As the name indicates, these drills are used for making holes in rocks
when blasting is to be done. The form and shapes most in use are shown
here.
The double drill, used so much in power drilling machines, is made in
the following way: Take a 1-in. octagonal bar of steel, any length.
Heat one end, and upset it to 1-1/4 in. in diameter. Put a small
1/2-in. fuller on the anvil and place the end of the bar lengthwise on
the fuller. Put the top fuller directly above this and strike with a
sledge hammer. The result is two fuller marks driven down within 1/2
in. of meeting. Reverse this and do the same on the other two sides.
This marks a cross.
[Illustration: Tools for rock drills used in machines]
[Illustration: Machine drill]
Place the forming tool in the anvil, and with a set hammer forge out
the wings as shown in the sketch. The diameter of the drill depends
upon the size of the hole to be drilled and is made accordingly. Use
a sharp chisel for cutting the cutting edges of the drill. It is now
ready for tempering. Drills are made in this way from the raw material.
When once made they are kept in shape by what is known as the drill
dressing tool shown here. These rock drills wear out on the sides,
thus reducing the diameter. When re-dressing they are always upset to
the required diameter. The re-dressing is done in the following way:
Heat the end of the drill in the fire and place the dressing tool so
that the cross impression fits the end of the drill. Drive on the end
lengthwise of the bar, upsetting the drill to the required diameter, at
the same time retaining the original shape of the cutting edge of the
drill.
_To temper rock drills:_ Take two small tubs of cold water, one clear
and the other made soapy by cutting up a bar of cheap soap into bits.
Heat the end of the drill to be tempered to a dull red and put it into
the tub of clear water, the cutting edge only under water until it is
cooled off. Take it out to polish. When the straw colour appears plunge
it into the tub of soapy water. This does away with any red heat that
might be in the bar when the temper colours appear. Soapy water is a
poor conductor of heat and it allows the steel to cool off without
suddenly chilling it.