Engineering 1253 – Technical Drawing P



DIMENSIONING TERMS AND PROCEDURES

Arrowheads are placed at the ends of dimension lines to show the limits of the dimension. They are drawn 3 mm long, 1 mm wide, and either filled or open. When room between extension lines for both the dimension number and the arrows is insufficient, the number is placed between the extension lines and the arrows are placed outside.

A dimension is the numerical value that defines the size, location, or geometric characteristic of a feature. Dimension numbers on the drawing are the actual size of a feature. These numbers are written 3 mm high. In metric dimensioning, when the value is less than one, a zero precedes the decimal point. This is not the case in Imperial measurements. Individual identification of linear units is normally not shown. The drawing contains a note stating, “UNLESS OTHERWISE STATED, ALL DIMENSIONS ARE IN MILLIMETRES” (or “INCHES”) or the word METRIC can appear in the upper right corner of the drawing instead.

A scale is a measuring tool used to calculate distances on a drawing. Scale can refer to the “ruler” or to the mathematical ratio used to calculate the size difference between the actual object and the drawn representation of the object. When an actual feature is not drawn to scale, the dimension is underlined or accompanied by the abbreviation “NTS” (not to scale).

A dimension line is a thin, solid line that shows the extent and direction of a dimension. Dimension lines are normally broken in the middle to insert the dimension number. Dimension lines should not cross over other dimension lines. When they do, they are unbroken. The space between the first dimension line and the object is at least 10 mm. Subsequent dimension lines are spaced at least 6 mm apart.

An extension line is a thin, solid line perpendicular to a dimension line indicating which feature is associated with the dimension. There is a visible gap of 2 mm between the object being dimensioned and the extension line, and the extension line extends 3 mm beyond the last dimension line.

A leader line is a thin, sold line used to indicate the feature with which a dimension, note, or symbol is associated. It is a straight line drawn at an angle and is neither horizontal nor vertical. Leader lines are terminated with an arrow that just touches the part of the object. On the end opposite the arrow, the leader line has a 3 mm long, horizontal shoulder. Text is extended from this shoulder and is centered on it. Leader lines are commonly used for sizing circles and arcs, and when they are, they are drawn as radial lines to the circle or arc, i.e., they point toward the centre of the circles or arcs.

The diameter symbol, the Greek letter “phi” [Φ] preceding a numerical value, indicates the diameter of a circle.

The radius symbol, a capital letter “R” preceding a numerical value, indicates the radius of a circle.

DIMENSIONING SIZE AND LOCATION

Location dimensions specify where a feature, such as the centre of a hole, is located with respect to the edge of an object or other feature.

Size dimensions describe the size of features: overall height, length, thickness, diameter of a hole, etc. They apply to straight lines or distances and specify the linear distance between two points. Dimensions can be in any direction. There are two basic methods of linear dimensioning: continuous and common point.

• Continuous dimensions are point-to-point dimensions. The starting point for one dimension is the end point of the previous dimension.

• Common point dimensions are referenced from some fixed point, usually on the left of the drawing.

DIMENSIONING FEATURES

Angular dimensions specify the angle between two lines. The dimension line is drawn as an arc with the centre of the arc at the apex of the angle. It terminates with arrowheads at extension lines (extensions of the two sides of the angle). Right angles are assumed to be 90( and are not dimensioned.

Circular dimensions specify the location of the centre of a circle and either a diameter or a radius.

• Diameter is the full distance across a circle, measured through the centre. The diameter measurement is used on full circles and arcs greater than 180°. Dimension the diameter of circular holes in their “circular” view. Dimension cylindrical objects on the “rectangular” view of the drawing, not the “circular” view.

• Radius is the distance from the centre of a circle to a point on the circumference of the circle (or the distance from the centre of an arc to a point on that arc). The radius measurement is used on arcs equal to or less than 180°. Dimension the radius of circular arcs in the view where the true shape of the arc is seen.

TEN (of many) BEST PRACTICES OF DIMENSIONING

1. Dimensioning must be complete. The user must not be required to guess or measure the drawing.

2. Each feature of an object is dimensioned once only. If extra dimensions are required for clarity, they are enclosed in brackets or have the letters “REF” (reference) printed after them.

3. Dimensions specify the size and location of a feature. Dimension all sizes first, then the location of features.

4. Dimensions are placed on the most descriptive view of the feature being dimensioned. Dimension the feature where it is visible. Never dimension to a hidden line.

5. Dimensions should by placed on a view that shows the true length of the feature rather than a foreshortened length. This is particularly important when dimensioning features on a surface that are at an angle to the plane of the orthographic view.

6. Dimensions applying to two adjacent views are placed between these views. Dimensions should be attached to only one view. Extension lines should not connect these two views.

7. Dimensions are aligned and grouped to promote clarity and uniform appearance.

8. Dimensions are located outside the boundaries of the object.

9. Dimensions should be stacked with the shortest dimensions placed closest to the object and longer dimensions further away. Dimension lines should not cross one another.

10. Dimension numbers are oriented so that they can be read when the drawing is seen horizontally. Overall lengths are always given. For subdivisions of the overall length, one subdivision of the overall length is given implicitly: the others, explicitly.

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