What does tolerance mean in design and technology?

Answer 1

Example: A an engineering company producing bolts for other industries will be given sizes that the bolt must be. It is nearly impossible for every bolt to be exact and therefore the designers allow a tolerance, basically a plus size or minus size that the bold can be.

In the case of a bolt. The overall length may be 50 millimetres - the size wll allow +/- 0.25 millimetres (quater of a millimetre) This is because the bolt may have to be part of another item and room is an issue.

The head of the bolt will be given where the size is measured across the "flats" Most bolts and tightened with a wrench or spanner. This will also have a +/- size allownce.

The length of the thread too will have a specific size and again an allownce (tolerance) of +/- X millimetres.

The thrards cut (turned) on the bolt will have a length and will conform to a an international size i.e. a gauge size example; 15 mill ISO.

This controls the diametre of the bolt threads, the depth of thret and how many threads there re to the millimetre.

If these sizes are not achieved then the bolt or bolts are not used. Therefore machine setters who set up the lathes that turn these will take a sample i.e. 1 in 50 and check all dimentions are correct. for the threads size they will use a gauge - This will tell them if the thread is to tight or to slack and then adjustments are made. Depending on the use of the bolt will depend on the rigor of the testing . I.e. Simple everyday use ill allow larger margins where in an aeroplane the tolerences are much smaller and the acceptance rate lower.