What is the tolerance in an engineering drawing?
The tolerance is the variation from the nominal dimension. The design engineer can provide tolerances in various types. Limit tolerances, unilateral tolerances, equal bilateral tolerances or unequal bilateral tolerances are the different ways.
The limit tolerance will not indicate nominal value however provide the maximum and minimum limit directly.
The variation permitted is in one direction for unilateral tolerance. There will not be any variation in other direction.
Equal bilateral tolerance permit variation in both directions. The tolerance value must be the same in both directions.
Equal bilateral tolerance
Unequal bilateral tolerance permit variation in both directions. However, the tolerance value must differ in both directions.
Unequal bilateral tolerance
Why do we need tolerance?
Suppose the part drawing has all specifications as per customer requirements. Part given to manufacturer, he will not be able to manufacture the part without variations. The variations could be from machine error, human error, environment effect, and skill error. There has to be always some tolerance for the specifications provided in the drawing. The manufacturer will always look more tolerance before rejecting any part. There is an inverse relationship between cost and tolerance. The cost will be high with reduced tolerance. Tolerance should be selected based on functional requirements.
Let’s take an example of a plate having the thickness of 15mm. The thickness of the plate has the variation of 1mm in both the directions. The width of the plate can vary from 14mm to 16mm if the manufacturer produces this part. If 15.5mm thick then it will be acceptable because it is in the range of allowed specification. On the other hand, if the manufacturer produces this part say as 16.1mm thick, it will be rejected as it is moving out of the allowed range. In general, the manufacturer can ask deviation if he is not able to meet the expected tolerance and design engineer may approve this deviation based on functional requirements.
Plate thickness with the equal bilateral tolerance