1. Tolerance
Tolerance is the amount of error that is allowed when manufacturing a product.
Very small differences are allowed in products because different machines, different machine operators or differences in material from batch to batch cause slight variations in products
Samples of products will be tested to make sure they fall within the tolerance allowed
e.g.
A bolt that is ideally 10mm in diameter can have a tolerance of + or – 1mm and still work as required. Therefore the bolt will be within tolerance if it is between 9.9mm and 10.1mm; any larger or smaller and the bolt will be out of tolerance and have to be discarded.
If a company is making lots of products out of tolerance, it will be more costly to run and therefore inefficient.
It must look into the reason for the inefficiency and resolve it e.g. material quality, machine maintenance , operator training

2. Tolerance diagram
The keyway in this cog has a + tolerance of .001mm and , this means that it can be very slightly bigger but no smaller to work