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106 THE DISTRIBUTOR’S LINK BRUNO MARBACHER ISO SYSTEM FOR LIMITS & FITS - INTERCHANGEABILITY OF METRIC MECHANICAL FASTENERS WORLDWIDE from page 12 How Does The System Work? The tolerance system is based on the fundamental tolerances, which are called IT-grades; they by themselves are not frequently used. IT grades merely indicate the total range of the tolerance. (See Table 1) The system works with tolerance zones, the zone symbols are comprised of letters and numbers e.g. H7, h13, h8, m6, The letters indicate the location of the tolerance in relation to the nominal dimensions of a given part. The numbers then indicate the actual tolerance, the actual limits; a bigger number calls out a bigger spread between the low limit and the high limit. Tolerances for internal feature are expressed with capital letters; tolerances for external feature are expressed in lower case letters. To express all the different tolerance zones, almost all the letters of the alphabet are used. The Tolerance zones are arranged in a certain sequence in relation to the zero line. That zero line represents the nominal dimension of a given part. (pin, shaft, bore, etc.,) For holes, the tolerance zone indicated by the letter “A” starts at the plus side of the zero line. The tolerance zones then gradually move to the zero line (at H) into the minus side to “Z”. TABLE 1 FIGURE 3 For shafts, the tolerance zone indicated by the letter “a” starts at the minus side of the zero line. The tolerance zones then gradually move to the zero line (at h) into the plus side as one moves to “z”. FIGURE 4 How does one choose the right number? The smaller the number, the smaller the tolerance. One should not use a too low number, as this would create a tighter tolerance, making it more difficult to produce a part. The result would be unnecessary high cost. At the same time, one does not want to use a too high number, as this may impair the usability of the parts being assembled. Following is a general guideline that can be applied for most cases. For gauges and instrument tolerances the numbers 01. 1, 2, 3, 4 are applied. For fits one typically utilizes the numbers 5, 6, 7, 8, 9, 10, 11. For more precise applications, the lower number, commonly chosen are 5 or 6. The higher numbers 12,13,14,15,16,17,18, are reserved for parts that are not meant to be fitted together or may only require a loose fit, such as head tolerance on a socket head cap so the key can easy be inserted. Making Them Fit Economically As mentioned above the system was created to establish a systematic approach. Depending on how one lays out the tolerance one either produces a loose or a tight fit. To accomplish this in a cost-effective fashion, two “fit systems” were created. One is the hole base system, the other the base shaft system. In the base shaft system, one gives the hole a fixed tolerance zone for example H7 or H8. To establish either a loose or a tight fit, one chooses the appropriate tolerance zone for the shaft. For a very loose fit one might choose “a6” for a very tight fit “z6” and so on. In the base shaft system, one uses a shaft with a fixed tolerance for example “h6”, “h8” or “h9”. To establish either a loose fit or a tight fit, one selects the appropriate tolerance for the hole. For a very loose fit one might decide on the “A6” for a very tight fit “Z6.” When the tolerance system was first introduced, there were a limited number of tools available to produce a precise hole. Even with modern manufacturing methods, it is still easier to make modifications on external features. Therefore, the preferred “fit system” is still the base hole system. CONTINUED ON PAGE 150

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