Page 15 - Electric Drives and Electromechanical Systems

Chapter 1 Electromechanical systems 7

The determination of the cutting forces is outside the scope of this book, because the
resolution of the forces along the primary axes is a function of the cutting angle and of
the path of the cutter relative to the material being milled, and hence reference should be
made to the data provided by the tool manufacturer.
The forces and powers required by the other three subtractive processes, drilling,
planing, and grinding, can be determined in a similar manner. The sizes of the drives for
the controlled axes in all types of conventional machine tools must be carefully deter-
mined to ensure that the required accuracy is maintained under all load conditions. In
addition, a spindle or axis drive unable to provide the required speed or torque will cause
a reduction in the surface quality, or, in extreme cases, damage to the machine tool or to
the workpiece.

1.2.2 Non-conventional subtractive machining processes

Non-conventional processes are widely used to produce products whose materials
cannot be machined by conventional processes, for example, because of the workpiece’s
extreme hardness or the required operation cannot be achieved by normal machine
processes (for example, if there are exceptionally small holes or complex profiles).
A range of non-conventional processes are now available, including;

Laser cutting and electron beam machining
Electrochemical machining (ECM)
Electrodischarge machining (EDM)
Water jet machining
In the laser cutting process Fig. 1.4A a focused high-energy laser beam is moved over
the material to be cut. With suitable optical and laser systems, a spot size with a diameter
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of 250 mm and a power level of 10 Wmm 2 can be achieved. As in conventional
machining the feed speed must be accurately controlled to achieve the required quality
of finish. In addition, the laser will not penetrate the material if the feed is too fast, or it
will remove too much material if it is too slow. Laser cutting has a low efficiency, but it
has a wide range of applications, from the production of cooling holes in aerospace
components to the cutting of cloth in garment manufacture. It is normal practice,
because of the size and delicate nature of laser optics, the laser is fixed, and the work-
piece is positioned using a multi-axis table. The rigidity of the structure is critical to the
quality of the spot, since any vibration will cause the spot to change to an ellipse, with an
increase in the cutting time and a reduction in the accuracy. It is common practice to
build small-hole laser drills or similar systems on artificial granite bed-plates since the
high density of the structure dampens any vibration.
In electron beam machining, a focused beam of electrons is used in a similar fashion
to a laser, however the beam is generated and accelerated by a cathode-anode
arrangement. As the beam consists of electrons it can be steered by the application of
a magnetic field. The beam can be focused to spot of 10e200 mm in diameter giving a