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150 THE DISTRIBUTOR’S

150 THE DISTRIBUTOR’S LINK BRUNO MARBACHER ISO SYSTEM FOR LIMITS & FITS - INTERCHANGEABILITY OF METRIC MECHANICAL FASTENERS WORLDWIDE from page 106 In both examples the holes are produced by reaming that produces a hole with an H7 tolerance. There are two common type of metric dowel pins. One that has an “m6 tolerance”, using this one would provide a drive fit (interference fit) The other has an h6 tolerance, which would create a sliding fit (clearance fit). Over the course of the years several combinations emerged that work best for certain application (see table 3). However, how well the combinations work, depends on the type of material, material thickness, the condition of the tool and machine and ultimately the machinist’s attention to detail. Therefore, one may have to adjust somewhat based on the circumstances, however, this should be kept to a minimum. Calculating Minimum & Maximum Dimensions To show this let us use an example: The part we use is a pin nominal size 5 (5 mm) The tolerance is m6. The size is diameter 5, therefore we choose line 3. The tolerance found under “m6” is 0.004 to 0.012. We apply this to the nominal size. 5 + 0.004 = 5.004 5 + 0.012 = 5.012 TABLE 2 TABLE 3 Tolerance Applied To Fasteners The tolerance system is applied to all sorts of mechanical components, including fasteners. The below illustration shows how it is applied to threaded fasteners. Here are some tolerance zones as they are applied to fasteners, hardware, machine elements: ¤ Shoulder diameter of socket shoulder screw (tolerance = f9) ¤ Width across the flats of hex cap screws (tolerance = h13) ¤ Wrench openings, sockets WAF (tolerance = G6, etc.) ¤ Allen wrenches (keys) (tolerance = h10, h11 etc.,) ¤ Width of parallel key (tolerance = h9) ¤ Keyways width (tolerance, shaft = P9, N9, bore = H9) ¤ Bore on tie end rod (tolerance = H7) ¤ Drill rods and cold drawn bars (tolerance = h8, h11, etc.) ¤ Cold Hex bars and square bars (tolerance = h8, h11, etc.) ¤ Dog point diameter on set screws (tolerance = h14) TABLE 4 FIGURE 5 CONTINUED ON PAGE 151

THE DISTRIBUTOR’S LINK 151 BRUNO MARBACHER ISO SYSTEM FOR LIMITS & FITS - INTERCHANGEABILITY OF METRIC MECHANICAL FASTENERS WORLDWIDE from page 150 ¤ Ball bearing shaft/bearing - shaft (tolerance = g5, g6, etc.), ¤ Ball bearing bore/bearing - bore (tolerance = G7, H7, etc.) ¤ Collar diameter of weld nuts (tolerance d11) ¤ Clevis pin diameter (tolerance = h11) ¤ Drilled hole diameters (tolerance = H11, H12) ¤ Standard punches diameter (tolerance = h6) ¤ Diameter on groove pins (area without the grooves (tolerance = h6) ¤ The groove on shafts for circlips, retaining rings, (tolerance = h11, h12, h13) ¤ The groove on bores for retaining rings (tolerance = H11, H12, H13) ¤ Bore of adjusting rings (tolerance = H11) Today’s metric standards often list the tolerances in mm, this makes it easier for inspection, nevertheless they are based on the system although sometimes rounded. Older standards may still show the tolerance zones. Tolerance References On Drawing Even though the system includes an almost endless amount of tolerance zones. There is not a given industrial field that utilizes them all. A company, making precision instruments uses different zones, than a company making big machinery. Hence, if one works in a certain field, one only must deal with certain zones. Many companies have therefore created tables of the tolerance they frequently use; others put the minimum and maximum dimension directly onto the drawing. A third possibility is to put a chart on the drawing, stating the applicable tolerance Conclusion The ISO system for limits and fits makes it possible for things to fit together and function properly. BRUNO MARBACHER