How to specify surface roughness requirement on a drawing

Every week a drawing reaches our quoting desk with the same note in the corner: Ra 0.8 all over. Every machined face, every cast pocket, every thread, every chamfer. That single line adds real money to the quote, and when I ask which face actually needs it, the answer is usually silence. The buyer copied it from an old print, or a CAD template inserted it by default.

I have quoted parts from customer drawings for close to twenty years, and the pattern rarely changes. A roughness note written without a function behind it either inflates the price or gets ignored, and both outcomes waste your time. This article shows how to specify surface roughness so the shop quotes the right thing and the lab measures the right thing. You never pay for polish that no function requires.

What Ra and Rz actually measure

Ra is the average absolute deviation of the surface profile from its mean line, defined in ISO 4287. Rz is the mean peak-to-valley height across the sampling lengths. Most drawings carry Ra, but the two numbers react differently to defects.

The difference matters. Ra averages away a single deep scratch. Five shallow grooves and one deep one can share the same Ra while their Rz values sit far apart. If your concern is a seal seat or a coating that has to bridge over peaks, Rz tells you more. On most machined faces, Rz lands at roughly four to seven times Ra, so a face at Ra 1.6 will often read Rz 7 to 10.

One more detail trips buyers: the cutoff. A profilometer filters the profile with a wavelength cutoff, 0.8 mm for Ra values between roughly 0.5 and 10 µm under ISO 4288. Measure the same face with a 2.5 mm cutoff and the number moves. I once lost the better part of a week to a dispute over a Ra 0.4 face because the supplier report used the wrong cutoff. Same part, different filter, numbers thirty percent apart. State the standard and the cutoff once on the drawing and this argument never starts.

Start from the function of the face

I ask one question when I review a print: what touches this face? The answer sorts every surface into a small number of groups.

Sealing faces need a defined finish, usually Ra 0.8 to 1.6 µm for static seals. Here is the part buyers find counterintuitive: smoother is not better. Below about Ra 0.4, a seal loses the micro-texture that holds lubricant, and the face can wear or weep. I have traced leaks to surfaces that were, on paper, better than spec.

Bearing seats and press fits want Ra 0.4 to 0.8 so interference stays predictable. Rougher faces scrub off their peaks during assembly and the press load drifts. Cosmetic faces follow the substrate: a Ra 3.2 face under clear anodize looks striped, so if appearance matters, say so on the print, because cosmetic finishing is priced by handling rather than by the micrometer. Weld bevels, mounting pads, and bracket faces are fine as-processed. Nobody should pay to polish a face nobody sees or measures.

For medical housings and instruments the stakes rise again. Implant and bone-screw surfaces follow their own roughness logic, and device makers lock finish into their ISO 13485 process controls alongside cleanliness. Buyers sourcing into that world can see how we pair finish with inspection on our medical device manufacturing page.

Write the callout so nobody can misread it

Surface roughness callouts follow ISO 1302. The check-mark symbol with the value inside, attached to the edge line of the face, is the entire system. A few habits keep it unambiguous.

Put the general requirement in the corner of the drawing with the all-over symbol, then call out exceptions on individual faces. Use one system throughout. If your company works in N grades, write Ra 1.6 (N7) once and stay with Ra numbers after that. Mixing systems across one drawing is how an N8 gets read as N7 and a 3.2 gets quoted as a 1.6.

Remember what the callout does not control. Roughness is not flatness. A face can hit Ra 0.8 and still be bowed beyond tolerance, which is why critical faces carry a roughness note and a geometry control under GD&T. Threads, knurls, and similar features sit outside the default requirement, so adding a number to a thread callout only invites questions.

One habit worth copying: a note that says how the finish will be verified. "Ra per ISO 4287, cutoff 0.8 mm, measured perpendicular to lay" is one line that removes every argument before it starts.

What each process holds without secondary operations

This is where quotes jump. A callout below the natural floor of the intended process turns every part into a two-operation job.

An investment casting carries the texture of the ceramic shell, so an as-cast pump housing reads Ra 3.2 to 12.5 depending on alloy and shell quality. Ask for Ra 0.8 on an as-cast face and you have ordered machining or polishing on every part. Our investment casting page lists the finishes we hold as-cast and after secondary work.

MIM behaves differently. Fine powder and a polished mold give an as-sintered surface near Ra 0.8 to 1.6, which is why small complex parts that need a decent finish come out of metal injection molding with no polishing at all. I like MIM for these parts, but I would not promise better than 0.8 without measuring the actual sintering batch. Below 0.8, plan a secondary operation and say so when you request the quote.

Machining sits at the flexible end. A carbide insert at the right speed and feed holds Ra 0.8 all day, and a careful setup holds 0.4 where the drawing needs it. The CNC machining page lists the finishes we quote by default.

Material plays a role too. Ductile alloys smear. 316L galls under heavy feeds and the surface turns shiny but irregular, while a free-machining grade shears clean. On corrosion resistant parts the finish also affects how the passive layer forms and how the part takes passivation. Our stainless steel 316L material page covers what buyers should expect from this grade.

How the finish gets measured

The standard tool is a stylus profilometer. It drags a diamond tip across the face perpendicular to the lay, over an evaluation length built from five sampling lengths, and a good report shows Ra, Rz, and the profile graph. A few things decide whether that number means anything.

Measure the part, not the coupon, when the drawing applies to the part. Casting skin varies across a mold, so agree on measurement locations for cast faces before the first shipment. Do not look for Ra on the CMM report. A coordinate measuring machine captures form and dimension, not texture. I still see buyers reject a shipment because the CMM report has no roughness data, when nobody ever told the lab it was required. If roughness is a release criterion, put it on the inspection plan with a sample size. Three to five parts per lot covers most production runs.

Mistakes that quietly inflate cost

The blanket note comes first. Ra 0.8 all over on a part that mixes cast and machined faces pays for polishing faces that touch nothing. When a quote jumps twenty percent after a finish note, this is the usual reason.

Numbers below the floor come next. Ra 0.1 on a milled face is a grinding or lapping job whether or not the drawing admits it, and verifying 0.1 reliably needs lab conditions on both ends. Finish on threads and knurls means nothing and slows the quote with questions. The opposite mistake is just as real: specifying smoothness where a seal needs texture produces a face that leaks, then a rework order to roughen what you paid to smooth.

I would rather see a drawing with five honest callouts on named faces than one heroic note in the corner. The first drawing quotes in a day. The second starts an email chain.

A short checklist before you release the drawing

• Name the faces that touch seals, bearings, or the product itself.
• Assign Ra per face group and leave the rest as-processed.
• Add the all-over symbol and mark exceptions on individual faces.
• State the standard, ISO 4287 and ISO 4288, and the cutoff once.
• Check each callout against the process you expect to use, or budget the secondary operation.
• Put roughness on the inspection plan with a sample size and measurement locations.

None of this takes long. Twenty minutes with the function of each face usually deletes the blanket note and trims the quote by more than the cost of the meeting. For reference, the how we work page describes how finish callouts, inspection, and quoting fit together in our workflow, and our FAQ collects the finish-related questions buyers raise most often.