Useful Hints and “Points.”
Many of these “points” are repetitions, with but little variation from the way they have been previously stated; they are thus repeated to emphasize their practical worth.
A good draughtsman leaves his work in such a state that any competent person can without difficulty ink in what he has drawn.
The criterion of a good set of drawings is that with a properly prepared specification they are complete in themselves and require no explanation.
A “break” in a figure or object in a drawing is shown in rough irregular lines, as in [fig. 134], on [page 131]; this is useful when the paper is not large enough to show the whole.
Never use a sloping line in writing fractions on a drawing. The objection arises from the fact that such a dimension as 13⁄16, if written with the inclined line, unless very distinctly executed, may be read as 13⁄16.
In inking do not draw the lines further than you wish them to go, but in penciling it is well to extend the lines, free up.
Never use a scale for a ruler.
Do not overload the pen with ink.
Having filled the pen, nearly close the nibs and try the width of the line on a piece of paper or the margin of the drawing.
Never refill or lay the pen aside without first cleaning it.
The application of the science of geometry to the drawing-board is absolutely necessary to success, for the reason that the whole fabric of mechanical drawing rests on the principles of geometry, which is well termed the science of measurements.
Section lines should be the last inked and always without previous penciling.
Center lines are necessary in working drawings.
In choosing T-squares, care should be exercised to see that the head slides up and down the left-hand side of the board easily, and that when pressed against the board with the left hand there is no “slogging” of the blade up or down, or in other words, that the head is bearing firmly for its whole length against the board.
The best place for the title of a drawing is said to be the upper left-hand corner; this facilitates the filing of the sheet.
Never use a soft pencil except for finishing in shadow lines.
The rubber should always be kept clean.
Great care should be taken to keep drawing boards out of the way of heat or damp, as these cause the wood to warp.
Circles and curves are to be “inked in” before straight lines. First ink the smallest and afterwards the larger curves.
Do not press heavily on the pencil so as to cut the paper, but draw lightly, so that the mark can be erased and leave no trace, especially if the drawing is to be inked.
The draughtsman should commence his work at the top of the paper, keeping the lower part covered over until he needs to use it.
Shade lines should be avoided in all working drawings, as their use interferes with accurate measurements.
To make ink stick to the tracing cloth, with a woolen cloth rub some powdered chalk or pounce over the surface on which the ink lines are to be drawn, then wipe the surface clean and use a good quality of ink.
For striking small circles a small bow pen should be used.
To fix lead pencil marks on sketches so that they cannot be readily erased, sponge them with milk carefully skimmed, then lay blotting paper over them and iron with a hot flat-iron.
To have the ink preserve its fluidity and to keep out all dirt and dust, keep the cover on the ink slab; the mistake is often made of putting too liberal a supply of water in ink well, which causes a waste of both time and ink; no more should be prepared than to meet immediate requirements.
Always draw on the right side of the sheet, which can be found by holding the sheet up to the light and looking across its surface with the eye nearly in the same plane as the paper; note which side is the smoothest and has the least number of blemishes on it; this is the right side to draw on.
As to sharpening pencils, it is always best to cut a chisel point on the pencil used for drawing, and put a circular point on the pencils in the bow pencil and pencil leg. The chisel point makes a finer line and lasts much longer than a round point.
The varnish used in many large drawing-rooms is simply white shellac dissolved in alcohol; it requires a little experience to mix these to a proper consistency, but this is soon acquired.
Never sharpen your pencil over the drawing.
A center line of a drawing is the line upon which the figure is to be constructed; the center line is the first line to be drawn.
The T-square belongs to the left side of the drawing-board, and is operated by the left hand. The right hand should be kept free for the purpose of picking up pencil, pen and bows, adjusting and marking off. The left hand controls the T-square and the triangle that slides along the upper edge of the square; the right hand is for the instruments.
The advantage of a paper rule or scale is that the paper will expand and contract under varying degrees of atmospheric moisture the same as the drawing does.
Avoid rubbing out and constantly cleaning the drawing with India rubber; if wrong lines are made or it is desired to make alterations, the part to be changed should be rubbed out and completely re-drawn.
When using the bows see to it that the steel-pointed leg that is put down first on the paper, to secure a center for a curve or a circle, is a trifle longer than the pencil or pen leg.
To clearly indicate the position of a center which is to be used again, lightly pencil a small circle about it; never put the point of a pencil in the center hole to enlarge or blacken it; the prick point made by the dividers and needle points should be no more than can be just seen, hence the circle to be made as advised above.
Fig. 294.
Be particular in having the legs of the dividers exactly the same length, and sharp, so that in pricking off distances, and dimensions, and centers, the indent or hole made in the paper is as small as possible.
The term “plane” means a perfectly flat surface; that is, something which has length and breadth but no thickness.
The best way to indicate on the drawing the surfaces which are to be finished is to write on the lines which represent the finished surfaces “finished,” tool-finish, or “faced,” according to the degree of finish required. The single letter f is frequently used.
Avoid fingering the drawing sheet as much as possible; in pointing to any part of the drawing use a pencil and not the finger.
Remember that a drawing is made to be read.
The skill in inking does not depend on the fineness of the line, but on its clearness.
A soft pencil should never be used on a mechanical drawing unless in rare cases when it is used for pencil shading; the hardness or softness of pencils is denoted by letters.
Never ink any portion of a drawing until the penciling is entirely finished.
Stretching or pasting the paper to the board is very seldom resorted to, for the reason that the mechanical drawings are to scale and the paper is natural when pinned to the board and more correct than if under a strain. Mechanical drawings are always required in practice right away, and time would be wasted and lost in damping and pasting and drying again.
A working drawing, whether made to a scale or not, must have all the dimensions plainly written upon it, for a workman should never be compelled to measure a drawing.
In marking off distances, centers, etc., a fine needle point is useful; the hole should not be punctured through the paper, merely a prick point, so that it will leave an impression, which will not be obliterated by the use of rubber; drawing-pens are often equipped with such a needle point in the end of the handle, that is visible only when the pen is unscrewed from the handle; but in the absence of one of this kind the point of the divider leg will be of use.
Mechanical construction drawings represent a large amount of mental and manual work, as well as a considerable cost in money; hence, they are of value quite as much as property which has been acquired by the expenditure of either labor or capital. It is wise to keep copies of original designs and sketches, as well as data and formulæ, for record and comparison.
The best system for keeping drawings is to make them of certain standard sizes, and to keep them flat, unrolled, in drawers, numbered, lettered and labeled.
In an office where space is limited and drawings have to be rolled it is well to use a number of pasteboard cases about three feet long and three inches in diameter. These are shown in [fig. 294].
A puncture can be made near the top and, when a new drawing or blue-print is inserted in this cylindrical case, a cardboard tag can be looped through the puncture. This label will give the title and number of drawings in that case.
A manuscript book methodically and neatly kept should tell immediately the number of the drawing and the case.
[Fig. 293] is good for practice in line drawing and also as an optical illusion. “You look and are deceived. At first glance you say, ‘Of course, those two lines are curved.’ You are mistaken. They are exactly parallel. In order to prove this hold them up edgewise to the eye. It is, of course, the subsidiary lines which lead the vision astray. It is a case of first impressions being quite wrong.”
Fig. 295.