6 Dimensioning

• Numerals
• Dimensions
• Extension Lines
• Arrowheads
• Dimension Figures
• Isometric Dimensioning
• Orthographic Dimensioning

If a drawing is to be complete, so that the object represented by the drawing can be made as intended by the designer, it must tell two complete stories. It tells this with views, which describe the shape of the object, and with dimensions and notes, which gives sizes and other information needed to make the object.

Therefore, your next step is to learn the basics of dimensioning. In that way you will understand not only how to interpret a drawing to get the information you need, but also how to dimension your sketches so that they can be used to communicate size information to others.

Numerals

It may seem a bit basic, but a few exercises with the shapes of numbers comes before dimensioning. The reason for such review is simply that incorrectly or carelessly made numbers on a drawing or sketch can easily be misinterpreted by someone on the job. That can be costly.

Therefore, the study of numbers forms is justified.

The number forms presented here have been determined to be the most legible, and are used by industry nationwide. The United States standardized 1/8” vertical numbers are correctly formed as follows:

Dimension Lines

The dimension line is a fine, dark, solid line with arrowheads on each end. It indicates direction and extent of a dimension. In machine sketches and drawings, in which fractions and decimals are used for dimensions, the dimension line is usually broken near the middle to provide open space for the dimension numerals. In architectural and structural sketches and drawings, the numerals are usually above an unbroken dimension line.

 

In either case, the dimension line which is closest to the object should be placed approximately

1/2″ away. The other dimensions beyond the first dimension (if any) should be approximately 3/8″ apart. You do not necessarily have to remember this, but you should remember not to crowd your dimension lines and to keep them a uniform distance apart.

The most important thing is that the drawing needs to be “clean” and dimensions need to be located in a space where they cannot be confused with a surface they are not intended to be used for.

Here is how dimension lines should be sketched:

 

Note: Dimensions less than six feet (72 in.) are given in inches. Dimensions over six feet are usually shown in feet and inches.  Be sure that it is clear how dimensions are called out.  When calling out dimensions that are over 12”, make sure ALL of dimensions are called out in total inches or feet inches throughout the entire drawing.  Either 4’-5” or 53”, they both mean the same thing but if there is a mix of dimensioning it can become easy to look at 4’-8” and see 48”.

Extension Lines

Extension lines on a drawing are fine, dark, solid lines that extend outward from a point on a drawing to which a dimension refers. Usually, the dimension line meets the extension line at right angles. There should be a gap of about 1 / 16″ where the extension line would meet the outline of the object, and the extension line should go beyond the outermost arrowhead approximately 1 /8″. Also, there should be not gaps where extension lines cross. Notice in this example the larger dimensions are correctly placed outside, or beyond the shorter dimensions, and that the dimensions are preferably not drawn on the object itself. Sometimes, however, it is necessary to dimension on the object.

 

 

It is important to remember to place dimensions on the views, in a two or three view drawing, where they will be the most easily understood. Avoid dimensioning to a hidden line and avoid the duplication of dimensions. Use common sense; keep dimensions as clear and simple as possible. Remember, the person reading your drawing needs to clearly understand, beyond question, how to proceed. Otherwise, costly time and material will be wasted.

There are two basic methods of placing dimensions on a sketch. They may be placed so they read from the bottom of the sketch (unidirectional dimensions) or from the bottom and right side (aligned dimensions). The unidirectional system is usually best, because it is more easily read by workmen.

 

 

When dimensions will not fit in a space in the usual way, other methods are used to dimension clearly, when those crowded conditions exist.

 

 

Arrowheads

Arrowheads are placed at each end of dimension lines, on leader lines, etc. Correctly made, arrows are about 1/8” to 3/16” in length, and are about three times as long as they are wide. Usually they have a slight barb, much like a fishhook.

To make your drawing look clean, use the same style throughout your drawing or sketch.

 

Dimension numerals

Numerals used to dimension an object are normally about 1/8” in height.

When a dimension includes a fraction, the fraction is approximately 1 / 4″ in height, making the fractional numbers slightly smaller to allow for space above and below the fractional line.

Again, it is particularly important that the numbers and fractions you may put on a sketch or drawing be legible. Sloppy numbers can cause expensive mistakes.

 

Notes

Notes are used on drawings to provide supplementary information. They should be brief and carefully worded to avoid being misinterpreted, and located on the sketch in an uncrowded area. The leader lines going to the note should be kept short. Notes are usually added after a sketch has been dimensioned to avoid interference with dimensions.

 

 

Quiz

 

Directions: Dimension the examples as indicated.

 

Dimension this 3 ¼ x 6 15/32 rectangle unidirectionally on the top and right sides.