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 and

replacement parts

• Accuracy is Expensive

Recall…Some Fundamental Rules

• Dimensions shall be toleranced

• Dimensions shall be complete with no more

dimensions than necessary

• Drawings shall define functional requirements

without 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 specific

dimension is permitted to vary (difference

between the maximum and minimum limits).

• The dimension below has a tolerance of

.0003.

Maximum Material Condition

• When specifying tolerance dimensions, the

maximum material condition (MMC) means

the product or part contains the maximum

amount of material specified by the tolerance.

• The heaviest part.

Allowance

• Allowance is the minimum clearance or maximum interference intended between the maximum material

condition (MMC) of mating parts.

• The allowance for the system below is: 25.000 -

24.890 = 0.110

More Terminology

• Nominal Size - General identification in

fractions (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 one

nominal 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 for Hole and Shaft

• Remember values are in thousandths of an inch.

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

Over To

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

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

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

Over To

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

Over To

Specifying a Fit - Inches

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

Over To

–0.4

–0.7

+0.4

–0

0.4

1.11.19-1.97

–0.3

–0.55

+0.4

–0

0.3

0.950.71-1.19

–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

0.1

0.450-0.12

Shaftg4

HoleH5

StandardLimits

Limitsof

Clear.

Class RC 1

NominalSize Range

Inches

Over To

Specifying Fits - Metric• Determine type of fit and

find corresponding table

• Determine basic size

• Find size range on table

• Determine tolerances for

Hole 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 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

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 one

point are additive

• Tolerances from different directions to the same

point become additive.

• Eventually when the stack-up exceeds feature

tolerances, the parts do not fit together

Tolerance Stack-up

Tolerance Stack-up

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 the creation of complete product models.

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

• The development of the STEP standard increasingly

supports product manufacturing information � use of the 3D database throughout the enterprise.

• Industry is moving towards model-based definition of critical product characteristics, including dimensioning

and geometric controls.

References

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

• Neumann, A. (1996). Geometric Tolerancing Fundamentals Workbook

http://www.geotol.com/details.htm#gtfw

• ASME Y14.5

• ASME Y14.41

Acknowledgments

The author wishes to acknowledge the support from

the Society for Manufacturing Engineers -

Education Foundation, SME-EF Grant #5004 for

“Curriculum Modules in Product Lifecycle

Management.”