Ra by machining process, the Ra / Rz / RMS / N-grade equivalents, and an honest note on why Ra to Rz is an estimate rather than a conversion. Written by the factory that cuts these surfaces, so the cost of each step down is stated too.
Ra is the arithmetic mean deviation of the roughness profile (ISO 4287) — the average height of the surface texture, in micrometres. It is the number nearly every drawing uses. Two things follow that most charts leave out: the process decides what Ra is achievable, and the cost does not rise smoothly — it steps, at the point where a second process has to be added.
Typical achievable ranges in normal production, not laboratory best cases. The typical column is what you get without asking; the best column is what the process can do when the part is set up for it.
| Process | Typical Ra (µm) | Best Ra (µm) | In µin | Notes |
|---|---|---|---|---|
| Rough turning / milling | 3.2–12.5 | 3.2 | 125–500 | Material removal stage; not a finish specification |
| Finish turning / milling | 1.6–3.2 | 0.8 | 63–125 | The default. Ra 1.6 falls out of a normal finishing pass |
| Fine turning (sharp insert, reduced feed) | 0.8–1.6 | 0.4 | 32–63 | One extra pass; the cheapest way to reach 0.8 |
| Drilling | 1.6–6.3 | 1.6 | 63–250 | Wall finish is rarely specified; use reaming or boring if it matters |
| Reaming | 0.8–3.2 | 0.4 | 32–125 | Usually paired with an H7 bore |
| Boring | 0.8–3.2 | 0.4 | 32–125 | Better roundness than reaming on larger diameters |
| Cylindrical grinding | 0.2–0.8 | 0.1 | 8–32 | Where the cost step is. A separate setup and process |
| Surface grinding | 0.2–1.6 | 0.1 | 8–63 | Flat faces, sealing lands, gauge surfaces |
| Honing | 0.1–0.4 | 0.05 | 4–16 | Bores that must hold oil — cross-hatch pattern is the point |
| Lapping | 0.05–0.2 | 0.012 | 0.5–8 | Metal-to-metal sealing faces, optical and gauge work |
| Polishing | 0.1–0.4 | 0.025 | 1–16 | Cosmetic or hygienic; does not by itself improve flatness |
| Wire EDM | 0.8–3.2 | 0.4 | 32–125 | Improves with each skim pass; each pass costs time |
| Sinker EDM | 1.6–6.3 | 0.8 | 63–250 | Finish follows the electrode and the setting used |
| Investment casting | 1.6–6.3 | 1.6 | 63–250 | As-cast; machined faces are specified separately |
| Sand casting | 6.3–25 | 6.3 | 250–1000 | Always machine any face that has to seal or locate |
EKINSUN LTD is a custom parts manufacturer in Guangdong, China, and supplies Ra 1.6 µm as machined on turned and milled parts unless a drawing says otherwise. Every finer value in the table above is available — state it at the feature that needs it, not across the whole part. From one piece, roughness measurement reported on request, quote in 12 hours.
The row you need when a drawing arrives in a different unit system to the one you work in.
| Ra (µm) | Ra (µin) | RMS (µin) ≈ | ISO 1302 grade | Rz (µm) ≈ | What it is used for |
|---|---|---|---|---|---|
| 0.012 | 0.5 | 0.6 | N1 | 0.05–0.08 | Gauge blocks, optical surfaces |
| 0.025 | 1 | 1.1 | N2 | 0.1–0.2 | Lapped seal faces, mirror polish |
| 0.05 | 2 | 2.2 | N3 | 0.2–0.4 | Honed bores, precision instruments |
| 0.1 | 4 | 4.4 | N4 | 0.4–0.7 | Fine ground bearing journals |
| 0.4 | 16 | 18 | N5 | 1.6–2.8 | Ground surfaces, gas-tight face seals |
| 0.8 | 32 | 36 | N6 | 3.2–5.6 | O-ring sealing lands, sliding fits, fine turned |
| 1.6 | 63 | 70 | N7 | 6.3–11 | General machined default — most parts, most faces |
| 3.2 | 125 | 139 | N8 | 12.5–22 | Non-critical machined faces, clearance surfaces |
| 6.3 | 250 | 278 | N9 | 25–44 | Rough machined, unimportant faces |
| 12.5 | 500 | 555 | N10 | 50–88 | Flame-cut, sawn, as-cast surfaces |
| 25 | 1000 | 1110 | N11 | 100–175 | Unfinished stock surfaces |
Conversions used above: Ra µin = Ra µm × 39.37, then rounded to the conventional pairs that appear on drawings — Ra 0.8 µm is exactly 31.5 µin but is universally written as 32, and Ra 3.2 µm is 126 µin but written as 125. RMS ≈ 1.11 × Ra is the customary factor for a machined profile, not an exact identity.
The honest part about Rz: Ra and Rz measure different things — Ra averages the whole profile, Rz measures peak-to-valley heights — so there is no exact conversion between them. Two surfaces with identical Ra can have quite different Rz, which is precisely why a drawing that specifies Rz is asking for something Ra does not describe. The customary estimate for a normally machined surface is Rz ≈ 4–7 × Ra, and the Rz column above uses that range. If the drawing calls out Rz, measure Rz — a calculated value is not the same specification.
This is the part a pure reference chart never tells you, and it is the reason so many drawings over-specify.
| Going from | To | What changes on the machine | Cost effect |
|---|---|---|---|
| Ra 6.3 | Ra 3.2 | Nothing — a finishing pass was going to happen anyway | Effectively free |
| Ra 3.2 | Ra 1.6 | Feed and nose radius chosen for finish; still one pass | Free in most cases |
| Ra 1.6 | Ra 0.8 | An extra spring pass at reduced feed, sharper insert | Modest — added cycle time only |
| Ra 0.8 | Ra 0.4 | A second process — grinding, with its own setup and fixturing | The real step. Far larger than every jump above it |
| Ra 0.4 | Ra 0.1 or finer | Honing, lapping or polishing after grinding | Another process again, plus measurement |
The practical consequence: going from Ra 3.2 to Ra 1.6 across a whole part usually costs nothing, while going from Ra 0.8 to Ra 0.4 on a single face can cost more than the rest of the part. Specify the fine value on the one face that needs it — a bearing journal, a sealing land — and leave the rest at the default.
| Feature | Usual Ra (µm) | Why |
|---|---|---|
| Static O-ring sealing land | 0.8 or better | Rougher and the elastomer cannot bridge the peaks; direction of the marks matters as much as the value |
| Dynamic seal running surface | 0.2–0.4 | The seal lip rides on it continuously; too rough abrades the lip, too smooth starves the film |
| Rolling bearing journal | 0.4 or better | Ground; the inner ring locates on this surface |
| Plain bearing / bushing bore | 0.4–0.8 | Honed or fine bored so the oil film survives |
| Sliding fit (H7/g6 pair) | 0.8 | A rough surface eats the clearance you calculated |
| Press fit (H7/p6) | 0.8–1.6 | Rough peaks shear off during assembly and the interference is lost |
| Gasketed flange face | 1.6–3.2 | A soft gasket wants some texture to grip |
| General machined faces | 1.6–3.2 | Default; specifying finer here is where money is wasted |
Roughness charts are normally written as if material did not matter. It does — the values in the process table above assume a well-behaved steel, and these are the deviations worth knowing before you specify a fine value.
| Material | Behaviour when cutting | Practical effect on Ra |
|---|---|---|
| Free-machining steel (12L14, 11SMn30) | Chips break cleanly, no built-up edge | The reference case — Ra 1.6 comes easily, 0.8 with a fine pass |
| Stainless 304 / 1.4301 | Work-hardens ahead of the tool and is prone to built-up edge | Needs sharper tooling and higher surface speed to reach the same Ra; a dull insert tears rather than cuts |
| Stainless 316L / 1.4404 | Gummier still, lower thermal conductivity | Hardest of the common stainless grades to finish; below Ra 0.4 plan on grinding rather than turning |
| 4140 / 42CrMo4, heat treated | Hard and stable, cuts predictably | Finishes well; ground surfaces below Ra 0.4 are routine on hardened journals |
| Aluminium 6061 | Soft, tends to smear and build up on the edge | Ra numbers look good but can hide smeared metal; polished aluminium is bright long before it is smooth |
| Aluminium 7075 | Harder and less gummy than 6061 | Cuts to a genuinely finer finish than 6061 with the same tooling |
| Brass CW614N (US ≈ C360) | The most forgiving of all | Reaches a fine finish with almost no effort — brass parts often measure better than specified |
| POM, PEEK and other plastics | Elastic recovery behind the tool | Ra measured immediately after cutting can differ from Ra a day later; specify the check condition if it is critical |
The takeaway for a drawing: Ra 0.8 on a brass part and Ra 0.8 on a 316L part are the same specification but not the same job. If a fine finish is on a stainless feature, say so early — it is the point where a second operation gets added to the quotation.
A roughness table gives values; it does not tell you what your part needs. EKINSUN machines to a stated Ra from one piece to production volume, and these are the cases where the number alone is not the whole specification.
| Your situation | Solved by the chart? | How we handle it |
|---|---|---|
| A standard part whose supplier already states its finish | Yes | Buy it. If a catalogue component meets the spec, that is cheaper than machining — honest recommendation |
| A different Ra on different features of the same part | No — one number per drawing note | Called out per feature; the fine value is cut only where it works |
| The drawing says Rz and your shop measures Ra | No — they are not interchangeable | We measure what the drawing specifies rather than converting between them |
| Lay direction matters — sealing lands, wiper faces | Ra says nothing about direction | Machining direction set so the marks run with the seal, not across it |
| Plating or anodising after machining | Chart values are for bare metal | Coating changes both the finish and the dimension — treatment choice covered here |
| A worn part to be reproduced, original finish unknown | Nothing to read it from | Finish inferred from the wear pattern and function, then confirmed with you — reverse engineering |
General CNC turning and milling land at Ra 1.6 to 3.2 micrometres with a normal finishing pass. A dedicated fine pass with a sharp insert and reduced feed reaches Ra 0.8, and below Ra 0.4 you are normally into grinding, honing or lapping rather than cutting. Roughing leaves Ra 3.2 to 12.5.
You cannot convert them exactly — Ra averages the whole profile while Rz measures peak-to-valley heights, so two surfaces with the same Ra can have different Rz. The customary estimate for a normally machined surface is Rz roughly 4 to 7 times Ra, often taken as about 4.5 times. If a drawing calls out Rz, measure Rz; substituting a calculated value is not the same specification.
Ra 3.2 micrometres equals 125 microinches. The conversion is Ra in microinches = Ra in micrometres multiplied by 39.37. The common pairs are Ra 0.4 = 16, Ra 0.8 = 32, Ra 1.6 = 63, Ra 3.2 = 125 and Ra 6.3 = 250 microinches.
EKINSUN supplies Ra 1.6 micrometres as machined on turned and milled parts unless the drawing states otherwise, which corresponds to N7 and 63 microinches. Ra 0.8 is a finishing pass, Ra 0.4 normally means grinding, and anything finer is honed, lapped or polished. Each step down adds time, so it is worth specifying only on the faces that need it.
A static elastomer seal face is generally specified at Ra 0.8 micrometres (32 microinches) or better, and gas-tight face seals are often tightened to 0.4. Direction matters as much as the number: turning marks that run around the sealing land will seal, while a radial scratch crossing it will leak at any Ra value.
Yes, and it steps rather than rises smoothly. Ra 3.2 to 1.6 is usually free because it falls out of a normal finishing pass. Ra 0.8 costs an extra pass at reduced feed. Ra 0.4 and below usually adds a second process — grinding, honing or lapping — with its own setup, so the jump from 0.8 to 0.4 is far more expensive than the jump from 3.2 to 1.6.
N grades are the ISO 1302 roughness grade numbers, each one a step on the Ra scale: N5 is Ra 0.4, N6 is Ra 0.8, N7 is Ra 1.6, N8 is Ra 3.2, N9 is Ra 6.3 and N10 is Ra 12.5 micrometres. Older drawings often use them in place of a numeric Ra callout, and they mean exactly the same thing.
Yes. State the Ra at the feature that needs it rather than across the whole part — a shaft can be Ra 0.4 at the bearing seat and Ra 3.2 elsewhere, and that costs far less than holding the fine value everywhere. EKINSUN machines from one piece to production volume, with a surface roughness measurement reported on request.
Tell us the feature and the value. A drawing, sketch or photo is enough. Answer within 12 hours.
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Name the feature and the value — we quote within 12 hours, from one piece, with the roughness reported on request.