Ch05

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

DIMENSIONS, TOLERANCES, AND SURFACES

1. Dimensions, Tolerances, and Related Attributes

2. Conventional Measuring Instruments and Gages

3. Surfaces

4. Measurement of Surfaces

5. Effect of Manufacturing Processes


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


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


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


Bilateral Tolerance

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


Unilateral Tolerance

Variation from the specified dimension is permitted in only one direction

Either positive or negative, but not both


Limit Dimensions

Permissible variation in a part feature size consists of the maximum and minimum dimensions allowed


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


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


Conventional Measuring Instruments and Gages

Precision gage blocks Measuring instruments for linear dimensions Comparative instruments Fixed gages Angular measurements


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


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


Micrometer

External micrometer, standard one‑inch size with digital readout (photo courtesy of L. S. Starret Co.)


Two sizes of outside calipers (photo courtesy of L. S. Starret Co.)

Calipers


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


Front view shows dial and graduated face; back view shows cover plate removed (photo courtesy of Federal Products Co.)

Dial Indicator


As part is rotated about its center, variations in outside surface relative to center are indicated on the dial

Dial Indicator Setup to Measure Runout


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


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


Gaging the diameter of a part (difference in height of GO and NO‑GO gage buttons is exaggerated)

Snap Gage


Plug Gage

Gaging of a hole diameter (difference in diameters of GO and NO-GO plugs is exaggerated)


Measurement of Angles

Bevel protractor with Vernier scale (courtesy L. S. Starrett Co.)


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


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


Surface Technology

Concerned with: Defining the characteristics of a surface Surface texture Surface integrity Relationship between manufacturing processes

and characteristics of resulting surface


Metallic Part Surface

Magnified cross section of a typical metallic part surface


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


Surface Texture

Repetitive and/or random deviations from the nominal surface of an object


Four Elements of Surface Texture

1. Roughness - small, finely‑spaced deviations from nominal surface Determined by material characteristics and

processes that formed the surface

2. Waviness - deviations of much larger spacing Waviness deviations occur due to work

deflection, vibration, tooling, and similar factors Roughness is superimposed on waviness



3. Lay - predominant direction or pattern of the surface texture



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


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


Surface Roughness

Average of vertical deviations from nominal surface over a specified surface length


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

dxL

yR

mL

ma ∫

0

=


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

n

i

ia n

yR

1


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


Surface Roughness Specification

Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels


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


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


Energy Forms that Affect Surface Integrity

Mechanical energy Thermal energy Chemical energy Electrical energy


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


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


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


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


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


Measurement of Surface Roughness

Three methods to measure surface roughness:

1. Subjective comparison with standard test surfaces Fingernail test

2. Stylus electronic instruments

3. Optical techniques


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


Stylus head traverses horizontally across surface, while stylus moves vertically to follow surface profile

Stylus Traversing Surface


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


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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Ch05

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