Fig. 47 shows a crane drum, grooved for chain, and carrying its driving gear and shaft. This is a very good illustration of the economy of grouping parts together, instead of detailing them separately. It is obvious that the drawing of each detail is just as clear as though it were separately drawn. In fact, the information conveyed is the most complete possible; for not only are the figures for each part clearly shown, but the exact manner in which the parts go together, thus enabling the workman to understand at a glance the assembling of the parts, and to make his fits accordingly. A general drawing for this purpose alone has to be made in the case of a complicated machine; and it would be confusing to put on such a drawing figures sufficient to detail each part. The draftsman who can properly judge when to use assembled drawings for detail dimensions, and when to avoid such use, will save a large amount of time and money in the production of drawings for shop use. A common rule that "every part shall be detailed separately" is in vogue in many drafting rooms; but it is seldom followed literally, and when so followed becomes a drag on office efficiency. A better rule is - "Detail every part separately when groups of parts cannot be clearly detailed together".
Fig. 46. Detail of Hoisting Drum to Cany Wire Rope.
Note on this drawing the method of indicating, by light diagonal lines across the shaft, the location of the bearings; also the enlarged view of a few teeth of the gears, with sufficient figures for the pattern maker to work out the teeth. These gears are "half-shrouded", or strengthened by a rim extending up to the pitch line.
Fig. 48 shows the detail of an ordinary shaft with a number of different bearings and fits upon it. It illustrates most of the common points which are necessary to be specified on shaft details. The distance, shoulder to shoulder, is usually given throughout the entire length of the shaft, and these distances summed up for the "over-all" dimension. The "over-all" dimension is important, because from it the stock is ordered and cut off; and the workman should not be required to add up a lot of figures to secure it. Fillets should be allowed at every shoulder, if possible, and their radius specified; in this way there is less liability to the formation of incipient cracks than if the corners are left sharp. Keyways should be carefully dimensioned and located. Bearings should be indicated by light lines running across the shaft diagonally; and it is good practice to print the name of each piece to which the shaft is fitted, just above the shaft at the point where such fit occurs, or the parts themselves are partially shown in light fines, as at A and B in the figure, thus enabling the workman to make the fits more intelligently. It is common practice to make all holes which receive shafts of exactly "gauge diameter", and to make the allowance for the fit in the shaft. For example, a "3" running fit" would mean that the hole in the piece to receive the shaft would be exactly 3" in diameter, while the shaft would be, say, "3" less .003"". Sometimes this allowance is indicated by giving the actual number of thousandths of an inch under size, as noted; sometimes by calling for a "running fit", or a "wringing fit", or a "pressed fit", or a "drive fit", or a "tight fit", as desired.
Shafts are often so long that it is difficult to represent their entire length on the sheet to the scale chosen. They are then "broken", as shown in the figure, and crowded up to a shorter length, the dimensions being depended upon to give the proper relation of the parts.
When there is occasion, because of some change, to alter a dimension on a finished drawing, it is usually permissable to change the dimensions without rubbing out the lines of the drawing, provided that no considerable number of other dimensions are affected, and provided that some sign or note is made on the drawing, calling attention to the fact that the dimension has been changed and that the drawing is "out of scale". Sometimes the dimension is placed in a circle thus o or a line drawn beneath it thus, 6"; or the words "out of scale" placed after it thus, 6" (out of scale). Although workmen are not allowed to "scale" drawings, yet it is dangerous to have dimensions which are out of scale on the drawings unless special attention is called to that fact.
The above remarks on "broken" pieces and "out-of-scale" dimensions are equally applicable to all details as to shafts, the points merely being illustrated by the figure under discussion.