Mechanical Design Drafting Basics Fundamentals Review

Engineering Drafting Guide and Resources

Drafting approach and philosophy.

The objective of dimensions are to convey to Manufacturing and Inspection the engineering requirements for the end item.  A given part can be dimensioned in many ways and although each method should yield the same requirements in practice this is rarely true. Careful consideration should be given to identifying a dimensioning scheme which conveys the form, fit, and function desired in the finished part. Before detailing an item consider the following:

1. Understand the requirements for the finished part and its interaction with the next level of assembly.

2. Work with Engineering and/or Design to define GD&T Datums which are consistent with the functionality of the finished part.

3. Define a dimensioning scheme to convey the finished part requirements.

4. Define a tolerancing scheme which insures the part will function throughout the range of manufacturing variability.  When adding tolerances, make sure they are achievable by the manufacturing process and that they can be inspected.

5. Don't ask manufacturing or inspection for more than is absolutely necessary to meet the design intent. 

Fundamental Rules

1. Show enough dimensions so that the intended sizes and shapes can be determined without calculating or assuming any distances.

2. State each dimension clearly so that it can be interpreted in only one way.

3. Show the dimensions between points, lines, or surfaces which have a necessary and specific relation to each other or which control the location of other components or mating parts.

4. Select and arrange dimensions to avoid the accumulation of tolerances that may permit various interpretations and cause unsatisfactory mating of parts and failure in use.

5. A feature shall not be located by more than one toleranced dimension in any one direction

6. A dimension shall be enclosed in parentheses "(     )" when it is (a) repeated on the same drawing, (b) specified on a subordinate document, (c) an accumulation of other dimensions, or (d) shown for informational purposes.

7. Where possible, dimension each feature in the view where it appears in profile and where its true shape is evident.

8. Do not show dimensions to lines representing hidden surfaces.

9. Where possible, express angular dimensions in degrees and decimal parts of a degree.

10. Where plus and minus tolerancing is used, the tolerance preference is bilateral and not unilateral. Equal plus and minus values are preferred.

11. Dimensions are shown outside the outline of the part unless clarity dictates otherwise.

12. Only end product dimensions and data are shown on drawings unless essential to the definition of design requirements.

13. Dimension, extension and leader lines shall not cross each other unless absolutely necessary. When it is unavoidable, a dimension line is never broken except for insertion of the dimension. An extension or leader line shall not run through a dimension nor shall it be broken except where it passes through or adjacent to arrowheads.

14. The word "TYPICAL" or the abbreviation "TYP" is not used. Indicate the number of places a dimension applies.

15. Use an industry standard dimensioning and tolerancing methods, such as ASME Y14.5-2009 GD&T.