This report documents how active structural control was used to significantly enhance the metal removal rate of a milling machine. An active structural control system integrates actuators, sensors, a control law and a processor into a structure for the purpose of improving the dynamic characteristics of the structure. Sensors measure motion, and the control law, implemented in the processor, relates this motion to actuator forces. Closed-loop dynamics can be enhanced by proper control law design. Actuators and sensors were imbedded within a milling machine for the purpose of modifying dynamics in such a way that mechanical energy, produced during cutting, was absorbed. This limited the on-set of instabilities and allowed for greater depths of cut. Up to an order of magnitude improvement in metal removal rate was achieved using this system. Although demonstrations were very successful, the development of an industrial prototype awaits improvements in the technology. In particular, simpler system designs that assure controllability and observability and control algorithms that allow for adaptability need to be developed.

This report documents how active structural control was used to significantly enhance the metal removal rate of a milling machine. An active structural control system integrates actuators, sensors, a control law and a processor into a structure for the purpose of improving the dynamic characteristics of the structure. Sensors measure motion, and the control law, implemented in the processor, relates this motion to actuator forces. Closed-loop dynamics can be enhanced by proper control law design. Actuators and sensors were imbedded within a milling machine for the purpose of modifying dynamics in such a way that mechanical energy, produced during cutting, was absorbed. This limited the on-set of instabilities and allowed for greater depths of cut. Up to an order of magnitude improvement in metal removal rate was achieved using this system. Although demonstrations were very successful, the development of an industrial prototype awaits improvements in the technology. In particular, simpler system designs that assure controllability and observability and control algorithms that allow for adaptability need to be developed.

This paper describes design methodologies for construction of an actuator that uses smart materials to provide hydraulic fluid power. In the class of actuators described, hydraulic fluid decouples the operating frequency of the output cylinder from the drive frequency of the piezoelectric or other smart material. This decoupling allows the piezoelectric to be driven at high frequency, to extract the maximum amount of energy from the material, and the hydraulic cylinder to be driven at low frequencies to provide long stroke. However, due to fluid compressibility and structural compliance, the fundamental impedance match between the fluid and the piezoelectric make it difficult to convert energy from the piezoelectric into pressurized hydraulic fluid flow. The basic design tradeoffs and major technical issues are discussed in the areas of materials, mechanical design, and fluid-mechanical interface. Prototype devices and component measurements are presented. Test methods are described, and test results quantifying pump pressure and flow, and actuator force and velocity are summarized. The series of tests show the potential of these devices for high force long stroke devices powered by smart materials.

IThe use of active feedback compensation to mitigate cutting instabilities in an advanced milling machine is discussed in this paper. A linear structural model delineating dynamics significant to the onset of cutting instabilities was combined with a nonlinear cutting model to form a dynamic depiction of an existing milling machine. The model was validated with experimental data. Modifications made to an existing machine model were used to predict alterations in dynamics due to the integration of active feedback compensation. From simulations, subcomponent requirements were evaluated and cutting enhancements were predicted. Active compensation was shown to enable more than double the metal removal rate over conventional milling machines.Keywords: milling, controls, chatter INTRODUCTIONMaximum metal removal rate is a quantitative measure of the productive capacity of a machine tool. This measure, which is machine as well as tool dependent, is limited by the onset of machining in~tabilities',~,~,~. Machining instabilities are minimized by enhancing the stability of structural vibratory modes significant to cutting dynamics.Active methods can be used to enhance the stability of pertinent structural vibratory modes for a variety of machining configurations. Although active methods are relatively immature5, potential performance enhancements resulting from active implementation overshadow performance enhancements resulting from current passive innovations.Active methods can be categorized into activelpassive, process control, and feedback compensation methods. Activelpassive methods actively tune or mimic stabilizing passive absorber^^^^^^. Process control methods alter or modulate operating parameters (spindle speed, feed rate, etc.) to locate, or oscillate between, predetermined stability domains9i10, and feedback compensation methods actively modify machine dynamics such that domains of stability are enlarged".'2713.Presented within this paper is the design of an active feedback compensator which will actively modify machine dynamics such that domains of stability are enlarged. This design process entails; 1) development and validation of a model, 2) modification of the model for active compensation, and 3) performance evaluation.In section 2, modeling and validation is discussed. In section 3, the modeling of dynamics essential for active compensation is discussed. And in section 4, performance earnings are presented. MODEL DEVELOPMENTA machine model is a set of mathematical relationships relating dynamic response parameters (displacement of tool tip center, force on tool, etc ...) to a set of process parameters (spindle speed, number of teeth on tool, etc ...). The goal of modeling is to create mathematical relationships which are sufficiently accurate for the purpose of prediction. Model accuracy is quantified by comparison with experimental data. The model discussed in this section is shown to compare well with experimental data.The machine model is the unification of a cutting model and a structural ...

This paper describes a design study to determine the feasibility of integrating active control into a milling machine to enhance milling-process performance. The study described herein focuses on the active suppression of chatter instabilities in an Octahedral Hexapod Milling (OHM) machine. Structural dynamics contributing to chatter instabilities were described using calibrated finite element models, which were coupled with a tool-workpiece interaction model for purposes of determining, by simulation, machine performance enhancement due to active control. An active vibration control design to minimize vibration at the tool tip was also integrated into the simulation. Active control subcomponent and actuator size requirements were determined from the modeling arid simulations. The study showed that active control is a feasible solution for suppressing chatter instabilities, allowing the metal removal rate of the OHM machine to be increased by roughly a factor of two. I . INTRODUCTIONIn machining, Metal Removal Rate (MRR) is limited by the power limit of the machine and by machining instabilities. The power limit of the machine is increased by increasing the horsepower of the motor. Typically, machining instabilities are minimized by stiffening machines and tools by adding reinforcing material. However, there are many tools and machines for which stiffening by material addition may not be practical. An example of such a machine is the Ingersoll Milling Machine Company's Octahedral Hexapod Milling (OHM) machine1 .As an alternative, stiffness may be increased in selected frequency ranges through the use of active control.The problem addressed in this paper is to synthesize an active control design that will make a flexible tool look stiff at the point of cutting. The target of the active control design is the Ingersoll OHM machine. This machine has been dynamically characterized, and essential dynamic characteristics are employed to study dynamic performance of the active control by simulation. The following sections of this paper describe the OHM machine model, development of an active control design to make a flexible tool used with the OHM machine look stiff at the point of cutting, and sizing of electrostrictive ceramic actuators for this active control design. OCTAHEDRAL HEXAPOD MILLING (OHM) MACHINE MODELThis section describes the development of a finite element model of the OHM machine with flexible and stiff milling tools. The model captures the local dynamic behavior of a diverse, but practical set of machine and tool configurations susceptible to chatter. Figure 1 shows an illustration of the machining head of the OHM machine, comprising a solid steel platform, spindle drive motor and spindle assembly, all supported by six servo struts. Some dynamics of this machine are inherently nonlinear and hysteretic due to the use of bolted connections, compression fittings, socketed joints and flexible 316 / SPIE Vol. 2721