Machined Surface Roughness on Castings: What Ra Values Mean

When a buyer asks about casting surface finish, they usually have two different things in mind: the as-cast texture and the machined roughness left after CNC work. This article is about the second one. Ra is the most common roughness value on engineering drawings, and it is also the most misunderstood. If you are sourcing cast parts with machined features, a few minutes spent understanding Ra will save you from expensive surprises.

What Ra actually measures

Ra stands for “roughness average.” It is the arithmetic mean of the absolute deviations of a surface profile from its center line over a sampling length. In plain language: the instrument drags a stylus across the surface, records every tiny peak and valley, and averages them into one number.

That averaging is the weakness. Two completely different surfaces can produce the same Ra. One part might have a smooth texture with a single deep scratch; another might have a uniformly fine finish. The Ra readings can be nearly identical. Ra does not tell you the height of the worst peak (that’s Rz), how far apart the peaks are (Rsm), or whether the surface has long-wavelength waviness. For a bearing seat or a seal face, a single scratch can cause failure even when the Ra looks good.

So treat Ra as a gate, not a complete description. It is the right first callout for most machined drawings. Just don’t assume a low Ra number means a defect-free surface.

Units matter too. Metric drawings use micrometres (µm); many US drawings use microinches (µin). A 1.6 µm finish equals about 63 µin. A 3.2 µm finish equals about 125 µin. Pick one system and stay with it. Mixed units cause exactly the kind of errors that show up at final inspection.

Castings put a different problem in front of you

Machining a casting is not the same as machining a piece of bar stock. Bar stock starts with a consistent wrought structure. A casting starts as a mold replica, and its surface carries history.

Investment castings — stainless steel, carbon steel, alloy steel — come out of the shell with a fairly smooth surface for a casting. But that surface is still mold skin, not a machined surface. It can carry tiny remnants of shell material, oxide, or scale. With aluminum, especially gravity or low-pressure castings, the as-cast skin can be sound and smooth one day and carry micro-porosity the next. You cannot see that by eye.

The real risk is what machining exposes. A porosity pocket just below the skin can sit there invisible through the whole casting process, then open up as a pit on a machined sealing face. This is why the internal inspection capability of the foundry matters, not just the CNC department. We screen for internal defects — porosity and shrinkage — with X-ray NDT in-house on our own XTH320 system. If you are buying machined castings for pressure-containing parts, ask what the supplier does to find subsurface porosity before the part goes to the machining line. And make sure the supplier runs leak testing on parts that need to hold pressure.

The practical point: a roughness callout only makes sense on surfaces that will actually be machined. Everything else should be described as as-cast finish, with the expected texture agreed at samples rather than by an Ra number.

Choosing an Ra value with intent

Industry practice, and it is fairly consistent across shops, runs something like this:

• 6.3 µm (250 µin) — general machining, non-critical surfaces.
• 3.2 µm (125 µin) — standard machined finish for most functional faces.
• 1.6 µm (63 µin) — fitted surfaces, light seals, bearing bores.
• 0.8 µm (32 µin) — sealing faces, dynamic surfaces, higher-cost machining.
• 0.4 µm or better — grinding territory, and it is expensive.

Each step down roughly doubles the cost or adds another operation. Tighter Ra means slower feeds, more tool changes, more inspection time, and more scrap when something drifts. The cheapest surface finish is the one you do not call out. Reserve tight roughness for the one or two faces that actually need it.

Casting alloys make this worse if you over-specify. Stainless castings such as 304 or 316 are gummy and work-harden as you cut them. Aluminum alloys are soft and prone to tearing. A 0.8 µm finish on a cast stainless part takes real process discipline, not just a sharp insert. If you don’t actually need it, don’t put it on the drawing.

How a supplier proves it hit the number

Ra cannot be checked with a fingernail, and roughness comparison specimens only give a rough feel — the word “rough” is doing a lot of work there. A real roughness measurement needs a contact profilometer or an optical equivalent, measuring across the machining lay direction, over a defined cutoff length. This is basic, and a surprising number of shops skip it.

When you evaluate a casting supplier, ask three things. First, what instruments do they have for roughness measurement? We measure roundness and profile in-house, alongside hardness testing, and we use CMM equipment from COORD3 and ZEISS for geometric checks. Roughness is a surface property; geometry is a different one. A CMM alone cannot give you Ra. A good supplier has both.

Second, where do they measure? Ra is not a property of the whole part. It is a property of one spot in one direction. If the drawing does not say where to measure, you and the supplier may measure different places and get different verdicts on the same part. Agree on the measurement location, direction, and cutoff length at the first article stage.

Third, do they machine in-house or subcontract? This matters more than buyers think. When machining is done in-house, the people who poured the casting control the tool path, the feeds, and the inspection. The roughness callout on your drawing stays in one building from melt to final check. Our 400+ CNC machines run in the same facility as our casting lines, so a surface finish problem is caught at the source rather than discovered on a truckload of finished parts.

What to put on the drawing

Writing a useful finish callout is not hard, and it prevents most arguments.

Specify the Ra value and the units. Specify which surfaces it applies to — do not put a global finish callout in the title block and let everyone guess. State the cutoff length if your drawing standard requires it. And make clear whether the requirement applies before or after coating. A part that gets anodized, plated, or passivated after machining will not have the same measured roughness afterward, because the coating changes the surface. If you want a certain final surface, say so explicitly and let the supplier pick the machining strategy that gets there.

Finally, treat Ra as a production control, not an inspection lottery. A single measurement on a finished part tells you very little about the batch. Require first article inspection, where the roughness measurement is made and recorded, and ask for periodic records during production. Our standard flow is straightforward: you send drawings and requirements, we do the engineering review and quote, then casting, machining, and delivery all happen in one facility with traceability back to the material certificates and inspection reports.

Ra is a useful number when it is specified deliberately, measured correctly, and interpreted honestly. The buyers who get burned are the ones who treat it as a magic smoothness value. Specify what the surface must do, not just what it should look like, and your casting supplier can hit the number every time.

Frequently Asked Questions

Why can a machined casting with a low Ra value still fail on a sealing face?

Ra is an arithmetic average, so a single deep scratch or a sub-surface porosity pocket opened by machining can hide within the number. We use in-house X-ray NDT with an XTH320 system to screen for porosity and shrinkage before machining, and we run leak testing on pressure-containing parts.

What Ra values should I specify for different machined features on cast parts?

Typical practice: 6.3 µm for general non-critical machining, 3.2 µm for standard functional faces, 1.6 µm for fitted surfaces and light seals, and 0.8 µm for sealing or dynamic surfaces. Finer than 0.8 µm usually means grinding and significant extra cost, so reserve tight callouts for the few faces that truly need them.

How does a supplier actually prove the Ra value on a cast surface, and what should I ask?

Ask what roughness instruments they have, where they measure, and whether machining is in-house. We measure roundness and profile in-house and use CMMs from COORD3 and ZEISS SPECTRUM for geometry, but Ra requires a profilometer or optical equivalent—a CMM alone cannot give Ra—so measurement location, direction, and cutoff length must be agreed at first article.

Does machining a casting create different surface finish risks than machining bar stock?

Yes. A casting starts with mold skin and can hide micro-porosity just below the surface that opens up as a pit after CNC machining. That is why pre-machining internal inspection and in-house control of both casting and machining matter; our 400+ CNC machines run in the same facility as the casting lines.

Key Parameters at a Glance