Active chatter suppression in an octahedral hexapod milling machine: A design study Jeffrey L. Dohner Sandia National Laboratories Dept. 1434/MS 0439, P.O. Box 5800, Albuquerque, New Mexico, 87185-0439 C. M. Kwan Intelligent Automation Incorporated 2 Research Place, Suite 202, Rockville, Maryland, 28050 Marc E. Regelbrugge Lockheed Martin Advanced Technology Center 325 1 Hanover Street, 0/93-30 B1255, Palo Alto, California, 94304 ABSTRACT 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 . INTRODUCTION In 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. 2. OCTAHEDRAL HEXAPOD MILLING (OHM) MACHINE MODEL This 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 0-8 194-2096-4/96/$6.O0 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx