CH 2: MANUFACTURING ASPECTSpecification• “Details and specific descriptions of the requirement of making or fabricating a product. Can be a written document describing in detail the scope of work, type of materials to be used, special construction techniques, dimensions, colors, or a list of the qualities and characteristics of a product”• Example of “specification” in a manufacturing drawing.
Standardisation• Standardization (or standardisation) is the process of agreeing on standards, which are (usually voluntary, written) agreements on technical specifications that define parameters and properties of products (goods and services).• In the context of technologies and industries, standardization is the process of establishing a technical specification, called a standard (eg: size, quality, dimension, testing, assembling, etc) among• The goals of standardisation – standardization can be to help with or ensure independence of single suppliers – Help to make easy in fabrication, compatibility, safety, repeatability, quality, interoperability of products, components or services in global.• Standardisation bodies eg: ISO, BS, ANSI, SIRIM etc
Tolerance• In order to ensure that assemblies function properly their component parts must FIT TOGETHER in a particular way.• This is defined as the permissible or acceptable variation in the dimensions (height, width, depth, diameter, and angles) of a part.• The root of the word “tolerance” is the Latin tolerare, meaning “to endure” or “put up with.”• No component can be manufactured to an exact size (called the nominal or basic size), so the designer has to decide on appropriate upper and lower limits for each dimension Refer Fig 2.2)• Accurately toleranced dimensioned features usually take much more time to manufacture correctly and therefore can increase production costs significantly.
ToleranceImportance of tolerance control• Dimensional tolerances become important only when a part is to be assembled or mated with another part.• Surfaces that are free and not functional do not need close tolerance control.• Fig below shows the basic size, deviation, and tolerance on a shaft, according to the ISO system.• Fig below shows the various methods of assigning tolerances on a shaft: (a) bilateral tolerance, (b) unilateral tolerance, and (c) limit dimensions.
Limits and fits• Limits and fits are essential in specifying dimensions for holes and shafts.• When parts are assembled together, engineers have to decide how they will fit together and the economics associated with it.• How they will fit together? – Clearance fit – Transition fit – Interference fit• Economics? – Interchangability
Limits and fits• There are two standards on limits and fits, as described by the American National Standards Institute (see ANSI B4.1, B4.2, and B4.3). One standard is based on the traditional inch unit.• The other is based on the metric unit and has been developed in greater detail. In these standards, capital letters always refer to the hole and lowercase letters to the shaft.
Limits and fits - definition• Tolerance is the difference between the maximum limit of size and the minimum limit of size.• Fit expresses the relationship between a mating parts with respect to the amount of clearance or interference which exists when they are assembled together.• Hole - designate all INTERNAL features of a part, including parts which are not cylindrical.• Shaft - designate all EXTFRNAL features of a part, including parts which are not cylindrical.
• Upper deviation - difference between the maximum limit of size and the corresponding basic size. This is designated ‘ES for a hole and es for a shaft.• Lower deviation- difference between the minimum limit of size and the corresponding basic size. This is designated ‘EI for a hole and ei for a shaft.• Grade of Tolerance - Group of tolerances with the same level of accuracy for all basic sizes.• Clearance - difference between the size of the hole and shaft (positive)• Clearance - difference between the size of the hole and shaft (negative)
Product Quality• Product quality always has been one of the most important aspects of manufacturing operations.• In view of a global competitive market, continuous improvement in quality is now a major priority, particularly for large corporations in industrialized countries.• In Japan, the single term kaizen is used to signify never-ending improvement.
Product Quality• Quality may be defined as a product’s fitness for use.• Several dimensions of quality generally are identified; these include characteristics such as performance, durability, reliability, robustness, and serviceability, as well as aesthetics and perceived quality.
Product Quality• Contrary to general public perception, high quality products do not necessarily cost more, especially when considering the fact that poor-quality products: – Present difficulties in assembling and maintaining components. – Result in the need for in-field repairs. – Have the significant built-in cost of customer dissatisfaction.
Quality Assurance• Quality assurance is the total effort made by a manufacturer to ensure that its products conform to a detailed set of specifications and standards.• It can be defined as all actions necessary to ensure that quality requirements will be satisfied; whereas, quality control is the set of operational techniques used to fulfill quality requirements.• An important aspect of quality assurance is the capability to (a) analyze defects as they occur on the production line and (b) promptly eliminate them or reduce them to acceptable levels.