Tool 4g handbook vukotaboljanovicsheetmetalformingprocesses

190 Appendix 2 Zero line. A straight line that, in a graphical interpretation of tolerance, corresponds with nominal Upper deviation. Algebraic difference between upper limit size and nominal size. Lower deviation. Algebraic difference between lower limit size and nominal size. True deviation. Algebraic difference between true size and nominal size. Tolerance. Tolerance is the algebraic difference between upper limit size and lower limit size. Fit. The relationship between mating parts of the same nominal size, which stem from the differences in the true sizes before assembling. Clearance. The relationship between mating parts of the same nominal size that stems from the dif- ference in their true sizes if the true size of the hole is bigger than the true size of the shaft before assem- bling. Accordingly, the clearance is always positive. size, so it is the beginning line for the calculation of deviations. Clearancefit. Fit that allows for rotation or sliding between mating parts. Interference. The relationship between mating parts of the same nominal size that stems from differ- ences in their true sizes: if the true size of the hole is smaller than the true size of the shaft, before assem- bling, the clearance is always negative. Interference fit. A fit that everywhere provides interference between the hole and the shaft when assembled, the maximum size of the hole is smaller than the minimum size of the shaft. Transitionfit. A fit that may provide either a clearance or interference between the hole and the shaft when assembled, depending on whether the hole and shaft overlap completely or in part. Hole-basis system. Tolerances are based on a zero line on the hole; also called the basic hole system. The tolerance position with respect to the holes is always represented by the capital letter lower limit size equals zero. The desired fit is achieved by selecting the correct tolerance position of the mating shaft indicated by a lower-case letter of the same nominal size. Shaft-basis system. Tolerances are based on a zero line on the shaft; also called the basic shaft sys- tem. The tolerance position is always represented by the lower-case letter h upper limit size equals zero. The desired fit is achieved by selecting the correct tolerance position of the mating hole indicated by a capital letter of the same nominal size. shows a graphical interpretation of nominal size, limit size, deviation, and tolerance on shaft and hole. shows tolerance positions with regard to the zero line. A2.2 SYSTEM OF LIMITS AND FITS Tolerances in the metric system are designated by the nominal size in millimeters, the letter designation of the tolerance position with regard to the zero line, and the grade of the tolerance position. Example: 50 1, H8. Designation of fits in the metric system of tolerances is expressed as the nominal size mm, followed by the tolerance position for the hole over the tolerance position for the shaft. Example: P6 or h7