Surface Roughness Ra Chart

What Ra Actually Measures

Ra is the arithmetic average of how far the surface deviates from its mean line, measured in micrometres (μm). A lower number means a smoother surface.

It's the most commonly specified roughness parameter, and it's also the most widely misunderstood — because it's an average. A surface can have a perfectly acceptable Ra and still have one deep scratch running across it. That matters, and we'll come back to it.

Process	Typical Ra (μm)	Achievable with care (μm)	Notes
Sand casting (as-cast)	12.5 – 50	—	Rough; machined if it matters
Sawing	6.3 – 25	—	Cut-off marks
Milling, roughing	6.3 – 12.5	—	Stock removal, not finish
Milling, general	1.6 – 3.2	0.8	The everyday shop finish
Turning, roughing	3.2 – 12.5	—	Feed marks visible
Turning, finishing	0.8 – 1.6	0.4	Sharp tool, light feed
Drilling	3.2 – 12.5	—	Bore finish from drilling alone is poor
Reaming / boring	0.8 – 3.2	0.4	Standard for a decent bore
Broaching	0.4 – 3.2	0.2	Good for keyways and profiles
Grinding	0.2 – 1.6	0.1	The route below Ra 0.4
Lapping / honing	0.05 – 0.4	0.025	Separate operation, real cost
Polishing	0.025 – 0.4	—	Labour-heavy, mirror finishes
Bead blasting	1.6 – 6.3	—	Uniform matte, cosmetic
Wire EDM	0.8 – 3.2	0.4	Depends on number of skim passes

Values are typical production ranges, not laboratory best cases. Material, tooling condition, machine rigidity and geometry all shift the result.

What Each Step Down Costs

Smoother is not linearly more expensive — it's roughly exponential, because at some point you stop cutting and start adding a whole new operation.

Ra (μm)	How you get there	Relative cost
6.3	Standard roughing, no extra passes	1.0× baseline
3.2	Normal finish, standard parameters	1.0–1.1×
1.6	Dedicated finishing pass, sharp tool	1.2–1.4×
0.8	Fine finishing pass, controlled feed and speed, good rigidity	1.5–2×
0.4	Grinding, or very careful finishing on suitable geometry	2.5–4×
0.2 and below	Lapping, honing or polishing — separate processes	4× and up

The step from 3.2 to 1.6 is cheap. The step from 0.8 to 0.4 is not, because you have changed processes. If your drawing calls for Ra 0.4 across every face, you have just added a grinding operation to a part that could have been finished on the mill. We covered this pattern in twelve design changes that cut machining cost.

Ra vs Rz: Why the Average Can Lie

Ra averages everything, so a single deep groove barely moves it. Rz takes the average of the peak-to-valley heights across several sampling lengths, which makes it far more sensitive to individual defects.

Why this matters in practice: a sealing face is defeated by one continuous scratch running across the seal line, not by general roughness. A surface can measure Ra 0.8 — perfectly respectable — and still leak, because the Ra figure averaged the scratch away.

Rule of thumb for conversion: Rz is roughly 4–7 times Ra depending on the process that made the surface. Don't rely on the conversion for a critical specification — measure what you actually care about.

When to specify both: sealing faces, hydraulic bores, bearing journals — anywhere a single deep mark kills the function. Ra for general texture, Rz (or Rmax) to catch the outliers.

Application Guide

What Roughness Does the Surface Actually Need?

Application	Sensible Ra (μm)	Why
Non-critical faces, cosmetic housings	3.2	Looks fine, costs nothing extra
General machined faces, mounting pads	1.6 – 3.2	The default for most parts
Bearing seats, press-fit bores	0.8 – 1.6	Needs consistency for grip
Sliding surfaces, linear guides	0.4 – 0.8	Friction and wear
Static sealing faces (gasket, O-ring groove)	0.8 – 1.6	Plus watch Rz for scratches
Hydraulic cylinder bores	0.2 – 0.4	Dynamic sealing, honing needed
Gauge blocks, precision bearings	0.1 – 0.2	Lapping territory
Optical or mirror finishes	0.025 – 0.1	Polishing, significant labour

How to Specify It on the Drawing

The single most useful thing you can do: don't put a roughness symbol on every surface. The same logic applies to geometric tolerances — our GD&T for buyers guide explains which ones actually earn their inspection cost.

Instead use the common convention — a general note covering everything not individually marked, plus specific callouts on the faces that matter:

General note: "Surface roughness Ra 3.2 unless otherwise specified" — or whatever your default is
Individual callouts: only on sealing faces, bearing bores, sliding surfaces, press fits
Add Rz or Rmax: where a single scratch would cause failure
Say whether it applies before or after coating — this catches people out. Anodising and plating change the surface (see our surface finish comparison chart for coating thicknesses)

Note the last point carefully. If you specify Ra 0.8 and the part is hardcoat anodised, the roughness you measure before and after coating will not be the same number. Say which one you mean.

Three Mistakes We See On Drawings

1. A blanket tight Ra across the whole part

The most common one. Ra 0.8 on every face means a finishing pass everywhere, and possibly grinding, for surfaces whose only job is to exist. Mark the critical faces and leave the rest at the general note.

2. Specifying Ra finer than the process can hold

Calling Ra 0.4 on a deep-pocket milled floor is asking for a grinding operation that may be geometrically impossible to perform in that pocket. If the value isn't achievable by the process you're specifying, it becomes an expensive negotiation.

3. Forgetting that finishing changes roughness

Anodising, plating, bead blasting and powder coating all alter the surface — usually making it rougher. If roughness is functionally critical, measure it in the final state, and say so on the drawing.

FAQ

Surface Roughness Questions

What Ra can CNC machining achieve?

Standard CNC milling and turning typically produces Ra 1.6 to 3.2 micrometres as-machined. With sharp tooling, appropriate speeds and a fine finishing pass, Ra 0.8 is routine and Ra 0.4 is achievable on many geometries. Below Ra 0.4 you are generally looking at grinding, lapping or polishing as separate operations.

What is the difference between Ra and Rz?

Ra is the arithmetic average deviation from the mean line, while Rz is the average of the peak-to-valley heights across several sampling lengths. Ra is the most commonly specified but it averages the surface, so it can hide an isolated deep scratch. Rz is more sensitive to individual peaks and valleys, which is why sealing faces often specify both.

How much does a smoother surface finish add to cost?

Going from Ra 3.2 to Ra 1.6 is usually modest. Going to Ra 0.8 requires a dedicated finishing pass with sharp tooling and controlled parameters. Below Ra 0.4, you are paying for a different process — grinding, lapping or polishing — and the cost rises steeply because it is additional handling, not just a slower cut.

Do I need to specify Ra on every surface?

No, and doing so is a common source of unnecessary cost. Leave non-critical faces at the standard as-machined finish and call out Ra only on sealing faces, bearing bores, sliding surfaces and press-fit interfaces. A blanket Ra 0.8 across a whole drawing means extra finishing passes everywhere for no functional benefit.

Can anodising change surface roughness?

Yes. Anodising builds an oxide layer that partly penetrates and partly grows outward, and it generally makes the surface slightly rougher, particularly with thicker hardcoat coatings. Bead blasting before anodising is often used deliberately to produce a uniform matte finish. Always measure roughness after finishing if the value is functionally critical.

How do you measure roughness?

Two ways in a normal shop. A contact profilometer drags a stylus across the surface and gives a direct reading, which is the standard method and what we use for inspection reports. Comparison samples — reference plates of known Ra — are faster for a quick visual and tactile check on the floor, and are usually enough to confirm a general machined finish.

Measured, Not Guessed

Send the Drawing With the Functional Requirement

Tell us what each surface has to do — seal, slide, locate, or just look right — and we'll recommend a roughness that achieves it without paying for more. If your drawing specifies a finish that's finer than the geometry allows, we'll flag it in the quote rather than after the order.

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