Microinches
Abbreviation/Symbol:
μin
Unit of:
Worldwide use:
A microinch (µin) is a millionth of an inch, which is 0.0254 micrometers. Surface roughness on US engineering drawings is still often given in microinches, where metric drawings use micrometers.
To convert microinches to micrometers multiply by 0.0254 – a 32µin surface finish is about 0.81µm.
Although the metric system offers alternative units for measuring surface finishes, such as micrometers (µm) or nanometers (nm), microinches remain popular due to their historical usage and familiarity in the industry. Additionally, many older machines and tools are calibrated in microinches, making it more convenient to continue using this unit rather than converting to metric units.
Definition:
A microinch (μin) is one millionth of an inch: 1in = 1,000,000μin, or 1μin = 0.0254μm. It is mostly seen on engineering drawings as a surface roughness figure.
Surface finish on machined parts such as engine components is quoted in microinches, as a smoother surface means less friction.
To put the size of a microinch into perspective, it is approximately 39 times smaller than a micron, which is already a very small unit of measurement. To convert from microinches to microns, divide by 39.37; to millimeters, divide by 39,370.
Origin:
A microinch is a millionth of an inch (0.0254μm), and the name is simply micro- plus inch. It was created for surface finish and fine machining tolerances.
In the US, the surface roughness of machined parts is still specified in microinches, because the inch is the working unit in the workshop. The metric equivalent is the micrometer, which is 39.37 microinches.
Common references:
1 million (1,000,000) microinches = 1 inch
Usage context:
A microinch is a millionth of an inch (0.0254 micrometers), and in the US it is used mainly for surface roughness. A typical machined finish is 125 microinches Ra and a smooth ground one 16 – in metric drawings, 3.2 and 0.4 micrometers.
Tolerances on precision machined parts are given in microinches on US engineering drawings. A microinch is 0.0254 micrometers, so 40 microinches is just over 1 micrometer.