Last week I started covering Section 2 of ASME Y14.5-1994, General Tolerancing and Related Principles, with the post Standards Wednesday – General Tolerancing Pt1. This week we will continue to explore this section of the ASME Y14.5 with Section 2.3, Tolerance Expression; Section 2.4, Interpretation of Limits and Section 2.5, Single Limits. Section 2 is a pretty hefty section and it will take us a few weeks to go through but we really need to cover it before we move on to GD&T.

Tolerance Expression

No, tolerances do not make faces. This section of ASME Y14.5 refers to the display of decimal places in tolerances. Depending on what side of pond you reside, the number of decimal places expressed in tolerances differs.

Tolerances in Millimeters

When creating drawings using millimeters, follow the guidelines shown below for tolerances.

Unilateral Tolerancing (millimeters)

Unilateral Dimensions in millimeters are shown with the zero value, in either the plus (+) of minus (-) value, without any additional decimal places.

Bilateral Tolerancing (millimeters)

Bilateral Dimensions are shown with the same number of decimal places on both the plus (+) and minus(-) value.

Limit Tolerancing (millimeters)

Limits Dimensions are shown with the same number of decimal places. If one of the values is a whole number then zeros are added for uniformity.

Basic Dimensions (millimeters)

When using Basic Dimensions the associated tolerance, such as the positional tolerance shown below, can have the number of decimal places required for the tolerance. However, the Basic Dimension must follow these guidelines:

• No Leading Zeros
• No Trailing Zeros
• No decimals added to whole numbers

Tolerances in Inches

When creating drawings using inches, follow the guidelines shown below for tolerances.

Unilateral Tolerancing (inches)

Unilateral Dimensions in inches must have the zero value matching the same number of decimal places as the other value ( either the plus (+) or minus (-) value) as well with the dimension value.

Bilateral Tolerances (inches)

Bilateral Dimensions in inches are displayed with the same number of decimal places in dimension value as well as the plus(+) and minus(-) values.

Limit Tolerances (inches)

In Limit Tolerances each value has the same number of decimal places…noticing a trend here?

Basic Dimensions (inches)

Same trend… When using Basic Dimensions the basic dimension has the same number of decimal places as the associated tolerance.

Angle Tolerances

On angles, the angle dimension and the tolerance must have the same number of decimal places regardless of drawing units.

Interpretation of Limits

This should go without saying but the limits expressed with the dimension tolerances are absolute. What does that mean? Well, no matter how many decimal places are shown on the print, it must be considered that the dimension has an infinite number of zeros trailing. If your print specifies a tolerance to be 0.125 and the actual part measures to be 0.12500000001, then guess what? It’s out of spec.

Plated or Coated Parts

If the part specified in the drawing is intended to be plated or coated, such as powder coating, the drawing must state one of the following in a general or local note:

• DIMENSIONAL LIMITS APPLY AFTER <insert process term>.
• DIMENSIONAL LIMITS APPLY BEFORE <insert process term>.

Single Limits

Single Limits refer to the practice of following a dimension with the term MIN or MAX. Single limits can be used only when the unspecified limit is defined somewhere else on the design. Using this approach is fine as long as the design can handle the specified dimension being zero or going out to infinity without hurting your design depending on the direction specified. In the example shown below, the radius is shown as R.250 MAX. This works because the radius cannot get any bigger and the designer has no problem with a sharp point on the corner.

By the way… I know your tempted to just write in MIN or MAX…Don’t you dare! In SolidWorks, you can select MIN or MAX in the Tolerance/Precision section of the Dimension PropertyManager.

Ya’ll Come Back Now, Ya’ Hear…

Make sure you come back next week. For sure I will be covering Tolerance Accumulation as per Section 2.6 and if I have time hopefully another section. Until then, make sure you keep asking me those questions…some of you have come up with some real head-scratchers.

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