There are various terms are there to understand the limits given to certain objects. These terms are explained by using simple shaft and hole assembly. The following are the terms related to limits.
Basic size: It is a standard size of hole or shaft which is decided during the designing process with which limits are to be decided.
Actual size: It is a final dimension of hole or shaft which is measured after manufacturing process.
Zero line: It is straight line which is drawn to represent the basic size. All limits and fits are explained with the reference of the zero line.
Tolerance: It is defined as permissible variation that are to be given onto the dimensions of the product.
It can be also defined as difference between the maximum size of the object and minimum size of the object.
Generally the tolerance is given in terms of Upper limit and Lower limit in between which final dimension of the object should be.
Upper limit: It is a limit which is given to the dimension of any object to determine the upper limit size or maximum size of that object.
Lower limit: It is a limit which is given to the dimension of any object to determine the lower limit size or minimum size of that object.
Tolerance zone: It defined as an area formed by the difference between maximum dimension and minimum dimension of the object.
Deviation: It is defined as the algebraic difference between limit of shaft or hole and basic size of the shaft or hole.
There are two types of deviations. They are i) Upper deviation ii) Lower deviaton
Upper deviation: It is defined as the difference between upper limit size and the corresponding basic size. It is represented by ES for hole and es for shaft. It is positive when the upper limit size is greater than the basic size. It is negative when the upper limit size is less than the basic size.
Upper deviation = Upper limit size - Basic size
Lower deviation: It is defIned as the difference between lower limit size and the corresponding basic size. It is represented by EI for hole and ei for shaft. It is positive when the lower limit size is greater than the basic size. It is negative when the lower limit size is less than the basic size.
Lower deviation = Lower limit size - Basic size
The tolerance is also defined in terms of upper deviation and lower deviation. It is given as
For shaft
IT = es - ei ( upper deviation - lower deviation)
For hole
IT = ES - EI
Fundamental deviation: It is a deviation either upper deviation or lower deviation which is closer to the zero line either for shaft or hole.
Basic shaft: A shaft is called as basic shaft when its upper limit size becomes equal to the basic size. It can be also defined as when upper deviation becomes equal to zero then that shaft is known as basic shaft.
Basic hole: A hole is called as basic hole when its lower limit size becomes equal to the basic size. It can also be defined as when lower deviation becomes equal to zero then that hole is known as basic hole.
Tolerance zone: It is the zone formed by the two limits of size of the part in the graphical representation of tolerance.
Tolerance grade: The tolerance grade is an indication of the degree of accuracy of manufacture and it is designated by the letters IT followed by a number, where "IT stands for International Tolerance grade".
Ex: IT01, IT0, IT1
Standard tolerance unit: It is a unit which is a function of basic size and it is used to find the tolerance value by different tolerance grades. It is denoted by 'i' and expressed in microns.
