Over the past few weeks I have written about the different ASME standards that impact areas of your drawing including the Drawing Title, General Notes & BOMs.  All of these pale in comparison to ASME Y14.5M-1994, Dimensioning and Tolerancing. I live by this standard; it has been both good to me and the bane of my existence. For the rest of you who actually have lives and don’t like to spend their free time reading standards, I am here to help. Over the coming weeks I will be covering various aspects of everyone’s favorite standard.

Fundamental Rules for Dimensioning and Tolerancing

1. All dimensions in a drawing must have a tolerance applied except when designated as MAX, MIN, REF or STOCK. Calling out tolerances globally in a title block or the general notes satisfies this requirement. Most companies use a combination of global tolerances and local tolerances applied to specific dimensions.
2. Dimensions created on a drawing must be clear and understandable without the need to scale the drawing. Scaling a drawing refers to using a scale, such as an engineering scale, to determine the size of a feature.
3. Each drawing must be fully defined, without overdefining the part, with using reference dimensions sparingly.
4. Drawings should not dictate the manufacturing process unless it it absolutely required to achieve the design intent.  No offense but despite how you feel about your machining skills most of the time the machinist would know more.
5. You can specify a dimension as NON-MANDATORY when they may be affected by finish, shrink, etc…
6. Arrange dimensions to be read easily and should only reference visible lines. Do not dimension to hidden lines, no matter how tempted you are by peer pressure, there is always a better way.
7. When items such as wires, sheet metal and rods use a gage or other code to to describe it size; use the linear dimension as the primary dimension and you may include the code in parenthesis as reference.
8. An angle of 90° is implied when two lines are perpendicular making a right angle that is not dimensioned. I have had many fun times trying to convince people of this. Take my advice, if you get disputed… bring out the book.
9. Unless otherwise noted on the drawing all dimensions apply at 20°C or 68°F. I hope your QC department brought their sweaters.
10. All dimension apply to free state condition.
11. Unless you specifiy otherwise on the dimension, it should apply to full depth, length or width of the feature.
12. Dimensions on a part apply only at the part level, it does not apply to the next assembly. If you want a dimension to also be applied at the assembly level you must include it in the assembly drawing.

This has probably been my shortest post since I started Standards Tuesday, don’t get used to it Next week We jump into the fun stuff…so get some rest and I will see you next week.

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