International tolerence (IT) grade

          IT Grade refers to the International Tolerance Grade of an industrial processes defined in ISO 286. This mechanical tolerence grade identifies what tolerences a given process can produce for a given dimension.

The specific tolerence for a particular IT grade is calculated by the following formula

Where :

               . T is the tolerence in micrometers {µm}

               . D is the geometric mean dimension in millimeters [mm]

               . ITG is the IT grade, a positive integer.

The larger the ITG, the looser the tolerence that can be achieved.

Application: 

          Almost all manufacturing processes have an IT grade associated with, indicating how precise it is. IT grade offer guidence for typical manufacturing method capability or how precise one can expect manufacture of a specific option or feature.

          When designing a part, an engineer will typically determine a key dimention (D) and Tolerence on that dimension. Using this formula, the engineer can determine what IT Grade is necessary to produce the part with those specifications. For example, if injection molding has an IT Grade of 13 and a part needs an IT Grade of 5, one cannot injection mold that part to those specifications. It is useful in determining the processes capable of producing parts to the needed specification.

LIMIT GAUGES AND THEIR APPLICATION

          Gauges are scale-less inspection tools. A limit gauge is not a measuring gauge, they are just used for inspection purpose. Two sets of limit gauges widely used for checking limit of dimension of various parts. There are two gauges : 'GO' limit gauge, and 'NOT GO GAUGE'.

          'GO GAUGE' should pass through or over a part while 'NOT GO GAUGE' should not pass through or over the part.

1. Go Limit:

          The GO limit applied to that of the two limits of size corresponds to the maximum material condition i.e., (1) an upper limit of shaft, and  (2) the lower limit of a hole. This is checked by GO gauge.

2. Not Go Limit:

          The Not Go Limit applied to that of the two limits of size corresponds to the minimum material condition, i.e., (1) lower limit of a shaft, and (2) the upper limit of a hole. This is checked by the Not Go gauge.

COMMON APPLICATIONS OF VARIOUS FITS

Some common applications of various fits are shown in below:

ACHIEVABLE TOLERENCE LEVELS BY VARIOUS MACHINING PROCESSES

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) recommends 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, broaching 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 obtaining 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 subtracted 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