Calculate the maximum and minimum interference between an H7 hole and a p6 pin using those limits: 0.001 / 0. For more information on performing tolerance analysis, please refer to Section 3.3. Now apply these interference values to the m6 pin and its limits of size to get the target tolerances for the locational interference fit hole. Use the tables to find the limits of size for the m6 pin with 0.25 inch diameter: +0.00059 /+0.00024. Because the least calculated interference should be 0, the upper limit for the hole to fit this oversized pin should be +0.00024. The upper limit of interference is 0.001, so the lower limit for the hole should be −0.00041.
If the designer prefers to use a tolerance designation instead of numerical values for the hole, a standard designation should be sought for limits of size of +0.00024/ −0.00041 for a diameter of 0.25 inches. Using the tables, the closest designation to those limits of size is K7 for that size of hole. Designation K7 at 0.25 inches diameter has limits of size of +0.0001 / −0.0005. A K7 hole combined with an m6 pin in that size will result in a fit that allows between 0.00014 and 0.00109 inches of interference in the joint. The standard fit allows between 0 and 0.001, so this modified fit should work acceptably. The advantage of using the system of limits and fits is that once a fit is calculated, the designations are easily remembered and reused. A standard dowel pin has an m6 designation, so the designer can easily remember (or record) that a K7 hole will yield a satisfactory interference fit. Numerical values need not be recalled once a fit is defined. When the designer has control over the tolerances applied to both parts, using standard fit designations can speed the process.
When using force fits, the pressure required to assemble the parts can be estimated using pressure factors. The stress resulting from force fits should be calculated for a more accurate result. It is essential that the elastic limit of the parts in a force fit assembly not be exceeded because that would result in a loosening of the fit. Consult the recommended resources for calculation guidance.
Table 3-1: Commonly Used Limits and Fits
Table 3-2: Selected ANSI Tolerance Grades
Table 3-3: Selected International Tolerance (IT) Grades
MACHINING TOLERANCES
Choice of tolerances should always take into account the manufacturing process capability as well as functional requirements. Every machining process has a tolerance capability. This can vary by machine and machinist. Some machining tolerances are given in ANSI B4.1. Table 3-4 illustrates some typical tolerance grades achieved by various machining processes. The values shown are intended only as a guide and vary depending on machine tool and operator. Tables 3-2 and 3-3 provide the numerical values for the tolerance grades as a function of part size.
Table 3-4: Manufacturing Process Average Tolerance Grades
LIMITS OF SIZE DATA
Tables 3-5 through 3-10 contain some of the more commonly encountered limits of size.
Table 3-5: Selected Limits of Size, Holes (Inch)
Table 3-6: Selected Limits of Size, Shafts (Inch)
Table 3-7: Additional Selected Limits of Size (Inch)
Table 3-8: Selected Limits of Size, Holes (Metric)
Table 3-9: Selected Limits of Size, Shafts (Metric)
Table 3-10: Additional Selected Limits of Size (Metric)
•Factors like long engagement lengths, temperature, and lubrication will affect fit.
•Standard fits are an excellent starting point, but are no substitute for careful analysis of tolerances.
•In a force fit assembly, the elastic limit of the parts must not be exceeded. Calculate the resultant stresses for all force fits to ensure proper grip.
•When choosing tolerances for holes, an H designation is preferred.
•Use the most generous grades and tolerances possible to ease manufacture.
•Standard fits can be used to calculate the total clearance or interference for a desired result. When designing around an item with given limits of size, those values can be applied to calculate the limits of size for the mating part.
TOLERANCES ON DRAWINGS, AND GD&T
Choosing tolerances and representing those values on drawings are critical steps of the design process. Choosing and analyzing tolerances is addressed in Sections 3.1 and 3.3 of this chapter. Communicating the desired tolerances and design intent can be simplified using implied tolerances, as well as Geometric Dimensioning and Tolerancing (GD&T).
