Dimensional Tolerances

All manufactured and installed things vary to some degree from their intended or specified dimensions or locations. Those variations affect how they fit together with other things, such as a door into its frame, or their strength or formability.

For relative large things like doors, the ‘industry’ has settled on ‘standard’ fitting tolerances (by consensus) and the tolerances do not matter too much. For installed things, the location tolerances are critical to how the building fits together and a certain tolerance must be allowed for just because it's hard to build to tight tolerances.

Thickness Tolerances:

Examples:

  • For thin things, especially when so thin that it is not possible to tell the difference by eye, such as sheet metal, the thickness tolerance means a lot. It also determines how much the product costs. It’s easy to get cheated and the product could fail (if it’s structural) or dent more easily (sheet metal roofing). ASTM standards were originally developed to standardize these things so that buyers and sellers could more easily agree on what they were talking about.
  • We hardly ever specify thickness tolerances, except for field fabricated or constructed things, because it is difficult to measure them. But we do specify many standards that include thickness tolerances - that is one particular reason for referencing the standard.
  • As with thickness tolerances, all things installed vary to some degree from their intended location, length, width, height, flatness, levelness, etc. Tolerances can be specified for how much variation is acceptable - less tolerance is referred to as ‘closer‘ or ‘tighter‘, more is ‘wider‘ or ‘looser‘. Such tolerances may be defined by a manufacturer because he does not intend to or cannot install to closer tolerances. Or the specifier can state the acceptable tolerance. Closer tolerances usually cost more, but wider tolerances can affect appearance (wavy concrete surfaces look bad) or function (automated warehouse floors must be very flat for the machinery to operate correctly). To much tolerance on the location of one item may make subsequently installed things not fit without adjustment.
  • This type of tolerance is usually stated as a variation from something. For location, the tolerance is usually absolute:
  • For flatness, levelness, etc., acceptable variation is usually stated in relation to another dimension:
  • Both dimensions are put inside the one choice so that they display better when both are shown. The second dimension is usually chosen to be some round number and may be different in metric - a computed conversion is not usually sensible.

Installation Tolerances:

Examples, with SpecLink-E choice coding:

  • Location: Maximum variation from specified location of plus/minus {{[[1/4] [[___]]] inch ([[6] [[___]]] mm)}}.
  • Flatness: Maximum variation of {{[[1/4 inch in 10] [[___]]] feet ([[6 mm in 3] [[____]]] m)}}.