DIMENSIONS, TOLERANCES, AND SURFACES

1. DIMENSIONS, TOLERANCES, AND SURFACES • Dimensions, Tolerances, and Related Attributes • Surfaces • Effect of Manufacturing Processes ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

2. Dimensions and Tolerances • In addition to mechanical and physical properties, other factors that determine the performance of a manufactured product 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

3. 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" • Dimensions on part drawings represent nominal or basic sizes of the part and its features • 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

4. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

5. Bilateral Tolerance Variation is permitted in both positive and negative directions from the nominal dimension • It is possible for a bilateral tolerance to be unbalanced; for example, 2.500 +0.010, -0.005 Figure 5.1 ‑ Ways to specify tolerance limits for a nominal dimension of 2.500: (a) bilateral ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

6. Unilateral Tolerance Variation from the specified dimension is permitted in only one direction, either positive or negative, but not both Figure 5.1 ‑ Ways to specify tolerance limits for a nominal dimension of 2.500: (b) unilateral ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

7. Limit Dimensions Permissible variation in a part feature size, consisting of the maximum and minimum dimensions allowed Figure 5.1 ‑ Ways to specify tolerance limits for a nominal dimension of 2.500: (c) limit dimensions ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

8. Surfaces • Nominalsurface - intended surface contour of part, defined by lines in the engineering drawing • The 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 it • The variety of manufacturing processes result in wide variations in surface characteristics ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

9. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

10. Surface Technology • Concerned with: • Defining the characteristics of a surface • Surface texture • Surface integrity • Relationship between manufacturing processes and characteristics of resulting surface ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

11. Figure 5.2 ‑ A magnified cross‑section of a typical metallic part surface ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

12. 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 produced it ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

13. Surface Integrity Concerned with the definition, specification, and control of the surface layers of a material (most commonly metals) in manufacturing and subsequent performance in service • Manufacturing processes involve energy which alters the part surface • The altered layer may result from work hardening (mechanical energy), or heating (thermal energy), chemical treatment, or even electrical energy • Surface integrity includes surface texture as well as the altered layer beneath ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

14. Surface Texture Repetitive and/or random deviations from the nominal surface of an object Figure 5.3 ‑ Surface texture features ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

15. Four Elements of Surface Texture • Roughness - small, finely‑spaced deviations from nominal surface determined by material characteristics and process that formed the surface • Waviness - deviations of much larger spacing; they occur due to work deflection, vibration, heat treatment, and similar factors • Roughness is superimposed on waviness ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

16. 3. Lay - predominant direction or pattern of the surface texture Figure 5.4 ‑ Possible lays of a surface ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

17. 4. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

18. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

19. Surface Roughness Average of vertical deviations from nominal surface over a specified surface length Figure 5.5 ‑ Deviations from nominal surface used in the two definitions of surface roughness ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

20. Surface Roughness Equation Arithmetic average (AA) is generally used, 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

21. An Alternative Surface Roughness Equation Approximation of previous equation is perhaps easier to comprehend: where Ra has the same meaning as above; yi = vertical deviations (absolute value) identified by subscript i; and n = number of deviations included in Lm ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

22. 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 width eliminates waviness deviations and only includes roughness deviations ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

23. Figure 5.6 ‑ Surface texture symbols in engineering drawings: • the symbol, and (b) symbol with identification labels Values of Ra are given in microinches; units for other measures are given in inches Designers do not always specify all of the parameters on engineering drawings ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

24. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

25. 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 metal forming (e.g., forging, extrusion), pressworking, and machining • Although primary function is to change geometry of workpart, mechanical energy can also cause residual stresses, work hardening, and cracks in the surface layers ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

26. Surface Changes Caused by Mechanical Energy • Residual stresses in subsurface layer • Cracks ‑ microscopic and macroscopic • Laps, folds, or seams • Voids or inclusions introduced mechanically • Hardness variations (e.g., work hardening) ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

27. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

28. Surface Changes Caused by 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

29. Surface Changes Caused by Electrical Energy • Changes in conductivity and/or magnetism • Craters resulting from short circuits during certain electrical processing techniques ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

30. Tolerances and Manufacturing Processes • Some manufacturing processes are inherently more accurate than others • Examples: • 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

31. 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 ©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”