ISO System of limits and fits for Gauges (ISO : 286-1 : 1998)
The present ISO Recommendation, ISO System of limits and Fits, Part - I, is based on the ISA system of limits and fits. It is the most comprehensive and detailed of all tolerance systems and the most modern one. It lists a full range of 28 holes and 28 shafts covering all possible requirements. It gives several qualities of tolerance for each grade of shaft and holes, and list all the possible combination and recommends for general use a few unilateral holes, an adequate range of shafts, the remainder being listed for special duty. It designates holes by capital letters and shafts by small letters. In numerical form, the tolerance size is defined by it's basic value followed by a symbol composed of a letter and a number (e.g : 50 g7); a fit is indicated by the basic size common to both components followed by symbol corresponding to each component, the hole being quoted first
e.g. 50 H8/g7 or 50H8 - g7 or 50 (H8/g7).
For the purpose of calculation of standard tolerance and fundamental deviations, it specifies 13 main diameter steps, and 22 intermediate steps, the results being computed on the basics of the geometric mean D of the extreme diameter of each step. The main diameter steps are above zero and up to 3, above 3 and up to 6 (represented as 3-6), 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180, 180-250, 250-315, 315-400, 400-500. The intermediate steps are used, in certain cases, when considered as necessary, for the deviations a to c and r to zc or A to C and R to ZC.
It provides 20 grades of tolerance (grades of manufacture) IT01, IT0, IT1, to IT18. the values of standard tolerances for grades 5 to 16 are determined from the tolerances factor i,
i = 0.45 3√D + 0.001 D microns (D is in mm)
The values of standard tolerance corresponding to grades 5 to 18 are as for IS : 919.
The standard tolerance corresponding to IT01, IT0, IT1, are 0.3 + 0.008D, 0.5 + 0.012 D, and 0.8 + 0.020D respectively. The values of IT2 to IT4 are scaled approximately geometrically between the values of IT1 and IT5.
The fundamental tolerance determine the dimensional difference between two limits, and the first are determined by defining one of the limits, i.e., the basic deviation or fundamental deviation. The other limits can be obtained by adding or subtracting the fundamental tolerance.
The formulae for fundamental shaft deviations are same as enumerated in IS : 919.
It specifies 28 grades of fits for both hole and shaft ranging from fits of extreme interference to fits of extreme clearance.
The present ISO Recommendation, ISO System of limits and Fits, Part - I, is based on the ISA system of limits and fits. It is the most comprehensive and detailed of all tolerance systems and the most modern one. It lists a full range of 28 holes and 28 shafts covering all possible requirements. It gives several qualities of tolerance for each grade of shaft and holes, and list all the possible combination and recommends for general use a few unilateral holes, an adequate range of shafts, the remainder being listed for special duty. It designates holes by capital letters and shafts by small letters. In numerical form, the tolerance size is defined by it's basic value followed by a symbol composed of a letter and a number (e.g : 50 g7); a fit is indicated by the basic size common to both components followed by symbol corresponding to each component, the hole being quoted first
e.g. 50 H8/g7 or 50H8 - g7 or 50 (H8/g7).
For the purpose of calculation of standard tolerance and fundamental deviations, it specifies 13 main diameter steps, and 22 intermediate steps, the results being computed on the basics of the geometric mean D of the extreme diameter of each step. The main diameter steps are above zero and up to 3, above 3 and up to 6 (represented as 3-6), 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180, 180-250, 250-315, 315-400, 400-500. The intermediate steps are used, in certain cases, when considered as necessary, for the deviations a to c and r to zc or A to C and R to ZC.
It provides 20 grades of tolerance (grades of manufacture) IT01, IT0, IT1, to IT18. the values of standard tolerances for grades 5 to 16 are determined from the tolerances factor i,
i = 0.45 3√D + 0.001 D microns (D is in mm)
The values of standard tolerance corresponding to grades 5 to 18 are as for IS : 919.
The standard tolerance corresponding to IT01, IT0, IT1, are 0.3 + 0.008D, 0.5 + 0.012 D, and 0.8 + 0.020D respectively. The values of IT2 to IT4 are scaled approximately geometrically between the values of IT1 and IT5.
The fundamental tolerance determine the dimensional difference between two limits, and the first are determined by defining one of the limits, i.e., the basic deviation or fundamental deviation. The other limits can be obtained by adding or subtracting the fundamental tolerance.
The formulae for fundamental shaft deviations are same as enumerated in IS : 919.
It specifies 28 grades of fits for both hole and shaft ranging from fits of extreme interference to fits of extreme clearance.
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