Machine Designers Reference. J. Marrs
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).
•L. Foster, Geo-Metrics III, Addison-Wesley Publishing Company, New York, NY, 1994
•Oberg, Jones, Horton, Ryffel, Machinery’s Handbook, 28th Ed., Industrial Press, New York, NY, 2008
•ANSI Y14.5M: “Dimensioning and Tolerancing”
•ANSI B4.1: “Preferred Limits and Fits for Cylindrical Parts”
IMPLIED TOLERANCES
To simplify drawings, most drawing title blocks contain a list of implied tolerance values. These tolerance values are applied to all dimensions unless otherwise specified. Tables 3-11 and 3-12 give some commonly used implied tolerances, but any tolerances may be applied to a drawing as implied tolerances. Wider tolerances generally reduce manufacturing cost, so generous implied tolerances are preferred if possible.
Table 3-11: Common Implied Tolerances (Inch)
Table 3-12: Common Implied Tolerances (Metric)
GEOMETRIC DIMENSIONING AND TOLERANCING (GD&T)
The use of Geometric Dimensioning and Tolerancing (GD&T) is the subject of many excellent books. GD&T allows the user to specify tolerances and relationships based on physical features and can yield drawings that are truer to the design intent that traditional Cartesian tolerancing. This section will serve as a basic introduction to the subject. Refer to the recommended resources for a full treatment of the proper use of GD&T and its symbols.
Datums
GD&T uses datums and a system of symbols to communicate the relationships between and tolerances of part features and surfaces. Datums are theoretically exact points, axes, or planes that are used as references to define the location and orientation of features on a part. Their typical appearance is illustrated in Figure 3-2. Although datums are usually associated with physical features, they are theoretical and have no tolerance or deviation from ideal even if the actual feature deviates. Datums should be selected to represent the function and mating relationship of a part. When selecting datums, it is helpful to use the following procedure:
1.Select datum A to be the primary constraining surface to contact the mating part when this part is placed into an assembly. This is the primary functional datum. For convenience, this surface is often a flat plane. Often this will be the bottom surface of a part, or a side surface if the part attaches on its side.
2.Select datum B to be the second constraining feature to locate the part to another part when the part is placed into an assembly. This datum is often a dowel hole or other locating feature. This is the secondary functional datum.
3.Select datum C to be the third constraining feature (if there is one) to contact the mating part when this part is placed into an assembly. This is the tertiary functional datum. This