Tolerances – A brief introduction

Engineering, Product Design

Tolerances – A brief introduction

As a product design consultancy who focus on design for manufacture, ensuring a product is suitably engineered for production is of the up most importance. Some of you who have been guided through the innovation process before will have heard the word ‘tolerance’ thrown about.

If the parts of a product don’t have to interact, there is simply no need for tolerances.  A tolerance is intrinsic to the mechanical function of product. Engineering tolerance is the permissible limit or limits of variation in a physical dimension. Tolerance is the difference between the maximum and minimum limits. This can be shown as upper and lower limits. E.g. Ø10.00 +0.20/-0.10.

In the example above, a finished part is acceptable when its dimension is anywhere between Ø9.90 and Ø 10.20. Outside of this range, a part would be rejected for either being too large or too small.

This range of allowable dimensions is known as the ‘tolerance band.’ The larger the difference between the upper and lower limits, the larger the tolerance band, referred to as a ‘looser’ tolerance. On the other end of the spectrum, a smaller tolerance band is considered a ‘tighter’ tolerance.

Tolerances should ALWAYS be used. If you leave a dimension without a tolerance, no on else will know the importance, or the unimportance of that dimension.


  1. Parts with proper tolerances will fit as desired (sliding fit, or a press fit).
  2. Well made parts contribute to reduced costs. Unnecessarily tight tolerances can cause parts to be more expensive.
  3. Manufacturers will apply their own set of standardised tolerances to non-toleranced dimensions, many manufacturers will not begin making parts until all features are defined. This can consume valuable time and quite possibly prolonging delivery time.
  4. By using proper tolerances, the liability of making parts correctly is passed on to the manufacturer.


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