Surface finish and texture

Two shafts measure Ra 0.4. One seals for 200,000 km. The other weeps oil from the first week. Nothing on the drawing distinguishes them, and that is the problem this topic is about.

A portable surface roughness tester resting on a shaft journal still held in a lathe, its stylus lowered onto the surface, with the fine machining marks running around the shaft catching the light along the measured band. — A roughness number comes out of this in about a second. What it does not report is the direction of those marks — and on a seal journal the direction is what decides whether it leaks.

What Ra is, and what it quietly loses

Ra is the arithmetic mean deviation of the profile from its mean line over the sampling length. Average the absolute height of every point, and there it is.

It is stable, repeatable and cheap to measure, which is why it became the default parameter on every drawing everywhere. It also averages away exactly the features that cause failures.

Same Ra, different surfaces, different lives. Ra is an average, so one deep scratch barely moves it — while Rz, and the bearing area curve, see it immediately.

Lay — the direction nobody specifies until the first leak

A diagram of surface lay and its effect on dynamic sealing. On the left a turned shaft journal is shown with a helical lay under a seal lip, with arrows indicating that the helix pumps lubricant along the shaft like a shallow screw thread. On the right a plunge-ground journal shows circumferential marks with no helix and no pumping action. A table beneath lists the lay left by turning, milling, grinding, honing and lapping and the symbol used for each on a drawing. — Turning leaves a helix. Under a seal lip a helix is a screw pump, and it will move oil regardless of how fine the Ra is. This is why seal makers specify plunge grinding, not a roughness number.

Every process leaves a direction:

Process	Lay	Typical Ra (µm)
Turning	Helical	1.6 – 6.3
Milling	Parallel or circular	1.6 – 6.3
Grinding	Parallel	0.2 – 1.6
Honing	Cross-hatched	0.1 – 0.8
Lapping / superfinishing	None	0.012 – 0.1

The bearing area curve, and why cylinder bores get two honing passes

The bearing area curve plots how much material is present at each depth from the highest peak. It distinguishes surfaces that Ra and Rz both call equal:

Sharp peaks — little material near the top. The peaks carry the whole load, wear off rapidly, and the debris goes into the oil.
Plateaus with deep valleys — material immediately available to carry load, valleys still open to hold lubricant.

That is exactly the difference between a plain honed bore and a plateau-honed one. The second honing pass cuts the peaks off in the factory rather than letting the piston rings do it over the first thousand kilometres.

Finish and fatigue

Fatigue cracks start at the surface, at the sharpest notch available. A roughness valley is a notch. A rough finish in a highly stressed fillet cuts endurance substantially — which is why crankshaft fillets are rolled or polished rather than left as machined, and why “as machined” in a fillet callout deserves a second look.

What to take away

Ra is an average, so it is stable, cheap — and blind to isolated deep features.
Rz sees the scratch that Ra averages away. Specify it on sealing and fatigue surfaces.
Lay is the direction of the marks, and it is decided by the process, not the Ra value.
A turned seal journal pumps oil along its helix and leaks at any finish. Plunge grind it.
The bearing area curve separates sharp peaks from load-carrying plateaus.
Plateau honing removes the peaks in the factory instead of in the customer’s engine.
Roughness valleys are notches; a rough stressed fillet fails early.
Specify finish for sealing, sliding, fatigue and interference fits. Leave the rest open.