Mechanical Drawing Self-Taught Part 11

A double thread, such as in Figure 205, is drawn in the same way, except that the slant of the thread is doubled, and the square is to be set for the thread-pitch A, A, both for the tops and bottoms of the thread.

[Ill.u.s.tration: Fig 207.]

A round top and bottom thread, as the Whitworth thread, is drawn by single lines, as in Figure 206. A left-hand thread, Figure 207, is obviously drawn by the same process as a right-hand one, except that the slant of the thread is given in the opposite direction.

For screw threads of a large diameter it is not uncommon to draw in the thread curves as they appear to the eye, and the method of doing this is shown in Figure 208. The thread is first marked on both sides of the bolt, as explained, and instead of drawing, straight across the bolt, lines to represent the tops and bottoms of the thread, a template to draw the curves by is required. To get these curves, two half-circles, one equal in diameter to the top, and one equal to the bottom of the thread, are drawn, as in Figure 208.

[Ill.u.s.tration: Fig. 208.]

These half-circles are divided into any convenient number of equal divisions: thus in Figure 208, each has eight divisions, as _a_, _b_, _c_, etc., for the outer, and _i_, _j_, _k_, etc., for the inner one.

The pitch of the thread is then divided off by vertical lines into as many equal divisions as the half-circles are divided into, as by the lines _a_, _b_, _c_, etc., to _o_. Of these, the seven from _a_, to _h_, correspond to the seven from _a'_ to _g'_, and are for the top of the thread, and the seven from _i_ to _o_ correspond to the seven on the inner half-circle, as _i_, _j_, _k_, etc. Horizontal lines are then drawn from the points of the division to meet the vertical lines of division; thus the horizontal dotted line from _a'_ meets the vertical line _a_, and where they meet, as at A, a dot is made. Where the dotted line from _b'_ meets vertical line _b_, another dot is made, as at B, and so on until the point G is found. A curve drawn to pa.s.s from the top of the thread on one side of the bolt to the top of the other side, and pa.s.sing through these points, as from A to G, will be the curve for the top of the thread, and from this curve a template may be made to mark all the other thread-tops from, because manifestly all the tops of the thread on the bolt will be alike.

For the bottoms of the thread, lines are similarly drawn, as from _i'_ to meet _i_, where dot I is marked. J is got from _j'_ and _j_, while K is got from the intersection of _k'_ with _k_, and so on, the dots from I to O being those through which a curve is drawn for the bottom of the thread, and from this curve a template also may be made to mark all the thread bottoms. We have in our example used eight points of division in each half-circle, but either more or less points maybe used, the only requisite being that the pitch of the thread must be divided into as many divisions as the two half-circles are. But it is not absolutely necessary that both half-circles be divided into the same number of equal divisions. Thus, suppose the large half-circle were divided into ten divisions, then instead of the first half of the pitch being divided into eight (as from _a_ to _h_) it would require to have ten lines. But the inner half-circle may have eight only, as in our example. It is more convenient, however, to use the same number of divisions for both circles, so that they may both be divided together by lines radiating from the centre. The more the points of division, the greater number of points to draw the curves through; hence it is desirable to have as many as possible, which is governed by the pitch of the thread, it being obvious that the finer the pitch the less the number of distinct and clear divisions it is practicable to divide it into. In our example the angles of the thread are spread out to cause these lines to be thrown further apart than they would be in a bolt of that diameter; hence it will be seen that in threads of but two or three inches in diameter the lines would fall very close together, and would require to be drawn finely and with care to keep them distinct.

[Ill.u.s.tration: Fig. 208 _a_.]

[Ill.u.s.tration: Fig. 209.]

The curves for a United States standard form of thread are obtained in the same manner as from the $V$ thread in Figure 208, but the thread itself is more difficult to draw. The construction of this thread is shown in Figure 208, it having a flat place at the top and at the bottom of the thread. A common $V$ thread has its sides at an angle of 60 degrees, one to the other, the top and bottom meeting in a point. The United States standard is obtained from drawing a common $V$ thread and dividing its depth into eight equal divisions, as at _x_, in Figure 208 _a_, and cutting off one of these divisions at the top and filling in one at the bottom to form flat places, as shown in the figure. But the thread cannot be sketched on a bolt by this means unless temporary lines are used to get the thread from, these temporary lines being drawn to represent a bolt one-fourth the depth of the thread too large in diameter. Thus, in Figure 208 _a_, it is seen that cutting off one-eighth the depth of the thread reduces the diameter of the thread.

It is necessary, then, to draw the flat place on top of the thread first, the order of procedure being shown in Figure 209. The lines for the full diameter of the thread being drawn, the pitch is stepped off by arcs, as 1, 2, 3, etc.; and from these, arcs, as 4, 5, 6, etc., are marked for the width of the flat places at the tops of the threads.

Then one side of the thread is marked off by lines, as 7, which meet the arcs 1, 2, 3, etc., as at _a_, _c_, etc. Similar lines, as 8 and 9, are marked for the other side of the thread, these lines, 7, 8 and 9, projecting until they cross each other. Line 10 is then drawn, making a flat place at the bottom of the thread equal in width to that at the top. Line 12 is then drawn square across the bolt, starting from the bottom of the thread, and line 13 is drawn starting from the corner _f_ on one side of the thread and meeting line 12 on the other side of the thread, which gives the angle for the tops of the thread. The depth of the thread may then be marked on the other side of the bolt by the arcs _d_ and _e_, and the line 14. The tops of all the threads may then be drawn in, as by lines 15, 16, 17 and 18, and by lines, as 19, etc., the thread sides may be drawn on the other side of the bolt. All that remains is to join the bottoms of the threads by lines across the bolt, and the pencil lines will be complete, ready to ink in. If the thread is to be shown curved instead of drawn straight across, the curve may be obtained by the construction in Figure 208, which is similar to that in Figure 207, except that while the pitch is divided off into 16 divisions, the whole of these 16 divisions are not used to get the curves, some of them being used twice over; thus for the bottom the eight divisions from _b_ to _i_ are used, while for the tops the eight from _g_ to _o_ are used. Hence _g_, _h_ and _i_ are used for getting both curves, the divisions from _a_ to _b_ and from _o_ to _p_ being taken up by the flat top and bottom of the thread. It will be noted that in Figure 207, the top of the thread is drawn first, while in Figure 208 the bottom is drawn first, and that in the latter (for the U.S.

standard) the pitch is marked from centre to centre of the flats of the thread.

[Ill.u.s.tration: Fig. 210.]

To draw a square thread the pencil lines are marked in the order shown in Figure 210, in which 1 represents the centre line and 2, 3, 4 and 5, the diameter and depth of the thread. The pitch of the thread is marked off by arcs, as 6, 7, etc., or by laying a rule directly on the centre line and marking with a lead pencil. To obtain the slant of the thread, lines 8 and 9 are drawn, and from these line 10, touching 8 and 9 where they meet lines 2 and 5; the threads may then be drawn in from the arcs as 6, 7, etc. The side of the thread will show at the top and the bottom as at A B, because of the coa.r.s.e pitch and the thread on the other or unseen side of the bolt slants, as denoted by the lines 12, 13; and hence to draw the sides A B, the triangle must be set from one thread to the next on the opposite side of the bolt, as denoted by the dotted lines 12 and 13.

[Ill.u.s.tration: Fig. 211.]

If the curves of the thread are to be drawn in, they may be obtained as in Figure 211, which is substantially the same as described for a V thread. The curves _f_, representing the sides of the thread, terminate at the centre line _g_, and the curves _e_ are equidistant with the curves _c_ from the vertical lines _d_. As the curves _f_ above the line are the same as _f_ below the line, the template for _f_ need not be made to extend the whole distance across, but one-half only; as is shown by the dotted curve _g_, in the construction for finding the curve for square-threaded nuts in Figure 212.

[Ill.u.s.tration: Fig. 212.]

[Ill.u.s.tration: Fig. 213.]

A specimen of the form of template for drawing these curves is shown in Figure 213; _g_ _g_, is the centre line parallel to the edges R, S; lines _m_, _n_, represent the diameter of the thread at the top, and _o_, _p_, that at the bottom or root; edge _a_ is formed to the points (found by the constructions in the figures as already explained) for the tops of the thread, and edge _f_ is so formed for the curve at the thread bottoms. The edge, as S or R, is laid against the square-blade to steady it while drawing in the curves. It may be noted, however, that since the curve is the same below the centre line as it is above, the template may be made to serve for one-half the thread diameter, as at _f_, where it is made from _o_ to _g_, only being turned upside down to draw the other half of the curve; the notches cut out at _x_, _x_, are merely to let the pencil-lines in the drawing show plainly when setting the template.

When the thread of a nut is shown in section, it slants in the opposite direction to that which appears on the bolt-thread, because it shows the thread that fits to the opposite side of the bolt, which, therefore, slants in the opposite direction, as shown by the lines 12 and 13 in Figure 210.

In a top or end view of a nut the thread depth is usually shown by a simple circle, as in Figure 214.

[Ill.u.s.tration: Fig. 214.]

To draw a spiral spring, draw the centre line A, and lines B, C, Figure 215, distant apart the diameter the spring is to be less the diameter of the wire of which it is to be made. On the centre line A mark two lines _a b_, _c d_, representing the pitch of the spring. Divide the distance between _a_ and _b_ into four equal divisions, as by lines 1, 2, 3, letting line 3 meet line B. Line _e_ meeting the centre line at line _a_, and the line B at its intersection with line 3, is the angle of the coil on one side of the spring; hence it may be marked in at all the locations, as at _e f_, etc. These lines give at their intersections with the lines C and B the centres for the half circles _g_, which being drawn, the sides _h_, _i_, _j_, _k_, etc., of the spring, may all be marked in. By the lines _m_, _n_, _o_, _p_, the other sides of the spring may be marked in.

[Ill.u.s.tration: Fig. 215.]

The end of the spring is usually marked straight across, as at L. If it is required to draw the coils curved instead of straight across, a template must be made, the curve being obtained as already described for threads. It may be pointed out, however, that to obtain as accurate a division as possible of the lines that divide the pitch, the pitch may be divided upon a diagonal line, as F, Figure 216, which will greatly facilitate the operation.

[Ill.u.s.tration: Fig. 216.]

Before going into projections it may be as well to give some examples for practice.

[Ill.u.s.tration: Fig. 219. (Page 169.)]

CHAPTER IX.

_EXAMPLES FOR PRACTICE._

Figure 217 represents a simple example for practice, which the student may draw the size of the engraving, or he may draw it twice the size. It is a locomotive spring, composed of leaves or plates, held together by a central band.

[Ill.u.s.tration: Fig. 217.]

Figure 218 is an example of a stuffing box and gland, supposed to stand vertical, hence the gland has an oil cup or receptacle.

In Figure 219 are working drawings of a coupling rod, with the dimensions and directions marked in.

It may be remarked, however, that the drawings of a workshop are, where large quant.i.ties of the same kind of work is done, varied in character to suit some special departments--that is to say, special extra drawings are made for these departments. In Figures 220 and 221 is a drawing of a connecting rod drawn, put together as it would be for the lathe, vise or erecting shop.

[Ill.u.s.tration: Fig. 218.]

[Ill.u.s.tration: Fig. 219.]

[Ill.u.s.tration: Fig. 220.]

[Ill.u.s.tration: Fig. 221.]

To the two views shown there would be necessary detail sketches of the set screws, gibbs, and keys, all the rest being shown; the necessary dimensions being, of course, marked on the general drawing and on the details.

In so simple a thing as a connecting rod, however, there would be no question as to how the parts go together; hence detail drawings of each separate piece would answer for the lathe or vise bands.

But in many cases this would not be the case, and the drawing would require to show the parts put together, and be accompanied with such detail sketches as might be necessary to show parts that could not be clearly defined in the general views.

The blacksmith, for example, is only concerned with the making of the separate pieces, and has no concern as to how the parts go together.

Furthermore, there are parts and dimensions in the general drawing with which the blacksmith has nothing to do.

Thus the location and dimensions of the keyways, the dimensions of the bra.s.ses, and the location of the bolt holes, are matters that have no reference to the blacksmith's work, because the keyways, bolt holes, and set-screw holes would be cut out of the solid in the machine shop. It is customary, therefore, to send to the blacksmith shop drawings containing separate views of each piece drawn to the shape it is to be forged; and drawn full size, or else on a scale sufficiently large to make each part show clearly without close inspection, marking thereon the full sizes, and stating beneath the number of pieces of each detail. (As in Figure 222, which represents the iron work of the connecting rod in Figure 220). In some cases the finished sizes are marked, and it is left to the blacksmith's judgment how much to leave for the finishing. This is undesirable, because either the blacksmith is left to judge what parts are to be finished, or else there must be on the drawing instructions on this point, or else signs or symbols that are understood to convey the information. It is better, therefore, to make for the blacksmith a special sketch, and mark thereon the full-forged sizes, stating on the drawing that such is the case.

[Ill.u.s.tration: Fig. 222]

As to the material of which the pieces are to be made, the greater part of blacksmith work is made of wrought iron, and it is, therefore, unnecessary to write "wrought iron" beneath each piece. When the pieces are to be of steel, however, it should be marked on the drawing and beneath the piece. In special cases, as where the greater part of the work of the shop is of steel, the rule may, of course, be reversed, and the parts made of iron may be the ones marked, whereas when parts are sometimes of iron, and at others of steel, each piece should be marked.

As a general rule the blacksmith knows, from the custom of the shop or the nature of the work, what the quality or kind of iron is to be, and it is, therefore, only in exceptional cases that they need to be mentioned on the drawing. Thus in a carriage manufactory, Norway or Swede iron will be found, as well as the better grades of refined iron, but the blacksmith will know what iron to use, for certain parts, or the shop may be so regulated that the selection of the iron is not left to him. In marking the number of pieces required, it is better to use the word "thus" than the words "of this," or "off this," because it is shorter and more correct, for the forging is not taken off the drawing, nor is it of the same; the drawing gives the shape and the size, and the word "thus" conveys that idea better than "of," "off," or "like this."

In shops where there are many of the same pieces forged, the blacksmith is furnished with sheet-iron templates that he can lay directly upon the forging and test its dimensions at once, which is an excellent plan in large work. Such templates are, of course, made from the drawings, and it becomes a question as to whether their dimensions should be the forged or the finished ones. If they are the forged, they may cause trouble, because a forging may have a scant place that it is difficult for the blacksmith to bring up to the size of the template, and he is in doubt whether there is enough metal in the scant place to allow the job to clean up. It is better, therefore, to make them to finished sizes, so that he can see at once if the work will clean up, notwithstanding the scant place. This will lead to no errors in large work, because such work is marked out by lines, and the scant part will therefore be discovered by the machinist, who will line out the piece accordingly.