take seal lips, dynamic O-ring grooves.
16 Ra (0.4 µm) — High-speed bearing seats, hydraulic sealing surfaces. Typically requires grinding.
8 Ra (0.2 µm) — Optical surfaces, very high-pressure seals. Lapped or polished.

How to Specify Finish on a Drawing

The ISO and ASME standard for surface finish callouts uses the “checkmark” symbol with the Ra value above. A few rules that catch new engineers:

Specify per surface, not globally. A blanket “all surfaces 32 Ra” callout is the single most expensive line on most drawings. Specify only the surfaces that need it.
Default the rest to “as machined.” A general note in the title block of “Unmarked surfaces 125 Ra max” is enough.
Distinguish “max” from “average.” “32 Ra” usually means “32 Ra max.” If you mean a target, say “32 Ra ±10”.
Match the process to the callout. Asking for 16 Ra on a milled surface forces a secondary grind. Asking for 63 Ra on a turned surface is free.

The Cost Impact of Surface Finish

A rough rule of thumb on a finish-milled aluminum surface:

250 Ra to 125 Ra: free (it’s the same finish pass)
125 Ra to 63 Ra: +20% cycle time (slower feed, smaller stepover)
63 Ra to 32 Ra: +60% cycle time, or a separate finish pass
32 Ra to 16 Ra: requires grinding or polishing — a secondary process. Often doubles total cost.
16 Ra to 8 Ra: lapping or polishing only. Sometimes triples cost.

The takeaway is simple. Every step finer than 63 Ra adds significant cost. Most parts do not need finer than 63 Ra on most surfaces.

Measuring Finish

A profilometer — typically a stylus dragged across the surface — is the standard tool. Modern shops use portable units that can be brought to the part. For high-volume work, optical profilometers (non-contact) are used. For first-article inspection on critical surfaces, the measurement should be part of the inspection report.

If the surface finish callout matters to your application, ask the shop to include a profilometer reading in the inspection report. Most shops will do it for free on first article and charge a small premium for serial inspection.

How DFW Machine Handles Surface Finish

We default unmarked surfaces to 125 Ra max — the standard for general machined parts. We treat tighter callouts as cost line items and we tell you up-front if a callout will require a secondary operation. If you over-specify finish on a drawing, we’ll flag it during DFM and offer a cheaper alternative.

Quote a CNC job: dfwmachine.com/contact. Send your drawing in PDF or STEP — we’ll review the finish callouts and quote both as-drawn and value-engineered.