James P. Lauffer

Milling machine for the 21st century: goals, approach, characterization, and modeling

Ingersolls Octahedral Hexapod--a milling machine for the future--is described. The specific target applications and the performance goals for an enhanced version of the machine are illustrated. The approach to achieving the goals by incorporating of advanced composites and active chatter and vibration control using smart materials is discussed. The machine characterization performed on an existing machine, the FE models developed and the plans to use the characterization and the validated models in designing an enhanced machine are described.

Active chatter control in a milling machine

The 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.

Mitigation of Chatter Instabilities in Milling by Active Structural Control

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.

Analysis of in-situ vibration monitoring for end-point detection of CMP planarization processes

This paper details the analysis of vibration monitoring for end-point control in oxide CMP processes. Two piezoelectric accelerometers were integrated onto the backside of a stainless steel polishing steel polishing head of an IPEC 472 polisher. One sensor was placed perpendicular to the carrier plate and the other parallel to the plate. Wafers patterned with metal and coated with oxide material were polished at different speeds and pressures. Our results show that it is possible to sense a change in the vibration signal over time during planarization of oxide material on patterned wafers. The horizontal accelerometer showed more sensitivity to change in vibration amplitude compared to the vertical accelerator for a given polish condition. At low carrier and platen rotation rates, the change in vibration signal over time at fixed frequencies decreased approximately 1/2 to 1 order of magnitude. At high rotation speeds, the vibration signal remained essentially constant indicating that other factors dominated the vibration signal. These results show that while it is possible to sense changes in acceleration during polishing, more robust hardware and signal processing algorithms are required to ensure its use over a wide range of process conditions.

On the use of active structural control to enhance the cutting performance of a milling machine

An active structural control system was developed and implemented on a hexapod milling machine to increase metal removal rate of the machine. The control system hardware consisted of dynamic actuators, sensors, processors and a telemetry system. The control law was implemented in software in the control processor. The objective of the control system was to reduce the dynamic response of the milling tool thereby improving its stability and its maximum depth-of-cut. System design including sensor and actuator development was guided using finite element modeling techniques. The components were constructed, and a successful experimental demonstration resulted.

Smart spindle unit for active chatter suppression of a milling machine: II. Dynamics and control

This paper addresses dynamics and control issues encountered in development of a smart spindle unit (SSU) for suppressing chatter in milling operations. The SSU comprises a suite of strain, displacement and force sensors coupled to four, high-force, electrostrictive ceramic actuators through a digital control processor. The operating principles of the SSU are discussed, and salient dynamics of the SSU and generic milling tools are explored in this context. Dynamic characteristics measured using the SSU sensors are presented and discussed relative to their influence on chatter and control of chatter.