Concept of Fundamental Deviation
The symbols of fundamental deviation for holes are A, B, C, D, E, F, G, H, JS,J, K , L, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC. The rest of the letters, i.e. I, L , O, Q, and W, are not used. For shafts the same symbols are used but in lower-case letters.
Fundamental deviation for holes
For holes of symbol among A to H, the lower deviation is above zero line, with lower deviation for hole H being zero. For holes having symbol among J to ZC, it is below zero line. In simple words, we can say that the lower deviation for holes A to H is the fundamental deviation and for holes J to ZC the fundamental deviation is the upper deviation.
Fundamental deviation for shafts
For shafts, the concept of fundamental deviation is opposite to holes. For shafts a to h, the upper deviation is below zero line and having the upper deviation being zero for shaft h. For, shafts having symbol in between j and zc, it is above zero line, i.e. the fundamental deviation for shafts a to h is the upper deviation and for shafts j to zc, it is lower deviation.
Let me show you a few examples of these symbols:
50H8/g7 or 50H8-g7, 40H7-g6
In these examples, 50 and 40 are the basic sizes. The symbol represents the fundamental deviation for holes and small letter ‘g’ represents the fundamental deviation for shaft. The numeric symbols represent the fundamental tolerance. You can get the table of ISO tolerance on the web as well as in the market and in some books, too. You just find the values of fundamental deviation and fundamental tolerance using those tables according to the symbol given. Now, we can easily calculate the actual size of shaft and hole only by small addition and subtraction.
There are a number of combinations of shafts and holes that may be used for a fit. It is better to select hole basis fit. Because it is better as the production of shafts to the required size is easier. But, the shaft basis system is very good for manufacturing bright drawn bars.