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OSCILLATIONS AND INSTABILITIES DURING WET FINE WIRE DRAWING
By Betzalel Avitzur
Metalforming Inc., Allentown, PA
Printed as ref. #190 of the publications listing
Abstract
Vibrations are a common occurrence during the drawing of ultra-fine
wire. Among the sources of these oscillations are the mechanical
design of the equipment, the properties of the wire and the lubricant,
and the processing parameters, such as reduction, die angle,
speed, friction, and temperature. Even alternating electric power
supply may generate vibrations. The vibrations are undesirable
and in the extreme, these vibration lead to wire breaks. Even
if the wire does not break, surface defects may be generated.
The vibrations cannot be eliminated, but they can be minimized
by process control.
This paper studies the phenomena of vibration experimentally
through the use of lube evaluation equipment, measuring the drawing
force and its fluctuations. The study is supported by the analysis
of the process of "wet wire drawing" and the phenomenon
of hydrodynamic lubrication.
Table of Contents
Introduction
Any wire making process experiences subtle, or not so subtle,
fluctuations in speed and in drawing force. The finer the wire
size gets the more excessive these disturbances become. Some
effects of these vibrations are:
- Wire tearing (breaking)
- Irregular wire size and surface finish
- Wire tangles
In this manuscript we will focus on the study of these fluctuations
during wet fine wire drawing where they are most prevailing.
We will observe the fluctuations using sensors that record speed
and drawing force. Through the study of these records we will
analyze the causes of the fluctuations. The knowledge of the
causes will assist us in providing remedies to diminish these
problems.
Equipment
Production vs. Laboratory
Equipment
There are compelling reasons for the study of the vibrations
to be made under controlled laboratory conditions with specially
designed, single pass, highly instrumented equipment. Some of
these reasons are:
- The production equipment cannot be instrumented.
- The production equipment is too expensive, both initially
and to run.
Determination of the vibration is best made in the laboratory
with highly instrumented equipment.
The Six Modules of the Equipment
Figure <1> presents a schematic description of a draw
bench for fine wire drawing. The equipment is an assembly of
six major distinct modules. (Module #6 is omitted.)
Module #1, the central module, is the frame and bath
combination, containing the die and die holder. The wire and
the die are immersed in the bath containing the lubricant. In
a more sophisticated unit, module #1 will also include a pump
to circulate the lubricant, a filter to clean the lubricant,
and when desired, a temperature control system.
Module #2 contains the pay-off spool which feeds the
wire into the drawing die.
Module #3 is the tensiometer, a standard sensor that
measures the tensile load on the emerging wire. All three rolls
are idling rolls mounted on a shaft with bearings of low friction.
The two end rolls are mounted on shafts that are clamped firmly
into the housing. The roll in the center is mounted on a spring-loaded
vertically mobile shaft, to which a potentiometer is attached
to measure displacements. The higher the tension on the wire
the larger the vertical displacement of the center roll. The
tension on the wire is the required drawing force. The potentiometer
measures the displacement which is then collected by the data
acquisition board inside the computer. A computer program accesses
the data acquisition board to capture the data.
The vertical displacement of the center roll is measured by
the potentiometer and converted to digital form by the data acquisition
board. It is then presented on the computer screen as a function
of the drawing speed of the motor in module 4.
Module #4 contains the entire spool pick-up system.
The spool is mounted directly on the shaft of a 'step' motor
that provides the moment (and force) to draw the wire. The speed
of the motor is controlled through a signal from the computer,
as provided by the operator. The speed is programmed to rise
monotonously up to a predetermined peak speed. The spool pick-up
motor, also a 'step' motor, is mounted on the transverse table
that can move horizontally parallel to the axis of symmetry of
the pick-up spool. The transverse motion table is driven by the
transverse motion motor, whose speed is controlled by the computer.
Limit switches reverse the direction of movement of the transverse
table, and a new layer commences automatically when the wire
reaches either end of the spool. The desired ratio of the speed
of the transverse motion motor to that of the motor of the pick-up
spool is determined through the pitch opted for the pick-up spool.
This ratio is programmed into the computer by the operator, and
transmitted as an analog signal to the transverse motion motor
control.
Module #5 comprises the computer control system, and
includes data collection, analysis, and display. The speed of
both motors and the measured tension are recorded into a file
together with other pertinent information for each run. Each
run is fully controlled through the computer. Data from each
file alone or from several files together can be manipulated
through the computer, analyzed, saved and displayed in tabular
and graphical forms.
Module #6 (Optional) comprises a lubricant circulation,
filtration and temperature control. For various uses the system
design may vary. This module is not presented in the schematic
of Fig. <1>.
Recording the Tests
Input Screen
A typical test of drawing of a wire through a die, while recording
the speed and drawing force at equal intervals in time, is presented
in Fig. <2>. The input for each run is provided through
the computer keyboard and displayed on the screen. The menu on
the screen is self-explanatory.
Output Screen
The results are presented on the screen, or in the printout
of Fig. <3>, together with the echoed input. On the abscissa
of Fig. <3> the sequential count of each reading is displayed,
from zero at start up to the total number of readings. The total
number of readings is selected as a menu item on the input screen.
The abscissa is often referred to as the time scale. The ordinate
presents the speed reading and the tension at each respective
moment. A legend of the output graphs is provided in the Appendix.
The results of each run can also be presented in tabular form.
Each run is stored as an electronic file.
Noise
There is a wide spread in the recording of tension and speed.
Two distinct sources for this spread are:
- Electronic signal noise.
- Actual system fluctuations in speed and tension.
It is vital to be able to distinguish between 'system' fluctuations
which we want to detect, and 'electronic line' noise which we
wish to filter out. We have employed the following two methods
to treat the fluctuations in the readings.
- Line filters to remove the electronic noise.
- Computer averaging of the readings.
The speed readings presented in Figs. <3> to <6>
are already filtered, hopefully removing the line noise. In Fig.
<4> the remaining spikes of the speed shown in Fig. <3>
were averaged and the same output file is presented. The user
may call for the graphs of any file while opting to average the
speed and specifying the number of readings over which the average
is requested. Presently we will address the phenomenon in detail.
Analysis of the Output
System Fluctuations
A draw bench is prone to vibrations, as is any mechanical
system. The amplitude and frequencies of these vibrations can
be controlled by the design of the equipment, the materials used,
and the processing parameters. For example, at a specific speed
the wire oscillations may enter a resonance frequency. The equipment
described may assist in studying the parameters that control
vibrations.
The length of the wire, suspended between the die and the
pick-up spool, behaves like a string of a musical instrument.
It oscillates naturally at primary and secondary frequencies.
When an excitation source of one of these frequencies is generated
by the system, resonance vibrations of excessive amplitude will
appear with dire consequences. Such a source of excitation may
be eccentricity in either the pick-up or the pay-off spools.
Another may be in the drive motor, either of a mechanical nature
or the frequency of the electrical power supply.
Kinks
Out of literally hundreds of files, Fig. <3> is one
with special significance. It displays a rare event that was
captured due to the sensitivity of the instrumentation and data
collection. At approximately reading number 175 on the abscissa
an abrupt step jump in tension, accompanied by an abrupt step
drop in speed is observed. When averaging is applied, Fig. <3>
transforms to Fig. <4>, and the abrupt nature of the event
is masked. A proposed interpretation of that event is that a
kink in the wire caused the abrupt rise in resistance when it
passed through the die, which in turn caused an abrupt small
drop in motor speed. The constant speed control of the motor
recovered the speed in a matter of a split second. Even with
a "stiff motor drive" speed will drop momentarily due
to elasticity of the system and due to "slip". The
level to which the recorded event is flattened depends on the
width over which we perform the averaging. The larger the number
of readings we average together the flatter the curve will be.
The more averaging we make the smoother the graph will be, and
the more likely it may be that we will miss an important event.
In Figs. <5> and <6> we display the characteristic
readings of another event similar to that of Figs. <3>
and <4>.
The manner in which the output is presented is fully under
the control of the investigator. First, the choice of 'sampling
rate' is made on the input screen. The higher the sampling rate
the more capable it will be to detect an instantaneous event.
The requested number of samples and the pick-up spool speed determine
the running time of each file. The events can be displayed by
the tabular presentation of the output file. It can also be detected
by the graphical presentation of the file without averaging.
The more detail we want to detect, the more tedious is the procedure.
It is however an excellent tool for troubleshooting.
The actual speed can easily be determined from Figs. <3>
and <5> despite the noise present. This noise should not
worry the investigator. However, for esthetic reasons the files
can be averaged to produce smoother figures such as Figs. <4>
and <6>. The choice of the style of the presentation can
be made at any time after saving the output file.
Kinks are severe defects in the smoothness of the incoming
wire. The abrupt rise in the resistance of the wire passing through
the die may, in the extreme, result in wire breaks. The less
severe the kink is, the lower the tension peak, and the less
the speed will be perturbed.
Bends and Other Irregularities in the
Pay-Off Spool
Short of a kink, we may observe other defects in the pay-off
spool. In the present sketch we observe a common bend over a
bundle of neighboring loops on the pay-off spool.
Fig. <7> Common Bends
It stands to reason that such a defect in the spooling may
give rise to periodic fluctuations in the tension readings. Other
irregularities may include surface, metallurgical, and strength
fluctuations in the incoming wire.
Summary
The use of a specially designed, stand-alone, highly instrumented,
computer controlled draw bench for the study of the vibrations
during fine wet wire drawing was demonstrated. The fluctuations
in the drawing force and speed were recorded and analyzed. The
causes for these fluctuations were revealed. (See Ref. [1] and
[3].)
Similar graphs with the averaging of speed may be practical
for the study of the effects of parameters such as temperature
and speed on the effectiveness of the lubricant in reducing friction.
(See Ref. [2].)
References
- Avitzur, B., "Lubricant Evaluation Laboratory for Fine
Wire Drawing", Proceedings of the Wire Association International,
1997.
- Avitzur, B., and Zimerman, Z., "From Bull-Block to Wet-Drawing",
The Wire Association International, Mordechi Award lecture, 1996.
- Avitzur, B., "Vibration", http://www.metalforming-inc.com/Seminars/Upcoming/
List of Figures
Fig. # Caption
- Schematic of the Draw Bench and Equipment
- Computer Input Data
- Raw Output Data
- Averaged Output Screen
- Another Raw Output Data
- Another Averaged Output Data
- Common Bends
Appendix
Echoing the Input * Every output graph echoes the input data
in a legend at its top. The legend reads:
First line of legend
File = File nameDate = Date of the test
avg = The average tension
std = The standard deviation of the the total number of tension
readings
Second line of legend
mat = Wire material
sd = Pick-Up spool diameter
wd = Initial wire diameter
lub = Identity code for the lubricant
Third line of legend
temp = Lubricant's temperature
r% = Percent reduction in area
 = Semi-cone angle of the die
Last line of legend
rem = This line is reserved for the user to make comments
of any sort on the dat
End of document, figure(s) follow.
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