§ 01

The Number on the Drawing Is Not What You Think It Is

When a Japanese precision machining drawing specifies Ra 0.4 μm on a mating surface, the engineer reading that number typically assumes they know what it means. They are almost certainly wrong about at least one of the following: what Ra actually measures, what it does not measure, why Ra 0.4 might be adequate for one application and completely wrong for another, and — most importantly — what happens at the contact interface between two surfaces that both nominally conform to the specification.

Surface roughness is one of the most routinely specified and most frequently misunderstood parameters in precision manufacturing. It is specified on nearly every machined surface drawing; it is measured on nearly every production line; and it is used as a proxy for functional performance in applications where it is, at best, an incomplete predictor and, at worst, actively misleading. This article explains what Ra and Rz actually measure, why Japanese manufacturers use them differently from their Western counterparts, and what the parameters tell you — and don’t tell you — about the surfaces that carry load, seal pressure, and transmit friction in precision components.

§ 02

What the Profilometer Actually Measures

The Stylus and the Profile

All standard surface roughness parameters — Ra, Rz, Rq, Rsk, Rku — are derived from a single measurement: a stylus trace across the surface. A diamond-tipped stylus (tip radius typically 2–5 μm for standard measurement, 0.5–2 μm for fine surfaces) is drawn across the surface at a constant speed (typically 0.06–0.5 mm/s), and the vertical displacement of the stylus is recorded as a function of horizontal position. The result is a profile — a 2D cross-section of the surface texture in the measurement direction.

This profile is then processed by the instrument’s software through a Gaussian filter that separates the roughness (short-wavelength component) from the waviness (medium-wavelength) and form (long-wavelength) of the surface. The roughness profile — the filtered signal — is what the standard parameters Ra and Rz are calculated from. The cut-off wavelength (λc) of the Gaussian filter determines where the boundary between roughness and waviness is drawn; ISO 4288 specifies the standard cut-off values for different Ra ranges.

Ra: The Arithmetic Mean Deviation

Ra is the arithmetic mean of the absolute values of the profile height deviations from the mean line, measured over the evaluation length:

Ra is the most widely specified surface roughness parameter globally — and it is widely specified precisely because it is easy to measure, easy to understand intuitively, and has a large legacy database of historical values. It is also the parameter most likely to be inadequate for functional specification, for exactly the same reason: its averaging nature suppresses the effect of isolated features — deep scratches, high spikes, burrs — that may be functionally critical while being statistically rare.

Rz: The Maximum Height Parameter

Rz (ISO 4287) is the sum of the maximum peak height and the maximum valley depth within a single sampling length:

Rz is more sensitive to outliers than Ra because it captures the extreme values rather than the average. A surface with Ra 0.4 μm might have Rz anywhere from 2 μm to 10 μm depending on whether the surface has isolated defects — the Ra cannot distinguish between a uniformly fine surface and one with rare but deep scratches. The Rz value reveals this distinction.

§ 03

Why Ra and Rz Tell Different Stories

Consider two surfaces with identical Ra = 0.4 μm:

Surface A is a turned steel shaft, regularly machined with a sharp insert at consistent feed rate. Its profile is a regular sinusoidal pattern — the tool nose radius traced across the surface at each pass, leaving a consistent scallop pattern. The peaks are regular and rounded; the valleys are consistent and shallow. The Ra and Rz ratio is approximately 4:1 (Rz ≈ 1.6 μm).

Surface B is a ground surface with occasional chatter marks from a worn grinding wheel. The background roughness is fine (Ra would be low if measured without the chatter marks), but at irregular intervals there are deeper grooves from the wheel chatter. The average Ra is 0.4 μm, but the Rz is 8 μm — five times higher than Surface A.

These two surfaces will behave completely differently in a sealing application. A rubber O-ring sealing against Surface A will conform to the regular 1.6 μm peaks without difficulty — the seal is effective. The same O-ring sealing against Surface B will bridge the fine background texture adequately but will be unable to conform to the 8 μm chatter grooves — the seal leaks along those grooves. Ra specification alone would accept both surfaces; Rz specification would reject Surface B.

§ 04

The JIS Standard and Japan’s Contribution to Surface Metrology

Japan’s industrial contribution to surface roughness standardisation is direct and traceable. JIS B 0601 — Japan’s surface roughness standard — preceded and informed the international ISO 4287 standard. The JIS standard introduced the concept of the ten-point mean roughness (Rz in the old JIS notation — distinct from the ISO 4287 Rz definition, which is a source of ongoing confusion in international engineering documents) as a complement to Ra, reflecting Japanese manufacturing practice that recognised the inadequacy of Ra alone for functional surface specification.

The historical JIS Rz (now called Rz JIS or sometimes Ry in legacy documents) was defined as the average of the five highest peaks and five deepest valleys within the evaluation length — a more statistically robust measure than the ISO 4287 Rz (which uses a single sampling length) and less sensitive to single outliers. Many Japanese engineering drawings produced before the 2001 JIS revision still specify this historical Rz definition — creating a compatibility problem when those drawings are read by engineers using ISO 4287-calibrated profilometers, which report a different Rz value for the same surface.

§ 05

Surface Texture and Functional Performance: What the Numbers Predict

Friction and Wear

In sliding contact between two surfaces, friction is generated by the asperity contacts between the surface peaks of the two mating parts. The real contact area — the sum of the individual asperity contact patches — is much smaller than the nominal contact area (typically 1–10% for engineering surfaces). The tribological behaviour of the contact is governed not just by Ra but by the shape and distribution of the asperities — their height distribution (captured by Rsk and Rku), their spatial frequency (captured by RSm, the mean spacing of profile elements), and the ratio of Rp to Rv (which determines whether the surface runs on peaks or on valleys).

For a bearing surface, the ideal texture is a plateau surface: fine peaks that wear quickly to a stable plateau during run-in, with deep valleys that retain lubricant. This texture has negative skewness (Rsk < 0) — more material below the mean line than above it. Japanese bearing manufacturers specify Rsk in addition to Ra and Rz for critical bearing surfaces precisely because Ra and Rz alone cannot distinguish between a plateau surface and a peaked surface with identical amplitude parameters.

Sealing

For elastomeric seals (O-rings, gaskets), the relevant parameter is the maximum peak height that the seal must conform to — which is Rp (maximum peak height), or conservatively Rz which includes both peak and valley. A rubber seal will conform to surface asperities up to its elastic limit; asperities higher than this limit create leak paths. Japanese hydraulic and pneumatic component manufacturers specify Rz rather than Ra for sealing surfaces, because Ra cannot predict the maximum asperity height that the seal must bridge.

Fatigue

Surface roughness is a primary initiator of fatigue cracks in cyclic loading. The stress concentration at a valley tip is approximately proportional to the depth/radius ratio of the valley — deeper, sharper valleys concentrate more stress. Rz captures valley depth (through Rv); Ra does not. For rotating shafts, gear tooth flanks, and other fatigue-critical surfaces, Japanese engineering standards specify both Ra and Rz, using Rz as the fatigue-relevant parameter and Ra as the process control parameter.

§ 06

The Mitutoyo SJ Series: How Japanese Profilometers Work

Mitutoyo’s SJ-series surface profilometers (SJ-210, SJ-310, SJ-410) are the reference instruments for surface roughness measurement in Japanese precision manufacturing. They implement ISO 4287, ISO 4288, and JIS B 0601 simultaneously — the operator selects the applicable standard, and the instrument applies the correct filter, sampling length, and evaluation length automatically.

The key engineering specifications of the SJ series that matter for measurement accuracy:

• Stylus tip radius: 2 μm (standard) or 0.5 μm (fine) — the smaller the tip radius, the smaller the features that can be resolved. A 2 μm tip cannot measure valley widths narrower than approximately 4 μm; a 0.5 μm tip resolves to approximately 1 μm. For surfaces ground to Ra 0.1 μm, the 0.5 μm tip is required.
• Stylus contact force: 0.75 mN (SJ-210) — low enough to avoid scratching soft materials (copper, aluminium) while maintaining adequate signal-to-noise ratio on the stylus displacement sensor.
• Measurement range: ±360 μm (SJ-210) — sufficient for Ra values from 0.05 μm (lapped surfaces) to Ra 12.5 μm (rough turned surfaces) without range switching.
• Drive unit straightness: <0.3 μm over 12 mm — the straightness of the traverse drive unit contributes to measurement uncertainty; a non-straight traverse introduces systematic errors into the profile that are indistinguishable from surface texture in the wavelength range of interest.

§ 07

Practical Guide: Specifying Surface Roughness Correctly

The following framework reflects Japanese precision manufacturing practice for surface roughness specification:

The most important practical point: Ra alone is almost never sufficient for functional surface specification. Japanese precision manufacturers specify Ra as the process control parameter (it is easy to measure and correlates with machining condition) but always add Rz for any surface with sealing, tribological, or fatigue requirements. This dual specification — rare in commodity Western manufacturing but standard in Japanese precision production — is one of the engineering practices that separates Japanese precision component quality from lower-tier production.