Why should the drawings be carefully checked? Can you guess? Yes, the dimensioning of the drawings should be done in a manner that will be most convenient and understandable to the production departments.

Then can you guess why tolerances must be placed on the dimensions of drawings? We use them to limit the permissible variations in size because it is impossible to manufacture part exactly to a given dimension.

In dimensioning a drawing, the numbers placed in the dimension lines represent dimension that are only approximate and do not represent any degree of accuracy. To specify a degree of accuracy, it is necessary to add tolerance figures to the dimension. Tolerance is the amount of variation permitted in the part or the total variation allowed in a given dimension.

We can take a shaft as an example .A shaft might have a nominal size of 63.5mm, but for practical reasons this figure could not be maintained in manufacturing without great cost. So, a certain tolerance would be added and, if a variation of ±0.08mm could be permitted ,the dimension would be stated 63.5mm±0.08mm.

Some dimensions are given close tolerances, which mean the part must fit properly with some other part. Generally speaking, the minimum cost of production and assembly will maximize profit. The cost of a part goes up as the tolerance is decreased. If a part has several or more surfaces to be machined, the cost can be excessive when little deviation is allowed from the nominal size.

There is another concept, which is sometimes confused with tolerance. It is called allowance. It is the minimum clearance space intended between mating parts and represents the condition of tightest permissible fit. We can understand this definition from the following example.

If a shaft, size  mm, is to fit a hole of size  mm, the minimum size hole is 1.500 mm and the maximum size shaft is 1.498 mm. Thus the allowance is 0.002 and the maximum clearance is 0.008 as based on the minimum shaft size and maximum hole dimension.