Study of Chatter Analysis in Turning Tool And Control Methods – A Review

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Study of Chatter Analysis in Turning Tool

And Control Methods – A Review

K. Reza Kashyzadeh

, Prof. Dr. M. J. Ostad-Ahmad-Ghorabi

1

Young Researchers Club, Semnan branch, Islamic Azad University, Semnan, Iran

2_{Advisor Professor, Department of Mechanical Engineering, Islamic Azad University, Semnan-branch, Semnan, Iran}

1_{[email protected]} 2

[email protected]

Abstract: Machine tool chatter is one of the major constraints that limit productivity of the turning process. It is a self-excited vibration that is mainly caused by the interaction between the machine-tool/workpiece structure and the cutting process dynamics. The frictional and impact chatter are mainly due to the nonlinearity of the dry friction and the intermittent contact between the cutting tool and the workpiece. There are some methods that can limit the chatter. In this paper we introduce and compare some of these methods.

Keywords:chatter, Regenerative, Turning tool, vibration.

I. INTRODUCTION

This document is template. We ask that authors follow Machining processes are often accompanied by self-excited relative vibration between the workpiece and the cutting tool, which is referred to as chatter. When chatter occurs, the amplitude of the self-excited vibration increases until nonlinearity limits any further increment [1]. Chatter results in rough surface finish, poor accuracy, shortened cool life and low metal-removal rate. Chatter becomes even more critical when machining materials that are difficult to cut. Some advanced cutting tool materials such as ceramic, silicon nitride and CBN require strict chatter control to prevent brittle breakage [2]. For high precision manufacturing, even mild vibration is undesirable. Furthermore, since modern machining systems have become more flexible, the frequently changing working conditions increase the possibility of bringing machining process into unstable operating regions [3]. And the productivity of expensive machining systems is often limited by chatter. It has defined chatter as self-generative vibrations that occur when the chip width is too great versus dynamic stiffness [1]. This phenomenon leads to a bad surface aspect and high noise level. As it reduces tool life, it increases production costs. For instance, the cost due to chatter is estimated to be around 0.35 h per piece on a cylinder block.

With such a cost, chatter prediction becomes highly necessary and a chatter criterion has to be chosen. First evocations of chatter are due to Taylor in 1907 and then to Schlesinger in 1936. A first comprehensive study was led by Doi in 1937 [2] and then with Kato in 1956 [3]. Tlusty and Polacek published their criterion the next year [4] and Tobias proposed his chatter maps the year after [5]. During the early 1960s, Pe´ters and Vanherck ran some tests and developed measurement techniques in order to discuss Tlusty and Tobias criterions [6]. The 1970s have shown some work on the dynamic parameters. Hanna and Tobias worked on the non-linearity of the stiffness [7] while the Pe´ters and Vanherck team produces highly interesting thesis on the identification of dynamic parameters during the cutting operations [8, 9]. At the end of 1970s, Tlusty presented his CIRP keynote paper on the topic [10]. Up to now major developments have been designed for aeronautic industry where tools are mostly more compliant than workpieces. In this way, Altintas and Budak have proposed an analytic method for computing stability lobes corresponding to Tobias’s chatter maps in 1995 [11]. This work has been extended in 1998 [12] by taking the workpiece’s behavior into account under the form of compliance-damping systems in two directions. A comprehensive summary of recent developments of the topic has been proposed by Altintas and Weck under the form of a CIRP keynote [13].

II. DIFFERENT TYPES OF CHATTER

A.Regenerative chatter

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If regenerative tool vibrations become large enough that the tool looses contact with the workpiece, then a type of chatter known as multiple regenerative chatter occurs. This mechanism has been the subject of studies by Shi and Tobias (1984), Kondo, Kawano, and Sato (1981), and Tlusty and Ismail (1982).

B. Mode Coupling

Mode coupling occurs whenever the relative vibration between the tool and the workpiece exists simultaneously in at least two directions in the plane of the cut. In this case, the tool traces out an elliptic path that varies the depth of cut in such a fashion as to feed the coupled modes of vibration. It is considered to be a factor when chatter develops in slender nearly symmetric tools, such as boring bars. We note the similarity between this mechanism and the phenomenon of aero elastic flutter. other mechanisms have been postulated. Arnold (1946) suggested that the cutting forces depend on the velocity in such a fashion as to produce negative damping. This chatter mechanism is essentially a frictional effect and has characteristics similar to that of the well-known Rayleigh oscillator (Nayfeh and Mook, 1979).

C. Thermo mechanical

Thermo mechanical chatter is due to temperature variations and the temperature dzstortion of the chip. the first approach to comprehensively describe the thermo. Mechanics was made by Hastings et al (Jang and Tarng, 1999).

The foregoing are all mechanisms that lead to self-excited oscillations. A common source of such vibrations in turning operations is rotating imbalance or misalignment of the workpiece. Tool run out and spindle errors also cause forced vibrations. Milling operations generally produce interrupted cuts as the cutters rotate in and out of the workpiece that these so-called.

D. Interrupted cuts

Interrupted cuts lead to impact oscillations, a form of forced machine-tool vibration that has been studied by Davies and Balachandran (1996).

III. CHATTER MODELLING THEORY

A. Simulation of Tool motion in one direction

Assume that a flat-faced orthogonal grooving tool is fed perpendicular to the axis of a cylindrical shaft held between the chuck and the tail stock center of a lathe (see Fig1 and Fig2).

Fig1. Turning Model [5, 14]

Fig2. Regenerative chatter vibration of chatter dynamics [6]

The equation of motion of the system can be expressed as:

The chatter vibration frequency is still close to the natural mode of the structure. For critical borderline stability analysis ( ), the characteristic function becomes

Where is the maximum axial depth of cut for

chatter vibration-free machining. The critical axial depth of cut can be found by equating the real part of the characteristic equation to zero:

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Substituting and rearranging

this equation yields

The stability lobes represent an unstable region and the points under the lobes are in the stable region. Controlling methods of chatter tries to shift the lobes above or to remove the unstable region.

Fig3. Typical stability lobe diagram [13].

B. Simulation of Tool motion in two separate directions

The tool is assumed to be compliant and experiences bending motion in directions x and y, while the workpiece is assumed to be rigid. The system can be modeled as a 2 DOF oscillator excited by the cutting force as it is shown in Fig. 4.

The governing equations read:

Where m, , , and are the modal mass, the

damping and the stiffness parameters in the x and y directions, respectively.

The cutting force is given in the form

Where and are the cutting coefficients, w is the depth of cut, h is the chip thickness and q is an exponent (q = 0.75 is a typical empirical value for this parameter).

In this model, it is assumed that the tool never leaves the workpiece, that is, h>0 during the cutting process.

Fig4. Model of regeneration in turning process [14]

If the tool were rigid, then the chip thickness would be a constant , which is just the feed per revolution. However, in practical cases the tool experiences vibrations that alter the cutting depth and, after one revolution of the workpiece, the tool cuts this wavy surface. Thus, the regenerative effect makes the chip thickness non-constant during machining. If the regenerative delay is , then the chip thickness can be given as:

Where is the speed of the feed. Here, the case corresponds to the loss of contact between the tool and the workpiece. In the current work only local bifurcation phenomena are analyzed and the effect of contact loss is not investigated. Therefore, in the

following analysis, we assume that during

machining.

Since the tool experiences vibrations in the x direction as well, the time delay is not equal to the rotation period of the workpiece, but it is determined implicitly by:

Here Ω is the spindle speed given in (rad/s) and R is the radius of the workpiece. Thus, the regenerative delay is a state-dependent delay since it depends on the state, both current (x(t)) and

Delayed ( ). Therefore, we will use the notation

, where describes the

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In order to reduce the number of parameters we assume

that the tool is symmetric, i.e., , .

The corresponding natural angular frequency is

and the damping ratio is [14].

The machining stability of the vertical milling machine can be predicted based on the analytical model developed by Alintas and Budak [11, 12, 15]. In their approach, the time-varying force coefficient of the dynamic milling process model was approximated by Fourier-series components. Following this, the stability relationship between the chatter-free axial cutting depths ( ) and the spindle speed (n) in end-mill operation were derived by Gagnol et al. [16] as follows.

The speed-dependent transfer function representing the ratio of the Fourier transform of the displacement at the tool tip over the dynamic cutting force can be expressed as:

In the above equations, and are, respectively, the real and imaginary part of the transfer function of the spindle tool tip. and are the cutting resistance coefficients in the tangential and radial directions to the cutter. N is the number of cutter teeth and k is the lobe number.

To predict the machining stability, a two-tooth carbide cutter was employed to machine the stock material of Al7075. The cutting resistance coefficients were calibrated

as and [17].

IV. CHATTER CONTROL

Over the years, various methods have been developed to avoid regenerative chatter without reducing the depth of cut. The basic principle of these techniques is to prevent the dynamic of the machining process from locking on the most favorable phase for chatter.

Slavicek and Vanherck proposed the use of milling cutters with non-uniform tooth pitch and Stone used end mills with alternating helix. Effectiveness of these methods in chatter suppression has been verified by simulation and experiments [18]. These techniques can be applied to the design of a non-uniform pitch cutter for a specific cutting condition, but cannot be applied to single point machining.

Weck et al. [19] utilized on-line generated stability lobes to select a spindle speed, so that maximizes the depth-of-cut limit. Later, Smith and Tlusty [20], Delio et al. [2] and Tarng et al. [21] avoided the need for the knowledge of the stability lobes and proposed that the best tooth passing frequency be made equal to the chatter frequency. This minimizes the phase between the inner and outer modulations.

This approach is adaptive in the sense that the spindle speed is changed based on feedback measurement of the chatter frequency. This method is practical for high spindle speed machining when the stability lobes are well separated.

Another technique to suppress regenerative chatter is sinusoidal spindle speed variation (S3V) around the mean speed to disturb the regenerative mechanism. Since this technique was introduced by Stoferle and Grab [22], there have been many research efforts to verify its effectiveness on machining stability by numerical simulation and experiments in turning [7, 8, 18, 23] and in milling [1, 8, 9].

Despite the above research efforts, this technique has not been implemented widely in industry because there is no systematic way to select the proper amplitude and frequency of the sinusoidal forcing signal. The selection of these parameters depends on the dynamics of the machining system and is constrained by the spindle-drive system response and its ability to track the forcing speed signal. In addition, variable speed machining can result in an adverse effect and may even cause chatter in an otherwise stable process [5, 10, 18].

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REFERENCES

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[18] Cuttino, IF., Miller, A. C. Schinstock, D. E., Performance Optimization of a Fast Tool Servo for Single-Point Diamand Turning Machines, IEEE/ASME Trans Mechatronics, Vol. 4, p169-178, 1999

[19] Fawcett, S. C., Small Amplitude Vibration Compensation for Precision Diamand Turning, Precision Eng., Vol. 12, No. 2, 1990

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[22] A. Yilmaz, E. Al-Regib, J. Ni, Machine-tool chatter suppression by multi-level random spindle speed variation, in: Symposium on Recent Advances in Machine Tool, ASME, Mechanical Engineering Congress and Exposition, Nashville ennessee, 1999