Dimensioning Drawings.
To “dimension” working drawings is to place measurements upon the parts represented, to enable the workman to proceed without measuring the drawing itself.
These dimensions should be placed so as not to interfere with nor crowd the lines of the drawing, nor yet interfere with one another.
Arrow-heads are used at the extreme points of measurement, the figures are generally inserted midway between the arrows; a dot and dash line reaches from the figure to the arrow-heads, as shown below.
Dimension
When the dimension is short these lines are omitted and the dimension is placed outside the drawing, thusDimensionand connected by a curved line; at other times it is found needful to place arrow-heads outside the drawing and the measurement inside.Dimension
When the dimension is long and narrow it is usual to carry the dimensions under the drawing by dotted and dash lines, as shown below.
Dimension
Arrow-heads and figures should be drawn free-hand with a common writing pen.
Usually dimensions are given in inches, up to 24 inches, as it is found less confusing; for instance, if written 1′ 1″ it may be mistaken for 11″; if written 13″ no mistake could be made.
Dimension
Again, 1′ 0″ may be mistaken for 10″; if written 12″ it would not; in addition to being more distinct, it occupies less space on the drawing. In large measurements there is more room for the figures, and, therefore, they can be spaced further apart—in feet and inches.
All figures should be made of a fairly large size. Vertical dimensions should read from the right hand, thus, as shown:
Measurements of importance, such as the diameter of a circle, the pitch or distance apart of rivets and bolts, etc., should be marked in figures on the drawing. When rough or unfinished work is mixed with machined or finished portions, it is usual to mark F, or “fin.,” after the latter dimension.
In practice, at times, instead of dimensions reference letters are used, thus:
Machine partD = diam. of shaft, 21⁄2inches.L = length of bearing, 33⁄4inches.T = thickness of collar,7⁄8inch.d = diam. of collar, 31⁄2inches.
D = diam. of shaft, 21⁄2inches.L = length of bearing, 33⁄4inches.T = thickness of collar,7⁄8inch.d = diam. of collar, 31⁄2inches.
D = diam. of shaft, 21⁄2inches.L = length of bearing, 33⁄4inches.T = thickness of collar,7⁄8inch.d = diam. of collar, 31⁄2inches.
Generally it is preferable to give the diameters of turned and bored work on a section, instead of an end drawn separately; confusion is sometimes caused by a number of radial dimensions.
Fig. 242 and fig. 243are introduced to show the principal measurements required in practical work, and the usual way in which such dimensions are marked when ordering parts of machinery.
Fig. 242is a pedestal, or metal frame; three views are shown, the center figure being an elevation, the lower figure is the plan of the base, the upper figure is a view of the top, on which is bolted the bearing block, it being on the outside of the center figure. The essential measurements are marked by letters.His the vertical height from base to the seat of bearing block:Lbeing the length, andWthe width of the base;Pis the length between checks, andBthe width of seat for bearing block;Cis the distance from center to center of the holding down bolt holes, andTis the depth of the holes in the base;Kis the distance from center to center of the bolt holes in the top for bearing block.
Fig. 243is a hanger, or metal bracket, and shows the center figure or elevation, the plan of the top and the plan of the seat for bearing block, which is bolted on the interior of the center figure.His the vertical distance from the top to the seat for bearing block; the other measurements required are marked by letters similar tofigure 242.
Now, one of the important matters in connection with dimensioning a drawing is the location of the figures. One rule, whose utility cannot be gainsaid, is that they should be so located that they can be altered or erased without damage to the lines of the drawing, as changes may be necessitated either by original errors in writing down the figures or by changes in the design being found desirable during the construction of the machine.
Pedestal and hangerFig. 243.—Fig. 244.
Fig. 243.—Fig. 244.