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LAURENCE CLAUS HOW FASTENERS ARE MADE - PART 2: HOT HEADING AND SCREW MACHINING from page 126
through a cutter which allows the part to fully or partially
feed through the die. These threads are most likely cut
threads. The other method is to use a pair of cylindrical
roll dies (see Figure 6). These form the threads and can
provide however many revolutions are needed to fully
form the thread.
FIGURE 5: EXAMPLE OF DIE SEAM
An additional challenge of an Upsetter is the
incidence of die seams under the head. These occur
when the clam shell dies do not close tightly enough or
perhaps, they are wearing out. If this occurs, the dies
separate a little and material is squeezed into this void
leaving a seam (see Figure 5). If this seam is small, it
may simply be ignored but if it is significant it will have
to be removed as its presence will cause the part not
to properly seat flat in the application. When they must
be removed, they will be machined off in a post forming
secondary operation.
Like cold headed blanks, threads must be put on.
Parts in the smaller diameter range will be flat die
rolled, just like their cold headed counterparts. However,
parts that are large in diameter or very long require a
different method. Long parts are most often threaded
FIGURE 6: EXAMPLE OF CYLINDRICAL DIE ROLLER
Screw Machining
Cold and hot heading are relatively new process
technologies. The earliest modern fasteners were made
on screw machines. This technology can be traced all
the way back to the early 1800s. A screw machine is
an automatic lathe that can perform cutting and other
machining operations from the front and sides of the
workpiece. Today most parts are produced from bar stock
that is automatically fed into the machine. However, the
lesser used process of feeding individual blanks into the
machine is also possible. When this feeding method is
employed the “screw machine” moniker is changed to
“chucking.”
Like cold heading, the screw machining process
has fundamental capabilities such as shaving, parting,
threading, etc. that are shared by all different screw
machining equipment. However, the process is defined
primarily by the machine. Screw machines can be
classified into several distinct categories, either by the
number of spindles , the feeding method, or the type of
drive. Like cold headers which are often defined by the
number of dies or blows they can impose on a part, screw
machines are defined by the number of spindles that are
turned past machining stations of the same number. A
single spindle screw machine has only one station to
accomplish the desired work in, while multi-spindle screw
machines move the workpiece through a sequence of
two or more stations. By moving the workpiece through
multiple stations, more complex parts can be created.
The number of spindles is usually, although not always,
an even number. The most common variations are 1, 4,
6 and 8 spindle screw machines. When more than one
spindle is incorporated, the machine must index to each
new station.
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