03-01-2013, 03:53 PM
SPECIAL-PURPOSE MACHINE TOOLS
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Introduction
In contrast to standard machine tools, special-purpose machine tools have their construction,
tooling, work location and kinematics designed for specific operations. Some of such
machines, e.g. railway-axle lathes and drilling machines with interchangeable drilling heads,
have been discussed in previous chapters as special versions of standard machines. In this
chapter, further special-purpose machine tools are described, but owing to the great variety of
such machines, this presentation does not claim to be complete.
10.2 Special-purpose machine tools for metal forming and metal separation
10.2.1 Straightening machines
For the manufacture of screws and springs, the raw material is bright round bar. The
straightness of these bars must be of a high order, as the machining processes (e.g. bar
turning or centreless grinding) are not by themselves able to correct bends in the material. The
straightness required is achieved by the use of a separate straightening process.
Figure 10.1 provides a survey of some straightening processes. These may be
fundamentally classified as either continuous or discontinuous. The latter include straightening
processes using a straightening jack or straightening roller, as well as those carried out in a
straightening press. A subdivision of the continuous process is based on whether the bar is
rotating. Machines where the bar is not rotating include multi-roller straightening machines and
those with a revolving straightening head. Technology using revolving bars are generally
known as diagonal-roller straightening or reeling.
Approximately 90% of bar straightening is carried out on double-roller straightening machines
(diagonal rolling or reeling with two rollers). The nature of the process (i.e. because of the bar
rotation) only permits bars with a circular cross-section to be straightened. This continuous
process is in particular suitable for the batch production of the long work.
Combined scrap-compacting and scrap-reduction installations
The scrap-processing plant described here is not truly a machine tool, as neither the metalforming
process nor the metal-separation operation produces a workpiece in the conventional
sense, i.e. no geometrically defined component. Machines of this type will assume increasing
importance in the future as the re-cycling of materials becomes more widespread.
Figure 10.4 (top) schematically illustrates the structural components of the machine for the
forming operation (i.e. compacting in this case) on the loose scrap in the loading bed. The
functional principles of the pre-compression unit may be seen in the lower part of Fig. 10.4.
The pressure beam is pressurized by the two hydraulic cylinders, which exert a force Fp
alternately. The final compacting prior to cropping is carried out by the vertical and horizontal
ram immediately in front of the guillotine.
Special-purpose machine tools for metal cutting
The machines described below cannot be fitted into the classifications of conventional metalcutting
machine tools owing to the arrangement or the types of their cutting tools. This group of
machines are nowadays rather a little bit historical residual. One-purpose machines such as
transfer line stations have been replaced by a modular building concept machine designed up
according to the costumer order. This concept has to be flexible without any redundancy to
avoid an idle investment.
Index-table machines for the production of rear-axle housings
The index-table machine described here is an example of a special-purpose machine for the
manufacture of complicated components. The precision machining of the bearing housings and
the flange faces, as well as the screw-cutting operation, is carried out in two table positions, but
the casting need only be clamped once.
The process is particularly economic due to the employment of multi-edged cutters, which
may be adjusted during the work cycle. Figure 10.8 shows in schematic form the cutting tools
and the order of operations for a typical example of work. The arrows indicate the feed motions
for machining. The cross slides (which may be seen on the vertical spindle in Fig. 10.9) are
moved hydraulically from within the spindle by employing helical-toothed driving rods.
Extruder-worm milling machines
The transportation, squeezing, kneading, plasticizing and extruding of pasty and plastic
materials, is increasingly carried out with the aid of an extruder worm. Most of such extruder
worms have, depending upon their function, a relatively complicated geometric form, e.g.
changing root diameters, changing helix angles or changing screw forms along the axis.
To produce such extruder worms on lathes is very difficult and hence special milling
machines have been developed for the machining of such profiles.