Convert Nanometers to Microinches (Ra)

The Nanometer (symbol: nm) is a unit of surface roughness. A surface-roughness unit equal to exactly 0.001 μm = 10⁻⁹ m. Nanometer-level roughness specification (typically reported as nm Rq RMS, not nm Ra) is required for: precision optical surfaces (mirrors for large-aperture telescopes per ESO E-ELT primary-mirror specification < 1 nm RMS over relevant spatial frequencies; EUV lithography photomask substrates for ASML scanners 0.2-0.5 nm RMS per SEMI P37 / P38 specifications), silicon wafer manufacturing (300 mm prime wafers achieve 0.1-0.3 nm Ra over the polished surface after CMP per SEMI M1 standard), and high-performance hydrodynamic / hydrostatic precision bearings (turbomolecular vacuum pump bearings, atomic-clock-grade reference flats). Atomic-Force Microscopy (AFM — Bruker Dimension Icon, Park Systems NX, Asylum MFP-3D) and white-light interferometry (Zygo, Bruker, Sensofar) resolve surface variations down to sub-nanometer (<0.1 nm RMS) over micrometer-scale fields. The IUPAC 2014 official roughness conversion convention recommends nm only when the underlying measurement instrument resolves <10 nm features — otherwise μm is preferred. Used heavily in EUV photomask qualification, optical-table flat-mirror specs, and semiconductor wafer-flatness GBIR/SFQR metrics.

It belongs to the metric measurement system.

Nanometers are commonly used in machining, quality control, and tribology.

The Microinch (Ra) (symbol: μin) is a unit of surface roughness. An imperial surface-roughness unit equal to exactly 0.0254 μm (1 μin = 25.4 nm). Microinches (μin) Ra is the traditional US unit on machining drawings per ASME B46.1 (Surface Texture — Surface Roughness, Waviness and Lay) and US-edition Machinery's Handbook. Typical achievable values per ASME B46.1 surface-finish charts and US machining-shop practice: sand-cast iron 500-1,000 μin Ra; rough turned/milled 125-500 μin Ra; fine turned/finish milled 32-125 μin Ra; ground 8-63 μin Ra; honed 4-16 μin Ra; lapped 1-8 μin Ra; mirror-polished/superfinished 1-4 μin Ra or less; precision optical mirror surfaces <1 μin Ra; silicon-wafer surfaces (after CMP polish) <0.1 μin Ra. US production-machining drawings still routinely call out '63' or '125' (μin Ra) for general-purpose finish; '32' for sealing surfaces; '16' for bearing fits per ANSI/ASME B46.1 + the unified-screw-thread standard ANSI/ASME B1.1. Converting μin Ra to μm Ra (multiply by 0.0254, or divide by 39.37) is a routine step when US-designed parts are manufactured in metric-default shops overseas (Mexico maquiladoras, China contract manufacturers).

It belongs to the imperial measurement system.

Microinches (Ra) are commonly used in machining, quality control, and tribology.