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Wednesday, 22 August 2018

THE BASIC SYSTEMS FOR DETERMINING FITS

There are two systems of fit for obtaining Clearance, Interference and Transition fit. These are 
1) Hole basic system
2) Shaft basic system

Significance of hole basic system:
          The Bureau of Indian Standards (BIS) recomends both hole basic system and shaft basic system, but this selection depends on production methods. In general for hole making we use different production methods like drilling, reaming, boring, broching etc., For shaft making either grinding or turning.
          If we use shaft basic system for specifying tolerence system, the number of holes of different sizes are required. For that we need different tools of different sizes.
          If we use hole basic system, we need only one tool for production of hole and the shaft can be machined easily for desired sizes. Because of this reason a lot of people use Hole basic system over shaft basic system.

1. Hole Basic System:
          In hole basic system the size of hole is kept constant and shaft sizes are varied for obtaing different types of fits. In hole basic system lower deviation of hole is zero. Minimum hole is taken as the basic size, an allowance is assigned, and tolerences are applied on both sides of and away from this allowance.

Example of Hole basis system:
1. The minimum size of the hole 15 mm is taken as the basic size.
2. An allowance of 0.5 mm is substacted from the basic hole size, making the maximum shaft as 14.5 mm.
3. Tolerence of 0.45 mm and 0.55 mm respectively are applied to the hole and shaft to obtain the maximum hole 15.45 mm and the minimum size of shaft 13.95 mm.

Maximum clearence = Max size of hole - Min size of shaft
                                      = 15.45 - 13.95 = 1.5 mm
Minimum clearence = Min size of hole - Max size of shaft
                                      =  15 - 14.5 = 0.5 mm

2. Shaft basis system:
          In shaft basis system, the size of the shaft kept constant and different fits are obtained by varying the size of the hole. Shaft basis system is used when the ground bars or drawn bars are readily available. These bars are not required further machining and fits are obtained by varying the sizes of the hole.
          In this system the basic size of the shaft is zero (the upper deviation of shaft zero) i.e the higher limit of shaft is basic size. Various fits are obtained by applying allowances and lower limit of shaft and upper limit of hole.

Example of Hole basis system:

1. The maximum size of the shaft 15 mm is taken as the basic size.
2. An allowance of 0.5 mm is added to the basic size of the shaft, making the minimum hole size as 15.5 mm.
3. Tolerence of 0.45 mm and 0.55 mm respectively are applied to the hole and shaft to obtain the maximum hole 15.95 mm and the minimum size of shaft 14.45 mm.

Maximum clearence = Max size of hole - Min size of shaft
                                      = 15.95 - 14.45 = 1.5 mm
Minimum clearence = Min size of hole - Max size of shaft
                                      =  15.5 - 15 = 0.5 mm
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