Modern Drafting Practices and Standards Manual

Chapter 6 - Dimensioning and Tolerancing

Section 6.0
Dimensioning and Tolerancing Introduction

Current dimensioning and tolerancing methods defined by the American Society of Mechanical Engineers (ASME) and approved by the American National Standards Institute (ANSI) can be utilized to reduce product cost. One reason cost can be reduced is that proper utilization of these methods will increase the clarity of the drawing requirements, which reduces ambiguity. It is much easier to achieve first-time quality when requirements are clearly stated. Another reason cost can be reduced is that all the functionally acceptable variation can be specified on the drawing. Permitting the maximum amount of variation has many manufacturing benefits that reduce part cost.

Section 6.1
Dimensioning and Tolerancing Symbols

This section is based on ASME Y14.5M-1994. Symbols unique to ANSI Y14.5M-1982 are also described for those who are contractually obligated to comply with the older standard.

Section 6.2
Dimensioning and Tolerancing General Dimension Application

This section is based on ASME Y14.5M-1994 and related standards. Properly applied dimensions define the size and location requirements of all features on a drawing. Compliance with dimensioning fundamentals make the requirements easy to read and result in a single possible interpretation of requirements.

Section 6.3
Dimensioning and Tolerancing General Tolerances

This section is based on ASME Y14.5M-1994.

Achievement of exact dimensional control is not possible due to fabrication processes being imperfect. To ensure that a design can be fabricated, it is necessary to permit variation on every dimension specification. The allowable amount of fabrication error (tolerance) specified on a drawing is normally calculated on the basis of functional requirements. The achievable level of manufacturing accuracy is determined by factors such as process capability. Product cost can be directly impacted by the specified allowable tolerances, and care should be taken to avoid specifying tolerances that are unnecessarily small.

Section 6.4
Dimensioning and Tolerancing Datums

This section is primarily based on the ASME Y14.5M-1994 standard. Figures have also been provided to illustrate the correct method for the application of symbols from the previous ANSI Y14.5-1982 standard. Symbols from the 1994 and 1982 standards should not be used on the same drawing.

Section 6.5
Position, Concentricity, and Symmetry

This section is based on the ASME Y14.5M-1994 standard. Any references to the ‘current standard’ or the ‘U.S. standard’ in this section is a reference to the ASME Y14.5M-1994 standard. Any explanations of past practices are clearly identified as past practices and a reference to the applicable standard indicated.

Section 6.5.1
Verifying Dimensional Requirements

Verification of dimension requirements can be accommodated through the use of coordinate measurement machines (CMM), functional gages, and open inspection setups. This section explains how dimensional requirements may be verified using functional gages and open setup methods.

Concepts included in this section are shown for verification of specific dimensional requirements and are written on the basis of ASME Y14.5M-1994. The gages and setups are based on the information given in the drawing of the part used in the example. Application of additional dimensional requirements on the parts would create additional acceptance criteria for the part and may alter the gage design or inspection setup. Care must be taken when extending the information in this section to parts that have different requirements specified. Particular attention should be given to datum simulation methods. Review Section 6.4 for detailed information regarding datums and datum simulation.

Functional gages shown in this section include dimensions that reflect the value that is being verified on the workpiece. Tool tolerances are not shown since each company imposes its own guidelines on tool tolerances.

There are multiple means of verifying most dimensional requirements. The means of verification shown in each figure of this section is intended to clarify the dimensional requirement created by tolerance specifications. It is not intended to indicate a single method that should be used to verify each tolerance type.

Accuracy of verification results is dependent on the accuracy of gage fabrication and inspection setup. Datum simulation methods impact the accuracy of the coordinate system relative to which measurements are made. It is important to realize that the tools, methods, and accuracy of the process are all factors in determining the accuracy of dimensional requirements verification.

Section 6.5.2
Dimensioning and Tolerancing (Advantages of Positional Tolerancing)

The purpose of this section is to explain the advantages of using positional tolerancing as described in Section K6.5.

Section 6.5.3
Dimensioning and Tolerancing (Formulas for Positional Tolerancing)

The purpose of this section is to present formulas for determining required positional tolerances or the required sizes of mating features to ensure that parts will assemble. The formulas are valid for all types of features or patterns of features and will give a “no interference, no clearance” fit when features are at maximum material condition with their locations in the extreme of positional tolerance.

Section 6.5.4
Dimensioning and Tolerancing Certification as a GD&T Professional

Certification of Geometric Dimensioning and Tolerancing Professionals, ASME Y14.5.2-1996 , is an American National Standard which establishes the body of knowledge required for certification as a Geometric Dimensioning and Tolerancing Professional (GDTP). The Y14.5.2-1996 document provides the basis for ascertaining an individual’s ability to read and interpret drawings prepared using the language of geometric dimensioning and tolerancing as defined in ASME Y14.5M- 1994, Dimensioning and Tolerancing. Certification is voluntary and establishes the qualifications for two levels of certification.

Section 6.6
Form, Orientation, Profile, and Runout

This section is based on the ASME Y14.5M-1994 standard on dimensioning and tolerancing.

Section 6.7
Form, Dimensioning and Tolerancing Decimal Inch System

This section covers the rules, requirements and considerations for use of the decimal inch system. Decimal inches have worked their way into the manufacturing industry from the fractional inch system, which was used since the start of the industrial revolution. The primary reason for the change over to the decimal inch was the move to manufacturing design and tolerances that the factional inch could not reasonably provide. Additionally, the decimal inch system is much better suited to industry automation, computer-aided design/drafting (CAD), computer-aided manufacturing (CAM), and numerical control equipment, including coordinated measuring systems. For example, in numerical positioning control of automatic machine systems, linear motions are expressed in inches and decimal parts of an inch.

Section 6.8
Dimensioning & Tolerancing (Dual Dimensioning)

With the rapidly growing use of metrication worldwide and the anticipated effect on drawings, it becomes necessary to establish certain practices relating to Inch/Metric Dimensioning - better known as Dual Dimensioning. The decision to use dual dimensioning practices should be premised principally on business considerations. For example, if a product appears to be destined for manufacture overseas as well as domestically, it may be advantageous to prepare the related drawings, both new and old, in the dual dimensioning system. This section establishes a uniform method of combining U.S. customary (inch) units of measure and those of the International System of Units (SI) on the same drawing. See Section K6.9, Inch/Millimeter Conversion.

Section 6.9
Dimensioning & Tolerancing (Inch/Millimeter Conversion)

The conversion of inches to millimeters is critical. Specific rules must be observed to ensure interchangeability of parts which are dual dimensioned and produced in both of these units. This section provides the ground rules, practices and other information to be applied in order to ensure the required interchangeability.

Section 6.10
Dimensioning & Tolerancing Dimensioning for Numerical Control

To facilitate information input into control media such as tapes used to control manufacturing machine movements and tool sequencing, it is essential that certain factors be observed. Some of these are already governed by basic dimensioning practices described elsewhere in this manual. Other factors must be identified, established and integrated with standard practices. Accordingly, these guidelines have been developed with the objective of:

a. Optimizing use of existing basic dimensioning practices

b. Integrating any special dimensioning requirements of numerical control machine planning and operation without compromising design requirements

c. Assuring that the drawing may be used for conventional manufacture as well as with numerical control equipment.

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