Mitigation of chatter instabilities in milling by active structural control

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Abstract

This paper documents the experimental validation of an active control approach for mitigating chatter in milling. To the authors knowledge, this is the first successful hardware demonstration of this approach. This approach is very different from many existing approaches that avoid instabilities by varying process parameters to seek regions of stability or by altering the regenerative process. In this approach, the stability lobes of the machine and tool are actively raised. This allows for machining at spindle speeds that are more representative of those used in existing machine tools.

An active control system was implemented using actuators and sensors surrounding a spindle and tool. Due to the complexity of controlling from a stationary co-ordinate system and sensing from a rotating system, a telemetry system was used to transfer structural vibration data from the tool to a control processor. Closed-loop experiments produced up to an order of magnitude improvement in metal removal rate.

Introduction

The following is a discussion of the experimental validation of an approach to enhance the productive capacity of a milling machine. This approach employs the use of active structural control technology to increase the performance of a machine by enhancing the stability of the cutting process. Unlike previous work, in this approach, the stability limits of the machine are altered. This approach was originally presented in 1997 in the paper by Dohner et al. [1], and, to the authors’ knowledge, the following describes its first successful hardware implementation.

Maximum metal removal rate (MMRR) is a quantitative measure of the productive capacity of a machine tool. MMRR is limited by several factors including the onset of machining instabilities that are a function of the vibratory modes of both the machine and the tool [2], [3], [4]. By altering the dynamic characteristics of these modes, instabilities could be mitigated and MMRR improved. This alteration could be achieved by physically modifying the structure of a machine; however, this would be very costly and time consuming.

Many researchers have developed methods to avoid these instabilities by seeking regions of exiting stability [5], [6]. Although promising for the high-speed machining of materials such as aluminum, for many ordinary materials, these approaches require running at speeds where tool wear can be excessive. To better illustrate this, consider Fig. 1, an illustration of the stability limits of a typical machine tool. As shown in this figure, lobbing effects produce regions of high stability in many machine tools. These lobes can be used to eliminate chatter by operating within a lobe. Nevertheless, most machines are not designed to operate at these speeds. Machines are designed to operate in a more confined section of the curve between ηmin and ηmax [ANSI/ASME standards]. In the approach described in this paper, it is assumed that the machine is designed to operate within this limited practical range.

Other approaches have been shown to mitigate chatter by varying spindle speed [7] or by modifying the periodicity of inserts within a tool. A cutting tool can have a number of inserts. As an insert cuts through metal it lays down a pattern, and this pattern affects the cut of the next insert on the tool. This interaction creates a dynamic feedback path between successive cuts, and, as in many feedback systems, can lead to instability. During cutting, energy is pumped into well coupled modes. If this energy pumping is great enough, energy gain will not be balanced by energy loss, and energy storage will grow; thereby, producing the dynamic instability known as regenerative chatter. Varying spindle speed or changing the period of inserts on the tool can modify this pattern and limit the flow of energy into the tool. Nevertheless, many machine tools are not designed to operate in such a mode, and the modification of a large set of machine tools can be very expensive.

An alternative approach would be to use an active structural control system to alter dynamics. By properly altering these dynamics, the stability lobes of the machine and tool (i.e. maximum depth of cut vs. r.p.m. as shown in Fig. 1) will increase. This is very different from previous approaches that avoid instabilities by varying process parameters to seek regions of stability [5], [6] or by altering the regenerative process [7]. Papers that discuss methods of avoiding instability are numerous, however, papers that discuss the use of active methods to improve stability, are relatively few [8], [9], [10], [11], [12], [13], and almost all of these address turning, not milling. Moreover, the difficulties of performing active control in milling are far greater than those in turning where actuators and sensors can be located in the same stationary co-ordinate system.

The goal of the approach described herein is to ultimately develop a chatter suppression system that could be used on a wide variety of machines and machine tools. This is similar to the approach presented in Ref. [12] where an adaptive tool holder was constructed for the purpose of mitigating chatter instabilities. Although conceptionally successful, presented experimental results showed little of the periodic character of chatter, and therefore, a demonstration of performance improvements due to control were questionable at best.

The following sections present a hardware implementation of the theoretical approach presented in the paper by Dohner et al. [1]. Chatter suppression is performed by using an active control system to drive up the stability limits of a machine and tool. The chatter response presented in this paper shows itself as a strong instability in a mode of vibration. By using active control to limit this instability, up to an order of magnitude improvement in cutting performance was achieved.

Section snippets

Hardware design

An illustration of the hardware designed and constructed to demonstrate the utility of active control is shown in Fig. 2. Vibration is sensed at the root of a rotating tool by strain gages that are arranged in half bridge configurations to sense bending in two lateral directions. Excitation voltages were supplied to the half bridges using commercial electronics. Power is supplied to these electronics via magnetic coupling between rotating and stationary wires, and a telemetry system is used to

Characterization

Without the benefit of previous design data, the SSU design relied heavily on the use of numerical analysis [1]. Although this allowed for enough insight to complete an initial design, a full characterization of dynamics was required upon fabrication.

Initial experimental analysis of the SSU showed that system dynamics were not controllable or observable [15]. Frequency Response Functions (FRFs) were measured between voltage inputs to power amplifiers and tool strain responses in stationary

Control design

Control design was performed as a two-step process: (1) the production of a reduced order realization of dynamics, and (2) the design of a robust controller.

Fig. 6 shows controller logic. Three voltage signals are fed into the controller—two voltage signals from the receiver and a voltage signal from the decoder. These signals are passed through anti-aliasing filters and are then sampled. The result is a numerical data train representing tool strain, in rotating co-ordinates (x,y,z). These data

Results

As discussed in the introduction, Fig. 1 is an illustration of the stability limits of a hypothetical machine and tool. The area below the curve is stable and the area above the curve is unstable. Notice, that at low spindle speeds, due to processing damping effects, large depths of cut can be taken; however, metal removal rate is also low. At higher rotational speed, lobbing exists. This lobbing represents regions of stability at larger depths of cut. Operating within these lobes will produce

Conclusions

The work presented in this paper was a validation of the theoretical approach presented in the paper by Dohner et al. [1] in 1997. The experimental results presented show that active structural control could be used to increase the MMRR of a milling machine by up to an order of magnitude. To the authors knowledge this was the first successful demonstration of this approach in milling.

Although the above experimental analysis was successful, the design of machines or tool holders that can

Acknowledgements

The authors would like to thank the following individuals for their support. Dennis Bray (Ingersoll Milling Machine Company), Bob Winfough (Ingersoll), David Martinez (Sandia National Laboratories), Leonard Haynes (IAI), James Handrock, Brian Driessen, Jim Redmond, and Karen Archibeque (Sandia National Laboratories).

Thanks also goes to DARPA for their funding and programmatic guidance, and to the USAF for their efforts as contract managers. Special thanks goes to Lockheed Martin Space Systems

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    Citation Excerpt :

    The research on the active control techniques shows great performance [3,4] but mostly stays in the lab level because sensors, controllers and actuators are needed, which means high costs and complexities in practical condition. Especially for end mill processes, active devices are often applied to the spindle bearing [5,6] and toolholder [7,8] but hardly conducted on the cutting tool directly as it is rotating. On the contrary, the passive control techniques which introduce damping elements passively have been applied to the slender cutting tool since Ormondroyd and Den Hartog in 1950s [9] and many commercial products like the Silent Tools from Sandvik have been proposed recently [10].

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