Tolerancing Chapter 10. 2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon,...

Tolerancing

Chapter 10

2Technical Drawing 13th EditionGiesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart

Objectives

• Describe nominal size, tolerance, limits, and allowance of two mating parts

• Identify a clearance fit, interference fit, and transition fit

• Describe the basic hole and basic shaft systems

Objectives (cont.)

• Dimension mating parts using limit dimensions, unilateral tolerances, and bilateral tolerances

• Describe the classes of fit and give examples of each

• Draw geometric tolerancing symbols

• Specify geometric tolerances

Understanding Tolerance

• Tolerancing is an extension of dimensioning

• It allows you to specify a range of accuracy for every feature of a product so the parts will fit together and function properly when assembled

Understanding Tolerance

• To provide tolerances in CAD, you must:• Understand the fit required between

mating parts• Have a clear picture of how inspection

measurements are performed• Be able to apply tolerance symbols to

a drawing

Tolerance

• Tolerance is the total amount a specific dimension is permitted to vary

• Use generous tolerances when possible because increased precision makes parts more expensive to manufacture

Quality Control

• Large batches of parts may use statistical methods to control quality where a sample of parts are inspected

• Specific tolerances are based on the part’s function and fit

Variations in Form

• Acceptable parts must not extend beyond boundary limits

Implied 90 Degree Angles

• When lines intersect on a drawing at angles of 90 degrees, it is customary not to dimension the angle

• Implied 90 degree angles have the same general tolerances applied to them as any other angles covered by a general note

Implied 90 Degree Angles

Fits Between Mating Parts

• Fit is the term for the range of tightness or looseness resulting from the allowances and tolerances in mating parts

Clearance Fit

Interference Fit

Transition Fit

Definitions for Size Designation• Nominal size – used for general

identification and usually expressed in decimals

• Basic size (basic dimension) – the theoretically exact size from which limits of size are determined

• Actual size – the measured size of a finished part

• Allowance – the minimum clearance or maximum interference specified to achieve a fit between two mating parts

Definitions for Size Designation

Basic Hole System

• Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size

Basic Shaft System

• In this system, the maximum shaft is taken as the basic size and is used only in specific circumstances

Specifying Tolerances

• The primary ways to indicate tolerances in a drawing are:• A general tolerance note• A note providing a tolerance for a

specific dimension• A reference on the drawing to another

document that specifies the required tolerances

Specifying Tolerances

• (cont.)• Adding limit tolerances to dimensions• Adding direct plus/minus tolerances

to dimensions• Geometric tolerances

General Tolerance Notes

Limit Tolerances

Plus-or-Minus Tolerances

Tolerance Stacking

Tolerances and Machining Processes

Metric System of Tolerances and Fits• The preceding material on limits

and fits applies to both systems of measurement

• The ISO has a system of preferred metric limits and fits• The system is specified for holes,

cylinders, and shafts and has similar definitions of terms

Geometric Tolerancing

• Geometric tolerances state the maximum allowable variations of a form or its position from the perfect geometry implied in the drawing• The term “geometric” refers to forms

such as planes, cylinders, squares, etc.

Geometric Tolerancing

• Tolerances of form and position (or location) control such characteristics as:• Straightness• Flatness• Parallelism• Perpendicularity (squareness)• Concentricity• Roundness• Angular displacement, etc.

Geometric Characteristics and Modifying Symbols

Symbols for Tolerances of Position and Form

Datum Surfaces and Features• Datum surfaces and features are

used as references to control other features

Maximum Material Condition• Maximum material condition

(MMC) means that a feature of a finished product contains the maximum amount of material permitted by the toleranced dimensions for that feature

Tolerances of Angles

• Bilateral tolerances have traditionally been given on angles

Using Geometric Dimensioning and Tolerancing• Geometric dimensioning and

tolerancing (GDT) considers an individual part’s dimensions and tolerances and that part’s relation to its related parts

• GDT also simplifies the inspection process

Using Geometric Dimensioning and Tolerancing• Applying GDT principles requires:

• Defining the part’s functions• Listing the functions by priority• Defining the datum reference frame• Control selection• Calculating tolerances

Tolerancing Chapter 10. 2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon,...

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