SPRING 2024

162
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS HOW FASTENERS ARE MADE - PART 2: HOT HEADING AND SCREW MACHINING from page 156
This usually means that everything associated
with the part in that spindle must be able to index.
As a result, bar fed, multiple spindle screw machines
vaguely resemble a gatling gun as each spindle must be
independently fed with bar stock that indexes in a rotary
motion around the centerline of the machine (see Figure
7). The feeding method also serves as a distinguishing
characteristic of the process. Screw machines may feed
long, continuous bars through a bar feeding mechanism
or load individual blanks into a collet or jaws that rotate
on the spindle. Finally, screw machines may be known
by the type of drive the machine incorporates. Traditional
machines are cam driven. This means that a series
of cams time when the machine performs its specific
functions such as moving the tools in and out of the
workpiece, indexing the machine, feeding new stock
into the machine, etc. In a cam driven machine, each
operation must be carefully planned and precisely timed
so that the entire process becomes an elegant and
finely choreographed dance. Traditional and Swiss screw
machines are cam driven. Most late model equipment is
numerically controlled by a computer (CNC machines).
The CNC controller executes a predefined program which
controls the overall machine and the precise timing,
placement, and movement of each tool.
Although each machine works a little differently,
the fundamental concept is universal. The workpiece
(continuous bar or individual blank) is fixtured into
a holder (usually a split collet or a set of jaws like a
drill chuck) that is integral to the spindle and grasps
the workpiece allowing it to spin at the speed of the
rotating spindle. Non spinning tools are then moved into
the spinning part. The tools are fixtured into holders
attached to moving slides. The slide may come in from
the front on what is called the axial slide (see Figure 8)
or from the sides on what is called the cross slide (see
Figure 9). In some cases, there can be more than one
cross slide that comes in from different radial directions
relative to the spindle. Common features that come from
tools on the axial slide include end drilled holes, end
tapped threads, and end chamfers. Common features
that come from the tools on the cross slide include
shaved features like grooves, turned down shoulders,
chamfers, faced bearing surfaces, burnished surfaces,
checkering or axial knurls, and parting (part cut-off), to
name just a few. The workpiece is indexed past each
station available and when it reaches the final station it
is cut-off from the continuous bar or released from the
collet.
FIGURE 7: EXAMPLE OF MULTI-SPINDLE SCREW MACHINE
(3 OF 6 SPINDLES PICTURED HERE) FIGURE 8: EXAMPLE OF AXIAL SLIDE
CONTINUED ON PAGE 166