Roughness and finishing: what Ra 0.8 costs

What does an Ra on the drawing say, and what not?

Ra is the average deviation of the surface from a mean line, in µm. Rz is the height from the highest peak to the deepest valley per sampling length, averaged. On the same face Rz reads higher; mix the two up and you price a surface roughness requirement several times too strict or too loose.

Sandvik Coromant shows that two very different-looking surfaces can share the same Ra. A single deep scratch weighs more in Rz, so a sealing face sometimes carries an Rz beside its Ra.

The surface texture symbol, a tick with the value beside it, points at the face it governs; a value by the title block covers every other face. The rules sit in ISO 21920-1, the successor to ISO 1302 (NEN-EN-ISO 21920-1:2022). Older drawings may show an N grade, such as N7 for Ra 1.6.

The drawing does not say how to get there: Ra 0.8 prescribes no grinding. The manufacturing engineer chooses the route; the cost engineer prices it, face by face.

What roughness do milling and turning reach?

In turning, feed and nose radius set the theoretical roughness. In Sandvik Coromant's table, 0.1 mm feed per revolution with a 0.4 mm nose radius gives Ra 0.63 µm in theory; at 0.2 mm, Ra 2.5. As a rule of thumb, the same source says a wiper insert holds that finish at twice the feed.

In face milling, the flat parallel land on the insert's cutting edge generates the surface. Sandvik Coromant keeps the feed per revolution below 80 per cent of the width of that land; once it exceeds the land, the cutter's runout shows in the face.

The table below is a shop-floor rule of thumb, not a standard: material, machine and workholding move the boundaries.

When does a roughness need an extra step?

In roughing and finishing, the roughness is part of the machining line for as long as the final pass reaches the value. An extra step only comes in when that pass cannot hit the requirement reliably enough to pass every part.

A familiar pattern: a sealing face polished by hand at the bench after turning. It costs skilled time, the finish varies from part to part and the face easily loses its flatness. Roller burnishing on the machine flattens the profile peaks in the same setup, without cutting. Wikipedia lists sealing surfaces among its typical uses, with very close control of size.

Around Ra 0.4 you reach grinding, honing or lapping: usually another machine or shop, so a line of its own with its own lead time. The next step: the measured time of such a step flows back into the next estimate for a similar face.

Is the dimension before or after anodising?

An anodised layer grows partly into the metal and partly out of it; Vereniging ION puts the outward share at about a half or a third in most processes. Hard anodising builds a far greater coating thickness: Wikipedia gives 13 to 150 µm.

The roughness changes too. According to Vereniging ION, a hard anodised layer usually ends up rougher than the metal beneath it. If the sealing face still has to be Ra 0.8 afterwards, polishing, honing or lapping on the coating follows, and it belongs in the price.

Mask threads and fits, or rework them afterwards, for instance by chasing the thread with a tap. Whether dimensions and roughness apply before or after the coating belongs in a note on the drawing. If it is not there, ask before you price the part.

Worked example: a sealing face from Ra 3.2 to Ra 0.8

A cover in EN AW-6082 with one sealing face, 30 parts, machine at € 80 an hour, then anodised, not hard. The table shows only the lines for the sealing face, plus the finish.

The finer requirement adds € 6.10 per part, € 183 over the batch. Put Ra 0.8 on the whole cover as a surcharge, six faces of the same size, and the customer pays for fine finish, measurement and tooling six times: more than € 36 extra per part.

Which finish belongs on its own line?

Anodising, blasting, passivation and electroless nickel plating are usually done by a subcontractor. Each treatment gets a subcontract line with that supplier's price, next to the machining.

A general surcharge for roughness hides the choice that sets the price. An ERP keeps what the last order cost, not why the sealing face got a burnishing step and the side face did not. Record that choice per face, and a repeat order or new revision shows which line changes.

No price from the finisher yet? Leave the line open rather than filling in a number from memory. How that works, and how much size an anodised layer takes, is covered in finishing without a price.

How to read roughness and finishing in a request

Run through these before you enter a time or a price.

• Which faces carry their own surface texture symbol, and which fall under the general value?
• Which faces are functional: sealing, bearing, sliding?
• Which finish follows, and who carries it out?
• Do dimensions and roughness apply before or after the coating?
• Which threads and fits must be masked?
• Does the customer want an inspection report, or will sampling do?
• Do you have a price from the finisher, or does that line stay open?