What would happen if your CAD system breaks down? Would you be able to put your engineering drawing on paper? These simplified geometrical constructions presented are those with which every engineer should be familiar with.
The purpose of this group of tutorials is to offer the reader an insight into the methods used in creating engineering drawings without the assistance of a computer aided design system. I will give details to various methods that illustrate the principles and applications of the fundamentals of engineering drawing.
How to bisect a straight line:
(a) With A and B as centers, Strike the intersection arcs as shown using any radius greater than one-half of AB. A straight line through points C and D bisects AB>
(b) Draw either 60⁰ or 45⁰ lines through E and F. Through their intersection draw the perpendicular GH that will bisect EF. Symbol
How to Trisect a straight line:
Given the line AB. Draw the lines AO and OB making a 30⁰ with AB. Similarly, dray CO and OD making 60⁰ with AB. AC equals CD equal DB
How to bisect and Angle:
(a) Given the angle BAC. Use and radius with the vertex A as a center, and strike and arc that intersects the sides of the angle at D and E. With D and E as centers and a radius larger than one-half of DE, draw intersecting arcs. Draw AF. Nagle BAF equals angle FAC
(b) Given an angle formed by the lines KL and MN having an inaccessible point of intersection. Draw BA parallel to KL and CA parallel to MN at the same distance MN as BA is from KL. Bisect angle BAC using the method explained in part (a). The bisector FA of angle BAC bisects the angle between the line KL and MN
How to draw Parallel Curved Lines about a Curved Center Line:
Draw a series of arcs having centers located at random along the given center line AB. Using a French curve, draw the required curved lines tangent to these arcs.
How to trisect an angle:
Given the angle BAC. Lay off along AB and convenient distance AD. Draw DE perpendicular to AC and DF parallel to AC. Place scale so that it passes through A with a distance equal to twice AD intercepted between the lines DE and DF. Angle HAC equals one-third of the angle BAC.
How to divide a straight Line into a given Number of Equal Parts:
Given the line LM, which is to be divided into five equal parts.
(a) Step off, with the dividers, five equal divisions along a line making any convenient angle with LM. Connect the last point P with M, and through the remaining points draw lines parallel to MP intersecting the given line. These lines divide LM into five equal parts.
(b) Some commercial draftsmen prefer a modification of the construction known as the scale method. For the first step, draw a vertical PM through point M. Place the scale so that the first mark of five equal divisions is at L and the last mark falls on PM. Locate the four intervening division points, and through these draw vertical intersecting the given line. The verticals will divide LM into five equal parts.
How to divide a line proportionally:
Given the line AB. Draw BC perpendicular to AB. Place the scale across A and BC so that the number of divisions intercepted is equal to the sum of the numbers representing the proportions. Mark off these proportions and draw lines parallel to BC to divide AB as required. The proportions in the figure are 1:2:3.
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