ENGINEERING DRAWING LECTURE 4
Conventions Convention or Code: The representation of any matter by some sign or mark on the drawing is known as convention or code. The convention make the drawing simple and easy to draw.
Convention for Lines Since, engineering drawing is the systematic combination of different types of lines, it is therefore, essential for the students to have clearly in mind the difference between the various types of lines.
Detailed Description and Uses of Various Lines 1. Visible Outline or Object Line: The outline or object line is represented by thick line and used to show the outer visible feature of the object in the drawing. Every edge or surface that is visible is represented by these lines. 2. Section Line or Hatching Line: It is a thin continuous line and is used for the purpose of sectioning an object. (Note: The section lines are drawn at angle of 45 to the horizontal line and are spaced uniformly from 2 to 4 mm apart depending upon the size of the object)
3. Centre Line, Locus Line, Pitch Circles, Extreme Position of Moveable Parts and Parts Situated Infront of Cutting Plane: These lines are represented by long and short dashes in proportion ranging from 6:1 to 4:1, closely and evenly spaced in any drawing. The Proportional once selected should be maintained through the drawing these are used to show the centre and location of cylindrical, conical and spherical object. The following rules should be kept in mind while drawing centre lines of various objects:- i. The centre lines should not end at out line representing surfaces but should extend approximately from 2 to 5 mm beyond the out lines of the object ii. Where centre lines cross, the short dashes should ntersect at symmetrical. In case of very small circles, the short dashes should be neglected while drawing centre lines.
4. Hidden Line: The hidden line is represented by short dashes, closely and evenly spaced. It is used to show the invisible for hidden parts on the drawing. 5. Construction Line: It is a thin continuous line and is used for constructing an object. These Lines do not appear in finished drawing except in geometrical drawing. 6. Dimension Line: It is thin continuous line, for giving dimension. This line terminates arrow heads where the dimension lines meet the extension lines. 7. Extension Line: It is a thin continuous line used for dimensioning an object. 8. Projectors Line: It is a thin continuation of outlines and is used for drawing projector.
9. Cutting Plane Line: The cutting plane line is represented by thick long line at the ends with thin long and short lines at the centre. It is used to show the edge of the cutting plane. 10. Short Break Line: The short break line is represented by thin free-hand and is used to show the break of an object for a short length. It results in a saving in space and time used for drawing without loss of any details. 11. Long Break Line: The long line is represented by thin ruled line provided with free-hand zig-zags at suitable intervals and is used to show the break for a considerable length of the object.
Conventions For Various Materials In engineering practice, there are different types of materials used for manufacturing the various parts of a machine. It is therefore, desirable that different conventions should be adopted to differentiate various materials for convenience on the drawing. The conventions materials thus save time and labour of the drawing work.
Dimensioning The art of writing the various sizes or measurements on the finished drawing of an object is known as dimensioning.
Notation of Dimensioning The notation of dimensioning consists of dimension lines, extension lines, arrow heads, dimension figures, notes, symbols, etc. These notations are explained below:- 1. Dimension Line: Dimension line is a thin continuous line used to indicate the measurement which is shown by figure in a space above the dimension line or space left in the dimension line. 2. Extension Line: Extension line is also a thin continuous line extending beyond the outline of the object. It should extend about 3 mm beyond the dimension line. There should be a visible gap of 1.5 mm between the feature s corners and the end of the extension line.
3. Arrow Heads: Arrow heads are used to terminate dimension lines. These touch the extension lines and indicates the extent of a dimension. The length of the arrow-head is about three times its width, the space in the arrow-heads should be filled in. The size of the arrow-heads should be proportionate to the thickness of the lines of the drawing. 4. Dimension Figure: A numerical that indicates the size of a particular feature of an object is called dimension figure. 5. Leader (Pointer Line): A leader is a thin continuous line drawn from note of the figure to show where it applies. It is terminated by an arrow-head or a dot. The arrow-head touches the outline, whereas the dot is placed within the outline of the object. The leader is generally drawn at any convenient angle, usually 30, 45 and 60 but of not less than 30. (Note: The use of long leaders should be avoided even if it is involves repetition dimensions or notes)
6. Notes: A note on drawing gives complete information regarding specific operation relating to a feature. It is generally placed outside a view and read in such a way that the drawing is viewed from the bottom edge. 7. Symbol: A symbol is the representation of any object by some mark on the drawing. It is used to save time and labour of drawing work.
Spacing of Dimensions
Theory of Dimensioning An object may be considered to be made up of a number of geometrical shapes such as prism, cylinder, pyramid, cone, sphere, etc. It then becomes very simple to dimension these geometric forms in a manner that will show their individual sizes and location to each other. Thus, the following two types of dimensions are commonly used in engineering drawing: 1. Size Dimensions. 2. Location Dimensions.
Types of Dimensions Size Dimensions: The dimensions which indicate the various sizes of the object such as length, breadth, diameter, etc. are known as size dimensions. Location Dimensions: The dimensions which locate the position of one feature with respect to the other feature are known as location dimensions. Distances between the centre lines of the holes from the edges of features are given by location dimensions.
System of Placing Dimensions Aligned System: In this system, all dimensions are so placed that they may be read from the bottom or the right hand edges of the drawing sheet. Here all the dimensions are placed normal and above the dimensions lines (Latest Method). Unidirectional System: In this system, all dimensions are so placed that they may be read from bottom edge of the drawing sheet. Note, there is no restriction on the controlling the direction of the dimension lines.
Unit of Dimensioning As far s possible, all dimensions should be given preferably in one unit only, i.e in millimeters. The symbol of unit mm can therefore be omitted while writing each dimensions but a note is added in a prominent place near the tittle block that All dimensions are in millimeters.
Exercise