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Surface roughness Ra: when it is worth specifying
Published 2026-09-17
Many drawings carry a roughness symbol in the title block with a value next to it. Ask where that value came from and the most common answer is that the last drawing had it. Unlike a dimensional tolerance, a roughness requirement does not look tight or loose at a glance, so it is the easiest thing to copy wholesale — and every requirement smoother than the as-machined state has to be paid for with finishing passes, slower feeds and extra inspection. This article is about which faces really need one, which faces are just burning money, and how to write it so it can be verified at first article.
What Ra describes, and what it does not
Ra is the arithmetic mean deviation of the profile: over a sampling length, take the absolute distance of the surface profile from its mean line and average it. It is an average, and that single fact explains both its strength and its blind spots — it reports reliably how rough a face is overall, but it cannot see anything isolated.
It says nothing about lay direction, nothing about one deep scratch running across the face, nothing about dents or clamp marks, and nothing about colour or gloss. Two faces with the same Ra reading — one covered in fine, even tool marks, the other mostly smooth with a single scratch across it — can behave completely differently once a seal is pressed against them.
So before deciding whether to specify it, be clear about what you are actually worried about: the overall texture of the face, a particular kind of defect, or appearance. Only the first is what Ra controls. The other two need a different kind of callout, covered below.
Faces worth a callout: the ones that work against something
The test is simple: does this face rely on its own surface condition to do a job in the assembly or in service? Few faces pass. They are usually one of the types in the table, and a given part tends to have one or two of them, not all of its faces.
Pay attention to the last column. In drawing reviews, the trouble is rarely whether the Ra value was set loose or tight — it is that nothing in that column was written: a sealing face with no lay direction, a contact face with roughness but no flatness. Ra is only part of what such a face needs.
| What the face does | Why roughness matters | What to write besides Ra |
|---|---|---|
| Sealing face under an O-ring or gasket | Peaks, valleys and scratches become leak paths | Lay direction; no scratches crossing the seal band |
| Sliding, guiding or reciprocating face | Affects friction, wear and smooth motion | Mating material and lubrication, in the notes |
| Dowel bores, press-fit bores and other fits | Peaks flatten on assembly and change the real fit | Which stage the size applies to, read with the tolerance |
| Heat sink contact face | Affects thermal contact resistance | Flatness often matters as much; is there an interface pad |
| Face to be bonded or painted later | Affects adhesion, but smoother is not always better | Cleaning and pretreatment, often more critical than Ra |
Faces where a callout is wasted money: four common cases
First, faces that touch nothing. The inside of a housing, the floor of a weight-reduction pocket, a face hidden once assembled — the normal as-machined condition is all these need. A value tighter than that buys nothing except machine time.
Second, the as-extruded surface. The outside of a profile is formed in the extrusion die, and its condition is set by the die and the extrusion process, not by machining. Put a machining-grade roughness on an extruded face and the only way to meet it is another cut — which eats wall thickness and alters section dimensions that EN 12020 was supposed to govern. If the appearance genuinely needs to be better, the conversation belongs to the finish or to an appearance sample, not to an extra machining operation.
Third, faces that will later be blasted, anodized or powder coated. Blasting resets the machined texture entirely and powder coating covers it; a machining-stage Ra on those faces means nothing by the time the part ships. Fourth, the copied general value in the title block. The general symbol is a perfectly standard notation; the problem is a value tighter than most faces on this part need, which forces a finishing pass on every face of the drawing.
Anodizing does not hide tool marks
A common assumption is that a slightly rough machined face does not matter because it will disappear under the anodize. Usually the opposite happens. The etching and chemical steps before anodizing change the surface texture, and the oxide layer grows out of the base metal — it tends to reveal tool marks, clamp marks and local material differences more clearly, not fill them in.
So when the same face carries a roughness requirement and a finish, the drawing has to say whether the roughness applies before or after finishing. Those are two different requirements with different machining and inspection behind them. A value without a stage gets read one way by the estimator and another way by whoever receives the parts.
If what you really want is appearance — uniform colour, no visible tool marks, no clamp marks on the show face — a number for Ra is the wrong tool. Designate the cosmetic faces, agree the viewing conditions, and sign off a reference sample at first article. How to choose the finish itself is covered in our article on anodizing versus powder coating.
How to write a callout the shop can execute
Put it only on functional faces, using the surface texture symbol defined in ISO 21920-1 (which replaced the older ISO 1302), pointed at the face itself rather than buried in a text note. The symbol can say whether material removal is required, what the lay direction is, and which parameter and limit apply — exactly what a phrase like smooth finish on this face cannot.
Derive the value from the function, not from an old drawing. When unsure, write the use in the notes: static O-ring seal on this face; sliding fit against the guide rail. With the use in hand, our review can tell you how the requirement will be met on the machine and fixture we plan to use, or where it can safely relax. For a sealing face, add a separate line prohibiting scratches across the seal band — the average that Ra reports will never catch that on its own.
Make the title-block default match normal machining, or simply state that unmarked faces are as machined. And remember that a fine finish is harder on thin walls, because vibration during cutting prints straight onto the surface. If a strict roughness requirement lands on a thin wall, raise it when you send the drawing so it can be assessed together.
Checking it at first article, and when to push back
Roughness is measured with a stylus profilometer, normally across the lay — measure along the tool marks and the reading flatters the part. The first article report should state which face, which direction and what reading, not just a tick in a pass column. If the drawing carries a dozen roughness callouts, the report needs a dozen measured lines, and that alone is a hint that too many were specified.
Before the drawing goes out, ask three questions: does this face touch another part once assembled? Will it be blasted, anodized or coated later? Did this value come from the function or from the previous drawing? If any one of them has no answer, that symbol deserves another look.
The reverse also holds. If a face on your drawing truly needs a controlled surface and the quotation says nothing about it, ask how that face will be machined and checked. Send the drawing with a note on what each face does, and our pre-quote engineering review will confirm it face by face.
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