Geometric dimensioning and tolerancing

Geometric dimensioning and tolerancing
Title	Geometric Dimensioning and Tolerancing
Organization	American Society of Mechanical Engineers, International Organization for Standardization
Related standards	ASME Y14.5, ISO 1101
Year	1966 (first ASME standard)
Status	Current

Geometric dimensioning and tolerancing is a symbolic language used on engineering drawings and computer-aided design models to precisely define the geometry of mechanical parts. It is a system for specifying and communicating engineering tolerances, controlling the form, orientation, and location of features on a part. The system is governed by standards published by organizations like the American Society of Mechanical Engineers and the International Organization for Standardization, ensuring a universal understanding in manufacturing and quality control.

Overview

Geometric dimensioning and tolerancing provides a comprehensive framework for defining the nominal geometry of parts and the permissible variation, or tolerance, from that geometry. It moves beyond traditional plus-minus tolerancing on dimensions to control the shape, profile, orientation, and runout of features. This system is critical in industries such as aerospace engineering, automotive industry, and medical device manufacturing, where precise fit and function are paramount. By using a standardized symbolic language, it reduces ambiguity in interpretation between design, manufacturing, and inspection teams at companies like Boeing and General Motors.

Fundamental concepts

The core principle is the concept of the datum, which is a theoretically exact point, axis, line, or plane derived from a datum feature on a part. A key rule is the envelope principle, which states that a feature of size must not violate its maximum material condition boundary. Other fundamental concepts include feature of size, virtual condition, and the bonus tolerance, which allows for additional geometric tolerance as a feature departs from its maximum material condition. These principles are foundational to standards like ASME Y14.5 and are taught in engineering programs globally.

Symbols and notation

The system uses a standardized set of symbols contained within a feature control frame, which is a rectangular box placed on the drawing. This frame contains a geometric characteristic symbol, such as for straightness, flatness, or position tolerance, followed by the tolerance value and applicable datum references. Modifier symbols, like the maximum material condition (M) or least material condition (L) symbols, are also used to further refine the tolerance application. This notation provides a compact and unambiguous instruction for machinists and coordinate-measuring machine programmers.

Datums and datum systems

Datums establish the coordinate system from which geometric tolerances are measured. A part is aligned to datum features (physical surfaces) which simulate the theoretical datum planes. A typical system involves a primary datum (A), a secondary datum (B), and a tertiary datum (C), which constrain the six degrees of freedom of the part. Proper selection of datums, often based on functional interfaces or assembly points, is critical for ensuring parts will assemble and function correctly in products like the F-35 Lightning II or automotive transmissions.

Tolerancing of form, orientation, and location

Form tolerances, such as cylindricity and circularity, control the shape of individual features without reference to datums. Orientation tolerances, like angularity and perpendicularity, control the tilt of features relative to datums. Location tolerances, most notably position tolerance, control the location of features of size, like holes, relative to a datum reference frame. Profile tolerances, controlling profile of a surface or profile of a line, are versatile controls used for complex contours, common in components for the Space Shuttle or consumer electronics.

Applications and benefits

The primary application is ensuring functional interchangeability of parts in mass production, a concept vital since the era of Eli Whitney and the American System of manufacturing. It allows designers to specify only the necessary constraints, often resulting in larger, more producible tolerances compared to traditional methods. This reduces manufacturing costs and scrap rates. It is essential in supply chain management, allowing parts from different suppliers, such as those for the Toyota Production System, to assemble without issue, and is integral to modern statistical process control.

Standards and history

The modern framework was largely developed during World War II, driven by the need for precision in military equipment. The United States Army and industry collaborated, leading to the first formal standard, published by the American Society of Mechanical Engineers as ASME Y14.5 in 1966. Major revisions occurred in 1982, 1994, 2009, and 2018. Internationally, the International Organization for Standardization publishes complementary standards like ISO 1101. The Society of Automotive Engineers and the American National Standards Institute have also played significant roles in its development and promulgation.

Category:Engineering disciplines Category:Technical communication Category:Industrial design