Practical Mechanics for Boys Part 10

MEMORIZING ANGLES.--It is well to try and remember these lines by fixing the angles in the memory. A good plan is to divide any of the quadrants into thirds, as shown by the points E, F, and then remember that E is 30 degrees from the horizontal line B, and that F is 60 degrees. Or, you might say that F is 30 degrees from the vertical line A, and E 60 degrees from A. Either would be correct.

[Ill.u.s.tration: _Fig. 112. Section Lining_]

SECTION LINING.--In representing many parts of a machine, or article, it is necessary to show the parts cut off, which must be ill.u.s.trated by what is called "section lining." Adjacent parts should have the section lines running at right angles to each other, and always at 45 degrees.

Look at the outside and then the inside views of Fig. 112, and you will see how the contiguous parts have the angles at right angles, and clearly ill.u.s.trate how every part of the wrench is made. Skill in depicting an article, for the purpose of constructing it from the drawing, will make the actual work on the bench and lathe an easy one.

[Ill.u.s.tration: _Fig. 113. Drawing an Ellipse_]

MAKING ELLIPSES AND IRREGULAR CURVES.--This is the hardest thing to do with drawing tools. A properly constructed elliptical figure is difficult, princ.i.p.ally, because two different sized curves are required, and the pen runs from one curve into the other. If the two curves meet at the wrong place, you may be sure you will have a distorted ellipse.

Follow the directions given in connection with Fig. 113, and it will give you a good idea of merging the two lines.

First. Draw a horizontal line, A, which is in the direction of the major axis of the ellipse--that is, the longest distance across. The narrow part of the ellipse is called the minor axis.

Second. Draw a perpendicular line, B, which we will call the center of the ellipse, where it crosses the line A. This point must not be confounded with the _focus_. In a circle the focus is the exact center of the ring, but there is no such thing in an ellipse. Instead, there are two focal points, called the _foci_, as you will see presently.

Third. Step off two points or marking places, as we shall term them, equidistant from the line B, and marked C, C. These marks will then represent the diameter of the ellipse across its major axis.

Fourth. We must now get the diameter of the minor axis, along the line B. This distance will depend on the perspective you have of the figure.

If you look at a disk at an angle of about 30 degrees it will be half of the distance across the major axis.

So you may understand this examine Fig. 114. The first sketch shows the eye looking directly at the disk 1. In the second sketch the disk is at 30 degrees, and now the lines 2 2, from the eye, indicate that it is just half the width that it was when the lines 3 3 were projected. The marks D D, therefore, indicate the distance across the minor axis in Fig. 113.

[Ill.u.s.tration: _Fig. 114. Perspection in Angles_]

Fifth. We must now find the focal points of the ellipse. If the line A on each side of the cross line B is divided into four parts, the outer marks E may be used for the foci, and will be the places where the point of the compa.s.s, or bow pen, is to be placed.

Sixth. Describe a circle F, so it pa.s.ses through the mark C, and move the point of the compa.s.s to the center of the ellipse, at the star, and describe a circle line G, from the mark C to the line B. This will give a centering point H. Then draw a line I from H to E, and extend it through the circle F.

Seventh. If the point of the compa.s.s is now put at H, and the pencil or pen on the circle line F, the curve J can be drawn, so the latter curve and the curve F will thus merge perfectly at the line I.

THE FOCAL POINTS.--The focal points can be selected at any arbitrary point, between C and the line B, and the point H may be moved closer to or farther away from the line A, and you will succeed in making the ellipse correct, if you observe one thing, namely: The line I, which must always run from H to E, and intersects the circle F, is the starting or the ending point for the small curve F or the large circle J.

[Ill.u.s.tration: _Fig. 115. Fig. 116. Fig. 117. Perspectives of Cubes_]

ISOMETRIC AND PERSPECTIVE.--A figure may be drawn so as to show an isometric or a perspective view. Thus, a cube can be drawn so as to make an isometric figure, as in Fig. 115, where the three sides are equal to each other.

Isometric means a method of drawing any object in such a manner that the height, length and breadth may be shown in the proportion they really bear to each other. Fig. 115 has the sides not only equal to each other, in appearance to the eye, but they have the same outlines and angles.

Contrast this figure with Figs. 116 and 117. In Fig. 116 two of the sides are equal in angles and outline; and in Fig. 117 each side has a different outline, and different angles. Nevertheless, all the cubes are, in reality, of the same dimension.

THE PROTRACTOR.--This is a most useful tool for the draughtsman. It enables the user to readily find any angle. Fig. 118 shows an approved form of the tool for this purpose.

[Ill.u.s.tration: _Fig. 118. Protractor. Section Lining Metals_]

SUGGESTIONS IN DRAWING.--As in the use of all other tools, so with the drawing instrument, it must be kept in proper order. If the points are too fine they will cut the paper; if too blunt the lines will be ragged.

In whetting the points hold the pen at an angle of 12 degrees. Don't make too long an angle or slope, and every time you sharpen hold it at the same angle, so that it is ground back, and not at the point only.

[Ill.u.s.tration: _Fig. 119. Using the Protractor._]

HOLDING THE PEN.--The drawing pen should be held as nearly vertical as possible. Use the cleaning rag frequently. If the ink does not flow freely, after you have made a few strokes, as is frequently the case, gently press together the points. The least grit between the tines will cause an irregular flow.

INKS.--As prepared liquid inks are now universally used, a few suggestions might be well concerning them. After half the bottle has been used, add a half teaspoonful of water, shake it well, and then strain it through a fine cotton cloth. This will remove all grit and lint that is sure to get into the bottle however carefully it may be corked.

[Ill.u.s.tration: _Fig. 120. Section Lining Metals_]

TRACING CLOTH.--It is preferable to use the dull side of the tracing cloth for the reasons that, as the cloth is rolled with the glossy side inside, the figure when drawn on the other side will be uppermost, and will thus lie flat; and on the other hand, the ink will take better on the dull side.

If the ink does not flow freely, use chalk, fine pumice stone, or talc, and rub it in well with a clean cloth, and then wipe off well before beginning to trace.

DETAIL PAPER.--The detail paper, on which the drawing is first made in pencil, should show the figure accurately, particularly the points where the bow pen are to be used, as well as the measurement points for the straight lines.

HOW TO PROCEED.--Make the circles, curves, and irregular lines first, and then follow with the straight lines. Where the point of the circle pen must be used for a large number of lines, as, for instance, in shading, the smallest circles should be made first, and the largest circles last, because at every turn the centering hole becomes larger, and there is liability to make the circles more or less irregular. Such irregularity will not be so noticeable in the large curves as in the smaller ones.

INDICATING MATERIAL BY THE SECTION LINES.--In section lining different materials can be indicated by the character of the lines, shown in Fig.

120.

CHAPTER IX

TREATMENT AND USE OF METALS

ANNEALING.--A very important part of the novice's education is a knowledge pertaining to the annealing of metals. Unlike the artisan in wood, who works the materials as he finds them, the machinist can, and, in fact, with many of the substances, must prepare them so they can be handled or cut by the tools.

Annealing is one of the steps necessary with all cutting tools, and it is an absolute requirement with many metals for ordinary use, as well as for many other articles like gla.s.s. This is particularly true in the use of copper.

TOUGHNESS AND ELASTICITY.--It means the putting of metals in such a condition that they will not only be less brittle, but also tougher and more elastic. Many substances, like gla.s.s, must be annealed before they can be put in condition for use, as this material when first turned out is so brittle that the slightest touch will shatter it, so that it must be toughened.

Malleable or wrought iron, if subjected to pressure, becomes brittle, and it is necessary to anneal it. Otherwise, if used, for instance, for boiler plates, from the rolled sheets, it would stand but little pressure.

The most immediate use the boy will have is the treatment of steel. He must learn the necessity of this process, and that of tempering, in all his cutting tools, and in the making of machinery where some parts are required to be constructed of very hard metal.

THE PROCESS.--To anneal steel it must be heated to a bright cherry red and then gradually cooled down. For this purpose a bed of fine charcoal, or iron filings and lime, is prepared, in which the article is embedded, and permitted to remain until it is cold.

There are many ways of doing the work, particularly in the use of substances which will the most readily give up their carbon to the tool.

Yellow prussiate of potash is an excellent medium, and this is sprinkled over the cherry-heated article to be annealed. The process may be repeated several times.

TEMPERING.--This is the reverse of annealing as understood in the art.

The word itself does not mean to "harden," but to put into some intermediate state. For instance, "tempered clay" means a clay which has been softened so it can be readily worked.

On the other hand, a tempered steel tool is put into a condition where it is hardened, but this hardness is also accompanied by another quality, namely, _toughness_. For this reason, the word _temper_, and not _hardness_, is referred to. A lathe tool, if merely hardened, would be useless for that purpose.

TEMPERING CONTRASTED WITH ANNEALING.--It will be observed that in annealing three things are necessary: First, heating to a certain temperature; second, cooling slowly; third, the particular manner of cooling it.

In tempering, on the other hand, three things are also necessary: