Transcript DIMENSIONS, TOLERANCES, AND SURFACES

DIMENSIONS, TOLERANCES,
AND SURFACES
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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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Unilateral Tolerance
 Variation from the specified
dimension is permitted in
only one direction
 Either positive or negative,
but not both
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Limit Dimensions
 Permissible variation in a
part feature size consists of
the maximum and minimum
dimensions allowed
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Conventional Measuring
Instruments and Gages
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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
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Calipers
 Two sizes of outside calipers (photo courtesy of L. S.
Starret Co.)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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
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, Principles of Modern Manufacturing 4/e SI Version
Snap Gage
 Gaging the diameter of a part (difference in height of
GO and NO-GO gage buttons is exaggerated)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Plug Gage
 Gaging of a hole diameter (difference in diameters of
GO and NO-GO plugs is exaggerated)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Measurement of Angles
 Bevel protractor with Vernier scale (courtesy L. S.
Starrett Co.)
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Why Surfaces are Important
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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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Metallic Part Surface
 Magnified cross section of a typical metallic part surface
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Surface Texture
 Repetitive
and/or random
deviations
from the
nominal
surface of an
object
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Four Elements of Surface
Texture
3. Lay - predominant direction or pattern of the surface
texture
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Four Elements of 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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Surface Roughness
Average of vertical deviations from nominal surface over a
specified surface length
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
Surface Roughness Equation
 Arithmetic average (AA) based on absolute values of
deviations, and is referred to as average roughness
Lm
y
Ra = ∫ dx
0 Lm
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, Principles of Modern Manufacturing 4/e SI Version
Alternative Surface Roughness
Equation
 Approximation of previous equation is perhaps easier
to comprehend
n
yi
Ra  
i 1 n
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
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Energy Forms that Affect
Surface Integrity
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Mechanical energy
Thermal energy
Chemical energy
Electrical energy
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
Measurement of Surface
Roughness
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Three methods to measure surface roughness:
1. Subjective comparison with standard test
surfaces
 Fingernail test
2. Stylus electronic instruments
3. Optical techniques
©2010 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version
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, Principles of Modern Manufacturing 4/e SI Version