The following illustrative drawings show the common practice in making working drawings, which it would be tedious and difficult to formulate as rules to guide the student. By a careful study of the illustrations, all of which are practical working drawings of a variety of pieces, and a close following of the description, more can be learned as to making a drawing than by adhering to a multitude of rules. After a study of the preceding pages, expounding the principles involved, the student will find most of his further questions answered by reference to Figs. 31 to 60 inclusive. While he may find that the methods of lines end dimensions shown in these figures are in some ways different from those he may see or hear of from other quarters, he should remember that the language of drawing differs widely in usage. The result desired, however, is always the same, namely, complete instructions to the workman. This is the sole test of a good drawing, however made, and the student's aim should always be to satisfy this requirement.
In making working drawings of a machine the detail draftsman must secure the several dimensions of the parts, either by measuring the general layout which has been made by the designing draftsman, or he may be given a rough sketch of the part, which he is to develop into an exact working shop drawing. Sometimes he may himself be called upon to go into the shop and measure up the parts of an existing machine, making his own sketches, and then detail drawings from them. Such sketches, while of the roughest kind, must be accurately and completely dimensioned at the time the sketch is made, as it is not always convenient or possible to make subsequent trips to the machine to fill in lacking dimensions on the sketch. The making of satisfactory sketches is not as easy as would at first appear, but is quite an art in itself, acquired by systematic action and experience.
An illustration of a sketch of this kind is shown in Fig. 31, the subject being a crank, keyed and clamped to a shaft. The first thing to do is to sketch the piece roughly but with sufficient care to enable the dimensions to be put on. (A soft or medium pencil is the best for such purpose and any scrap of paper or a sketching pad will suffice.) Each portion of the piece should then be separately considered and carefully gone, over to see that it is not only properly located but that it has the three dimensions, length, breadth, and thickness properly noted.
Thus in Fig. 31, the large hub should first be located by giving its distance 101/8" from the center of the smaller hub, then its diameter 5 3/4" should be given, the distance between its faces 3½", and 6" radius between them; next, the diameter of the hole to receive the shaft 21/1 5/6", after which it is noted that it has a keyway, the dimensions of which are necessary. Then the boss for the clamping bolt should be located by the figure 2 3/8" to its center, its location in the other direction being on the center line of the arm. This boss has a diameter of 2 3/8" and a length of 2 ¼" each side of the center to the finished surface, the depth of the counterbore being 1/16", and the width of the slot to the bore ¼, all of which dimensions should be carefully put on. The boss has a hole in it tapped at one end for a V bolt and drilled at the other end l 1/32". This completes the figuring of the large hub and we can proceed to dimension the other end of the arm. This has a diameter of 4 3/8", the thickness of the arm being 2", and there are facing pads on either side ¼" high, bringing the total distance from face to face 2½";.in order to show positively that these portions are central with the faces of the large hub, the figure ½" is put at one side; the diameter of these facing pads is 3½", and the hole through the head of the crank is 1 1/1 1/6" diameter. Having put on the above figures we now have to provide a connection between the head of the crank and the hub, and it therefore becomes necessary to give figures, for the size of the arm; the thickness of the arm has been already given as 2", and the width being the same as the diameter of the hub, the side lines are simply drawn tangent to the same; at the smaller end the width may be conveniently given along a line tangent to the facing pad as 3¾". The arm is filleted into the hub by 1" radius. The only thing now uncertain is whether the corners of the arm are sharp or rounded, and this is shown by the little section of the corner giving ¼" radius.
Fig. 31. Preliminary Sketch of Crank.
The above description is tedious and the dimensions can probably be put on more quickly than the discussion of them can be read, but it should be especially noted that the systematic method has been followed of taking each part of the piece separately and dimensioning it before taking up any other part. While this is not always entirely possible to do in complicated pieces, yet it is absolutely necessary that in general this principle be always followed; otherwise it is impossible to be sure that all dimensions are on.
The description above also applies to the dimensioning of the piece after it is drawn in detail, this being represented in Fig. 32.
Referring to this figure, the bold character of the drawing should be noted, the solid lines being strong and of absolutely different character from the center or dimension lines. There is no uncertainty about the direction or termination of the lines; the figures are bold, plainly made, and absolutely clear; there can be no possible excuse for the workman to read any of the lines or dimensions wrongly. In other words, the drawing satisfies the definition of a working drawing, as previously given, in that it conveys absolutely definite instructions to the workman, expressed in the simplest and most straightforward way.
On most machine parts a portion only of the surfaces are finished; and these are usually indicated, as previously noted, by the small letter f placed across the line representing the surface; this indicates to the pattern maker that he is to allow extra stock on the pattern, so that when the rough casting is made there will be sufficient metal to enable the finishing cut to be taken to the proper dimension. These finished surfaces are the most important surfaces of the piece, to which all the other parts have to be related. In order that the several parts of the machine may properly go together, it is necessary for the draftsman, in putting on the figures, to start from some one finished surface, and so arrange the figures that the machinist can readily work from one finished surface to another. In Fig. 32 the dimensions of the rough parts as given may not be exactly maintained in the casting, but the distances between the finished surfaces must be exactly secured. The method of figuring a keyway is illustrated in this figure, and it should be carefully noted that the depth of the keyway (5/16") is given from the corner where the side of the keyway intersects the bore; this is because the depth of the keyway is readily measured by scale from this point.
Fig. 32. Detail Drawing of Crank.
The thread for the 1" bolt is indicated in this case by a double line, the inside line representing the bottom, the outside representing the top of the thread, while the lines of the helix are entirely omitted. This is not as common a method of representing a thread as the conventional method previously described.