1. DIMENSIONS, TOLERANCES, AND SURFACES
Dimensions, Tolerances, and Related Attributes
Surfaces
Effect of Manufacturing Processes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

2. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

5. Bilateral Tolerance
Variation is permitted in both positive and negative directions from the nominal dimension
Possible for a bilateral tolerance to be unbalanced; for example, ,
Figure 5.1 Ways to specify tolerance limits for a nominal dimension of 2.500:
(a) bilateral
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

7. Limit Dimensions
Permissible variation in a part feature size consists 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

8. Surfaces
Nominal surface – designer’s 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
Variety of processes result in wide variations in surface characteristics
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

11. Metallic Part Surface
Figure 5.2 A magnified cross‑section of a typical metallic part surface.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

13. Surface Texture
Repetitive and/or random deviations from the nominal surface of an object
Figure 5.3 Surface texture features.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

14. 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, heat treatment, and similar factors
Roughness is superimposed on waviness
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

15. Four Elements of Surface Texture
Lay - predominant direction or pattern of the surface texture
Figure 5.4 Possible lays of a surface.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

16. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

17. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

18. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

19. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

20. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

21. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

22. Surface Roughness Specification
Figure 5.6 Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

23. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

24. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

25. Energy Forms in Surface Integrity
Mechanical energy
Thermal energy
Chemical energy
Electrical energy
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

26. Surface Changes 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

27. Surface Changes 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

28. Surface Changes 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

29. Surface Changes by Electrical Energy
Changes in conductivity and/or magnetism
Craters resulting from short circuits during certain electrical processing techniques such as arc welding
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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 ( in.) or better
Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e