Tolerances. EML 2023 Computer Aided Design Introduction tolerancing –technique of dimensioning parts within a required range of variation to ensure interchangeability

Tolerances

EML 2023Computer Aided Design

Introduction

• tolerancing– technique of dimensioning parts within a required

range of variation to ensure interchangeability

• A tolerance should be as large as possible without interfering with the function of the part to minimize production costs.

2


Tolerance Dimensions

• unilateral tolerance

• bilateral tolerance

• limit tolerance

2.250+.000-.008

2.250 ± .004

2.2552.245

3


Mating Parts

4


Cylindrical Fits

5


Cylindrical Fits – English Units

6



7



8



9


Cylindrical Fits – English Unitstolerance - difference between the

limits of size for a single feature ; shaft .0025 ; hole .0040

limits of tolerance – the max and min sizes of a single feature

allowance – the tightest fit between two mating parts ; .0050

nominal size – the general size of a shaft or hole ; 1.5

basic size – the size to which the plus-and minus is applied

actual size – the measured size of the finished part

fit - the degree of tightness or looseness between two assembled parts

10



• clearance fit – gives a clearance between two assembled mated parts ;

• min clearance .005 ;

• max clearance .0115

• transition fit

• interference fit

• line fit

11



• clearance fit

• interference fit – a binding fit that requires the parts to be forced together, much as if they were welded

• transition fit – may range from a interference fit to a clearance fit between the mated parts

• line fit – results in surface contact or clearance when limits are reached

12



• The ANSI B4.1 standard defines a series of fits between cylindrical features in inches for the basic hole system.

• The types of fits covered in the standard are:– RC: running or sliding clearance fits– LC: clearance locational fits– LT: transition locational fits– LN: interference locational fits– FN: force and shrink fits

13



• RC: running or sliding clearance fits– RC1 to RC9

• LC: clearance locational fits• LC1 to LC11

• LT: transition locational fits• LT1 to LT6

• LN: interference locational fits• LN1 to LN3

• FN: force and shrink fits• FN1 to FN5

14


Example – RC9 Fit

• hole and shaft have a basic diameter of 2.5”• From Tables in Appendix 29

thousands of an inch

15


Example – LT3 Fit



2.50122.5000

2.50082.5001

16


Example – LN2 Fit



2.50122.5000

2.50212.5014

17

Example Problem

18


Cylindrical Fits – Metric Units

• ANSI B4.2 standard • basic size – the diameter from which limits are calculated

• upper and lower deviation – the difference between the hole or shaft size and the basic size

• tolerance - the difference between the maximum and minimum sizes

19



20



• fundamental deviation – a letter grade that describes the deviation closest to the basic size

• International Tolerance (IT) grade – a series of tolerances that vary with the basic size to provide a uniform level of accuracy within a given grade

• there are 18 IT grades: IT01, IT0, IT1, …, IT16

21



• Hole basis– a system of fits based on the minimum hole size as the basic

diameter– the fundamental deviation for a hole-basis system is “H”– Appendices 35 and 36 give hole-basis data for tolerances

• Shaft basis – a system of fits based on the maximum shaft size as the basic

diameter– the fundamental deviation for a hole-basis system is “h”– Appendices 37 and 38 give shaft-basis data for tolerances

22

23



24



25


Example 1

• determine the shaft and hole limits for:– hole-basis system– a close running fit– a basic diameter of 49 mm

26

27


Example 1

• use a preferred basic diameter of 50 mm

• use a fit of H8/f7

28


Example 2

• determine the shaft and hole limits for:– hole-basis system– a location transition fit– a basic diameter of 57 mm

29


30


Example 2


• use a fit of H7/k6

31


Example 3

• determine the shaft and hole limits for:– hole-basis system– a medium drive fit– a basic diameter of 96 mm

32

33


Example 3


• use a fit of H7/s6

34


Nonstandard Fits, Nonpreferred Sizes

• determine the shaft and hole limits for:– hole-basis system– a close running fit– a basic diameter of 45 mm

(do not change to a preferred size)

35

36


Nonstandard Fits, Nonpreferred Sizes

• use a fit of H8/f7

37

Documents

Tolerances. EML 2023 Computer Aided Design Introduction tolerancing –technique of dimensioning parts within a required range of variation to ensure interchangeability