Earnest T. Child.

Incidental to the practice work which the student must not neglect, it will be well to study as to the detail and general make-up of drawings. The first question that will naturally occur is : the class of line to use. The first tendency will be to use very light lines; but this should be avoided, especially in case the drawing is to be blue-printed, as fine lines are very apt to print out. The full lines should be about one-fiftieth of an inch wide, and the dotted lines about two-thirds as wide as the full. Those shown on page 41 in the December number are of the proper weight. It will be seen that shade lines are not used in this case. For regular shop work it is not generally found expedient to use shade lines, as they add to the cost of the drawing, and do not ordinarily add enough to the clearness of the drawing to pay for the outlay. For this reason, the drawings which will be given hereafter will omit shade lines. It will not do to entirely condemn the use of shade lines, as there are instances where a drawing will be incomplete without them. Our work will not be complete without a few words on this subject.

Mechanical Drawing III Working Drawings 67

Fig. 1.

In using shade lines on a drawing, it is generally assumed that the light is coming from the upper left-hand corner of the sheet. This will cause shadows to be cast on the lower and right-hand lines. In case one surface is nearer the observer than another, the lower and right-hand lines of the nearest surface will be shaded. When two surfaces representing different pieces are flush, the dividing line should not be shaded. In some instances it will be found very difficult to determine just which lines should be shaded, but the general rule given above will almost always apply. Shade the lower right-hand quadrant of outside circles and the upper left-hand quadrant of inside circles, and always keep the shading outside of the surface which they bound. Shade lines or circles should be tapered off gradually to the regular weight of line. When shade lines are used, the light lines should be much finer than one-fiftieth of an inch. (See Fig. 1.) The various classes of conventional lines are shown by Fig. 2. No. 1 shows a fine full line suitable for drawing when using shade lines- of the width shown by No. 2. The latter is the proper width of line where shading is omitted and the work has to be blue-printed. No. 3 shows a dotted line, No. 4 a dash line, No. 5 a dot-and-dash line, and No. 6 a double dot-and-dash line. It will be noted that Nos. 3 to 6 are finer than No. 2. This adds clearness and strength to the drawing.

Mechanical Drawing III Working Drawings 68

Fig. 2.

In expressing on the drawing the character of a surface other than a plane, it is often necessary to use what is called line shading. This is particularly effective in showing curved surfaces. To show a cylindrical surface (Fig. 3), draw parallel lines quite close together at the outside edge, but rapidly growing farther apart, until they stop about one-third way across the cylinder. The effect is strengthened by using heavier lines on the lower portion of the cylinder. In a similar way a sphere or a cone may be shown. A plane at an angle may be shown by parallel lines spreagd equidistant, one from the other, but this is seldom used to any great extent, as it is very apt to confuse the drawing.

A knurled surface is one which is cut by diagonal cross-grooves, to enable a firm hold to be secured, and is used particularly on hand check-nuts and cylindrical nuts. This is shown by diagonal cross-lines, and if the surface is a cylinder, he spacing is changed to give the effect of a round object (Fig. 4).

Working drawings are of several classes.

First.- Outline drawings, giving the general outline dimensions and space occupied, but not showing any detail or dotted work to speak of.

Mechanical Drawing III Working Drawings 69

Fig. 3.

Second. - Assembly or erection drawings, showing the entire machine, with all its parts in their proper positions. This class of drawings will contain a great deal of detail and dotted work, as often the greater part of the working mechanism will be concealed behind some part of the frame or some other part.

Mechanical Drawing III Working Drawings 70

Fig. 4.

Third. - Detail drawings, showing to a large scale the individual parts of the machine, with all the information necessary for their completion. It is allowable to show several details on a single sheet, and in the case of steel forgings it is customary to show several on one sheet.

Fourth. - Some machines are so complicated that it is necessary to make a separate sheet showing all the special bolts used in their construction, together with a bolt list, and in some drawing-rooms it is customary to make motion diagrams for each machine; but these are rather outside of our present field, and are only mentioned here incidentally.

In making drawings it is often impossible to draw them full, or finish, size, and accordingly we must adopt some scale, so that a large object may be shown in a comparatively small space. That is, one inch on the drawing will be made equal to so many feet of the actual object.

Mechanical Drawing III Working Drawings 71

Fig. 5.

Mechanical Drawing III Working Drawings 72

Fig. 6.

Mechanical Drawing III Working Drawings 73

Fig. 7.

Mechanical Drawing III Working Drawings 74

Fig. 8.

Referring to the November issue, page 15, we find given the common scales.

It is customary to make outline drawings in the proportion of 1/4" to 1', to 1/2" to 1'; erection drawings on 1" to 1', to 11/2 to 1'; and detail drawings on 3" to 1', to full size; but these scales are often varied, a great deal depending on the size of the sheet, the number of screws, and the size of the object to be shown. The delineation of nuts, bolts and screw threads varies greatly, some draftsmen always showing the threads in detail, and others merely showing them by conventional lines.

The student should first be made acquainted with the various styles of thread used, and also with the different classes of bolts, screws, etc.

The most common thread is the V type. A right-hand thread is one which,"when turned clock-wise, will screw in, and a left-hand thread screws in when turned to the left. A double thread is one having two distinct threads following each other around the screw, and the pitch, or distance which the screw moves forward with one revolution, is double that of a single thread of the same size.

Mechanical Drawing III Working Drawings 75

Fig. 5 shows a V thread drawn out in detail, one end showing a right-hand thread, the other a left-hand thread.

Fig. 6 shows the conventional method used for showing a V thread. If the screw is concealed, the lines will be shown dotted.

Fig. 7 shows a conventiona double thread. The ends of the screws are either rounded, as in Fig. 6, or chamfered, as in Fig. 7.

Fig. 8 shows a conventional square thread. A thread of this kind is extremely difficult to draw in detail, as it requires a full knowledge of descriptive geometry, and it will not be necessary to show it here, as it is rarely used in practice. If any long piece is to be threaded throughout its entire length, the threading may be shown at the ends only, and the fact that it is to be threaded throughout, covered by a note or by the use of a dimension line with "Thread " marked in place of a figure. Sometimes the outline of a screw is shown, as in Fig. 5, with the cross-lines omitted, but Fig. 6 is the most common method used. The great majority of bolts are fitted with V threads.

A tap-bolt is screwed direct into a tapped hole, and generally does not extend through the material into which it is screwed.

A through or nut bolt passes entirely through the material which it holds, being secured by a nut. It is customary to allow a thickness of nut equal to the diameter of bolt, and the extreme diameter of a hexagonal nut is approximately twice the diameter of the bolt.

A set-screw has a square head, the short diameter of which is equal to the diameter of the screw. The end of the screw is made nearly flat, to insure a good hold, as it is used for securing pulleys, collars, etc., to moving shafting. There are other types of bolts and screws used, which need not be described here. The method used in showing tapped holes is practically the same as for screws. The end view of a tapped hole may be shown by two concentric circles: by a full circle with a dotted one outside of it, or by a single circle with the word "Tap " written in, giving the size of the hole. It will be well for the student to make a practice sheet covering the work described above, and shown by the accompanying figures, in order to become familiar with the various classes of work described.

As stated in the December editorial, it is the intuition to use the details of a small upright engine, and finally the assembly of same as practice sheets, to be used in connection with our talks on mechanical drawing. One or more of the details will be presented with each number, giving the student sufficient work to occupy his spare time between the successive issues. These working drawings will be introduced in connection with the descriptive text, and will be illustrative of the work required in actual practice.

The detail given herewith shows the crank shaft with governor pulley on the left of the crank, and the eccentric on the right of the crank. The former is shrunk on to the shaft, and the latter is secured by means of two 1/2" round-head countersunk screws. It will be seen that the crank pin is slightly larger in diameter than the shaft.