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Mechanical Drawing Tips Sitemap

01. Introduction - Mechanical drawing is entirely different from freehand drawing. In freehand drawing the artist takes a pencil and some paper and sketches roughly what he sees—whether it be a portrait, a landscape or a nude figure. In doing this he makes use of his creative ability—his artistic sense—as well as his knowledge of design and anatomy. If he is sufficiently creative and gifted, his drawing will be a "work of art" even though it will be far from an accurate representation of the real thing.

02. Your Equipment - As a beginner you need only the bare essentials; and there is no need to buy the very highest grade instruments until you are thoroughly familiar with their use. Here is a complete list of first essentials for everyone who is going to do mechanical drawing. The number after each article refers to the number on the drawing opposite.

03. T-Square - Follow these steps carefully and do everything that you are told to do:

FIRST:

Place a sheet of paper on your drawing board and secure it to the board by means of the tape or the thumb tacks (one tack in each corner). There should be about an inch margin all around the paper; or, in other words, the paper should be 2 inches shorter and 2 inches narrower than the drawing board.

04. Two Triangles - Next to the T-square the two triangles shown in Figure 2 are your most useful drawing board tools. All lines that you draw, with the exception of horizontal lines, are to be drawn with these triangles. Never use a ruler or a scale to draw lines—use the triangles.

05. Practise Drawings - With the aid of the T-square, the two triangles and the ruler, draw the following figures carefully and accurately, just the size that they are shown here:

Save your drawings of these diagrams to be inked in later.

06. The Protractor - The protractor, Figure 5, is used to measure angles just as the ruler is used to measure lengths.

Most protractors are in the form of a semicircle, marked off into 180 degrees, every ten degrees being numbered as 0, 10, 20, 30, etc. The marking usually goes from left to right, just the way an angle moves through an arc, but many protractors have a reverse scale beneath the main one, this reverse scale reading from right to left. Figure 5 shows these two scales on the protractor. Note also the vertical line in the center of the base. We'll call this line X.

07. Use Protractor - 1. Draw the following angles to a horizontal line:

14° 25° 39° 47° 54° 68° 77° 81°

2. Draw the following angles to a vertical line:

16° 33° 49° 66° 71° 80°

3. To any line drawn at any angle, draw a line at 48 degrees to the line at any point.

4. The base of a triangle is 3 inches long. The angle at the left is 124 degrees, and the left side is also 3 inches. What are the other two angles?

08. Scale Drawings - Because everything you draw must be drawn accurately to some scale, it is essential that you become thoroughly familiar with scale drawing.

A scale is a ruler marked off conveniently to represent proportionally larger or smaller distances. All mechanical drawings are drawn to some convenient scale.

09. Scale Drawings - 1. What is the length and the width of a rectangle 6 inches long and 4 inches wide when drawn to the half-size scale? The half-inch scale? The quarter-size scale?

2. Draw the following:

a. A square 8' 6" on a side to the half-inch scale.
b. A rectangle 29' 5" by 53' 4" to the eighth-inch scale.
c. A triangle whose sides are 16', 32' and 28', respectively, to the eighth-inch scale.

10. Drawings To Scale - From the following rough sketches make good, clear, accurate scale drawings, selecting the scale best suited for the size of your sheet of paper:

Save your drawings of these sketches to be inked in later.

11. The Instruments - Up to this point we have: been considering only the so-called board equipment which is made up of the drawing board, the T-square, two triangles, some good scales and some 2H pencils. The only "outside" implement we have discussed is the protractor, but nothing has been said about the set of instruments.

12. Geometric Figures - It is important for the student of mechanical drawing to become thoroughly familiar with many fundamental geometric figures, because they play a vital part in all branches of the subject. Most nuts, for example, are either hexagonal or octagonal, and a knowledge of regular polygons is necessary to facilitate the drawing of these machine parts. The same is true for cylinders, cones, pyramids, triangles and quadrilaterals. Undoubtedly you have already become acquainted with these geometric figures in your elementary high school geometry, but a review here will help to refresh your memory and show you their application to mechanical drawing.

13. Using Geometric - 1. What is the difference between a trapezoid and a rhomboid?

2. Define: Isosceles triangle, pentagon, regular pentagon, square.

3. What is the difference between a rhombus and a rhomboid?

4. Define: Parallelograms, quadrilaterals.

5. Which of the following statements are true?

14. Draftsman - In Figure 19 we see the line A-B. It is required to draw a line through A-B at right angles to it, cutting it in half.

METHOD: With B as a center and any radius greater than half A-B, draw an arc above and below the line A-B. With A as a center and the same radius, do the same thing. The two arcs will meet above and below A-B in the points O and 0'. Now join O with O'; this line bisects A-B at right angles.

15. Lines + Working - In every branch of drafting and mechanical drawing lines are standardized. A particular kind of line always means a certain thing, regardless of whether it is straight or curved or pencilled or inked. These lines are shown below, together with their application in a simple drawing. Line A is a medium-heavy solid line, always denoting the visible outline of any object.

16. Drawing Designs - If you have followed the foregoing instructions carefully, you should be able to construct the nine designs shown here. Numbers 1, 3, 6 and 8 are drawn with the compass only; the other designs require the additional use of the ruling pen. All these designs are easy to do, once you have laid them out in pencil. Number 1, for example, is merely a series of six incomplete circles centered on the diagonals of a regular hexagon. The rest are equally simple.

17. Shop Drawings - Now we come to a very important part of this book—the working drawing or shop drawing. As their name implies, these drawings are carefully drawn to scale, and are used by the mechanic or carpenter in making the actual tool or machine part. Every tool or machine part is made from a shop drawing, and hence, a knowledge of shop drawings is essential to every draftsman, or for that matter, to anyone else who wants to do mechanical drawing.

18. Hand Lettering - If you examine any type face, regardless of its size or particular style, a number of characteristics will become apparent. For one thing, you will notice that the letters M and W are wider, and that the I and small 1 narrower, than any of the other letters in the alphabet. In the case of the M and W, the width is slightly more than the height, particularly in the lower case letter (not capitals).

19. Perspective - Every draftsman must know the fundamentals of perspective. The architectural draftsman must be thoroughly familiar with the intricate details of the subject, and the mechanical draftsman should be able to make perspectives from shop drawings just as readily as he makes shop drawings from perspectives.

20. Isometrics - Nearly all isometric drawings are made with the 30-60 triangle, and most of them are done in angular "perspective" where the two sides make an angle of 120 degrees with each other and 30 degrees with the horizontal. In isometrics all sides that are parallel stay parallel and will not vanish to a vanishing point if prolonged. There are neither vanishing points nor foreshortening in isometric drawing. The difference between true perspective and isometric drawing is shown in the diagram, Figure 34, where A is the perspective of a block and B is the isometric drawing of it.

21. Sections - We have already seen the importance of perspectives, isometrics and shop drawings. Let us now consider a new and equally important subject known as SECTIONS.

A section of any object, such as a machine or machine part or a tool or anything else, is what we would see if we could slice or saw that object into two parts at the point where the section is to be taken. Everyone is familiar with the appearance of a baseball, but few of us know what a section through the ball looks like. From all outside appearances, the ball could be hollow and we would not know the difference.

22. General Review - 1. Divide a line 7 11/16 inches long into 9 equal parts.

2. A circle has a diameter of 7 inches. What is the length of one side of a regular polygon of 13 sides inscribed in it?

3. Put a circular plate on your paper and carefully draw a pencil line around it. Now ink in this circle with you pen compass.

4. With the aid of the compasses and French curve, draw the following ellipses:

Major axis 6 inches, minor axis 4 inches. Major axis 8 inches, minor axis 3½ inches.

THE END