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Oscillations and Instabilities During Wet Fine Wire Drawing

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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:

  1. Wire tearing (breaking)
  2. Irregular wire size and surface finish
  3. 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:

  1. The production equipment cannot be instrumented.
  2. 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:

  1. Electronic signal noise.
  2. 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.

  1. Line filters to remove the electronic noise.
  2. 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

  1. Avitzur, B., "Lubricant Evaluation Laboratory for Fine Wire Drawing", Proceedings of the Wire Association International, 1997.
  2. Avitzur, B., and Zimerman, Z., "From Bull-Block to Wet-Drawing", The Wire Association International, Mordechi Award lecture, 1996.
  3. Avitzur, B., "Vibration", http://www.metalforming-inc.com/Seminars/Upcoming/

 

List of Figures

Fig. # Caption

  1. Schematic of the Draw Bench and Equipment
  2. Computer Input Data
  3. Raw Output Data
  4. Averaged Output Screen
  5. Another Raw Output Data
  6. Another Averaged Output Data
  7. 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

Symbol: alpha (lc) = 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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Last Modified:
Monday April 26 2010

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