Surface roughness is the one thing on a CNC drawing that almost nobody specifies deliberately, and it is frequently the reason a part that passed dimensional inspection still gets rejected. Ra 3.2 gets copied from the previous drawing because it looked right, without anyone asking whether the function needs it, whether the process can reach it, or what it will cost. CNC surface roughness deserves better — it is one of the few specifications that directly links design intent, process choice, and unit price.
This guide explains how to read a surface finish callout, what Ra, Rz and Rmax actually measure, which finishes each process reaches without extra cost and which require a second operation, how much tighter finishes really cost, and how to specify it so that inspection and manufacturing agree.
For the process side rather than the specification side, see our CNC parts finishing guide.
Reading a surface finish symbol
A modern callout draws meaning from up to four positions around the basic tick symbol, and the compact form (fine) hides most of them. Once you know the layout the callout reads like a sentence:
| Symbol position | What it specifies | Common values |
|---|---|---|
| Main tick with a number | Upper limit of the roughness parameter, default Ra | Ra 3.2, Ra 1.6, Ra 0.8 |
| Upper left | Method — material removal required | Circled C, M, none |
| Upper right | Machining allowance or lay direction | "=" parallel, "⊥" perpendicular |
| Lower right | Additional parameter, sampling length, or lay | Rz 6.3,fillet radius |
| Circled value | Machining required vs prohibited | Circle around the tick |
Two conventions decide most confusion. First, a plain value means an upper limit: "Ra 3.2" means 3.2 µm or smoother, and a smoother part is acceptable. Writing "Ra 3.2 max" is redundant but harmless. Second, the default parameter is Ra, so "3.2" alone means Ra 3.2 µm. If you mean anything else, you must write the parameter name, because a bare number is not a specification of Rz or Rmax.
Units also catch people out. Ra in micrometres versus microinches is a 25.4× error, and imperial drawings using the old microinch RMS callouts (such as "32 micro finish") do not convert cleanly to Ra — 32 µin RMS corresponds roughly to Ra 0.8 µm, but the two parameters are computed differently, so it is an approximation rather than an identity.
Ra, Rz and Rmax: why the parameter choice changes everything
Ra — the arithmetic average height — is the most cited and the least informative. A turned surface and a ground surface can share an Ra value while behaving completely differently in a seal or a bearing.
| Parameter | What it measures | Sensitivity | Typical use |
|---|---|---|---|
| Ra | Arithmetic mean of absolute profile deviation | Blind to isolated peaks and valleys | General machining default |
| Rz | Mean height of the five highest peaks to five deepest valleys | Catches individual deep scratches | Sealing surfaces, coating prep |
| Rmax / Rt | Single largest peak-to-valley in the sample | Highly sensitive to defects | Critical sealing and fatigue surfaces |
| Rsk (skew) | Symmetry of the profile about the mean | Determines lubricant retention | Bearing surfaces, running-in behaviour |
There is no exact conversion between them, and any table claiming one is doing you a disservice. As an order-of-magnitude guide on a normal milling or turning profile, Rz is roughly 4–8× Ra, and Rz may be 6–10× Ra on a ground finish. That spread is *why* specifying Rz alongside Ra is useful rather than pedantic: it constrains the peak population that Ra averages away.
What each process actually reaches
Capability is a function of feed rate, nose radius, tool condition and material — not of the machine's price tag. The following reflect what we hold routinely rather than laboratory best cases. Note the column on *extra cost*: everything above about Ra 0.8 stops being free.
| Process | Ra readily achieved (µm) | With extra effort (µm) | Extra cost? |
|---|---|---|---|
| As-cast / as-extruded | 6.3 – 25 | — | No |
| Saw cut | 6.3 – 12.5 | — | No |
| Turning, rough | 3.2 – 6.3 | — | No |
| Turning, finish | 1.6 – 3.2 | 0.8 | No, up to 0.8 |
| Milling, face mill | 1.6 – 3.2 | 0.8 – 1.6 | No |
| Milling with ball nose, fine stepover | 0.8 – 1.6 | 0.4 | Slightly — longer cycle |
| CNC turning with wiper insert | 0.4 – 0.8 | 0.2 | Partly |
| Reaming | 0.8 – 1.6 | — | No |
| Five-axis machining, fine strategy | 0.8 – 1.6 | 0.2 – 0.4 | Yes |
| Grinding | 0.2 – 0.8 | 0.05 – 0.1 | Yes — separate process |
| Polishing / lapping | 0.05 – 0.4 | < 0.05 | Yes — manual |
Three practical rules fall out of that table. Ra 1.6 is free from most chip-making processes; asking for it costs nothing. Ra 0.8 usually costs extra only because of cycle time, not because a second machine is needed. Ra 0.4 and below is a different process — grinding, lapping or polishing — and should be quoted as one.
Before arguing over a number, check the measurement: our gage R and R guide separates equipment variation from appraiser variation, and neither costs much to fix.
Material matters as much as the process. Aluminium alloys generally finish better than stainless at the same parameters; see our guides to aluminium and stainless steel for the machinability differences that translate directly into achievable Ra.
Why Ra alone will not save you: lay, waviness and coating
A surface has three superimposed components, and a drawing that specifies only one is leaving the others uncontrolled:
- Roughness — the fine tool marks, controlled by feed and nose radius.
- Waviness — the longer-wavelength error from deflection, chatter or machine geometry. It is usually what makes a flat face leak even at excellent Ra.
- Lay — the direction of the predominant pattern, which decides whether a seal wipes or channels.
For sealing faces specifically, specify lay direction plus flatness, not Ra alone. A circular lay on a gasket face is a genuine functional requirement, and a part with perfect Ra 0.8 in the wrong direction will still leak. The same logic appears when the finish must survive a subsequent coating — anodizing changes the surface texture measurably, so specify roughness before or after coating explicitly, based on which surface actually does the work. Our brushed aluminium panels are a good example of a finish that is specified for appearance and inevitably changes the measured Ra.
Specifying roughness so that inspection agrees
Most disputes about finish are not disagreements about the part. They are disagreements about the measurement conditions, which the drawing never stated. Include these:
- The parameter. Ra is a choice, not a default you should leave implicit. Add Rz if the peak population matters.
- The limit type. Upper limit is the norm; state it if you mean a two-sided band.
- The sampling length. Ra 3.2 measured over one millimetre of profile is a different result than over eight.
- The direction of lay, where it is functional.
- Before or after coating — the single most commonly forgotten clause.
- Which surfaces. A global "Ra 1.6 unless noted" prices every cosmetic face like a sealing face.
Measurement itself is then straightforward: a contact stylus profilometer is the norm for anything below Ra 0.8, while modern pieces reach down to about Ra 0.5 μm. Comparison blocks are adequate for shop floor checks. The relationship between roughness and dimensional tolerance is covered in our CNC machining tolerance guide; they are independent requirements, and specifying a tight tolerance does not give you a smooth surface.
Which method proves each of those numbers is a separate decision, and it is the one most drawings leave open: our CNC inspection methods guide compares CMM, air gauging, vision and hand tools by tolerance band.
Turning intent into a callout
Map function to number first, then check that the intended process can reach it. In practice almost every callout on a CNC drawing falls into one of four buckets:
- General machined surfaces: Ra 3.2. Reached by any finishing pass, costs nothing extra, improves cosmetics over 6.3 substantially.
- Cosmetic surfaces that are seen or touched: Ra 1.6, sometimes 0.8 on audio or consumer products. Usually free of extra cost, occasionally slower cycle time.
- Mating and locating surfaces: Ra 0.8 to 1.6 with flatness or position controls — the numbers matter less than the geometric control.
- Sealing faces and sliding surfaces: Ra 0.4 to 0.8 with specified lay, and often Rz alongside Ra. Expect this to touch cost.
Two habits save real money. Default to Ra 3.2 and tighten only where function demands it — an entire drawing at Ra 0.8 forces fine finishing passes on surfaces nobody will ever look at. And state lay rather than tighter Ra wherever the function is directional, because lay costs nothing where a lower Ra would cost a lot.
Roughness and tolerance interact, and that interaction is where cost hides. Our tolerance guide covers the dimensional half; if you are choosing between processes for the surface you need, the comparison table in our milling capability guide shows where different strategies land.
Send the drawing with finish callouts attached and we will quote it as drawn plus our recommended relaxation, with a 24-hour turnaround. Our surface treatment line handles the finishing in the same building as machining, and the audio equipment sector page shows why cosmetic versus functional finish specification matters to the customers we serve most. Request a quote.
FAQ
What CNC machining services does Ruijin CNC offer?
Ruijin CNC provides CNC turning, CNC milling (3-axis and 4-axis), 5-axis CNC machining, sheet metal fabrication, metal stamping, and surface treatment including anodizing, plating and powder coating.
What tolerances and certifications does Ruijin CNC hold?
We hold tight tolerances down to ±0.01mm across all processes, and are certified to IATF 16949, ISO 9001 and ISO 13485.
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We accept prototypes from 1 piece with no strict MOQ, provide free DFM review, and return a quote within 24 hours of receiving your CAD drawing and requirements.
How do I get a quote from Ruijin CNC?
Send your STEP file and drawing to gongtianbao@xhlmarketing.com or via WhatsApp +86 17620300785. We reply with a DFM review and a firm quote within 24 hours.