9.
Inspection and Gaging Tools
Nageswara Rao Posinasetti
1 October 11, 2020
Inspection
Nominal dimension
Tolerance
Use
Appearance
Cost
Datum or Reference
2 October 11, 2020
Basic principles of gaging
Measurement
Measuring can be defined as the determination of
a dimension.
Gaging
Gauging is defined as the acceptability of a given
dimension whether it lies in its specified or
allowable limits or not.
Gage tolerance
10% work tolerance
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Gage maker tolerances
Class XX – closest tolerances – master gages
Class X – next best – master and inspection
gages
Class Y – next best – inspection and working
gages
Class Z - lowest – working gages with coarse
tolerances
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Table 9-1 Standard Gage Maker’s Tolerances
Above To and Class (in)
(in) including (in) XX X Y Z
0.010 0.825 0.00002 0.00004 0.00007 0.00010
0.825 1.510 0.00003 0.00006 0.00009 0.00012
1.510 2.510 0.00004 0.00008 0.00012 0.00016
2.510 4.510 0.00005 0.00010 0.00015 0.00020
4.510 6.510 0.000065 0.00013 0.00019 0.00025
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Gage tolerances
Plug gage for 1 0.0006 inches
Gage tolerance = 0.00012 inch
From table Z class
If Plug gage for 1 0.0006 inches
Gage tolerance = 0.00006 inch
From table X class
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Gage tolerances
Plug gage for 1 0.0075 inches
Gage tolerance = 0.0015 inch
From table Z class
Smaller degree of gage tolerance means more
expensive
Gage tolerances should be realistically applied.
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Allocation of gage tolerances
Bilateral system
The GO and NOGO gage tolerance zones are
divided into two parts by the high and low limits
of the workpiece tolerance zone
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Maximum
Work
Tolerance
limit
Work tolerance zone
Bilateral
Minimum
Work
Tolerance
9
limit October 11, 2020
Allocation of gage tolerances
Hole to be gaged 1.2500 0.0006 inch
Work tolerance = 0.0012 inch
Hole size from 1.2506 to 1.2494 inch
Gage tolerance (10%) = 0.00012 inch
Z-class tolerance zone
GO gage size 1.2494 0.00006 inch
NOGO gage size 1.2506 0.00006 inch
10 October 11, 2020
Table 9-1 Standard Gage Maker’s Tolerances
Above To and Class (in)
(in) including (in) XX X Y Z
0.010 0.825 0.00002 0.00004 0.00007 0.00010
0.825 1.510 0.00003 0.00006 0.00009 0.00012
1.510 2.510 0.00004 0.00008 0.00012 0.00016
2.510 4.510 0.00005 0.00010 0.00015 0.00020
4.510 6.510 0.000065 0.00013 0.00019 0.00025
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Allocation of Gage Tolerances
Unilateral system
The work tolerance zone entirely includes the
gage tolerance zone
Work tolerance smaller by the sum of the gage
tolerance
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Maximum
Work
Tolerance
limit
Work tolerance zone
Unilateral
Bilateral
Minimum
Work
Tolerance
13
limit October 11, 2020
Allocation of gage tolerances
Hole to be gaged 1.2500 0.0006 inch
Work tolerance = 0.0012 inch
Hole size from 1.2506 to 1.2494 inch
Gage tolerance (10%) = 0.00012 inch
Z-class tolerance zone
GO gage size 1.24940 + 0.00012 inch
NOGO gage size 1.25060 - 0.00012 inch
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Allocation of gage tolerances
Commercial gages
Allocate gage tolerance negatively with reference
to both the maximum and minimum limits of the
workpiece tolerance (C)
NOGO gage tolerance is divided by the maximum
limit of the workpiece tolerance and the GO gage
tolerance is held within the minimum limit of the
workpiece tolerance.
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Maximum
Work
Tolerance
limit
Work tolerance zone
Commercial
Plain plug
Commercial
Plain ring
Minimum
Work
Tolerance
16
limit October 11, 2020
Allocation of gage tolerances
The objectives in choosing an allowance
system should be the economic production of
as near 100% usable parts as possible, and the
acceptance of the good pieces and rejection of
the bad.
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Gage Wear Allowance
Wear allowance is an amount added to the
nominal diameter of a GO-plug and
subtracted from that of a GO-ring gage.
It is used up during the gage life by wearing
away of the gage metal.
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Gage Wear Allowance (5%)
Hole diameter 1.5000 0.0006 inch
Work tolerance = 0.0012 inch
Gage tolerance (10%) = 0.00012 inch
GO gage size 1.49940 + 0.00006 = 1.49946
inch
Add gage maker tolerance as shown
previously, 1.49946 +0.00012 and –0.00000 as
limits
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Gage Materials
Chrome plated
Tungsten carbide tipped
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Surface Plate
Main horizontal reference plane
Rigid block of granite or cast iron
Generally have three-point suspension to
prevent rocking when mounted on uneven
surface
Two types
Cast-iron plates
Granite surface plates
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Surface Plate
Cast-iron plates
Well ribbed and high strength
Good wear-resistance qualities
After machined, surface scraped by hand to flat
Operation long and cost high
Granite surface plates
Manufactured from gray, pink, or black granite
Several degrees of accuracy
Extremely flat finishes produced by lapping
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Advantages of Granite Plates
Not appreciably affected by temperature
change
Will not burr, therefore, accuracy not impaired
Nonmagnetic
Rustproof
Abrasives will not embed themselves as easily
in the surface
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Template
Specified profile
Guide to the location of workpiece features
with reference to a single plane.
A straight edge is a template for checking
straightness.
Radius gages
Screw pitch gages
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Plug Gages
A plug gage is a fixed gage usually made up of
two members
One member is called the GO end and the
other the NO-GO or NOT-GO end
AGD – American Gage Design standards
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Ring gages
Ring gages are usually used in pairs, consisting
of go member and no-go member.
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Snap Gages
For measuring length, diameter, thickness or
width
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Amplification and Magnification of Error
As the measurable error becomes small, it is
difficult for direct reading and hence some
means of magnification is required.
Dial indicators
Optical
Pneumatic gauging
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Geometric Dimensioning and
Tolerancing
ASME Y14.5-1994 (Rev. 2009)
ISO 1101, 129, 3040
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Modifiers
Maximum Material Condition (MMC) -
Minimum diameter of a hole and maximum
diameter of a shaft
Least Material Condition (LMC)
Maximum diameter of a hole and minimum
diameter of a shaft
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Flatness checking
Checking process depends upon the accuracy
required
Using a dial indicator
After the surface is leveled, the indicator
explores the entire area and the full indicator
movement is a measure of flatness.
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Checking a Profile
Follow a master part as shown in Fig 9-42.
Number of dial indicators set in a plane
confirming the contour as shown in Fig 9-43.
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Checking for
Parallelism/Perpendicularity
Parallelism can be checked by placing a part on
a surface plate and searching for out of parallel
condition with an indicator.
Some times a special fixture may be required
as shown in Fig 9-49.
Perpendicularity of a cylindrical feature and it
MMC can be checked with a functional gage as
shown in Fig 9-51.
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Checking a Runout
Total runout consists, by definition, of two
concentric cylinders that encompass a feature
about a defined axis.
Generally, total runout is measured by taking
single runout at intervals along an entire
cylinder.
The extremes of needle deflection for the
entire set of intervals yields total runout.
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Checking a Position
Shown is a hole relation gage to check hole-
to-hole relationship.
If RFS (Regardless of Feature Size) is used in
place of MMC, gaging will become difficult.
To maintain the positional tolerance of 0.10 in
eleven gage pins will be required for all sizes
between MMC and LMC.
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Positional Tolerance Rules
For parts with internal features, the nominal
gage feature size is directly determined by
subtracting the total positional tolerance
specified at MMC from the specified MMC
size of the feature to be gaged for location.
For parts with external features, the nominal
gage feature size is directly determined by
adding the total positional tolerance specified
at MMC size of the feature to be gaged for
location.
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