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Dimensioning & Tolerancing Rules

TECH 4472

Y14.5M

ASME Standard.

Voluntary Consensus standard.

The Subcommittee consists of individuals from industry,

academia, National Laboratories,

self-employed individuals and the Department of Defense.

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Dimension

A numerical value that defines the size, location, orientation, form or other geometric characteristic.

Tolerance

The total amount a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.

Fundamental Rules

Which dimensions should have a tolerance?

All dimensions must have a tolerance.

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Exceptions:

1) Reference dimensions (minimize these)

2) Commercial stock size (specify as a decimal with gage or code numbers in parentheses)

3) Dimensions identified as “MAX” or “MIN”

Dimensioning Must Be Complete

No scaling

No assumptions

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No extra dimensions allowed. Only those required to define the part.

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Dimension with the end in mind

Dimensions will control the FUNCTION and MATING RELATIONSHIP.

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Shall not be subject to more than one interpretation.

Don’t tell the machinist how to do his job.

“The drawing should define a part without specifying manufacturing methods.”

Section 1.4e

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...unless it is essential to the engineering requirements.

If it looks square, it is.

90 degree angles are not specified.

Identify the Units

UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS (or INCHES).

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Difference between INCH and MM

MM

1) Use a leading zero

2) Omit decimal point and trailing zeros for whole numbers

3) Omit trailing zeros

INCH

1) Do not use leading zero

2) Number of digits matches tolerance. Use trailing zeros as necessary

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