Bung-head wire nails, or brads as the smaller sizes are called, have very small heads, which allow the nail to be sunk below the surface. This is done by means of the nail punch, or set, and is necessary when the surface is to be planed after the nailing.
26. Screws. Screws are much used, and allow the pieces to be readily taken apart. They are divided into two classes, flat head and round head, and are of steel or brass. Steel screws are either blued or bright. Bright screws are polished and blued screws are produced by treating the bright ones with heat or an acid.
Fig. 55. Methods of using Screws
[Fig. 55] shows a flat-head screw at a and a round-head at b. Flat heads are used for the more common work where it is desirable to have the screw head flush (even) with the surface or below it, while round heads are used where this is not necessary. In the latter case round heads are used partly because they are more ornamental. Flat heads must always be flush or below the surface, and in all but the softest woods it is necessary not only to bore a hole for the screw, but also to countersink it with a countersink bit in order that it may receive the head. Two methods of fastening with flat-head screws are shown in [Fig. 55].
Sketch A shows the two pieces of wood in position, the hole bored in upper piece (only) and countersunk; B shows the screw in position. In this case the screw head is visible. It is occasionally desirable to hide the screw entirely. Sketch C shows the hole prepared for the screw; D shows the screw in position and a circular wooden plug driven in over it. The plug is then leveled with the surface and the screw completely hidden.
Fig. 56. The Difference between Perspective and Mechanical Drawing
27. Mechanical Drawing. A mechanical, or working, drawing is quite different from a pictorial drawing such as an artist produces. The artist’s drawing represents objects as they appear, while the mechanical drawing represents them as they really are. Things in nature do not look as they are. For example, when we stand on a railroad track the rails appear to converge until they seem to meet in the distance. We know that this is not the case, that the rails are really everywhere equally distant. The optical illusion of the rails meeting at the horizon is called perspective. Mechanical, or constructive, drawing takes no account of perspective. In [Fig. 56] a is the perspective representation of a track, while b shows a track by mechanical drawing.
In a working drawing more than one view is necessary to show the true shape of an object.