  Drawing Instruments.—For working the exercises in this book the student should be provided with the following:—A well-seasoned yellow pine drawing-board, 24 inches long, 17 inches wide, and 3/8 inch or 1/2 inch thick, provided with cross-bars on the back to give it strength and to prevent warping. A T square, with a blade 24 inches long attached permanently to the stock, but not sunk into it. One 45° and one 60° set square. The short edges of the former may be about 6 inches and the short edge of the latter about 5 inches long. A pair of compasses with pen and pencil attachments, and having legs from 5 inches to 6 inches long. A pair of dividers, with screw adjustment if possible. A pair of small steel spring pencil bows for drawing small circles, and a pair of small steel spring pen bows for inking in the same. A drawing pen for inking in straight lines. All compasses should have round points, and if possible needle points. A piece of india-rubber will also be required, besides two pencils, one marked H or HH and one marked HB or F; the latter to be used for lining in a drawing which is not to be inked in, or for freehand work. Pencils for mechanical drawing should be sharpened with a chisel point, and those for freehand work with a round point. Drawing-paper for working drawings may be secured to the board by drawing-pins, but the paper for finished drawings or drawings upon which there is to be a large amount of colouring should be stretched upon the board. The student should get the best instruments he can afford to buy, and he should rather have a few good instruments than a large box of inferior ones. Drawing-paper.—The names and sizes of the sheets of drawing paper are given in the following table:—
The above sizes must not be taken as exact. In practice they will be found to vary in some cases as much as an inch. Cartridge-paper is made in sheets of various sizes, and also in rolls. Hand-made paper is the best, but it is expensive. Good cartridge-paper is quite suitable for ordinary drawings. Centre Lines.—Drawings of most parts of machines will be found to be symmetrical about certain lines called centre lines. These lines should be drawn first with great care. On a pencil drawing centre lines should be thin continuous lines; in this book they are shown thus — - — - —. After drawing the centre line of any part the dimensions of that part must be marked off from the centre line, so as to insure that it really is the centre line of that part: thus in making a drawing of a rivet, such as is shown at (a) fig. 1, after drawing the centre line, half the diameter of the rivet would be marked off on each side of that line, in order to determine the lines for the sides of the rivet. Inking.—For inking in drawings the best Indian ink should be used, and not common writing ink. Common ink In cases where there are straight lines and arcs of circles touching one another ink in the arcs first, then the straight lines; in this way it is easier to hide the joints. Colouring.—Camel's-hair or sable brushes should be used; the latter are the best, but are much more expensive than the former. The colour should be rubbed down in a dish, and the tint should be light. The mistake which a beginner invariably makes is in having the colour of too dark a tint. First go over the part to be coloured with the brush and clean water for the purpose of damping it. Next dry with clean blotting-paper to take off any superfluous water. Then take another brush with the colour, and beginning at the top, work from left to right and downwards. If it is necessary to recolour any part let the first coating dry before beginning. Engineers have adopted certain colours to represent particular materials; these are given in the following table:— Table showing Colours used to represent Different Materials.
Printing.—A good drawing should have its title printed, a plain style of letter being used for this purpose, such as the following:— The following letters look well if they are well made, but they are much more difficult to draw. For remarks on a drawing the following style is most suitable:— All printing should be done by freehand. Border lines are seldom put on engineering drawings. Working Drawings.—A good working drawing should be prepared in the following manner. It must first be carefully outlined in pencil and then inked in. After this all parts cut by planes of section should be coloured, the colours used indicating the materials of which the parts are made. Parts which are round may also be lightly shaded with the brush and colours to suit the materials. The centre lines are now inked in with red or blue ink. The red ink may be prepared by rubbing down the cake of crimson lake, and the blue ink We may here point out the importance of putting the dimensions on a working drawing. If the drawing is not dimensioned, the workman must get his sizes from the drawing by applying his rule or a suitable scale. Now this operation takes time, and is very liable to result in error. Time is therefore saved, and the chance of error reduced, by marking the sizes in figures. In practice it is not usual to send original drawings from the drawing office to the workshop, but copies only. The copies may be produced by various 'processes,' or they may be tracings drawn by hand. Many engineers do not ink in their original drawings, but leave them in pencil; especially is this the case if the drawings are not likely to be much used. Scales.—The best scales are made of ivory, and are twelve inches long. Boxwood scales are much cheaper, although not so durable as those made of ivory. If the student does not care to go to the expense of ivory or boxwood scales, he can get paper ones very cheap, which will be quite sufficient for his purpose. The divisions of the scale should be marked down to its edge, so that measurements may be made by applying the scale directly to the drawing. For working such exercises as are in this book the student should be provided with the following scales:—
A scale of 1 is spoken of as 'full size,' and a scale of 1/2 as 'half size.' Engineers in this country state dimensions of machines in feet, inches, and fractions of an inch, the latter being the 1/2, 1/4, 1/8, 1/16, &c. In making calculations it is generally more convenient to use decimal fractions, and then substitute for the results the equivalent fractions in eighths, sixteenths, &c. The following table will be found useful for this purpose:— Decimal Equivalents of Fractions of an Inch.
Engineers use a single accent (´) to denote feet, and a double accent (´´) to denote inches. Thus 2´ 9´´ reads two feet nine inches. |
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