DIMENSIONS, TOLERANCES, AND SURFACES

DIMENSIONS, TOLERANCES, AND SURFACES • Dimensions, Tolerances, and Related Attributes • Conventional Measuring Instruments and Gages • Surfaces • Measurement of Surfaces • Effect of Manufacturing Processes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dimensions and Tolerances • Factors that determine the performance of a manufactured product, other than mechanical and physical properties, include : • Dimensions - linear or angular sizes of a component specified on the part drawing • Tolerances - allowable variations from the specified part dimensions that are permitted in manufacturing ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dimensions (ANSI Y14.5M‑1982) A dimension is "a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature" • The dimension indicates the part size desired by the designer, if the part could be made with no errors or variations in the fabrication process ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Tolerances (ANSI Y14.5M‑1982): A tolerance is "the total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits" • Variations occur in any manufacturing process, which are manifested as variations in part size • Tolerances are used to define the limits of the allowed variation ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Variation is permitted in both positive and negative directions from the nominal dimension Possible for a bilateral tolerance to be unbalanced Ex: 2.500 +0.010, -0.005 Bilateral Tolerance ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Variation from the specified dimension is permitted in only one direction Either positive or negative, but not both Unilateral Tolerance ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Permissible variation in a part feature size consists of the maximum and minimum dimensions allowed Limit Dimensions ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Accuracy and Precision Accuracy - the degree to which a measured value agrees with the true value of the quantity of interest • A measurement procedure is accurate when it avoids systematic errors (positive or negative deviations that are consistent from one measurement to the next) Precision - the degree of repeatability in the measurement process • Good precision means that random errors in the measurement procedure are minimized ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Conventional Measuring Instruments and Gages • Precision gage blocks • Measuring instruments for linear dimensions • Comparative instruments • Fixed gages • Angular measurements ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Precision Gage Blocks Standards against which other dimensional measuring instruments and gages are compared • Usually square or rectangular blocks • Surfaces are finished to be dimensionally accurate and parallel to  several millionths of an inch and are polished to a mirror finish • Precision gage blocks are available in certain standard sizes or in sets, the latter containing a variety of different sized blocks ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Measurement of Linear Dimensions • Measuring instruments are divided into two types: • Graduated measuring devices include a set of markings on a linear or angular scale to which the object's feature of interest can be compared for measurement • Nongraduated measuring devices have no scale and are used to compare dimensions or to transfer a dimension for measurement by a graduated device ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Micrometer • External micrometer, standard one‑inch size with digital readout (photo courtesy of L. S. Starret Co.) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Mechanical Gages: Dial Indicators • Mechanical gages are designed to mechanically magnify the deviation to permit observation • Most common instrument in this category is the dial indicator, which converts and amplifies the linear movement of a contact pointer into rotation of a dial • The dial is graduated in small units such as 0.01 mm or 0.001 inch • Applications: measuring straightness, flatness, parallelism, squareness, roundness, and runout ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dial Indicator • Front view shows dial and graduated face; back view shows cover plate removed (photo courtesy of Federal Products Co.) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dial Indicator Setup to Measure Runout • As part is rotated about its center, variations in outside surface relative to center are indicated on the dial ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Electronic Gages Family of measuring and gaging instruments based on transducers capable of converting a linear displacement into an electrical signal • Electrical signal is amplified and transformed into suitable data format such as a digital readout • Applications of electronic gages have grown rapidly in recent years, driven by advances in microprocessor technology, and are gradually replacing many of the conventional devices ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

GO/NO‑GO gages So-named because one gage limit allows the part to be inserted while the other limit does not • GO limit - used to check the dimension at its maximum material condition • Minimum size for internal feature such as a hole • Maximum size for external feature such as an outside diameter • NO‑GO limit - used to inspect the minimum material condition of the dimension in question ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surfaces Nominal surface – designer’s intended surface contour of part, defined by lines in the engineering drawing • Nominal surfaces appear as absolutely straight lines, ideal circles, round holes, and other edges and surfaces that are geometrically perfect • Actual surfaces of a part are determined by the manufacturing processes used to make them • Variety of processes result in wide variations in surface characteristics ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Why Surfaces are Important • Aesthetic reasons • Surfaces affect safety • Friction and wear depend on surface characteristics • Surfaces affect mechanical and physical properties • Assembly of parts is affected by their surfaces • Smooth surfaces make better electrical contacts ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Technology • Concerned with: • Defining the characteristics of a surface • Surface texture • Surface integrity • Relationship between manufacturing processes and characteristics of resulting surface ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Texture The topography and geometric features of the surface • When highly magnified, the surface is anything but straight and smooth • It has roughness, waviness, and flaws • It also possesses a pattern and/or direction resulting from the mechanical process that producedit ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Four Elements of Surface Texture • Roughness - small, finely‑spaced deviations from nominal surface • Determined by material characteristics and processes that formed the surface • Waviness - deviations of much larger spacing • Waviness deviations occur due to work deflection, vibration, tooling, and similar factors • Roughness is superimposed on waviness ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Four Elements of Surface Texture • Flaws - irregularities that occur occasionally on the surface • Includes cracks, scratches, inclusions, and similar defects in the surface • Although some flaws relate to surface texture, they also affect surface integrity ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Roughness and Surface Finish • Surface roughness - a measurable characteristic based on roughness deviations • Surface finish - a more subjective term denoting smoothness and general quality of a surface • In popular usage, surface finish is often used as a synonym for surface roughness • Both terms are within the scope of surface texture ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Roughness Equation • Arithmetic average (AA) based on absolute values of deviations, and is referred to as average roughness where Ra = average roughness; y = vertical deviation from nominal surface (absolute value); and Lm = specified distance over which the surface deviations are measured ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Alternative Surface Roughness Equation • Approximation of previous equation is perhaps easier to comprehend where Ra has same meaning as above; yi = vertical deviations (absolute value) identified by subscript i; and n = number of deviations included in Lm ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Cutoff Length • A problem with the Ra computation is that waviness may get included • To deal with this problem, a parameter called the cutoff length is used as a filter to separate waviness from roughness deviations • Cutoff length is a sampling distance along the surface • A sampling distance shorter than the waviness eliminates waviness deviations and only includes roughness deviations ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Roughness Specification • Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Integrity • Surface texture alone does not completely describe a surface • There may be metallurgical changes in the altered layer beneath the surface that can have a significant effect on a material's mechanical properties Surface integrity is the study and control of this subsurface layer and the changes in it that occur during processing which may influence the performance of the finished part or product ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Changes Caused by Processing • Surface changes are caused by the application of various forms of energy during processing • Example: Mechanical energy is the most common form in manufacturing • Processes include forging, extrusion, and machining • Although its primary function is to change geometry of workpart, mechanical energy can also cause residual stresses, work hardening, and cracks in the surface layers ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Changes Caused by Mechanical Energy • Residual stresses in subsurface layer • Example: bending of sheet metal • Cracks ‑ microscopic and macroscopic • Example: tearing of ductile metals in machining • Voids or inclusions introduced mechanically • Example: centerbursting in extrusion • Hardness variations (e.g., work hardening) • Example: strain hardening of new surface in machining ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Changes Caused by Thermal Energy • Metallurgical changes (recrystallization, grain size changes, phase changes at surface) • Redeposited or resolidified material (e.g., welding or casting) • Heat‑affected zone in welding (includes some of the metallurgical changes listed above) • Hardness changes Wikipedia.org – root weld ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Changes by Caused Chemical Energy • Intergranular attack • Chemical contamination • Absorption of certain elements such as H and Cl in metal surface • Corrosion, pitting, and etching • Dissolving of microconstituents • Alloy depletion and resulting hardness changes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surface Changes Caused by Electrical Energy • Changes in conductivity and/or magnetism • Craters resulting from short circuits during certain electrical processing techniques such as arc welding ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Measurement of Surfaces • Two parameters of interest: • Surface texture - geometry of the surface, commonly measured as surface roughness • Surface roughness • Surface integrity - deals with the material characteristics immediately beneath the surface and the changes to this subsurface that resulted from the processes that created it ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Measurement of Surface Roughness • Three methods to measure surface roughness: • Subjective comparison with standard test surfaces • Fingernail test • Stylus electronic instruments • Optical techniques ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Stylus Instruments • Similar to the fingernail test, but more scientific • In these electronic devices, a cone‑shaped diamond stylus is traversed across test surface at slow speed • As the stylus head is traversed horizontally, it also moves vertically to follow the surface deviations • The vertical movement is converted into an electronic signal that can be displayed as • Profile of the actual surface • Average roughness value ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Stylus Traversing Surface • Stylus head traverses horizontally across surface, while stylus moves vertically to follow surface profile ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Tolerances and Manufacturing Processes • Some manufacturing processes are inherently more accurate than others • Most machining processes are quite accurate, capable of tolerances = 0.05 mm ( 0.002 in.) or better • Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Surfaces and Manufacturing Processes • Some processes are inherently capable of producing better surfaces than others • In general, processing cost increases with improvement in surface finish because additional operations and more time are usually required to obtain increasingly better surfaces • Processes noted for providing superior finishes include honing, lapping, polishing, and superfinishing ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

DIMENSIONS, TOLERANCES, AND SURFACES