Magnetorheological fluid-controlled boring bar for chatter suppression
Introduction
The vibration of tools used in machining operations plays a key role in hindering the productivity of those processes. Excessive vibrations accelerate tool wear, cause poor surface finish, and may damage spindle bearings (Altintas, 2000). Chatter is a self-excited vibration phenomenon common in machining. In deep hole boring, the long, cantilevered boring bars have inherently low stiffness. This makes them prone to chatter, even at very small cutting depths. Chatter during the boring process directly influences the dimensional accuracy, surface quality, and material removal rate (Tlusty, 2000). Suppressing the chatter effectively in deep hole boring is important.
Research in boring chatter suppression has been conducted during the past several decades. Tewani et al. (1995) used an active dynamic absorber to suppress machine tool chatter in a boring bar, the vibrations of the system are reduced by moving an absorber mass using an active device such as an piezoelectric actuator. Tanaka and Obata (1994) suppressed the chatter of slender boring bar using piezoelectric actuators, chatter vibration signals detected by a pickup are fed to a computer. Marra et al. (1995) designed a H∞ controller to eliminate the vibrations in cutting operations with a active piezoelectric actuators. Li et al. (2006) investigated the effects of varying spindle speed. Pratt and Nayfeh (2001) suppressed chatter with a magnetorestrictive actuator, and Pan et al. (1996) proposed actively controlling such an actuator. Wong et al. (1995) applied actively controlled electromagnetic dynamic absorbers. Li and Hu (1997) suppressed chatter by using a dynamic damper. Rivin and Kang (1992) described a systemic approach to the development of cantilever boring bar tooling structures. Finally, Wang and Fei (1999) developed an on-line chatter detection and control system utilizing electrorheological (ER) fluid.
Based on the mode of chatter suppression employed, suppression methods can be classified into three types: passive, active, and semi-active. Passive chatter suppression is simple in structure, but the dynamic parameters of the damper used cannot be adjusted, leading to poor performance when the working condition changes. Li and Hu (1997) suppressed chatter by using a dynamic damper, and Rivin and Kang (1992) developed a cantilever boring bar tooling structures, belong to the passive chatter suppression method. Active chatter suppression allows for continuously adjustable dynamic parameters based on the feedback signals. The chatter suppression methods based on the effects of varying spindle speed (Li et al., 2006), piezoelectric actuator (Tewani et al., 1995), magnetorestrictive actuator (Pratt and Nayfeh, 2001) and actively controlled electromagnetic dynamic absorber (Wong et al., 1995) belong to active chatter suppression method. This type is more effective than passive methods, but it requires high power and is expensive. Finally, semi-active chatter suppression can improve stability by changing the inherent stiffness and dynamic damping parameters of a system (Wang and Fei, 1999). Semi-active chatter suppression not only has better damping effectiveness than the passive mode, but also has lower power and cost requirements than active suppression (Lam and Liao, 2001). A semi-active chatter suppression method employing the MR fluid is investigated in this study. MR fluid exhibits some advantages over typical ER materials. Compared to ER fluids (Wang and Fei, 1999), which have high working voltages (2–5 kV) and narrow working temperatures (10–70 °C), the power (1–2 A or 50 W) and voltage (12–24 V) requirements for MR fluid activation are relatively small, and the working temperatures (−40 to 150 °C) of MR fluid are relatively broadened. So MR fluids are more practical and suitable for machine tool applications. In addition, ER fluids are sensitive to impurities, which is not a problem for MR fluids (Srinivasan and McFarland, 2001).
In this study, a MR fluid-controlled chatter suppressing boring bar is proposed. The theory of chatter suppression using an MR fluid-controlled boring bar is first introduced. Next, the design and fabrication of a MR fluid-controlled boring bar is described. Finite element modeling (FEM) of the magnetic system is then detailed, the Euler–Bernoulli dynamic beam model of the MR fluid-controlled boring bar is created, and the stability of the MR fluid-controlled boring system is analyzed. Finally, a series of machining experiments in different spindle speeds to demonstrate chatter suppression based on MR fluid control of a boring bar are conducted for validation.
Section snippets
Theory of chatter suppression based on the MR fluid-controlled boring bar
Most chatter occurring in practical machining processes is what is termed regenerative chatter, which is usually caused by instability of the cutting process in combination with the mechanical structure of machining system. The frequency of regenerative chatter is close to the natural frequency of the machine tool (Merrit, 1965). The system dynamics of regenerative chatter is illustrated by the block diagram shown in Fig. 1 (Yang and Tang, 1983). The tool tip displacement Y(s) is generated by
MR fluid and design and fabrication of a MR fluid-controlled boring bar
Magnetorheological materials are a class of material whose rheological properties may be rapidly varied by applying a magnetic field. Most commonly, these materials are fluids that consist of micron-sized, magnetically polarizable ferrous particles suspended in a carrier liquid. When exposed to a magnetic field, the suspended particles polarize and interact to form a structure aligned with the magnetic field that resists shear deformation or flow. This change in the material appears as a
FEM analysis of magnetic system for the MR fluid-controlled boring bar
The magnetic system for the MR fluid-controlled boring bar is important for energy transformation efficiency, as well as the chatter suppression capability of the system. FEM analysis was applied to analyze and design the magnetic field. The boring bar and support sleeve are made of AISI 1020 low-carbon steel with a magnetic conductivity of 1000 H/m. The non-magnetic sleeve and excitation coil are made of non-magnetic mallet alloy with a magnetic conductivity of only 1.5 H/m. The MR fluid is
Dynamic modeling of the MR fluid-controlled boring bar
In this research, the ratio of length to diameter of the MR fluid-controlled boring bar is 6, the workpiece has a stiffness value much greater than that of the boring bar, so the bending mode of the boring bar need to be considered. Bending mode includes tangential vibrations and radial vibrations, tangential vibrations are in the direction of speed, so they will not affect regeneration of the chip load significantly but only the phase, but radial vibrations will change the depth of cut.
Stability analysis and simulation of the MR fluid-controlled boring bar
Previous studies showed that usually only one dominating mode exists in the chatter exhibited by most machining processes (Yang and Tang, 1983). By assuming n = 1 and multiplying m = ρll to Eq. (13):where u = cos(β − α)cos γ4, , Ks is base stiffness, and ΔKm is the variable stiffness generated by the MR fluid.
The system in Eq. (14) consists of three parts: (i) the stiffness and damping inherent in the system, (ii) the equivalent
Experimental validation
Experiments were carried out on a lathe. A schematic diagram and photograph of the experimental setup are shown in Fig. 9(a) and (b), respectively. The horizontal vibration of the tool tip was measured by a piezoelectric accelerometer placed at the free end of the boring bar. A computer, connected to a power amplifier, controls the current of the MR fluid-controlled boring bar. When chatter is detected, as measured by the behavior of the output signal produced by the accelerometer, the control
Conclusions
A chatter suppression method based on a MR fluid-controlled boring bar was presented and validated experimentally. The magnetic system inside the boring bar was designed using the FEM analysis. A dynamic Euler–Bernoulli beam model of the MR fluid-controlled boring bar was derived to analyze the regions of operating stability, and was also used to show that chatter could be suppressed by adjusting the damping and natural frequency of the system. Experimental results in different spindle speeds
Acknowledgements
The authors would like to acknowledge the supports from National Natural Science Foundation of China (grant nos. 50405036, 50775203), Hi-tech Research and Development Program of China (grant no. 2006AA04Z329) and Natural Science Foundation of Zhejiang Province (grant no. Y104462).
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