Tolerancing Chapter 11. 2 Technical Drawing 13 th Edition Giesecke, Mitchell, Spencer, Hill Dygdon,...

Tolerancing

Chapter 11

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

© 2009 Pearson Education, Upper Saddle River, NJ 07458.

All Rights Reserved.

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• Where the internal part fits into an external part with

space between the parts.




Interference Fit• Where the internal part will have to be forced into an

external part.

• Produces a negative clearance or allowance.




Transition Fit

• Where there the smallest shaft will fit into the largest hole, but the largest shaft will have to be forced into the smallest hole. Refers to either a tight clearance or interference.

• May or may not produce a negative clearance or allowance.




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 NOTESGENERAL TOLERANCE NOTES

General notes are usually located in the lower right corner of the drawing sheet near the title block. Often, general tolerance notes are included in the title block itself.

“ALL TOLERANCES ±1 mmUNLESS OTHERWISE NOTED.

ANGLES 1 DEGREE.”

“ALL TOLERANCES ±1 mmUNLESS OTHERWISE NOTED.

ANGLES 1 DEGREE.”




LIMIT TOLERANCESLIMIT TOLERANCESLimit tolerances state the upper and lower limits for the dimension range inplace of the dimension values.

… is the preferred method of stating tolerances.

Method of Stating Limits

Note: The upper value is always placed abovethe lower value.

Note: The upper value is always placed abovethe lower value.




Plus-or-Minus TolerancesPlus-or-Minus Tolerances

• Allows variations in only one direction from the nominal size.

• Allows variations in both directions from the nominal.




Tolerance StackingTolerance Stacking• The variations produced by tolerances are stacked in

chain dimensioning




Fits between Mating PartsFits between Mating Parts

Fit is the range of tightness or looseness resulting from the allowances and tolerances in mating parts. The loosest fit, or maximum clearance, occurs when the smallest internal part (shaft) is in the largest external part (hole),




Fits between Mating Parts continued…Fits between Mating Parts continued…

Clearance Fit A clearance fit occurs when an internalpart fits into an external part with space (or clearance) between the parts.

Interference Fit An interference fit occurs when the internal part is larger than the external part, so the parts must be forced together.




USING AMERICAN NATIONALUSING AMERICAN NATIONALSTANDARD LIMITS AND FIT TABLESSTANDARD LIMITS AND FIT TABLES

Portion of RC8 Fit Table. The InternationalOrganization for Standardization (ISO) publishes a similar series of fit tables for metric values.

The American National Standards Institute has issued ANSI B4.1, Preferred Limits and Fits for Cylindrical Parts, defining terms and recommending preferred standard sizes, allowances, tolerances, and fits in terms of the decimal inch. This standard gives a series of standard classes of fits on a unilateral-hole basisso that the fit produced by mating parts of a class of fit will produce approximately similar performance throughout therange of sizes.




TOLERANCES AND MACHINING TOLERANCES AND MACHINING PROCESSESPROCESSES

Tolerances Related to Machining Processes




• 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

50H8/f7Basic Size

Hole Tolerance

Shaft Tolerance

Fit

Metric System of Tolerances and FitsMetric System of Tolerances and Fits




• 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 TolerancingGeometric Tolerancing




GeometricGeometric Characteristics and Modifying Characteristics and Modifying SymbolsSymbols• Tolerances of form and position (or location) control such

characteristics as:




Symbols for Tolerances of Position and FormSymbols for Tolerances of Position and Form

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