3-5Selected Limits of Size, Holes (Inch)
3-6Selected Limits of Size, Shafts (Inch)
3-7Additional Selected Limits of Size (Inch)
3-8Selected Limits of Size, Holes (Metric)
3-9Selected Limits of Size, Shafts (Metric)
3-10Additional Selected Limits of Size (Metric)
3-11Common Implied Tolerances (Inch)
3-12Common Implied Tolerances (Metric)
3-13ANSI and ISO Geometric Symbols
3-14Area Under the Standard Normal Curve
3-15Effect of Material Modifier on Stack-Up
3-16Effect of Least Material Condition Modifier on Stack-Up
3-17Boundary Calculations for Positioned Features of Size
3-18Stack-Up Method Application Matrix
LIMITS, FITS, AND TOLERANCE GRADES
All parts and features have some variation from ideal size, and this variation is controlled by the designer through the application of dimensional tolerances. The system of limits and fits allows the designer to quickly tolerance parts that fit together with a predetermined clearance or interference. Limits of size refer to the two tolerances or deviations applied to a dimension that set the upper and lower limits for that dimension. These tolerances are meant to be applied to nominal parts that were designed ‘line to line’ (without clearance when nominal size) in assemblies. The standards governing limits and fits are specific to cylindrical features and parts (holes and shafts), but these fits can also be applied to non-cylindrical features and parts like rectangular slides and pockets. When selecting fits, one must consider the loading, speed, length of engagement, temperature, and lubrication conditions of the assembly. The designer is free to use the standard fits or specify different tolerance combinations to achieve the desired result.
•Oberg, Jones, Horton, Ryffel, Machinery’s Handbook, 28th Ed., Industrial Press, New York, NY, 2008
•ANSI B4.1: “Preferred Limits and Fits for Cylindrical Parts”
•ISO 286: “ISO System of Limits and Fits”
TYPES OF FITS AND THEIR LIMITS
Fits refer to the amount of clearance or interference between mating parts. There are three basic types of fits: clearance fits, transition fits (chance of either clearance or interference), and interference fits. Each fit specifies two sets of tolerances, or limits of size: one for the hole or external feature, and one for the shaft or internal feature. These tolerances are applied to the nominal feature size when parts are designed ‘line to line’ (without clearance at nominal size), and can be either positive or negative. Tolerance designations are represented by a class letter followed by a tolerance grade number. The hole or internal feature’s tolerance class is represented by a capital letter, and the shaft or external feature’s tolerance class is represented by a lowercase letter. The larger the grade number, the wider the tolerance range. On part drawings, tolerances are given using the class letter and grade number designation, the numerical tolerances themselves, or both. For example, a hole and pin are designed nominally ‘line to line’ and the designer wishes to apply a press fit to the joint. The pin is a commercial item with an m6 tolerance. The designer would then need to find out what tolerance designation to apply to the hole. This will be solved in the next few paragraphs.
ANSI B4.1 governs preferred limits and fits in inch units. The ANSI system uses descriptive two-letter designations to represent fits. Each type of ANSI fit has a series of possible grades, each represented by a number. The grade indicates the degree of tightness of the fit. ANSI fit grades are not the same as tolerance grades. A graphical representation of the ANSI standard fits is shown in Figure 3-1. ISO 286 governs preferred limits and fits in metric units. The ISO system of fits specifies preferred tolerance combinations for each fit type. Table 3-1 lists some commonly used fits, their typical use, and their graded tolerance designations from both the ANSI and ISO standards. Revisiting the example with the m6 tolerance pin, it can be seen from Table 3-1 that a locational interference fit joint can have a tolerance combination of H7/p6. Because the pin has an m6 tolerance rather than the standard p6, some calculation will be required to adjust the tolerances. That will be discussed further in the next paragraph.
Figure 3-1: Graphical Representation of ANSI Standard Fits
Tolerance grades, or limits of size, are graduated based on the size of the feature being toleranced. Larger features will have larger tolerance ranges. Standard ANSI tolerance grades and their numerical values are shown in Table 3-2. IT tolerance grades and their values are given in Table 3-3. The graded tolerance values corresponding to a given fit letter and grade number combination are normally obtained either through the use of charts or are built in to drafting (CAD) software. Some selections from these charts are provided in Tables 3-5 through 3-10. To use the tolerance charts, the designer must look up first the value corresponding to the letter designation. The tolerance values are then located on the letter designation chart at the intersection of the feature size row and number designation column.
Again revisiting the example with the m6 pin, the numerical limits of size for each tolerance designation can be found using Tables 3-5 through 3-10. First look up the limits of size for the diameter of the hole and pin, assuming that they are the standard locational interference fit tolerances of H7/ p6. If the pin diameter is 0.25 inch, the chart limits of size values for H7 in that size are +0.0006 / −0. For p6, the limits of size
