for the features bearing position tolerances. Figure 3-15 illustrates a typical position callout for holes, and Figure 3-16 shows some possible variations allowable within position tolerance zones. Compound tolerance frames are an advanced technique that may be used to control position of features within a pattern while allowing a separate tolerance to control the location of the pattern center relative to the datums. This powerful technique is illustrated in the recommended resources as well as in Chapter 4 of this book.
Figure 3-15: Position Callout and Tolerance Zones
Figure 3-16: Position Tolerance Zones and Allowable Variation of a Hole Axis
Modifiers
Tolerance modifiers include maximum material condition, minimum material condition, and regardless of feature size. Modifers may not be added to runout, concentricity, or symmetry specifications. Maximum material condition (MMC) is the condition in which a feature is at the limit of size corresponding to the maximum material left on the part. For a hole, MMC corresponds to the smallest hole within the stated limits of size. For an external diameter, MMC corresponds to the largest diameter within the stated limits of size. Least material condition (LMC) is the condition in which a feature is at the limit of size corresponding to the least material left on the part. LMC for a hole is the largest hole within the limits of size, and for an external diameter is the smallest diameter within the limits of size.
MMC and LMC are used to modify a tolerance or datum reference based on the size of the feature as produced, rather than its theoretical size. Regardless of feature size (RFS) indicates that the tolerance or datum reference applies to a nominal feature and not to the feature as produced. RFS is assumed in all cases unless otherwise stated. MMC is commonly used, whereas LMC is seldom used. When the MMC modifier is present, the tolerance is read as “tolerance when feature is at maximum material condition.” The use of MMC permits additional tolerance when the considered feature, as produced, departs from its maximum material condition. Consult the recommended resources and Section 3.3 for guidance on the proper use of modifiers.
•All dimensions on a drawing must have tolerances specified, either implied or explicit.
•Dimensions and tolerances should convey design intent and functional relationships between features and surfaces.
•Analyze all critical tolerances to ensure proper fit and function.
•Use standard symbols when applicable rather than notes. Standard symbols have clear and universal meaning, whereas notes may be misunderstood.
•Specify the loosest tolerances possible to save cost and enable a choice of manufacturing methods. This may require explicit tolerances that are looser than the drawing’s implied tolerances in some cases.
•When checking drawings, a useful technique with paper drawings is to use a highlighting marker for checked dimensions and a red pen to make changes. The title block should also be checked, including any implied tolerance information.
•Apply position tolerance to all holes, and dimension their locations with basic dimensions.
•Threaded features and tapped holes are normally dimensioned with the modifier “Regardless of Feature Size (RFS),” and the tolerance is applied to the axis of the thread derived from the pitch cylinder. Exceptions to this practice must be noted on the drawing.
•When applicable, apply the modifier “Maximum Material Condition (MMC)” to allow more deviation when parts to be fit together are not produced at the maximum material condition limits of size. This can save cost by allowing more deviation while ensuring proper fit.
Written by Charles Gillis, RE.
Every dimension on every feature on every mechanical component has variation. The allowable variation is specified by the designer through tolerances associated with each dimension. Understanding the effects of these variations on the assembly and assigning appropriate tolerances to dimensions requires performing tolerance stack-ups. Sometimes referred to as tolerance analysis or tolerance assignment, performing stack-ups bring together understanding of manufacturing processes and dimensioning standards (e.g. ASME Y14.5) to meet the assembly’s functional requirements; they are a critical element of good design practice.
The choice of tolerance is as important as any other design choice. Tolerances must not be chosen arbitrarily, but rather with good understanding of assembly requirements, manufacturing process capabilities, and cost. Good designs allow the largest tolerances possible to achieve the functional requirements. Manufacturing methods evolve and improve over time, and larger tolerances give manufacturers greater flexibility to choose methods: part routing, machine tool choice, setups, etc. Overly restrictive tolerances tie the hands of manufacturers and drive costs up. Excessive precision may meet the functional requirements, but is a very poor design choice.
Tolerance stack-up calculations are performed during the design phase to understand sources of variation within a physical assembly, for sensitivity analysis, and to verify that the design intent has been captured on dimensional specifications (detail drawings). Performing stack-up calculations allows designers to assign tolerances based on manufacturing capability, to determine assembly process capability, and to implement process control. This section presents the reasons and methods for conducting tolerance stack-up calculations, including several different mathematical approaches and the appropriateness of each approach. The purpose is to enable the reader to understand the effects of tolerance stack-ups on design choices, ultimately enabling better design choices.
•D. Madsen and D. Madsen, Geometric Dimensioning and Tolerancing, 8th Edition, Goodheart-Willcox, Tinley Park, Il, 2009
•A. Newmann and S. Newmann, GeoTol Pro - A Practical Guide to Geometric Tolerancing per ASME Y14.5-2009, Society of Manufacturing Engineers, Dearborn, MI, 2009
•ASME Y14.5 - 2009: Dimensioning and Tolerancing
DESIGN PRACTICE
Good design practice involves an iterative process:
1.Determine which component dimensions contribute to critical assembly dimensions
