WINTER 2024

176
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS HOW FASTENERS ARE MADE - PART 1: COLD HEADING from page 168
As a result, Open Extrusion processes are only able
to give about 30-35% reductions in area. In the second
forward extrusion, the part is entirely confined in the die
before any force is exerted on it. Since it is supported
by this confinement in the tool it can be pushed much
harder and cold headers can achieve 70 to 75%
reductions of area. This is known as a Trapped or Impact
Extrusion (See Figure 8). Finally, when a part is confined
and a pin is shoved into it, the result is that material will
flow backward along the pin. This is known as Reverse
or Backward Extrusion. Reverse Extrusion is how cold
headers can form hollow parts, nuts, and internal drive
recesses (See Figure 9).
FIGURE 7: EXAMPLE OF OPEN EXTRUSION
FIGURE 8: EXAMPLE OF TRAPPED EXTRUSION
FIGURE 10: EXAMPLE OF TRIMMED SQUARE HEAD
Piercing and Trimming
Although the cold heading process is inherently
free of scrap, there are two exceptions. Parts with nonround
heads such as hex or square heads are created
by forming a fully round disk and then trimming it with a
tool that shears the desired shape (See Figure 10). On
hex and square head bolts and screws the advantage of
a trimmed head is better engagement with the drive tool.
The disadvantage of trimming, however, is the generation
of some scrap and the necessity to have a station in the
cold heading machine to accomplish the trimming. In a
similar vein when creating a part with a through hole or
passage, such as a nut, it is most common for the hole
or hollow cylinder to be formed from both faces of the
part, resulting in a center web that separates the two
sides. In the final forming station, a pin is sent through
the part shearing this web and piercing a small slug
of scrap material (See Figure 11). Once again, a small
amount of scrap is generated but it is far less than if a
hole were to be drilled through the part.
FIGURE 9: EXAMPLE OF REVERSE EXTRUSION IN A NUT
FIGURE 11: EXAMPLE OF PIERCED SLUG – UPPER LEFT SHOWS
BEFORE PIERCING AND UPPER RIGHT SHOWS AFTER PIERCING
CONTINUED ON PAGE 180