Page 230 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Examples

25 H7 - is the tolerance size of a hole whose basic size is 25. The fundamental deviation is represented by the
letter symbol H and the grade of tolerance is representated by the number symbol 7. (Fig 11)
25 e8 - is the tolerance size of a shaft whose basic size 25. The fundamental deviation is represented by the letter
symbol e and the grade of tolerance is represented by the number 8. (Fig 12)

Fig 11 Fig 12

A very wide range of selection can be made by the combination of the 25 fundamental deviations and the 18
grades of tolerances.
Example
In figure 13, a hole is shown as 25 ± 0.2 which means that 25 mm is the basic dimension and ± 0.2 is the deviation.
As pointed out earlier, the permissible variation from the basic dimension is called `DEVIATION'.
The deviation is mostly given on the drawing with dimensions.
In the example, 25 ± 0.2, ± 0.2 is the deviation of the hole of 25 mm diameter. (Fig 13) This means that the hole
is of acceptable size, if its dimension is between
25 + 0.2 = 25.2 mm
or 25 - 0.2 = 24.8 mm.
25.2 mm is the maximum limit. (Fig 14)
Fig 13 Fig 14

24.8 mm is the minimum limit. (Fig 15)
The difference between the maximum and minimum limit of size is the TOLERANCE. Tolerance here is 0.4 mm.
(Fig 16)

Fig 15 Fig 16

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CITS :Engineering Drawing (Mechanical) - Exercise 9 CITS :Engineering Drawing (Mechanical) - Exercise 9