Chapter 16
Tolerancing
Objectives
• Describe the purpose of conventional
tolerancing and its limitations
• Use standard tables to specify an
appropriate fit between two mating parts
• Explain the advantages of using geometric
dimensioning and tolerancing (GD&T) over
conventional tolerancing
Objectives (cont’d.)
• Recognize the datum reference frame on a
drawing with geometric dimensions and
tolerances
• Describe the tolerance zone shape for
each geometric tolerance
• Correctly read the feature control frames
on a drawing with geometric dimensions
and tolerances
Introduction
• Relationships between different parts
– Necessary to specify intended fit between
parts
• Problems with inexperience in new
engineers
– Lack of knowledge in history, materials
– Inappropriate tolerance values
Formats for Tolerances
• Displayed in several common formats
FIGURE 16.03. Formats for tolerance dimensioning in millimeters and inches.
Tolerance Buildup Problems
• Can be minimized depending on type of
dimensioning
– Chain: yields largest tolerance buildup
– Baseline: can eliminate some accumulation
– Direct: best way to eliminate tolerance
accumulation
• Single dimension placed between two key points
Statistical Tolerance Control
• Based on sound statistical practices
• Can be applied only when appropriate
statistical process control methods used
FIGURE 16.07.
Tolerancing with
statistical process
control.
Use of Tables for Fits
• Types of fits
FIGURE 16.08. Specifying a
CLEARANCE FIT with
limit dimensioning.
FIGURE 16.09. Specifying an
INTERFERENCE FIT with limit
dimensioning
FIGURE 16.10. Specifying a TRANSITION
FIT with limit dimensioning.
Fit Terminology
FIGURE 16.11.
Clearance fit
terminology.
FIGURE 16.12.
Interference fit
terminology.
English Fits
•
•
•
•
•
Running or sliding clearance
Locational clearance
Locational transition
Locational interference (see next slide)
Force or shrink (see next slide)
English Fits (cont’d.)
FIGURE 16.18. Locational interference and force fits.
Metric Fits
FIGURE 16.19. Metric fit table.
Fits Tables
• To specify an inch fit between holes and
shafts from a standard table
– Determine type of fit appropriate for the design
and locate corresponding table
– Determine basic size of the parts
– Find size range on the table
– Determine tolerances for hole and shaft
– Remember that values on the English tables
are in thousandths of an inch
Fits Tables (cont’d.)
FIGURE 16.23. A close sliding fit.
Fits Tables (cont’d.)
• To determine metric fits
– Determine type of fit appropriate for the design
and locate corresponding table
– Determine basic size of the parts
– Find the size range on the table
– Determine the tolerances for the hole and the
shaft
Conventional Tolerancing versus
Geometric Tolerancing
• Feature with size
– Cylindrical or spherical surface or set of two
opposed elements or opposed parallel
surfaces associated with size dimension
• Feature without size
– Planar surface or a feature where the normal
vectors point in the same direction
Conventional Tolerancing versus
Geometric Tolerancing (cont’d.)
FIGURE 16.29. Conventional
tolerance dimensioning of a block.
Conventional Tolerancing versus
Geometric Tolerancing (cont’d.)
FIGURE 16.30. How
conventional tolerancing
controls surfaces.
Location of Holes and Pins with
Conventional Tolerancing
FIGURE 16.35. Square
tolerance zones from
conventional tolerancing.
Geometric Dimensioning and
Tolerancing (GD&T)
• GD&T is a 3-D mathematical system for
describing the form, orientation, and
location of features on a part within precise
tolerance zones
– Better communication throughout design
process
– Almost nothing can be interpreted in more
than one way
The Datum Reference Frame
FIGURE 16.40. Components of the theoretical datum system.
The Datum Reference Frame
(cont’d.)
FIGURE 16.41.
Datum terminology.
Geometry Characteristic Symbols
and Feature Control Frames
FIGURE 16.42.
Geometric characteristic
symbols.
Geometry Characteristic Symbols
and Feature Control Frames
(cont’d.)
• Feature control frame
– Contains geometric characteristic symbol, the
geometric tolerance, and the relative datums
FIGURE 16.44. A feature
control frame with the
perpendicularity tolerance.
FIGURE 16.45. A feature control
frame with the position tolerance.
Order of Precedence for Datums
FIGURE 16.47. The
sequence of datum
features
Position Tolerances versus
Conventional Tolerances
FIGURE 16.49. Geometric dimensioning and tolerancing of the PLATE.
Position Tolerances versus
Conventional Tolerances (cont’d.)
• Basic dimensions
– Theoretically exact
• Maximum material condition modifier
– Size of zone changes if size of hole changes
• Square vs. cylindrical tolerance zones
FIGURE 16.53. Cylindrical tolerance
zones for the position tolerance.
Form Tolerances
• For individual features and not related to
datums
– Straightness
– Flatness
– Circularity
– Cylindricity
Profile Tolerances
FIGURE 16.63.
Profile of a line.
Profile Tolerances (cont’d.)
• Profile of a surface
• Inspection of profile tolerances
– Optical comparitors
– Overlay charts
– Mechanical gaging
• Used when datum reference frame applied
Orientation Tolerances
• Parallelism tolerance
– Can be used to control a surface
FIGURE 16.69. Parallelism
tolerance used to control a
surface.
Orientation Tolerances (cont’d.)
• Perpendicularity
• Angularity
FIGURE 16.78. Inspecting the
angularity between two surfaces.
Location Tolerances
• Position
– Can be used to locate axis of a hole
FIGURE 16.79. Using the position
tolerance to locate the axis of a hole.
Location Tolerances (cont’d.)
• Concentricity
– Applied to cylinder
FIGURE 16.81.
Concentricity
applied to
a cylinder.
• Symmetry
Runout Tolerances
• Circular runout
• Total runout
• Multiple datums
– Tolerances applied based on function, so
careful specification of datums is necessary
Examples of Specifying Fits and
Geometric Tolerances
• Specifying the fit between two parts
FIGURE 16.99. Coupling assembly.
Examples of Specifying Fits and
Geometric Tolerances (cont’d.)
FIGURE 16.101. Limit dimensions for the STUD and
BUSHING.
Adding Geometric Dimensions
FIGURE 16.102.
PLATE ASSEMBLY.
FIGURE 16.103. Establishing
the datums on the PLATE.
FIGURE 16.104.
Controlling the datums
on the PLATE.
FIGURE 16.105. Positioning the hole on the PLATE.
FIGURE 16.106. Controlling the other surfaces on the PLATE.
Summary
• Covered basic information related to
conventional tolerancing and GD&T
• Discussed interchangeable manufacturing
and explained why it is important to the
way modern industry functions
Summary (cont’d.)
• Discussed conventional tolerancing,
including how to specify fits and how
tolerance dimensions control form and
location
• Covered the basics of geometric
dimensioning and tolerancing
Summary (cont’d.)
• Discussed the advantages of geometric
tolerancing over conventional tolerancing
• Explained the importance of the datum
reference frame to establish a coordinate
system for design manufacturing and
inspection
• Explained how to read feature control
frames
• Described each geometric tolerance