Fig. 44 shows the cylinder head of a steam engine. Although in the drawing the entire circle is shown, it would have been just as clear if only one-half had been shown, similar to the manner of showing the side views of the gears just discussed. In the plan view, it should be noted that the tapped holes are indicated by double circles, while the drilled holes show a single circle. The inner circle for the tapped holes is intended to represent the bottom of the thread, while the outer circle represents the top of the thread. Another conventional method for a tapped hole is to fill in the circle entirely with black ink; the method illustrated, however, is the most common.
Fig. 45 shows a water cylinder for a triplex pump, and is an excellent illustration of many of the points heretofore brought out, combined on a single drawing. This drawing should be carefully studied in detail. Note the general boldness of the lines, and the sharp contrast between the full-line work and the center, dimension, and dotted lines. Note that in the cross-sectional views the dotted lines have been almost entirely eliminated, thereby leaving the section work clear and plain. There is no necessity of showing parts in dotted lines which are beyond the plane of the section, but in several places just enough simple dotted work is shown to convey the relation of the parts.
Fig. 45. Detail of Water Cylinder for Triplex Pump.
Each sectional view is made for a specific purpose, and that purpose is never allowed to be obscured. The top plan is mainly to show the upper portion of the water cylinder; the cross section below it gives the principal interior view; the right-hand half-elevation and cross section are for the purpose of showing the face of the valve chamber and the interior of the valve chamber; the horizontal cross section in the upper right-hand corner of the drawing shows not only the interior of the barrel and valve chamber at two different points, but also a clear outline of the base of the entire casting. Each view must be used with the other to get a clear idea of the construction; but each view is so simple in itself that no confusion arises in the mind as to what its lines mean; one view is readily associated with the other, and the grouping of the four views is such that the eye passes easily over all of them.
Note the grouping of the dimensions, following in general the purposes of each of the views as explained above. On the top plan are given the dimensions affecting the top of the casting only. On the principal vertical cross section are given the greater part of the dimensions for the entire piece. This is as it should be, for the dimensions should always be grouped as much as possible on the principal view of an object, provided they can be clearly put on that view and not become so numerous as to cause confusion. A drawing over which the eye has to wander widely in search of the several dimensions of the same portion, is slow and difficult to read. On the right-hand half-elevation and cross section are the figures for the outline of the face of the valve chamber, and the location of the tapped holes for the hand-hole cover-bolts. On the horizontal cross section are given the figures for the interior dimensions of the valve chamber, and a complete dimensioning of the base of the casting. The special attention of the student is called to this systematic grouping of the dimensions on the view which will most clearly show them. A glance at this drawing is sufficient to suggest what a confusion of figures there would have been, had it been attempted to place them all on two views, and if, instead of cross sections, full and dotted lines had been used.
On this drawing is indicated a new method of showing finished surfaces. Each surface which is intended to be finished in the machine shop, has drawn next to it a medium-weight line consisting of a long dash and two dots. This method of showing finished surfaces is not as common as the one heretofore used, of writing the letter f across the line; it has the advantage, however, of conveying an absolutely definite idea of the extent of the surface to be finished, and in some instances is especially valuable on this account. It is a good way of specifying the finish; but for general practice the letter / is simpler and perhaps more readily and universally understood.
This drawing, while not complicated, contains quite a large number of dimensions, and is a good example of the principle of systematic figuring. The student's attention, therefore, is called to the following analysis of the dimensions on the drawing.
The casting consists of a barrel, in which the plunger slides, with a stuffing box at the top and a waterway at the bottom leading into the valve chamber; attached to this barrel is the valve chamber, consisting of two compartments, the lower one for suction, the upper one for discharge; to support both barrel and valve chamber and permit of their being bolted to the water-supply casting, a rectangular base is provided.
Beginning at the top of the casting, the figures for the stuffing box, inside and outside dimensions and thicknesses are given, and note made that the outline is square. The tapped holes for the gland studs, and bosses for the drips, are shown most clearly in the top plan, and are therefore dimensioned there. Next we come to the bore of the barrel to receive the plunger, and here the square shape of the casting changes to a round, the diameter and thickness of metal being given. Below this cylindrical part is the waterway, the height of which (2½") is given, and then the V thickness of the base below. This completes practically all the dimensions of the barrel and stuffing box.
Passing to the valve chamber, it is first necessary to locate the center line of same in reference to the barrel (10¾" centers). This being done, the arrangement of valves is dimensioned, and figures given for the valve chambers, thus - length, breadth, depth, thickness of metal, fillets, etc.; then follow the location of the face of the valve chamber, 5¾" from the center line, and the layout for the hand-hole cover; then the location of the upper face of the valve chamber., 14¼" from the base, and the layout for the flange of the discharge pipe, which is shown on the top plan.
Few figures as yet have been placed on the base of the water cylinder; these are now completed by starting at one side of the base and going completely around same, giving not only external dimensions and radii, but also location of bolt-holes and their sizes - all of which are shown in the horizontal section.
This completes the dimensions; and if the student has carefully adhered to each particular part of the casting until completely dimensioned, and has not passed in haphazard fashion from one portion of the casting to another, he will have succeeded in dimensioning the piece with absolute completeness. No part will have escaped being dimensioned, and no part will be dimensioned twice. It would be a good plan for the student to copy this drawing, using a scale of 3 inches to the foot, and, in making the drawing, to follow the description as given above in reading the figures from the cut. He will thus more clearly realize the systematic progress from one part of the casting to the other, and will himself check the figures shown.
Fig. 46 shows the detail of a hoisting drum to carry wire rope. Attention is called in this detail to the enlarged cross section of the rim, conveniently placed to show clearly the style of the groove. It should also be noted, that, instead of drawing the grooves the entire length of the drum, but a few are drawn at each end of the drum and a note placed against same to indicate that the grooves are to be cut the entire length. This is another "short cut" consistent with the definition of a working drawing. The breaking away of a portion of a view is illustrated in the right-hand elevation, in which a small section is exposed to show the method of fastening the end of the wire rope.