1. DIMENSIONS, TOLERANCES, AND SURFACES
Dimensions, Tolerances, and Related Attributes
Conventional Measuring Instruments and Gages
Surfaces
Measurement of Surfaces
Effect of Manufacturing Processes
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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
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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"
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
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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
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5. Bilateral Tolerance
Variation is permitted in both positive and negative directions from the nominal dimension
Possible for a bilateral tolerance to be unbalanced
Ex: ,
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6. Unilateral Tolerance
Variation from the specified dimension is permitted in only one direction
Either positive or negative, but not both
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7. Limit Dimensions
Permissible variation in a part feature size consists of the maximum and minimum dimensions allowed
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8. Measurement
Procedure in which an unknown quantity is compared to a known standard, using an accepted and consistent system of units
Measurement provides a numerical value of the quantity of interest, within certain limits of accuracy and precision
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9. 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
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10. 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, Principles of Modern Manufacturing 4/e SI Version

11. 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
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12. 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
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13. Micrometer
External micrometer, standard one‑inch size with digital readout (photo courtesy of L. S. Starret Co.)
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14. Calipers
Two sizes of outside calipers (photo courtesy of L. S. Starret Co.)
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15. 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 inch
Applications: measuring straightness, flatness, parallelism, squareness, roundness, and runout
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16. Dial Indicator
Front view shows dial and graduated face; back view shows cover plate removed (photo courtesy of Federal Products Co.)
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17. 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
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18. 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, Principles of Modern Manufacturing 4/e SI Version

19. GO/NO‑GO gages Minimum size for internal feature such as a hole
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
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20. Snap Gage
Gaging the diameter of a part (difference in height of GO and NO‑GO gage buttons is exaggerated)
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21. Plug Gage
Gaging of a hole diameter (difference in diameters of GO and NO-GO plugs is exaggerated)
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22. Measurement of Angles
Bevel protractor with Vernier scale (courtesy L. S. Starrett Co.)
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23. 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
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24. 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
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25. Surface Technology Defining the characteristics of a surface
Concerned with:
Defining the characteristics of a surface
Surface texture
Surface integrity
Relationship between manufacturing processes and characteristics of resulting surface
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26. Metallic Part Surface
Magnified cross section of a typical metallic part surface
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27. Surface Texture It has roughness, waviness, and flaws
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
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28. Surface Texture
Repetitive and/or random deviations from the nominal surface of an object
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29. 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
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30. Four Elements of Surface Texture
Lay - predominant direction or pattern of the surface texture
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31. 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
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32. 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
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33. Surface Roughness
Average of vertical deviations from nominal surface over a specified surface length
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34. 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
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35. 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, Principles of Modern Manufacturing 4/e SI Version

36. 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
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37. Surface Roughness Specification
Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels
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38. 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
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39. 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
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40. Energy Forms that Affect Surface Integrity
Mechanical energy
Thermal energy
Chemical energy
Electrical energy
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41. 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
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42. 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
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43. 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
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44. 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
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45. 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
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46. Measurement of Surface Roughness
Three methods to measure surface roughness:
Subjective comparison with standard test surfaces
Fingernail test
Stylus electronic instruments
Optical techniques
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47. 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
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48. Stylus Traversing Surface
Stylus head traverses horizontally across surface, while stylus moves vertically to follow surface profile
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49. 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 ( in.) or better
Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified
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50. 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
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