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Assembly Fits

Nathan W. Hartman, Ed.D.

Department of Computer Graphics Technology

Purdue University

Some material provided by Dr. Theodore Branoff, NC State University



Tolerance Dimensioning

• Why do we need tolerance dimensioning?

– Interchangeable parts manufacturing

– Parts are manufactured at widely separate localities

– Effective size control

– Modern industry relies on it for subcontracting andreplacement parts

• Accuracy is Expensive



Recall…Some Fundamental Rules

• Dimensions shall be toleranced

• Dimensions shall be complete with no moredimensions than necessary

• Drawings shall define functional requirementswithout specifying manufacturing methods.

• Decimal dimensions shall be used.

Per ASME Y14.5M-1994



Specification of Tolerances

Bilateral-EqualLimit Dimension

Bilateral-UnequalUnilateral



Tolerance

• Tolerance is the total amount a specificdimension is permitted to vary (differencebetween the maximum and minimum limits).

• The dimension below has a tolerance of.0003 .



Maximum Material Condition

• When specifying tolerance dimensions, themaximum material condition (MMC) meansthe product or part contains the maximum

amount of material specified by the tolerance.• The heaviest part.



Allowance

• Allowance is the minimum clearance or maximuminterference intended between the maximum materialcondition (MMC) of mating parts.

• The allowance for the system below is: 25.000 -

24.890 = 0.110



More Terminology

• Nominal Size - General identification infractions (ex. 1-1/2 for 1.500).

• Basic Size - General identification in decimal

(ex. 1.500).• Actual Size - Measured size.

• Limits - Maximum and minimum sizes

indicated by the tolerance dimensions.



Clearance Fit• Space is always left between parts.

• What is the allowance in this case?

• 1.5000 – 1.4988 = .0012



Interference Fit• Always an interference of material.

• What is the allowance in this case?

• 1.5000 – 1.5013 = -.0013 or just .0013



Transition Fit• Fit might result in clearance or interference.



Line Fit• Clearance or surface contact may result at assembly.



Basic Hole System (Hole Basis)• The minimum size hole is taken as the basic size.

• Used when standard tools are used to produce holes(reamers & broaches).



Basic Shaft System (Shaft Basis)• The maximum shaft size is taken as the basic size.

• When several parts having different fits, but onenominal size are required on a single shaft.



Specifying a Fit - Inches

• Determine type of fit and

find corresponding table• Determine basic size

• Find size range on table

• Determine tolerances forHole and Shaft

• Remember values are inthousandths of an inch.

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25

–0.45

+0.3

–0

0.25

0.750.40-0.71

–0.2

–0.35

+0.25

–0

0.2

0.60.24-0.40

–0.15

–0.3

+0.2

–0

0.15

0.50.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25

–0.45

+0.3

–0

0.25

0.750.40-0.71

–0.2

–0.35

+0.25

–0

0.2

0.60.24-0.40

–0.15

–0.3

+0.2

–0

0.15

0.50.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To



Specifying a Fit - Inches• RC1 - Close Sliding Fit

• Basic size of 1.500• Upper tolerance on hole is

+0.4 which is really+0.0004

• Lower tolerance on hole is-0.

• Upper tolerance on shaft is

-0.0004• Lower tolerance on shaft is

-0.0007

–0.4 –0.7+0.4 –00.41.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25 –0.45

+0.3 –0

0.250.75

0.40-0.71

–0.2 –0.35+0.25 –00.20.60.24-0.40

–0.15 –0.3

+0.2 –0

0.150.5

0.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To

–0.4 –0.7+0.4 –00.41.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25 –0.45

+0.3 –0

0.250.75

0.40-0.71

–0.2 –0.35+0.25 –00.20.60.24-0.40

–0.15 –0.3

+0.2 –0

0.150.5

0.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To



Specifying a Fit - Inches

–0.4 –0.7+0.4 –00.41.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25 –0.45

+0.3 –0

0.250.75

0.40-0.71

–0.2 –0.35+0.25 –00.20.60.24-0.40

–0.15

–0.3

+0.2

–0

0.15

0.50.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To

–0.4 –0.7+0.4 –00.41.11.19-1.97

–0.3 –0.55

+0.4 –0

0.30.95

0.71-1.19

–0.25 –0.45

+0.3 –0

0.250.75

0.40-0.71

–0.2 –0.35+0.25 –00.20.60.24-0.40

–0.15

–0.3

+0.2

–0

0.15

0.50.12-0.24

–0.1 –0.25

+0.2 –0

0.10.45

0-0.12

Shaftg4

HoleH5

Standard

Limits

Limitsof

Clear.

Class RC 1

Nominal

Size RangeInches

Over To



Specifying Fits - Metric• Determine type of fit and

find corresponding table• Determine basic size

• Find size range on table

• Determine tolerances forHole and Shaft

0.3700.110

24.89024.760

25.13025.000

25 MaxMin

0.3700.110

19.89019.760

20.13020.000

20 MaxMin

0.1800.060

0.9400.880

1.0601.060

1 MaxMin

FitShaftc11

HoleH11

Loose RunningBasicSize

0.3700.110

24.89024.760

25.13025.000

25 MaxMin

0.3700.110

19.89019.760

20.13020.000

20 MaxMin

0.1800.060

0.9400.880

1.0601.060

1 MaxMin

FitShaftc11

HoleH11

Loose RunningBasicSize



Specifying Fits - Metric• Loose Running Fit

• Basic size of 25

0.3700.110

24.89024.760

25.13025.000

25 MaxMin

0.3700.110

19.89019.760

20.13020.000

20 MaxMin

0.1800.060

0.9400.880

1.0601.060

1 MaxMin

FitShaftc11

HoleH11

Loose Running

BasicSize

0.3700.110

24.89024.760

25.13025.000

25 MaxMin

0.3700.110

19.89019.760

20.13020.000

20 MaxMin

0.1800.060

0.9400.880

1.0601.060

1 MaxMin

FitShaftc11

HoleH11

Loose Running

BasicSize



Functional Dimensioning

• Functional features come into contact with other parts

• Dimension and tolerance these features first



Tolerance Stack-up

• Tolerances taken in the same direction from onepoint are additive

• Tolerances from different directions to the same

point become additive.

• Eventually when the stack-up exceeds featuretolerances, the parts do not fit together



Tolerance Stack-up



Design Tolerance Distribution

Design Tolerance Distribution

70%

15%

10%5%

Manufacturing

Tooling

Inspection

Wear Allowance



How is this related to PLM?

• Understanding how parts fit together is critical for thecreation of complete product models.

• The ASME Y14.41 standard is being used morefrequently by industry.

• The development of the STEP standard increasinglysupports product manufacturing information use of the3D database throughout the enterprise.

• Industry is moving towards model-based definition ofcritical product characteristics, including dimensioningand geometric controls.



References

• Bertoline, G.R. & Wiebe, E.N. (2007). Fundamentals ofGraphic Communications (5e). McGraw-Hill: New York.

• Neumann, A. (1996). Geometric TolerancingFundamentals Workbookhttp://www.geotol.com/details.htm#gtfw

• ASME Y14.5

• ASME Y14.41



Acknowledgments

The author wishes to acknowledge the support fromthe Society for Manufacturing Engineers -Education Foundation, SME-EF Grant #5004 for

“Curriculum Modules in Product LifecycleManagement.”

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