Convert Microinches (Ra) to Micrometers (Ra)

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.

The Micrometer (Ra) (symbol: μm) is a unit of surface roughness. The SI unit of surface roughness — most commonly reported as the arithmetic mean deviation Ra (formerly CLA — Center Line Average) per ISO 4287 / ISO 21920-2 (the 2021 revision that replaced ISO 4287) in micrometers (μm). μm Ra is the global manufacturing standard for specifying surface finish on engineering drawings per ISO 1302 (Geometrical product specification — surface texture indication) and the unified GD&T notation per ISO 1101. Typical achievable values per Machinery's Handbook and Sandvik Coromant cutting-tool data: sand-cast iron surface 12.5-25 μm Ra; rough-turned/milled with carbide tool 3.2-12.5 μm Ra; fine-turned/finish-milled 0.8-3.2 μm Ra; ground 0.2-1.6 μm Ra; honed 0.1-0.4 μm Ra; lapped 0.025-0.2 μm Ra; mirror-polished/superfinished <0.025 μm Ra (1 μin or less); silicon-wafer CMP polished <0.001 μm Ra (atomic-scale). Measurement instruments per ISO 25178 (areal-roughness) and ASME B46.1: stylus profilometers (Mitutoyo SJ-410, Taylor Hobson Form Talysurf, Mahr MarSurf XR20) for contact methods; optical confocal microscopes (Sensofar S-Neox, Zeiss Smartzoom) and white-light interferometers (Zygo NewView, Bruker ContourGT) for non-contact methods.

It belongs to the metric measurement system.

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