Tolerance

Why are Tolerances used?

We have to say how accurately we want to make an object. Nowadays objects can be measured to very high levels of accuracy, down to the size of an atom.

Inspectors do not check what the actual measurements are but whether they are within tolerance.

In manufacturing, the designer has to work out how accurate he wants to make each part of the object. It will depend on how it is made, materials and how it fits to other components.
Materials have problems in that they expand and contract.
Timber expands a lot across the grain but less along the grain. You cannot accurately make objects in timber. Their measurements change with humidity.
You can cut Steel more accurately by hand than timber. Steel can be cut very accurately using a process called “Grinding”.
Some parts of an object will have to be accurately made if they are to be fitted to another component. Other parts of the object can then be made to a lower level of accuracy.

What is Tolerance?

Tolerance is the range of measurements of an object which can be accepted as being correct. Comes from the meaning “tolerate, allow, permitted variation”.

10mm is the nominal length of the object.

10+/- 1mm This means the acceptable measurement is + or – 1mm of the nominal length 10mm. i.e. from 9mm (10 - 1) to 11mm (10 + 1)

The symbol ± is used to say ‘Plus’ or ‘Minus’.

General Tolerance is ± 1mm

For hand work is ± 0.5mm

General machining is ± 0.01mm

Fine machining is ± 0.001mm

We have to be able to inspect to these levels of accuracy.