This first semi-annual technical report covers the activities between
October 1995 and March 1996. For wires below 1.5 mil, present day conventional
drawing technology is limited to approximately 4% to 6% reduction in area
per die. For smaller wire sizes both the reduction per die and speed become
smaller, and the wire breaks more often. With "Augmented Hydrostatic
Extrusion" (HYDRAW) Ref. [1] and Ref. [2], as described in the figure we are able to extend beyond some
of present day limitations of the conventional processes. Reductions up
to 25% are accomplished and wire breaks rarely occur. In this paper we
will focus on the implementation of a concept that is essential to the
success of the project. According to this concept we substitute a built-in
automatic control system, to replace the more cumbersome closed loop sensor
and control system.
The closed loop control system. In the original HYDRAW design
(Ref. [2]) the pressure in the chamber supplies the bulk of the motivation
power for the extrusion. The pressure is generated to a level below that
required to push the wire out. While the pressure increases the liquid
compresses and acts like a spring. The liquid in the chamber therefore
cannot establish a positive speed control for the emerging wire.
The pick-up spool augmented the pressure with the required remaining
minimal draw force that is essential to provide the speed control. The
tension sensing device between the die and the pick-up spool (not shown
here) provided the signal to the pressure pump. One option of the mode
of this control is that if the tension rose, the pump was instructed to
increase the pressure, and vise versa. In another mode the signal instructed
a pressure control valve to respond.
The system works but it is sluggish. The response may be too slow or
it may overshoot the target. Speed and tension fluctuate excessively and
at higher and higher drawing speeds the wire breaks more and more often.
The finer the wire is, the harder it becomes to control the system.
Built-In Clutch Control System. In the present design we introduced
a clutch between the pick-up spool and the tension producing system. The
characteristics of the clutch are such that when the difference between
the speed of the emerging wire and the speed of the shaft increases the
drawing force increases, and vice versa. Thus, for example, when the wire
speed slows down, the differential speed rises, automatically providing
the required increase in tension.
The clutch also provides the two required control limits on the emerging
wire, i.e., tension and speed.
Tension. The wire can only sustain a drawing force that is below
its strength. The motivation draw force is designed so that even when the
wire is slowed down to a standstill, the tension will not reach the tensile
strength of the wire. Thus wire break due to overload is deterred.
Speed. The pinch-off phenomena due to excessive speed is described
in Ref. [2]. When control is maintained, the speed of the driving motor
is the upper limit on the speed of the emerging wire. When the wire speed
approaches that limit, the pulling force drops to zero before reaching
the speed limit. Thus, run-out and pinch-off are prevented as long as the
speed of the driving motor is maintained below the run-out speed.
