SURFACE FINISH (RA, RZ, PROCESS MAP)

Surface roughness is quantified by parameters extracted from a profilometer trace. Ra (arithmetic average) is most common; Rz (max peak-to-valley over five sample lengths) is more sensitive to outliers. Each manufacturing process delivers a typical Ra range: turning 0.8-6.3 µm, milling 0.4-6.3, surface grinding 0.1-1.6, lapping 0.012-0.4, polishing <0.05. Tighter finishes cost exponentially more; specify only what the function requires.

• Ra = (1/L) · ∫₀ᴸ |y(x)| dx
• Rz = avg of 5 max peak-to-valley
• Rq (RMS) = sqrt((1/L) · ∫ y² dx)
• Rt = max(peaks) - min(valleys)

References & StandardsASME B46.1, ISO 4287, ASME Y14.36M (symbols)

Pick surface finish based on function: dynamic seal contact (Ra 0.2-0.4 µm for O-rings), static seal (Ra 1.6 µm), bearing journal (Ra 0.4 max), fatigue-loaded surface (smoother is better; below 0.4 µm for high-cycle), or cosmetic. Cross-reference to standard symbols on drawings (per ASME Y14.36M) and ensure the spec is achievable by the available process.