The flange face is not just flat metal. It is a controlled texture that the gasket has to bite into. Pick the wrong texture for the gasket and the joint either leaks immediately or leaks within a few thermal cycles.
I get called to flange leaks more than any other class of repair, and the diagnosis is usually one of two things — the geometry is out of spec (covered in RTJ grooves) or the finish is wrong for the gasket the project specified. The second case is the one most crews never check.
Here is what you need to know.
What “Ra” actually measures
Surface roughness is quoted in Ra — the arithmetic average of the peak-to-valley height of the texture, in micrometres (µm) or microinches (µin). The conversion is roughly:
| Ra µm | Ra µin |
|---|---|
| 0.8 | 32 |
| 1.6 | 63 |
| 3.2 | 125 |
| 6.3 | 250 |
| 12.5 | 500 |
For flange faces you mostly care about three bands: 3.2 to 6.3 µm (stock), under 1.6 µm (smooth), and 3.2 to 12.5 µm with a controlled spiral lay (serrated).
The three finishes you will see
Stock finish (Ra 3.2 – 6.3 µm)
The default raised-face finish on most ASME B16.5 carbon-steel flanges ex-works. A controlled concentric or spiral pattern, deliberately rough enough to grip a soft-cut gasket.
- Use with: compressed non-asbestos sheet (CNAF), graphite sheet, PTFE-faced gaskets, most spiral-wound gaskets in standard service.
- Avoid with: soft elastomer gaskets, metal-jacketed gaskets, any gasket that needs to flow into a smoother face.
The roughness is what makes it seal. A “perfectly smooth” face on a stock-finish gasket will leak — the gasket has nothing to bite.
Smooth finish (Ra ≤ 1.6 µm)
Mirror-grade, no visible tool marks at arm’s length.
- Use with: elastomer O-rings, metal-jacketed gaskets, soft aluminium or copper gaskets, food / pharma sanitary joints.
- Avoid with: soft sheet gaskets in heavy service — they will extrude under bolt load.
In the field, smooth finish is what you cut for RTJ groove interior walls (Ra ≤ 1.6 µm is in API 6A) and for sanitary tube ends on high-purity tube facing.
Spiral serrated (Ra 3.2 – 12.5 µm, controlled spiral)
A continuous spiral groove cut into the face, typically 0.5 mm pitch. Looks like a vinyl record. The walls of the groove grip soft gasket material under compression and prevent it from being squeezed out laterally.
- Use with: most raised-face gasket installations on ASME B16.5 flanges in process service. CNAF, graphite, PTFE sheet, spiral-wound.
- Avoid with: anything that needs a smooth face. The serrations will cut soft metal gaskets.
The spiral direction matters less than the consistency. The pitch and depth do matter.
What ASME B16.5 actually requires
ASME B16.5 specifies a stock finish of Ra 3.2 to 6.3 µm for standard raised-face flanges and allows alternative finishes by agreement. Other key codes:
| Standard | Application | Finish requirement |
|---|---|---|
| ASME B16.5 | RF pipe flanges | Stock: Ra 3.2 – 6.3 µm |
| ASME B16.20 | Spiral-wound gasket flanges | Stock or spiral; Ra 3.2 – 6.3 µm typical |
| API 6A | Wellhead flanges (RTJ) | Smooth: Ra ≤ 1.6 µm in groove |
| ASME BPE | Sanitary process | Smooth: Ra ≤ 0.8 µm wetted, ≤ 0.5 µm electropolished |
The number on the drawing is the number that has to come off the machine. Not “about that,” not “approximately.” If the gasket manufacturer published a finish range for their product, use that range.
How the field machining produces each finish
A portable flange facer controls finish through three variables:
- Feed rate per revolution — slow feed = smooth, fast feed = rougher pattern.
- Tool nose radius — large radius smooths the pattern, small radius leaves a visible spiral.
- Cutting speed — too slow leaves chatter, too fast wears the insert and roughens the finish.
For a stock finish on carbon steel, typical settings: feed 0.3 to 0.5 mm/rev with a 0.4 mm nose radius insert. For smooth (≤ 1.6 µm): feed 0.08 to 0.15 mm/rev with a 0.8 mm nose. For spiral serrated: a profile tool that cuts the groove directly, single pass.
Measure the result with a surface roughness tester (cheap handheld units start around USD 200 and pay for themselves on the first verification) or a comparator block. Do not eyeball it. A face that looks smooth to the eye is routinely Ra 3 µm, twice the limit for an RTJ groove.
Three field failures I see most often
1. Smooth finish cut for a stock-finish gasket
Crew over-finishes the face because “smooth is better.” The CNAF gasket has nothing to grip, the joint weeps at first thermal cycle, and the conclusion is “we need a better gasket.” Wrong diagnosis. The gasket was fine. The face was wrong.
2. Stock finish left on an RTJ flange seat
When the face around an RTJ groove is left at stock finish (3.2+ µm), the groove walls can still be smooth but the ring sits on the rough face under bolt load. Result: the line contact band is contaminated by the rough face and the seal does not develop. Cut the face area around the groove to Ra ≤ 1.6 µm with the groove.
3. Spiral serration with wrong pitch
When a flange face is re-cut after damage and the new spiral pitch does not match the original, the gasket impressions from previous bolt-up sit between the new grooves. The face looks fine; the joint leaks because half the gasket bite is into old impressions, not new grooves. Cut deep enough to fully remove the previous pattern.
What this means for tooling
Holding Ra under 1.6 µm in the field needs:
- A rigid machine — vibration adds 1 to 2 µm to whatever the geometry would otherwise produce.
- Sharp tooling — a worn insert produces a wavy finish that comes back as a leak.
- Steady drive — air or hydraulic supply must not fluctuate during the finishing pass.
Most general-purpose flange facing machines will hit Ra 1.6 µm reliably. For sanitary work below Ra 0.8 µm, the machine needs more rigidity than the bore size alone suggests. Send the gasket spec along with the flange OD and we will match the right machine — quote form.
For related flange work, see also how to machine RTJ grooves.
About the author — Dongmei Guo is founder of Kedes Machine. On the shop floor since 2010, she builds portable on-site machining and cold cutting equipment in Shenzhen and works directly with field crews in oil & gas, power, shipbuilding and high-purity tubing. She writes here when a question comes up on the workshop floor that is worth answering in full.