In Fig. 7 at (a) is shown what is termed a square-head bolt. These are used for all common connections, such as the joining of two flanges or other surfaces which are not exposed to view. At (b) is shown a screw-head bolt, which is used for work where a countersunk head is necessary in order to obtain a smooth surface, the slot in the head being filled with putty after the work is in place.

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Fig. 7.

At (c) is shown a counter sunk-head bolt used for the same purpose as a screw head, and also for securing wooden stair treads to wrought-iron strings, so as to permit the head to finish flush with the tread. A button-head bolt with a square shoulder is also used for the same purpose, the shoulder aiding in the tightening of the nut. The countersunk bolt is also used in joining flat surfaces where a smooth finish is required. At (d) is shown a tap bolt. This is used for all connections where it would be impossible to get a nut on the end. The hole into which this bolt is to be screwed has a thread cut in it, or is tapped, as the technical phrase would express it.

At (e) is shown a countersunk tap bolt which is used where very heavy work is to be joined, and requires a smooth-finished surface. After the bolt has been tightened up by the aid of a wrench, the square head is cut off.

Lagscrews, shown at (f), are used to secure iron to stonework or woodwork; a plug of soft metal or wood is driven in a hole drilled in the stonework and into this the screw is inserted.

At (g) is shown a double-expansion bolt, which is used for fastening work to smooth surfaces, as brick or stone walls. This bolt consists of four pieces and a nut. The bolt is inserted in a hole drilled in the wall for it, and over it is slipped the expansion piece j; the expansion piece k is then put in place, and finally the expansion socket l. The nut is then turned down against the article to be secured, and the expansion socket l is forced down, while the bolt draws up, and the prongs on j and k expand, taking hold in the stonework and binding in the strongest manner possible. The single-expansion bolt (h) acts in the same way, and is used for the same purpose. The chief advantage of these bolts is that they may be taken out without damage to the surrounding stonework.

Expansion bolts are also made with screw heads, for use where the surface will be exposed to view.