Engineering tolerances

Engineering tolerances are how far a real part may differ from its drawing and still be accepted. No machine can make a part of exactly the stated size. So the drawing says, for example, 25.00 mm plus or minus 0.02 mm, and any part between 24.98 mm and 25.02 mm is good. Tolerances are often a few hundredths of a millimetre, and for precise parts a few thousandths.

Here is how it works in general. The designer decides how much error the product can accept, because a tighter tolerance costs more to make. The factory chooses a machine and a process that can stay within that tolerance. After making a part, an inspector or a measuring machine checks it against the drawing. Parts inside the limits pass. Parts outside are repaired or rejected. When parts from different makers must fit together, tolerances make sure that any one part fits any other.

A tolerance has two parts: the nominal size and the allowed difference. The difference can be on both sides or on one side only. For a shaft that must go into a hole, the designer may say it can be smaller than 25.00 mm but never larger. Tolerances also decide how two parts fit. A clearance fit is always loose, so the shaft turns freely. An interference fit is always tight, so the shaft must be pressed in. Cost rises steeply as tolerance gets tighter, because it needs a better machine, slower cutting and more inspection. Errors also add up. Ten parts that are each off by 0.05 mm can together be off by 0.5 mm, so designers check the total.

Take, for example, a piston in an engine. If it is too loose in the cylinder, the engine loses power, and if it is too tight, it jams. So its size is held to a few thousandths of a millimetre. Consider a smartphone, where hundreds of parts from different suppliers must fit into a thin case. Say a bolt is made in one factory and a nut in another. Tolerances make sure they match.

Tolerances connect many pages. The designer sets them in CAD, the shop makes parts on a CNC machine, and measuring is done by industrial CT scanners, machine vision and leak testing. The requirement is part of the digital thread.

How is it different from quality in general? Quality can mean many things. A tolerance is a number: the exact range of sizes that is accepted.