Chapter 10

Tolerancing

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TolerancingTolerancing Tolerances are used to control the

variation in size that exists on all manufactured parts.

The amount that a size is allowed to vary depends on the function of the part & its assembly.– electric drill vs. jet engine

The more accuracy required in a part (smaller tolerance) the greater the cost.

Tolerances allow for interchangeable parts, which permits the replacement of individual parts in an assy. instead of replacing the whole system if a part goes bad or fails.

ToleranceTolerance Tolerance is the total amount a

dimension may vary. It is the difference between the maximum size and the minimum size.

Tolerances can be expressed on a drawing in several ways: – direct limits, or as tolerance values

applied directly to a dimension. (Fig. 10.8)

– geometric tolerances (Fig. 10.25)– notes referring to specific

conditions– A general tolerance note in the

titleblock. (Fig. 10.7)

DimensionsDimensions Limit dimensions:

Plus/minus dimensions:– unilateral

– bilateral

Tolerancing TermsTolerancing Terms Nominal size: the “name” or general

size. Often expressed as a fraction. Basic size: the theoretical size used as a

starting point for the application of tolerances. Nominal size in decimal format.

Actual size: the measured size of the finished part.

Limits: the maximum & minimum sizes shown by the toleranced dimension.

Allowance: the min. clearance or max. interference between 2 parts.

MMC: the condition of a part in which it contains the most amount of material.EX: biggest shaft or smallest hole.

LMC: the condition of a part in which it contains the least amount of material.

Tolerance, MMC, LMC?Tolerance, MMC, LMC?

Limit dimensions:

Plus/minus dimensions:– unilateral

– bilateral

Complete Worksheet TOL-1

Complete Worksheet TOL-1

Remember x.xxxx– I.e. 0.1234– I.e. 2.1200

FitFit Fit: the degree of tightness between

mating parts. The three most common types of fit

found in industry are:– Clearance fit - there is always a

space between the 2 mating parts. (shaft is smaller than the hole)

– Interference fit - the 2 mating parts always interfere with one another in assembly. (shaft is bigger than the hole)

– Transition fit - sometimes a clearance fit & sometimes an interference fit between the mating parts.

Clearance FitClearance Fit

Interference FitInterference Fit

Do on ownpage 335, Figure

10.1a

Do on ownpage 335, Figure

10.1a

•Hole Tolerance?

•Shaft Tolerance?

•Minimum clearance?

•Maximum clearance?

•Allowance?

Complete Number 1 of Worksheet 66

Complete Number 1 of Worksheet 66

ANSI Standard FitsANSI Standard Fits A group of English usit tolerance

relationships called preferred precision fits have been developed. They are specified in ANSI B4.1.

The five classes are:– Running & sliding fits (RC)– Clearance locational fits (LC)– Transition locational fits (LT)– Interference locational fits (LN)– Force or shrink fits (FN)

These tolerances are specified in Appendix 5 through 9, pages a25-a32. The values in these tables are given in THOUSANDTHS of an inch. Example: 1.2 = 0.0012

Class RC9: Running & Clearance Fit

Basic DIA = 2.0000

Class RC9: Running & Clearance Fit

Basic DIA = 2.0000

Hole

Shaft

Shaft Tolerance = Hole Tolerance = Max. Clearance = Min. Clearance =

Class FN2:Medium Drive fit

Basic DIA = 0.5000

Class FN2:Medium Drive fit

Basic DIA = 0.5000

Hole

Shaft

Shaft Tolerance = Hole Tolerance = Max Interference = Min Interference =

Complete Worksheets Tol-2, Tol-3 and 66

Complete Worksheets Tol-2, Tol-3 and 66

Metric Fit - TermsMetric Fit - Terms Basic size: size to which limits of

deviation are assigned. (Fig.10.1, p 343)– Must be same for both parts– Basic sizes selected from chart in Table

10.2 Deviation: difference between the

actual size and the basic size. (Fig. 10.14)

Upper deviation: difference between the max. size limit & the basic size.

Lower deviation: difference between the min. size limit & the basic size.

Fundamental deviation: the deviation closest to the basic size. (Fig. 10.18)– Capital H indicates Hole – Lowercase letter, I.e. f , indicates shaft

Metric Fit – Terms cont’dMetric Fit – Terms cont’d

International Tolerance Grade (IT): a group of tolerances that vary depending upon the basic size, but have the same level of accuracy within a given grade. (Fig 10.18)– There are 18 IT grades. The

smaller the grade number, the smaller the tol. zone.

– See Appendix 10 for IT grades.

Preferred Metric FitsPreferred Metric Fits There are 2 systems used to indicate

preferred fits in the metric system.– Hole basis: the system of fits where

the min. hole size is the basic size. The fundamental deviation for a hole basis system is indicated by a capital “H”.

» EX. 50H8

(50 = DIA in mm) (Basic size)

(H = using Basic Hole system) (fundamental deviation)

(8 = closest running fit) (IT grade)

» For mating part 50H8/f7 (Appendix 11)– Hole Limits?– Shaft Limits?– Fit?

Preferred Metric FitsPreferred Metric Fits The second system used to indicate

preferred fits in the metric system.– Shaft basis: the system of fits where

the max. shaft size is the basic size. The fundamental deviation for a shaft basis system is indicated by a lowercase letter.

» EX. 50h7

(50 = DIA in mm) (Basic size) (h = using Basic Shaft system)

(fundamental deviation)

(7 = closest running fit)(IT grade)

» For mating part 50F8/h7 (Appendix ?)» Limits and Fits?

– Hole Limits?– Shaft Limits?– Fit?

Try the following problemTry the following problem

Basic DIA 41mm Use Hole Basis System Sliding

Try the following problemTry the following problem

Basic DIA 41mm Use Shaft Basis System Sliding

Try the following problemTry the following problem

Basic DIA 58mm Use Shaft Basis System Force Fit

Look UpLook Up

What type of fit is a 6 H7/s6?

Hole or Shaft Basis System?

Complete Worksheets Tol-4 and 67

Complete Worksheets Tol-4 and 67