Dimension Units In Technical Drawings

Technical drawings are a common communication language in different professional areas such as architecture and engineering. A technical drawing must be understood and commented on by another reader of it, from another side of the world in the same profession. For example, a technical drawing that is prepared to explain the manufacturing instructions of a designed part must be understood by all manufacturers in the world. 

Technical Drawing with Engineering Graphics

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So, a technical drawing must be prepared according to defined standards. These standards are generally made by ANSI or ISO. Using and indicating dimensions and dimension units in technical drawings must be according to the standards. In this article, these standards are explained. 

Dimensions In Technical Drawings

Image Source: FREDERICK E. GIESECKE, SHAWNA LOCKHART, Technical Drawing With Engineering Graphics 15th Ed., Pearson, Pg.512.

In technical drawings in mechanical engineering, inches or millimeters are used as primary units. According to the size of the part or assembly, other kinds of units such as feet or meters can be used. 

The most important rule is, the common unit that is used for that technical drawing must be indicated at the drawing letterhead table on the right-down side. For example, you need to indicate the unit as Unit: mm. 

Millimeters are shown as ‘mm’ and inches are shown as ” at the end of the value. Examples: 10mm or 10″. Feet are indicated by ‘ and meters is indicated by ‘m’. And do not forget that 1 mm is equal to .04 in. 

Rounding The Dimensions In Technical Drawings

In technical drawings, rounding the decimal dimensions is a common practice. According to that practice, you need to follow these rules both for inches or millimeters. 

If the last number on the decimal side is smaller than 5, delete the last number then make no change in the previous number. For example, 3.563 mm can be rounded to 3.56. 

If that number is bigger than the 5, round up the decimal side. For example, 3.566 mm can be rounded up to 3,57 mm.

In general, even numbers are preferred for last numbers in technical drawings. Because of this, if the last number is 5, round this dimension to an even number. For example, 3.655 mm can be rounded to 3.66 mm. 

Showing The Two Of Units In Dimensions

Image Source: FREDERICK E. GIESECKE, SHAWNA LOCKHART, Technical Drawing With Engineering Graphics 15th Ed., Pearson, Pg.513

In technical drawings, both metric and imperial units can be shown in dimensions. It can be shown the metric is at up and imperial is at the downside of the dimension line. Also, if the inches are given as dimensions, the metric value of it can be shown in parentheses. 

Conclusion

These are the general rules to apply and follow the dimension in technical drawings. If you apply all of these rules in your technical drawings, your drawing will show its high quality. 

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