Line types used in technical drawing are used for different purposes to provide specific information to the people looking at the drawing. Drafting students or those reading the drawings have to learn what they mean, just as one learns a new language. It is a basic requirement and learnt early in drafting instruction.
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Object lines are solid heavy lines, .7mm to .9mm. These lines define the shape of the object portrayed and are the outermost outline of the object. A round bar is shown as a circle in one view and a rectangle in the other. Both would be drawn with object lines.
A centre line is a .3mm to .5mm line that alternates between short and long dashes. It is used to identify a hole as shown from the side. If a hole were in a plate, the centre line would locate the centre in the view where the feature isn't shown.
A hidden line is a .3mm to .5mm dashed line. It shows features, such as holes, in a view they are not in. The feature will be shown in another view of the drawing.
A break line is a .3mm to .5mm or .7mm to .9mm line with "Z" breaks, for a flat object, and "S" breaks, for a round object. These are used to show that a portion of the part is not shown. The area left out will not have any features that are unique yet is the same as what is shown. An example would be a rod that is threaded on both ends. Break lines would be used to eliminate the section between the threaded sections to shorten the object.
A section line is a .7mm to .9mm line drawn at angles, normally 45, 30 or 60 degrees, to show a feature more clearly. The cutting plane line is a .5mm dashed line with arrows on the end to show where it slices through the material.
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