Technical Drawing for Product Design

The technical product documentation that is currently drawn up in many companies is unfortunately still ambiguous and contains many errors, such as erroneous datums, imprecise or missing distances, as well as incongruent and difficult to check tolerances. For about 150 years, a tolerancing approach called “coordinate tolerancing” was the predominant tolerancing system used on engineering drawings. This methodology in fact no longer results to be the most suitable for the requirements of the modern global productive realty, in which companies, for both strategic and market reasons, frequently resort to suppliers and producers located in different countries, and it is therefore necessary to make use of communication means in which the transfer of information is both univocal and rigorous. GD&T or GPS is a symbolic language that is used to specify the limits of imperfection that can be tolerated in order to guarantee a correct assembly, as well as the univocal and repeatable functionality and control of the parts that have to be produced.

The main concepts proposed in this chapter have the aim of expressing the fundamental rules on which the geometrical specification of workpieces can be based through a global approach that includes all the geometrical tools needed for GPS. Indeed, in an increased globalisation market environment, the exchange of technical product information and the need to unambiguously express the geometry of mechanical workpieces, are of great importance. The symbols, terms, and rules of the GPS language that are given in the ISO 17450, ISO 1101 and ISO 14660 standards are presented in this chapter through tools and concepts that allow an engineer to perfectly specify the imperfect geometry of a component, and to understand the impact of the drawing specifications on inspection.

The advantages of the geometric specification of a product are here illustrated by converting a traditional 2D coordinate tolerancing drawing using the geometrical tolerance method. The new language of symbols permits the functional requirements of the products to be fully expressed in technical documentation. Moreover the dimensioning of a workpiece according to the geometrical tolerance method can reduce the ambiguity in the indications and in the interpretation of the dimensional and geometrical requirements of the products in order to achieve not only unambiguous communication between the design, production and quality control entities, but also with the clients and suppliers of the outsourced processes.

This chapter is focused on the main differences between the ISO and ASME standards in the geometrical specification domain of the industrial products, and it starts with details on the historical evolution of the two standards. The main principles of the ISO GPS and ASME GD&T standards, such as the principle of independence and the envelope requirement, are illustrated. Designers are recommended to always indicate the reference standard in the technical drawings of companies, as the interpretation of drawing specifications and the relative inspection may lead to two different results. Finally, the main novelties of the new ASME Y14.5:2018 standard and the new ISO 22081 standard on general tolerances are shown.

In ISO GPS terminology, the maximum material requirement, (MMR) and least material requirement (LMR) represent two of the fundamental rules on which geometrical dimensioning with tolerances is based, and which are the subject of the ISO 2692 standard. The designer, when establishing a maximum or least material requirement, defines a geometrical feature of the same type and of perfect form, which limits the real feature on the outside of or inside the material. MMR is used to control the assemblability of a workpiece while LMR is used to control a minimum distance or a minimum wall thickness. This chapter also introduces the Reciprocity requirement (RPR) and the “zero tolerance” concepts and offers practical examples to guide a designer in his/her choice of the correct requirement from the geometrical tolerance specifications.

A datum system has the purpose of defining a set of two or more ideal features established in a specific order (for example, a system made up of a triad of mutually orthogonal planes) that allows not only the tolerance zones to be orientated and located, but also their origin to be defined for the measurement, and the workpiece to be blocked during the control. When it is not desirable to use a complete integral feature to establish a datum feature, it is possible to indicate portions of the single feature (areas, lines or points) and their dimensions and locations using datum targets. This chapter illustrates the main differences between the ISO 5459 and ASME standards for the specification of datums and highlight some theoretical and mathematical concepts. This section also provides simple rules to follow whenever choosing functional datums to ensure the part will function as intended, with the least possible amount of variation.

It is possible to control four form tolerance types: straightness, flatness, roundness and cylindricity. The chapter covers all the concepts necessary to define specification operators (according to ISO 17450-2) and some procedures to establish the reference elements in order to define the deviation errors. Some terms related to form parameters are described such as peak-to-valley, peak-to-reference and reference-to-valley deviations. The ASME standards use the envelope requirements or Rule #1, according to which form tolerances are contained within the dimensional ones, and these tolerances are therefore only used with the purpose of limiting the error when the workpiece is produced with dimensions close to the least material condition.

Orientation tolerances (parallelism, perpendicularity and angularity) are used to control the orientation of a feature (surface or feature of size) with respect to one or more datums. When the orientation tolerances are applied to a feature of size and an MMR or LMR is added, the control of the orientation deviation no longer refers to the median line, but to the entire extracted feature (MMVC boundary), and it should not violate the MMVC virtual condition. The ISO and ASME standards use two different approaches to control the orientation of a feature of size: in order to orient a feature of size, the ISO standards define the concept of extracted median line or median surface. Instead, in the ASME standards, the axis or median plane is used to control the orientation of a feature of size.

This chapter describes how to use location tolerances to specify an allowable location error. The location tolerances that should be used, in function of the feature of the workpiece that has to be located, are in particular presented. The effects of the material condition requirement and the correct choice of the modifiers for position tolerances (axis and surface interpretation) are illustrated. Particular emphasis is given to the location of the patterns, as introduced with the new rules of the ISO 5458:2018 standard (multiple indicator pattern specification and simultaneous requirement). When geometrical position tolerances are applied, the value of the tolerance is calculated from the mating conditions (fixed and floating fastener conditions). The ASME standards specify, without any shadow of doubt, that the position tolerance symbol should only be utilised for a “feature of size”, while the ISO standards allow it to be used to position a planar surface. The ISO standards are defined as “CMM Friendly”, that is, the preferred control system is the coordinate measurement machine, while the ASME standards are based on functional gauges that represent a physical representation of the tolerance zone. A special functional application of location tolerances is to control concentricity and symmetry. These controls have been removed from the new ASME standard.

A geometrical tolerance on a profile is one of the most versatile and powerful instruments that can be used for functional dimensioning, and is the tolerance most frequently used by designers. A profile may in fact be used to control the size, form, orientation and location of a feature. Because of the flexibility in the level of control that can be achieved with the profile tolerance, this control may be used to substitute the classical coordinate dimensioning method. The present chapter covers the ISO rules on profile tolerances in order to appropriately specify the control of a profile with a combination of the SZ, CZ, UF symbols and the bidirectional and unidirectional zones that are specified with profile tolerances. A composite profile tolerance is used in the ASME standards when the design applications require stricter tolerances on the form than is needed for the orientation or location of the same feature. The new ASME symbols, that is, From-To and dynamic profile, are presented.

A run-out error is the surface variation that occurs relative to a rotation axis. There are two types of run-out controls: circular run-out (2D) and total run-out (3D). Run-out tolerances are composite controls that define the requirements for the permissible coaxiality, orientation and form deviations of a surface element in relation to a datum axis. The application of the run-out control to an assembly and its combination with the tangent plane symbol, are illustrated in this chapter as main novelties of the new ASME Y14.5-2018 standard.

In the absence of other indications, all the specified dimensions and tolerances are applied and controlled without the action of any force, except gravity, but components such as thin metal and flexible parts, rubber gaskets, and flexible parts in general, can deform or bend, even as just a result of their weight. These components should be inspected under a restrained condition in order to simulate their shape in the installed condition. The restraint or force on the non-rigid parts is usually applied in such a manner as to resemble or approximate the functional or mating requirements. According to the ISO standards, the non-rigid parts should be identified on a drawing by means of “ISO 10579-NR” (Non Rigid) obligatory indications within or near the title block; such a note indicates that the part is not considered rigid and it indicates the clamping forces or other requirements necessary to simulate the assembly conditions. In order to invoke a restrained condition in the ASME standards, a general note, or a local note, should be specified or referenced on the drawing to define the restraint requirements. When a general note invokes a restrained condition, all the dimensions and tolerances apply in the restrained condition, unless they are overridden by a “free state” Ⓕ symbol

In the past, the verification of the local size of workpieces was carried out using traditional metrological instruments, such as callipers and micrometres, or using hard gauges to check for conformance with the global size tolerances. Today, in order to remove any ambiguities that existed in the traditional size tolerance specifications, it is necessary to both exploit the benefits of CMMs and other coordinate measuring systems that collect several points from the surface of a feature and, at the same time, to expand the concept of size tolerancing in order to communicate the function of a part more clearly. The specification operators defined in the ISO 14405 standards provide mechanisms that can be used to expand the domain of size specifications by means of a rich, new set of size modifiers, which provide new capabilities that can be used to address the requirements that arise from many industrial applications.