Tolerances for Fasteners

Why are specified tolerances necessary?

It's the only way of making sure that everyone who sees the specification or drawing knows the importance or unimportance of a particular dimension.

For example, if an end-user needs a fastener that fits inside a pre-drilled hole, then the dimensions of the fastener need to account for that hole size, otherwise, a manufacturer may decide it ok to make a larger fastener which results in an improper fit and thus increased costs and time wasted to remanufactured goods.

Likewise, there is no need to specify very tight tolerances for fasteners where this is unnecessary. Tighter tolerances are harder to achieve and thus the manufacturing cost goes up.

There has to be a common-sense balance between requirement and specification.

Various international standards exist and excerpts from those standards which relate specifically and generally to nuts are detailed below:
ISO 4759

ISO 4759 consists of the following parts, under the general title Tolerances for Fasteners:

Part 1 - Bolts, Screws, Studs and Nuts - Product grades A, B and C

Part 2 - Bolts, Screws and Nuts with Thread Diameters from 1 up to 3mm and Product Grade F, for Fine Mechanics

Part 3 - Plain Washers for Bolts, Screws and Nuts - Product grades A and C

We're concerned with Part 1 because we're a nut manufacturer.

Note the following:

  • Tolerance except for threads, are selected from a system of limits and fits as specified in ISO 286-2.
  • Tolerance of metric threads are taken from the series of tolerance classes specified in ISO 965-3.
  • Tolerance of form and position are specified and indicated in accordance with ISO 1101, ISO 2692 and ISO 8015.
  • Tolerances apply to uncoated fasteners unless otherwise specified
  • Deviations are only permitted in product standards where there are technical valid reasons.
  • Tolerances specified in product standards take precedence over ISO 4759
  • Dimensions and tolerances are given in millimetres