Lines and Dimensions

Lines 
Lines is one important aspect of technical drawing.  Lines are always used to construct meaningful drawings. Various types of lines are used to construct drawing, each line used in some specific sense.  Lines are drawn following standard conventions mentioned in BIS (SP46:2003). A line may be curved, straight, continuous, segmented. It may be drawn as thin or thick. A few basic types of lines widely used in drawings are shown in Table 1.  

Table 1. Types of letters used in engineering drawing.

Line Strokes 
Line strokes refer to the directions of drawing straight and curved lines. The standards for lines is  given in  BIS : SP-46,  2003 
Vertical and inclined lines are drawn from top to bottom, horizontal lines are drawn from left to right. Curved lines are drawn from left to right or top to bottom. The direction of strokes are illustrated in figure 1.

Conventions used in lines

·       1. International systems of units (SI) – which is based on the meter.

·       2. Millimeter (mm) - The common SI unit of measure on engineering drawing.

·    3. Individual identification of linear units is not required if all dimensions on a drawing are in the same unit (mm).

·       4. The drawing should contain a note: ALL DIMENSIONS ARE IN MM. (Bottom left corner outside the title box)

5.Typical figures showing various lines used in the construction of engineering drawing is shown in figure 2.

A typical use of various lines in an engineering drawing is shown in figure below:

Dimensioning
The size and other details of the object essential for its construction and function, using lines, numerals, symbols, notes, etc are required to be indicated in a drawing by proper dimensioning. These dimensions indicated should be those that are essential for the production, inspection and functioning of the object and should be mistaken as those that are required to make the drawing of an object.  The dimensions are written either above the dimension lines or inserted at the middle by breaking the dimension lines.

Normally two types of dimensioning system exist. i.e. Aligned system and the unidirectional system.These are shown in figure 3.

In the aligned system the dimensions are placed perpendicular to the dimension line in such a way that it may be read from bottom edge or right hand edge of the drawing sheet. The horizontal and inclined dimension can be read from the bottom where as all the vertical dimensions can be read from the right hand side of the drawing sheet.
In the unidirectional system, the dimensions are so oriented such that they can be read from the bottom of the drawing.

Rules to be followed for dimensioning. Refer figure 4.

1. Each feature is dimensioned and positioned only once.

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

·        3Size dimensions – give the size of the component.

·       4. Every solid has three dimensions, each of the geometric shapes making up the object must have its height, width, and depth indicated in the dimensioning.

Dimensioning consists of the following:

·        A  thin, solid line that shows the extent and direction of a dimension.  Dimension lines are broken for insertion of the dimension numbers

·        Should be placed at least 10 mm away from the outline and all

·        other parallel dimensions should be at least 6 mm apart, or more, if space permits

The important elements of dimensioning consists of extension lines, leader line, arrows and dimensions.

Extension line – a thin, solid line perpendicular to a dimension line, indicating which feature is associated with the dimension. There should be a visible gap of 1.5 mm between the feature’s corners and the end of the extension line.Figure 5 shows extension lines.
Leader line 
A thin, solid line used to indicate the feature with which a dimension, note, or symbol is associated.   Generally this is  a straight line drawn at an angle that is neither horizontal nor vertical.  Leader line is terminated with an arrow touching the part or detail.  On the end opposite the arrow, the leader line will have a short, horizontal shoulder.  Text is extended from this shoulder such that the text height is centered  with the shoulder line

·        Arrows –  3 mm wide and should be 1/3rd as wide as they are long - symbols placed at the end of dimension lines to  show the limits of the dimension.  Arrows are uniform in size and style, regardless of the size of the drawing.Various types of arrows used for dimensioning is shown in figure 6.

Dimensioning of angles: The normal convention for dimensioning of angles are illustrated

Few examples during dimensioning of solids are shown below:

·        Prism –    This is the most common shape and requires three dimensions. Two dimensions shown on the  principal view and  the third dimension on the other view.

·        Cylinder – Cylinder is the second most common shape. It requires  two dimensions: diameter and length, both shown preferably on the rectangular view.

·        Cone – requires two dimensions – diameter of the base and altitude  on the same view and length. Both shown on the rectangular view is preferred.

·        Right pyramids – requires three dimensions – dimensions  of the base and altitude.

·        Spheres – requires   only one dimension. i.e. diameter. However in case of extra features, those dimensions are required to be provided.

RULES OF DIMENSIONING

1.    Between any two extension lines, there must be one and only one dimension line bearing one dimension.

2.    As far as possible, all the dimensions should be placed outside the views. Inside dimensions are preferred only if they are clearer and more easily readable.

3.    All the dimensions on a drawing must be shown using either Aligned System or Unidirectional System. In no case should, the two systems be mixed on the same drawing.

4.    The same unit of length should be used for all the dimensions on a drawing. The unit should not be written after each dimension, but a note mentioning the unit should be placed below the drawing.

5.    Dimension lines should not cross each other. Dimension lines should also not cross any other lines of the object.

6.    All dimensions must be given.

7.    Each dimension should be given only once. No dimension should be redundant.

8.    Do not use an outline or a centre line as a dimension line. A centre line may be extended to serve as an extension line.

9.    Avoid dimensioning hidden lines.

10.  For dimensions in series, adopt any one of the following ways.

i.        Chain dimensioning (Continuous dimensioning) All the dimensions are aligned in such a way that an arrowhead of one dimension touches tip-to-tip the arrowhead of the adjacent dimension. The overall dimension is placed outside the other smaller dimensions.

ii.        Parallel dimensioning (Progressive dimensioning) All the dimensions are shown from a common reference line. Obviously, all these dimensions share a common extension line. This method is adopted when dimensions have to be established from a particular datum surface

iii.        Combined dimensioning.  When both the methods, i.e., chain dimensioning and parallel dimensioning are used on the same drawing, the method of dimensioning is called combined dimensioning.