Chapter 11

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 assembly instead of replacing the whole system if a part goes bad or fails.

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: – a general tolerance note in the

titleblock. – direct limits, or as tolerance values

applied directly to a dimension. (Fig. 11.8)

– geometric tolerances (Fig. 11.31a)– notes referring to specific

conditions (page 364)

ToleranceTolerance

2.500 ±0.004 0.503 ±0 .003

0.5002.496+0.008-0.000

+0.006-0.000

.500

.506

2.5042.496

Limit dimensions:

Plus/minus dimensions:– unilateral

– bilateral

DimensionsDimensions

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.

MMC (maximum material condition): the condition of a part in which it contains the most amount of material.

EX: biggest shaft or smallest hole. LMC (least material condition): the

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

0.5000.506

2.5042.496

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

Limit dimensions:

Plus/minus dimensions:– unilateral

– bilateral2.500 ±0.004 0.503 ±0 .003

0.5002.496+0.008-0.000

+0.006-0.000

Tolerance? Width Hole

MMC?

Complete Activity 10 TOL-1

Complete Activity 10 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.

Tolerancing TermsTolerancing Terms Basic size: the theoretical size

used as a starting point for the application of tolerances.

Nominal size in decimal format. Allowance: the min. clearance or

max. interference between 2 parts.

Clearance

Interference

ClearanceInterference

Minimum

Maximum

Min or Max?

Clearance FitClearance Fit

Interference FitInterference Fit

ALLOWANCE =

0.001

0.001

Page 362 - Hands On 11.1a & b in groups on

the board and c by yourself

Page 362 - Hands On 11.1a & b in groups on

the board and c by yourself

Hands-On (not Figure) 11.1 Are the fits clearance, transitional or interference? Use categories below (Mistakes in textbook)

A B C

Hole Tolerance?

Shaft Tolerance?

Minimum clearance/ interference?

Maximum clearance/ interference?

Allowance?

Complete Activity 10 TOL-3 #s 4, 5, & 6

Complete Activity 10 TOL-3 #s 4, 5, & 6

Which is between

Largest Hole / Smallest Shaft?

Largest Hole / Largest Shaft?

Smallest Hole / Largest Shaft?

Smallest Hole / Smallest Shaft?

Shaft (Tab)Information given?

Size of tab, clearance fit, allowance0.005, and tolerance on slot is .002.

Minimum or maximum interference?

Basic SystemsBasic Systems Basic Hole System

– Standard reamers, etc., are used to produce holes and standard gauges used to check sizes. Shafts are machined.

» Determine Minimum hole size, apply allowance, then apply tolerances.

Basic Shaft System – Not used often. However,

advantageous when shafts cannot be easily machined to size or when several parts fit onto one shaft.

» Determine Maximum shaft size, apply allowance, then apply tolerances.

ANSI Standard FitsANSI Standard Fits A group of English unit tolerance

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

The five classes are: Table 11.1 pg 367– 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 Appendixes 5 through 9, pages a23-a30. The values in these tables are given in THOUSANDTHS of an inch.

Example: 1.2 = 0.0012

Class RC9: Basic DIA = 2.0000

Class RC9: Basic DIA = 2.0000

Hole limits =

Shaft limits =

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

RC means?

9 means?

Class FN2: Basic DIA = 0.5000

Class FN2: Basic DIA = 0.5000

Hole limits =

Shaft limits =

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

FN means?

2 means?

Complete Activity 10 Tol-2 and Finish Tol-3

(#s 1, 2, & 3)

Complete Activity 10 Tol-2 and Finish Tol-3

(#s 1, 2, & 3)

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

assigned. (Fig.11.9, p 365)– Must be same for both parts– Basic sizes selected from chart in Table 11.2 (p

371) Deviation: difference between the actual size and the

basic size. (Fig 11.14, p. 369) (Tolerance) Upper deviation: difference between the max. size

limit & the basic size. – Hole system: Upper deviation (tolerance) is the

difference between the largest shaft size and the basic size (minimum clearance or maximum interference, also known as allowance).

– Shaft system: Upper deviation (tolerance) is the difference between the largest hole and the basic size (maximum clearance or minimum interference).

Lower deviation: difference between the min. size limit & the basic size. – Hole system: Lower deviation (tolerance) is the

difference between the smallest shaft size and the basic size (maximum clearance or minimum interference).

– Shaft system: Lower deviation (tolerance) is the difference between the smallest hole and the basic size (minimum clearance or maximum interference, also known as allowance).

Metric Fit - TermsMetric Fit - Terms

Fundamental deviation: the deviation closest to the basic size. (Fig 11.18, p 370)

Preferred Fits: There are 2 systems used to indicate preferred fits in the metric system. (Table 11.3, page 372)– Hole basis

» Capital H indicates Hole Basis System. – Shaft basis

» Any other capital letter, I.e. S, indicates Shaft Basis System

Basic Shaft SystemBasic Shaft System

Lower deviation (Maximum clearance)

Upper deviation (Minimum clearance / allowance)

Fundamental deviation for the hole equals the allowance

Basic size applied to shaft – the type of fit will determine whether the basic size will be the largest shaft or the smallest shaft. Fundamental deviation

for the shaft equals the basic size

Basic Hole SystemBasic Hole System

Basic size applied to hole – the type of fit will determine whether the basic size will be the largest hole or the smallest hole.

Upper deviation (Minimum clearance / allowance)

Fundamental deviation for the shaft equals the allowance

Lower deviation (Maximum clearance)

Fundamental deviation for the hole equals the basic size

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. (Page 370, Fig 11.17 & 11.16)

– There are 16 IT grades. The smaller the grade number, the smaller the tolerance zone.

– Tolerance Value. See Appendix 10, page a31, for IT grades.

Preferred Metric FitsPreferred Metric Fits Hole basis: the system of fits where the minimum

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

EX. 50H8– Basic size is 50 which is the DIA in mm– Tolerance Grade:

» Hole Symbol: H8» Fundamental deviation is H - capital H

refers to the Basic Hole system» Fundamental deviation value is the

nominal size: » IT grade is 8: Close Running fit

– Tolerance value for 8 is (Append. 10)

– Mating part is f7 (Append. 11-12)

» Shaft Symbol: f7» Fundamental deviation is f

– Fundamental deviation value is the allowance

– Hole Limits? (Append. 11-12)

– Shaft Limits? (Append. 11-12)

– ISO Symbol: = 50H8/f7

Preferred Metric FitsPreferred Metric Fits 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– Basic size is 50 which is the DIA in mm)– Tolerance Grade:

» Shaft Symbol: h7 » Fundamental deviation is h - lower case

h refers to the Basic Shaft system » Fundamental deviation value is the

nominal size: » IT grade is 7: Close Running fit

– Tolerance value for 7 is (Append. 10)

– Mating part is F8 (Append. 13-14)

» Hole Symbol: F8 – Fundamental deviation value is the

allowance (Append. 13-14)

– Hole Limits? (Append. 13-14)

– Shaft Limits? (Append. 13-14)

– ISO Symbol: 50h7/F8

Look UpLook Up

What type of fit is a 6H7/s6? (text has a typo – find H7/a6 and fix your text. It should be s instead of a)

– Hole or Shaft Basis System?– Type of Fit?

6S7/h6?– Hole or Shaft Basis System?– Type of Fit?

Shaft Basis System, DIA 32, Locational Interference– Write the specification (ISO Symbol)

for the above information.

Try the following problem in groups on

the board

Try the following problem in groups on

the board

Given: Basic DIA 41mm, use Hole Basis System, Sliding Fit

Write Specification:

Hole:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Maximum Clearance / Minimum Interference? =

Minimum Clearance / Maximum Interference? =

Allowance =

Shaft:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Try the following problem in groups on

the board

Try the following problem in groups on

the board

Given: Basic DIA 41mm, use Shaft Basis System, Sliding Fit

Write Specification:

Maximum Clearance / Minimum Interference =

Minimum Clearance / Maximum Interference =

Allowance =

Hole:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Shaft:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Try the following problem in groups on

the board

Try the following problem in groups on

the board

Given: Basic DIA 58mm, use Shaft Basis System, Force Fit

Write Specification:

Maximum Clearance / Minimum Interference =

Minimum Clearance / Maximum Interference =

Allowance =

Hole:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Shaft:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Complete Activity 11 Pages Tol-4 & 5

Complete Activity 11 Pages Tol-4 & 5