Understanding Industrial Blueprints 169
Die.
A die is a tool used to form or stamp out metal parts or to cut external threads. Dies can be simple ob-
jects or made of a series of jigs and fixtures to ensure that the die makes contact with the metal stock at
the correct place and angle.
Tempering.
Tempering is a process of heat treatment to change the physical characteristics of ferrous alloys. The
object of tempering or drawing is to reduce the brittleness in hardened steel and to remove the internal
strains caused by the sudden cooling in the quenching bath. The tempering process consists of heating
the steel by various means to a certain temperature and then cooling it. The rate of cooling usually has
no effect on the metal structure during tempering. Therefore, the metal is usually allowed to cool in still
air. When steel is in a fully hardened condition, its structure consists largely of martensite. On reheating
the steel to a temperature of about 300 to 750 degrees F, a softer and tougher structure known as troos-
tite is formed. If the steel is reheated to a temperature of 750 to 1290 degrees F, a structure known as
a sorbite is formed, which, while lacking the strength of troostite, has far greater ductility. High-speed
steel is one of the few metals that will become harder instead of softer after it is tempered.
Drill.
A drill is a pointed tool that is rotated to cut holes in material. Figure 7.7A shows a typical drill with its re-
lated nomenclature and two holes,one 0.75 deep and the other through, as well as the operations of up-
right drilling machines.
Boring.
To bore is to enlarge and finish the surface of a cylindrical hole by the action of a rotating boring bar (cut-
ting tool) or by the action of a stationary tool pressed (fed) against the surface as the part is rotated. Bor-
ing produces a hole with a continuous inside diameter or a tapered or contoured diameter. When a
straight and smooth hole is required that is too large or odd-sized for drills or reamers, a boring tool can
be utilized to bore any size hole into which the tool holder will fit by inserting it into the drilling machine.
A boring bar with a tool bit installed is used for boring on larger drilling machines. To bore accurately,
the setup must be rigid, the machine sturdy, and a power feed used.
Reaming.
Reaming consists of enlarging a hole. It is similar to boring but is more precise and is implemented af-
ter boring or drilling. Reaming is performed with a drill machine or lathe; it is difficult if not impossible,to
drill a hole to an exact standard diameter. Where extra accuracy is specified, the holes should first be
drilled slightly undersized and then reamed to size. The majority of hand and machine reamers have a
slight chamfer at the tip to aid in alignment and starting (Figure 7.8).
Tapping
Tapping is the process of cutting a thread in a drilled hole and cutting internal threads. Tapping can be
accomplished on a lathe or drilling machine. On a drilling machine it is done by selecting and drilling the