tenth

A tenth is a unit of linear measurement equal to 0.0001 inches, or one ten-thousandth of an inch. In machining, it sits at the boundary between tolerances that can be held on manual equipment and those requiring CNC machines or specialized tooling. Because it equals 2.54 micrometers, tenths appear frequently in prints for parts where dimensional control matters but extreme precision is not economically necessary.

The term emerged from the imperial system's decimal subdivisions. A mil is 0.001 inches; a tenth of a mil is 0.0001 inches. Machinists also call this a tenth mil or sometimes a half micron colloquially, though the metric conversion is approximate. The name reflects the mental math required on the shop floor: if a tolerance is given in tenths, the machinist divides by ten to understand the scale of error allowed.

Tolerances of ±0.0005 inches, ±0.001 inches, and ±0.002 inches all work out to whole numbers of tenths. A manual milling machine with a worn spindle or dull cutter cannot reliably hold tenths; neither can hand reaming. A surface grinder with a properly dressed wheel, a CNC mill with rigid toolholding, or a precision lathe with a tuned spindle can. In assembly work, a hole drilled to ±0.0005 inches is holding tenths on both sides of nominal.

Tenth-level tolerances appear on automotive components, hydraulic fittings, bearing bores, and optical instrument bodies. They also appear on gages and fixtures themselves, where repeatability of measurement or setup is critical. Exceeding a tenth tolerance often triggers scrap, rework, or selective assembly. Holding tenths consistently requires stability: coolant temperature control, tool offsets verified against gage blocks, and machine warm-up cycles before production runs.

In shops that work in metric exclusively, the equivalent concept is the micrometer (µm), and tenths correspond loosely to 2 to 3 micrometer tolerances. However, drawings still sometimes specify tenths on legacy tooling or when interfacing with imperial-design equipment. Understanding tenth-level tolerances is a threshold skill that separates general machining from precision work.