Technology & Engineering
Tolerance in Engineering
Tolerance in engineering refers to the permissible limit of variation in a dimension, structure, or material. It is crucial for ensuring that components fit and function properly within a system. Engineers use tolerance analysis to determine the acceptable range of variations and to design and manufacture products that meet specified requirements.
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7 Key excerpts on "Tolerance in Engineering"
eBook - ePub- Peter Childs, Peter R. N. Childs(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
A solid is defined by its surface boundaries. Designers typically specify a component's nominal dimensions such that it fulfills its requirements. In reality, components cannot be made repeatedly to nominal dimensions due to surface irregularities and intrinsic surface roughness. Some variability in dimensions must be allowed to ensure that manufacture is possible. However, the variability permitted must not be so great that the performance of the assembled parts is impaired. The allowed variability on the individual component dimensions is called the tolerance.The term “tolerance” applies not only to the acceptable range of component dimensions produced by manufacturing techniques but also to the output of machines or processes. For example, the power produced by a given type of internal combustion engine varies from one engine to another. In practice, the variability is usually found to be modeled by a frequency distribution curve, for example the normal distribution (also called the Gaussian distribution). The modeling and analysis of this kind of variable output are outlined in Section 19.3 . One of the tasks of the designer is to specify a dimension on a component and the allowable variability on this value that will yield an acceptable performance. A general introduction to the specification of component tolerances is given in Section 19.2 .19.2 Component Tolerances
Control of dimensions is necessary in order to ensure the assembly and interchangeability of components. Tolerances are specified on critical dimensions that affect clearances and interference fits. One method of specifying tolerances is to state the nominal dimension followed by the permissible variation. So a dimension could be stated as 40.000 ± 0.003 mm. This means that the dimension should be machined so that it is between 39.997 and 40.003 mm. The tolerance is called bilateral when the variation can vary on either side of the nominal dimension. For unilateral tolerance, one tolerance is zero, e.g.Most organizations have general tolerances that apply to dimensions when an explicit dimension is not specified on a drawing. For machined dimensions, a general tolerance may be ±0.5 mm. So a dimension specified as 15.0 mm may range between 14.5 and 15.5 mm. Other general tolerances can be applied to features such as angles, drilled and punched holes, castings, forgings, weld beads, and fillets.
- Sung H Park, Jiju Antony;;;(Authors)
- 2008(Publication Date)
- WSPC(Publisher)
268 Robust Design for Quality Engineering and Six Sigma problems. Most engineers tend to go straight from system design into tolerance design, probably because they do not know how to use experimental design and statistical optimization. In doing so, they miss out on the most important engineering activity. After parameter design, tolerance design is necessary in most cases. Good engineering procedure requires that a sensitivity analysis be performed on a design to determine the relative contribution to the overall variation made by each component. Then, the cost of controlling the overall variation must be determined by taking control of these individual contributors to the variation. This is why a trade-off is made between reduction in the quality variation and increase in manufacturing cost. Figure 9.1 Specification and tolerance. A technical specification consists of two parts, nominal size and tolerance, as shown in Figure 9.1. The nominal size is the standard target value for quality, and the tolerance is the difference between the nominal size and the specification limits. There are two specification limits : the upper specification limit ( USL ) and the lower specification limit ( LSL ). Suppose the specification for a part dimension is 1.870 ± 0.005 cm. Then, the nominal size, LSL and USL are 1.870, 1.865 and 1.875 cm respectively. The tolerance is ± 0.005 cm, and the specification tolerance is 0.01 cm. Very often the specification tolerance is simply referred to as the “ tolerance ” . Tolerance Design 269 9.2 DETERMINATION OF TOLERANCES For a manufacturer of components, parts or materials, the specification limits are usually set by the firm buying the products. For a manufacturer of end products, the product specifications are often set by the customer or the planning department of the manufacturer. There are two specific sets of characteristics which determine the quality of a given product.
- Alex Krulikowski(Author)
- 2012(Publication Date)
- Cengage Learning EMEA(Publisher)
Dimensions and tolerances play a major role in the function, production, and inspection costs of a part. This chapter will familiarize you with some of the fundamental concepts related to dimensions and tolerances. Since most engineering drawings today are used internationally, all of the drawings in this book use metric units. New Terms Goal Dimensions,Tolerances, and Notes Used on Drawings 2 • Tolerance • Unequal bilateral tolerance • Unilateral tolerance • General note • Limit dimensioning • Local note • Nominal size • Plus and minus tolerancing Understand the types of dimensions, tolerances, and notes Performance Objectives Upon completing this chapter, you should be able to: Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. The Purpose of Dimensions and Tolerances Dimensions and tolerances are the elements that create clarity and precision on an engineering drawing. Without them, a drawing is no more than an “artist’s depiction” of a part. Three common purposes of dimensions and tolerances are shown in Figure 2-1 and described below: Communicate and document functional relationships Influence manufacturing choices Influence inspection choices Functional Relationships Dimensions and tolerances communicate/document func-tional relationships by providing a toolset of symbols and rules that can be used to convert product requirements into drawing specifications. The relationships between part sur-faces are defined based on how they function in the product. The amount of tolerance represents the amount of variation that will not harm the functional requirements.
eBook - PDF- P.R.N. Childs, T.H.C. Childs(Authors)
- 2003(Publication Date)
- Butterworth-Heinemann(Publisher)
d 299 15 ENGINEERING TOLERANCING The aims of this chapter are to introduce the concepts of component and process variability and the allocation of component tolerances. The ability to select suitable tolerances for the assem-bly of components should be developed, along with an understanding of geometric tolerancing and the capability to determine the quantitative variability of a process or assembly. LEARNING OBJECTIVES At the end of this section you should: • be able to select a tolerance band for differ-ent practicable applications; • be able to determine an interference fit torque capacity; • be able to recognize a geometric tolerance symbol; • be able to determine the sure fit variability of a process; • be able to determine statistical variability of a process. 15.1 Introduction A solid is defined by its surface boundaries. Designers typically specify a component’s nom-inal dimensions such that it fulfils its requirements. In reality,components cannot be made repeatedly to nominal dimensions due to surface irregular-ities and the intrinsic surface roughness. Some variability in dimensions must be allowed to ensure manufacture is possible. However, the variability permitted must not be so great that the perform-ance of the assembled parts is impaired.The allowed variability on the individual component dimen-sions is called the ‘tolerance’. The term tolerance applies not only to the acceptable range of component dimensions pro-duced by manufacturing techniques, but also to the output of machines or processes. For example, the power produced by a given type of internal combustion engine varies from one engine to another. In practice the variability is usually found to be modelled by a frequency distribution curve, for example the normal distribution (also called the Gaussian distribution).The modelling and analysis of this kind of variable output is outlined in Section 15.3.
eBook - PDFDesign of Electromechanical Products
A Systems Approach
- Ali Jamnia(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
Section VI Best Practices and Guidelines 327 16 Tolerance Specification and Analysis Introduction I mentioned various techniques for tolerance analysis in Chapter 7 with an indication that more details would be found here in this chapter. In the early days of modern engineering, there were two groups of people responsible for developing the detailed engineering drawings—the so-called blue prints. The first group was the engineers responsible for the design of the product from the concept down to each component. Once the design was on paper, it was handed over to the second group often referred to as drafts- men. This second group was responsible for finishing the design by pro- viding specifications for the required hardware (such as screws, nuts, bolts, etc.), specifying tolerances on nominal dimensions provided by engineers, and creating the physical 2D drawings. It was not uncommon that an engi- neer-in-training spend time working as a draftsman and learning from the old-timers. This did not happen in the early days of my career. Even though my engi- neering schooling included the old-fashion training in engineering graphics, by the time I began my work as an engineer computer-aided design (CAD) softwares were used commonly and shortly after, just about all engineers I knew were experts in using 3D CAD software to develop concepts. And, by touch of a button, 2D drawings could be created from the 3D files. The task of placing nominal dimensions on the prints was simplified tremendously. Tolerances were identified based on the design engineer’s (or his supervi- sor’s) knowledge—typically, focused on the interaction of neighboring com- ponents. Gross tolerance issues or problems were often identified in the first and second builds and corrected. However, no one would predict manu- facturing yield failures due to tolerance mismatch. In a way, relatively low manufacturing yields (and rework) were accepted as a fact of life.
eBook - PDFComputer-Aided Design, Engineering, and Manufacturing
Systems Techniques and Applications, Volume VI, Manufacturing Systems Processes
- Cornelius T. Leondes(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
Specifically, these tolerance bounds represent the economical precision that can be held by a specific machine, and the engineers should design products with these tolerances in mind. Process tolerances are usually designed based on intuition and best guess of the engineers, thus being vulnerable to overestimation or underestima-tion. Loosely estimated tolerances may cause problems in the succeeding operations, while unneces-sarily tight tolerances, on the other hand, escalate the manufacturing cost. One of the techniques used to widen the overestimated tolerances is called tolerance chart balancing, which can be performed either manually [11, 12, 13] or mathematically [2, 4, 14, 15, 16, 17, 18, 19, 22]. Among several other techniques, Nagarwala et al. [2] have proposed a slope-based method for solving the tolerance-process selection problem, wherein the cost function is approximated by a sequence of linear functions. Ngoi and Seng [4] designed tolerances by concurrently minimizing the nonlinear manufacturing cost and maximizing the working dimensions. Wei and Lee [10] have included the process capability index of the machinery in their tolerance design problem; therefore, the scrap rate of each operation and the entire process can be predicted with respect to the tolerances assigned. Ngoi [14] has proposed a linear programming model, coupled with identification schemes of tolerance chain, to determine the total additional tolerances to be added to the existing process tolerances. A weighting system is involved to evaluate the relative importance of operations. Unfortunately, this model does not consider the process capability of the equipment, thereby rendering the allocated tolerance to be uneconomic. Lee et al. [18] have used nonlinear design functions to determine the coefficients of the nonlinear cost equation.
eBook - PDF- Connie Dotson(Author)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
MEASUREMENT AND TOLERANCES CHAPTER THREE Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 3 Measurement and Tolerances 41 feature) could invalidate the effort. In contrast, exces-sive precision could price the research out of consider-ation. The second case that is even more common is in the production of goods. Too little precision produces bad products. Too much precision causes the goods to be priced out of the market. Either of these situations is made worse by the number of individuals involved and particularly by their separation, often at distant fa-cilities. An unambiguous expression of “good enough” is clearly needed. This expression is called tolerance. Tolerance is the total amount that a specific dimension is permitted to vary. In dimensional metrol-ogy, tolerances are applied to both position (where) and size (how big) dimensions. Both types of dimen-sions must have tolerances for economical manufacture (see Figure 3–1). 3–2 GEOMETRIC DIMENSIONING AND TOLERANCING The authoritative document governing the use of geo-metric dimensioning and tolerancing (GD&T) in the United States is ASME Y14.5M, “Dimensioning and Tolerancing.” Outside the United States, a series of standards under the general heading of ISO (Inter-national Standards Organization) standards governs geometric dimensioning and tolerancing. There are many standards that address specific requirements such as screw threads, drills, gears, and so forth. The United States is a member of the ISO and has con-tributed greatly to the documents.
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