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 16.10. Specifying a TRANSITION FIT with limit dimensioning.

FIGURE 16.08. Specifying a CLEARANCE FIT withlimit dimensioning.

FIGURE 16.09. Specifying an INTERFERENCE FIT with limit dimensioning

Fit Terminology

FIGURE 16.11. Clearance fitterminology.

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.

FIGURE 16.42. Geometric characteristic symbols.

Geometry Characteristic Symbols and Feature Control Frames

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 featurecontrol frame with theperpendicularity 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

• Symmetry

FIGURE 16.81. Concentricity applied to a cylinder.

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 andBUSHING.

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