Chapter8_Dimensioning

CHAPTER 8

DIMENSIONING

8.1 THEORY OF DIMENSIONING

After the shape of an object has been described by ortographic views, the value of the drawing for the construction o fthe object depends upon dimensions and notes that describe the size. In general, the description of shape and size together gives complete information for producing the object represented.

The dimensions put on the drawing are those required for the proper functioning of the part after assembly. Before dimensioning the drawing of the functional requirements of the machine has to be studied and understood. These dimensions has to be selected so as to be readily usable by the workers who are to make the piece.

In dimensioning two vital rules will be folowed:

1. Each feature is dimensioned and positioned only once.

2. Each feature is dimensioned and positioned where its shape shows.

8.2 TECHNIQUES AND CONVENTION

A dimension is used to give the distance between two points, lines or planes, or between some combination of points, lines and planes. Extension lines lead the eye to the particular feature of described by the dimension. Notes are word statements giving information that cannot be given by the views and dimensions.


Figure 8.1

8.2.1 EXTENSION LINES

Extension lines should not touch the outline of the view but should start about 1.5 mm (1/16 in) from it.
Arrowhead lengths vary somewhat depending upon the size of the drawing as shown in the Fig.8.2. Dimension lines should be placed
in general 12 mm (1/2 in) away from the outlines of the view.

Figure 8.2

8.2.2 DIMENSION PLACEMENT

-Dimensions should be applied to one view.
-Dimension outside the view are preferred.
-Parallel dimension lines should be placed uniformly (shown in Fig.8.3a).
-Dimensions may also terminate at center lines or visible outlines of the view. Where a measurement between centers is to be shown, as at Fig.8.3b), the center lines are continued to serve as extension lines.
-Usually the outline of the view becomes the terminal for arrowheads, as at Fig.8.3c, when a dimension must be placed inside the view.
-Extension lines for an angular dimension are shown in Fig.8.3d, with one of the extension lines used for a linear dimension.
-Where a point is located by extension lines alone, the extension lines should pass through the point as in Fig.8e.


(a)	(b)

(c)	(d)

(e)
Figure 8.3

8.2.3 LEADERS

Leaders are straight (not curved) lines leading from a dimension value or an explanatory note to the feature on the drawing to which the note applies (Fig.8.4). An arrowhead is used at the pointing end of the leader. The note end of the leader should terminate with a short horizontal bar at the midheight of the lettering and should run to the beginning or the end of the note, never to the middle.
Leaders should be drawn at an angle to contrast with the principal lines of the drawing, which are mainly horizontal and vertical. Thus leaders are usually drawn at 30°, 45°, 60° or horizontal. When dimensioning a circular feature with a note, make the leader radial.

Figure 8.4

8.2.4 READING DIRECTION

Engineering drawings use two systems for reading direction, the aligned or the unidirectional. In the aligned system the figures are oriented to be read from a position perpendicular to the dimension line as shown in Fig.8.5, thus the guied lines for the figure will be parallel to the dimension line. The figures should be arranged so as to be read from the bottom or right side of the drawing.

Figure 8.5

The unidirectional system originated in the automotive and aircraft industries, and is sometimes called the "horizontal system". All figures are oriented to read from the bottom of the drawing as shown in Fig.8.6. Notes must be lettered horizontally and read from the bottom of the drawing in either system.

Figure 8.6

8.2.5 SELECTION OF DISTANCES

Any object can be broken down into a combination of basic geometric shapes like prisms and cylinders. If the size of each of these elementary shapes is dimensioned and the relative position of each is given, the dimensioning of any piece can be done systematically. Dimensions can thus be classified as dimensions of size and dimensions of positions.

8.2.5.1 DIMENSIONS OF SIZE

Since every solid has three dimensions, the object must has its height, width and depth.

	Prisms (square, rectangular or triangular ) requires 3 dimensions


	For regular hexagon or octogonal type prisms require 2 dimensions
	Cylinders require 2 dimensions
	Right cone requires 3 dimensions
	Right pyramid requires 5 dimensions
Figure 8.7

8.2.5.2 DIMENSIONS OF POSITION

After the basic shapes have been dimensioned for, size the position of each relative to the others must be given. Again position must be established in height, width and depth directions.

Figure 8.8

8.2.6 SELECTION OF DIMENSIONS

Selecting dimensions of position ordinarily requires more consideration than selecting dimensions of size, because there are several ways to give a position. In general positionla dimensions are given between the finished surfaces, center lines or combination of both as shown in Fig.8.9.


Figure 8.9

8.2.7 UNNECESSARY DIMENSIONS & DIMENSION CLARITY

In all cases of selecting dimensions the important consideration is clarity. One view of a part will usually describe the shape of some feature better than other views. A feature is best dimensioned in the view showing the shape of the feature. This principle is called "Contour Principle".
Use of the contour principle implies that duplicate or unnecessary dimensions are to be avoided. An unnecessary dimension, other than a duplicate, is a dimension that is not essential to make a part. Unnecessary dimensions always occur when all the individual dimensions are given in addtition to the overall dimension as shown in Fig.8.10.

Figure 8.10

Datum points, lines and edges of surfaces of a part are features that are assumed to be exact for purposes of computation or reference, and from which the positions of other features are established. In Fig.8.11a, the left side and bottom surfaces of the part are the datum surfaces, and at b the center lines of the central hole in the part are datum lines. Where apositions are specified by dimensions from a datum, different features are always positioned from this datum adn not with respect to one another.


(a)	(b)
Figure 8.11

8.2.8 PLACEMENT OF DIMENSIONS

In reading a drawing, it is natural to look for the dimensions of a given feature where that feature appears more characteristic and an advantage in clarity and in ease of reading will certainly result if the dimension is placed there. Some cautions should be taken into consideration while dimensioning:

1. Dimensions outside the view are preferred.
2. Dimensions should be applied to one view.
3. Dimensions should be placed on the view that shows the distance in its true length.
4. Dimension lines should be placed, in general 10 mm (1/2 in) away from the outlines of the view.
5. Parallel dimension lines should be placed uniformly.
6. Values should be midway(autocentered) between the arrow heads, except when a centerline interferes.
7. Dimensions arranged in continous form are preferred upon readibility.
8. Place a longer dimension line outside a shorter one.
9. Dimensions should never be crawded.
10. Center lines are used to indicate the symmetry, they should be considered as part of dimensioning.
11. All notes must be read horizontally.
12. Never use a center line, a line of a view, or an extension line as a dimension line.
13. Never allow outline of any kind to pass through a dimension line.
14. Avoid dimensioning to dashed lines if possible.