Of the two methods of making bolts, either by upsetting or by welding the heads, the first method is more common on small bolts and machine made bolts. The welded head is more commonly used for heavy hand forged bolts. The upset head is the stronger provided both are equally well made. The size of the bolt is always given as the diameter and length of shank or stem. Thus a bolt known as 1/2 inch by 6-inch, or 1/2-inch bolt 6 inches long, would mean a bolt having a shank 1/2 inch in diameter and 6 inches long from the under side of the head to the end of the stem, having the dimensions of the bolt shown in Fig. 72. The dimensions of the bolt heads are always the same for the same sized bolt, and are determined from the diameter of the shank. The diameter of the head at D, Fig. 72, is the distance across the head from flat side to flat side, and is known as the diameter across the flats. The thickness of the head is taken as shown at T. If S equals the diameter of the shank of the bolt, the dimensions of the head would be as follows:

Fig. 72. Bolts with Square and Hexagon Heads.

D = 1 1/2 x S+ 1/8 inch.

T = 8.

For a two-inch bolt the dimensions would be as follows:

Diameter of head D = l 1/2 x 2+ 1/8=3 1/3 inches.

The thickness of head T would be equal to diameter of the shank, or 2 inches. These dimensions are for rough or unfinished heads. Each dimension of a finished head is 1/16 inch less than the same dimension of a rough head. Bolts generally have the top corners of the head rounded or chamfered off. This may be done with a hand hammer; or with a cupping tool, which is simply a set hammer with the bottom face hollowed out into a cup shape.

Where large quantities of bolts are to be made, the bars are heated in a furnace and headed by special machinery. Where the work is done by hand the tools are of the simplest character. The method of upsetting is shown in Fig. 73. The header consists of a disc in which a hole has been drilled to correspond to the diameter of the bolt. A handle 12 or 15 inches in length is welded to the disc. Such a tool is shown in Fig. 74. The hole should be about 1/32 inch larger than the nominal size of iron. To make a bolt with this tool: First cut off the iron to the required length; then heat the end to be headed, to a dull straw color; strike the end with a hammer or against the anvil and upset it so that the portion intended for the formation of the head will not pass through the header. Then place the hole of the header over the square hole in the tail of the anvil and drop the cold end of the bolt through it.