A DEMONSTRATION OF ACTIVE SUPPRESSION OF MILLING-TOOL CHATTER
M. E. Regelbrugge* J. P. Lauffer†
Rhombus Consultants Group, Inc. Sandia National LaboratoriesMountain View, CA Livermore, CA
J. L. Dohner‡ C.M. Kwan¶
Sandia National Laboratories Intelligent Automation, Inc.Albuquerque, NM Rockville, MD
N. Shankar#
Lockheed Martin Missiles & Space CompanyPalo Alto, CA
* Vice President, AIAA Associate Fellow† Principal Member of the Technical Staff, SEM Member‡ Senior Member of the Technical Staff, ASME Member¶ Director of Research & Development, IEEE Senior Member# Senior Staff Scientist
ABSTRACT
This paper presents the overall approach taken in, andkey results obtained from, an effort to apply solid-state,active materials to suppress chatter in machiningoperations using milling tools. A proof-of-conceptprototype device, called the “Smart Spindle Unit,” wasbuilt and refined with the objective of increasing stabledepths of milling cuts. The paper describes the overallchatter-control system architecture, its realization in theSmart Spindle Unit and the performance of the systemas measured in a series of live cutting tests. Overall,the Smart Spindle Unit was able to increase stabledepths of full-immersion test cuts by factors in therange 2-20. Stability of partial-immersion test cuts wasimproved by 85-220% as measured by depth-of-cut atconstant chip loading.
INTRODUCTION
The particular system described herein is the SmartSpindle Unit (SSU), which was designed and builtunder the DARPA Advanced Reconfigurable Machinefor Flexible Fabrication (ARMF3) Program, andinstalled on the Ingersoll Horizontal OctahedralHexapod Milling (HOHM) machine. The HOHMmachine is depicted in Figure 1. The SSU wasdesigned specifically to allow active control of milling-tool dynamics for purposes of suppressing chatter of
flexible milling tools. Machining processes targeted bythe ARMF3 SSU involve slender milling tools such asthose typically used to machine deep-pocket features inmolds, dies and several types of aerospace structures.These slender tools are relatively flexible, and are proneto chatter when metal removal rates are increased. TheSSU was intended to improve metal removal rates by75% in these processes, resulting in an equivalentimprovement in machine productivity.
Regenerative tool chatter is a well-knownphenomenon1–4 that occurs when machining forcesexcite dynamic motions of the tool in such a way as toleave variations of the cut surfaces that further excitethose dynamic motions. Usually, the vibration modesresponsible for the onset of milling-tool chatter areassociated with the highest dynamic flexibility betweenthe tool and workpiece. When these modes are excitedby machining forces, their responses will cause thegreatest relative motions between the tool andworkpiece. Primary excitations occur as shaped forceimpulses caused by the entry of a milling tooth into theworkpiece material. Dynamic responses to these forcesleave small spatial variations in the newly cut surfacethat produce an additional, periodic force between thetool and workpiece as the milling teeth pass through theworkpiece. This periodic force occurs at a frequencycorresponding to the excited vibration. Chatter occurswhen the damping in the responding modes isinsufficient to reduce the vibratory motions of the tool
43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Con22-25 April 2002, Denver, Colorado
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or workpiece caused by these periodic forces betweencuts made by successive teeth. Damping arises fromtwo principal sources; natural damping of the tool andworkpiece structures, and so-called “process damping”occurring as a result of interactions between the cuttingteeth and workpiece surface. Thus, the tool orworkpiece will be able to cut in a stable manner only upto a certain depth or chip load, whereupon the forcesassociated with the periodic excitation overwhelm thedamping in the system and lead to a limit-cycleoscillation of the tool relative to the workpiece.
The ARMF3 Smart Spindle Unit (SSU) is designed tocontrol milling-tool chatter by modifying the bending
dynamics of the tool. This is done by changing thedynamic boundary condition at the root of the tool insuch a way as to minimize strain energy in the toolcaused by dynamic bending vibrations. The boundarycondition is changed by moving a spindle cartridge inthe plane normal to the rotational axis of the tool andspindle. Control signals are derived from measurementsof tool bending strain, which are transmitted off therotating tool via FM telemetry. The spindle cartridgeholds the rotating spindle and tool holder in such a wayas to admit lateral motion as commanded through four,high-stiffness ceramic actuators. Axial motion isprevented by use of an hydrostatic thrust bearing.Figure 2 illustrates the key components of the SSU.
Figure 1. Rendering of the Horizontal Hexapod Machine
Chatter control is accomplished by measuring thebending strain in the milling tool during cutting, andfeeding that signal back to the control actuators to movethe base of the tool in such a way as to minimizebending strain energy. Since the actuators act in afixed, nonrotating coordinate system, the control signals
derived from the tool bending strain measurementsmust also be expressed in those coordinates. This isdone by transmitting the strain signals via FM telemetryfrom the rotating spindle to a control unit that resolvesthe rotating strain signal into the stationary, actuatorframe of reference using information from a spindle-
Work-piece
HexapodStrut
SSU
Frame
Platform
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mounted, digital rotary encoder. Control laws are thenexpressed in the stationary actuator frame of reference,and control signals are sent to the actuators through apower amplification system. Figure 3 illustrates thesignal paths employed to affect chatter control using theSSU.
Figure 4 shows a photograph of the assembled SSU, inwhich the four actuator housings, instrumented tool andFM strain telemetry system are clearly visible.Development of the SSU is further described in Refs.5–7. Details of the SSU chatter-control algorithms anddynamics and controls issues are discussed in Ref. 8.
CeramicActuator (4)
ActuatorHousing
Spindle
Tool Holder
Milling Tool
SSU Housing
SpindleCartridge
FM TelemetryAntennae Hydrostatic
Thrust Bearing
StrainGages
Figure 2. SSU Components and General Arrangement
MDSP-2000PROGRAMMER
INTERFACE
TOOL
MACHININGFORCES
MECHANICAL
ANALOG
DIGITAL
FM RADIO
SPINDLEMOTOR(25 KW)
FANUCSPINDLE
CONTROL
SPINDLE/BRGCARTRIDGE
DIGITALSHAFT-ANGLE
ENCODER
STRAINGAGES
STRAINMOD/XMIT
STRAINRCV/DEMOD
STRAIN ROTARYTRANSFORMATION
(4) ACTUATORPOWER AMPSFILTERS
A/DD/A DSP FILTERS
CONTROLSYNTHESIS
(4) PMN STACK
ACTUATORS
1010
1
10101
1010
1
10101
10101
KEY:
10101
Figure 3. Signal Paths for SSU Chatter Control
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Figure 4. Photograph of Assembled SSU
EXPERIMENTAL RESULTS
Extensive tests were made of the SSU and chattercontrol system under various machining conditions.The methodology of Ref. 9 was employed for all tests.All cutting tests were made using the 15 mm-diameter,2-tooth, instrumented tool cutting into an aluminumalloy block at 0.1 mm/tooth chip load in the range3000-4200 RPM.
Chatter was identified both by the characteristic audibleresponse and by observing the strain response of thetool. Figure 5 shows typical time histories of tool-strainreadings during stable (top) and unstable (bottom) cuts.Figure 6 shows these data in the form of strain powerspectral densities (PSDs). The excited tool bendingmode is clearly evident in the PSD for unstable cuttingshown in Figure 6. Figures 5 and 6 correspond to full-immersion cuts made with no active control.
Figures 7 and 8 show similar tool-strain responses forcuts made at the same depth and speed (3600 RPM,0.01 mm depth, full immersion) with control off (top)and control on (bottom). Clearly, the uncontrolled toolexperiences chatter in this condition, and the controller
has restored chatter-free operation of the cutting toolwhen active. Both the qualitative, strain magnitudeplot, and the more quantitative PSD plot indicate thatstrain response during controlled cutting is like thatobserved for stable, uncontrolled cuts. Surface finishquality is also restored in the controlled, chatter-freecut.
Nonetheless, as depth-of-cut is increased, even the SSUcontrol system will no longer be able to prevent toolchatter. The reason is that harmonic responses of thetool’s tooth-passing frequency will also grow withincreasing cutting depth, and responses occurring in thefrequency range near the tool’s bending resonances willbegin to excite those resonances and lead to instabiltiy.
This effect is captured in Figures 9 and 10. Figure 9shows strain response PSDs resulting from an unstable,uncontrolled cut at 0.01 mm depth, and a controlled,nearly unstable cut at 0.30 mm depth. Note theproximity of the high harmonic responses to theinstability frequency (excitation of the tool’s bendingmodes proved uncontrollable as the depth wasincreased to 0.35 mm at these conditions). Figure 10shows the evolution of strain response as a function of
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depth of cut. Note that the 3600 RPM tooth-passingharmonic responses shown at 120 Hz, 240 Hz and 360Hz grow linearly with depth of cut, whereas the
response of the tool’s bending resonance growssuperlinearly as it is driven by the harmonic response at480 Hz.
Figure 6. PSDs of Open-Loop Strain Response for Stable (lower) and Unstable (upper) Cuts.
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Figure 7. Strain Response for Uncontrolled (top) and Controlled (bottom) Cuts.
Figure 8 Controlled (blue) and Uncontrolled (green) Strain Response
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Figure 9. Tool-Strain Response for Maximum Stable Depth of Cut and for Instability.
Figure 10. Strain Response Spectrum vs. Depth of Cut (controlled)
Finally, to demonstrate the robustness of the controlapproach, hardware and software, we turned the controlsystem on and off during cutting and we were able todemonstrate not only control of the system, but alsorecovery of control and cutting stability from anunstable cut. Figure 11 shows a time response of the
strain measured for a 0.2 mm-deep, half immersion cutat 3600 RPM spindle speed and 1440 mm/min spindlefeedrate (0.2 mm/tooth chip load). This figure clearlyshows tool strain levels were quickly reduced to thestable range when the control was active.
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Figure 11. Strain Response during Off-On-Off Modulation of Controller
Figures 12 and 13 present the most compellingsummary indication of the efficacy of the SSU’schatter control system. Figure 12 shows the increasein stable depth-of-cut obtained for quarter-, half- andfull-immersion cuts according to the terminology ofRef. 8. Stable cutting depths are increased by 85-220% by the chatter control for fractional-immersioncuts (by far the most common kind of cut made inproduction milling processes. Depending on spindlespeed, stable depths of full immersion cuts wereincreased by factors of 2-30 with application of thechatter control.
Figure 13 shows a photograph taken of a cut surfacein the test aluminum alloy. The cut path is from rightto left in this photograph, and the first visible regionwas cut with the chatter control disabled. After thechatter control was turned on, the photograph showsa high-quality cut-surface finish was restored.
CONCLUSION
The SSU has demonstrated the feasibility of activelycontrolling and suppressing chatter of milling tools.This demonstration represents a fundamental advancein machine-tool applications of active control as ithas successfully addressed the myriad challenges
associated with controlling flexible dynamics ofrotating tools. Thus, the sensing, control andactuation technologies embodied in the SSU havebeen shown, through the results presented here, to beprototypical of a milling-tool chatter control andsuppression system.
By stabilizing the milling process at increased depthsof cut, devices like the SSU can be expected toimprove the productivity of high-quality millingoperations by factors of 2-20, depending on cuttingconditions. Furthermore, the SSU was shown to becapable of restoring stable cutting from an unstablecutting condition, offering the potential to relaxrestrictions on feed rates and cutting depths in themilling of complex part shapes.
While the demonstrated improvements in cuttingperformance are promising and, in some cases, quitedramatic, the SSU’s capabilities must necessarily bequalified by its nature as a developmental, proof-of-concept prototype. Quite naturally, the SSU’s chattercontrol system is not capable of controlling orsuppressing tool chatter under all machiningconditions, or with all types of milling tools.Accordingly, further development of such systems isneeded to extend their practical range of applicabilityfrom the level of the proof-of-concept prototype.
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Nonetheless, the SSU has proven to be a legitimateproof-of-concept demonstrator that proved thepossibility of chatter control of rotating milling tools,and showed that active control using telemetric
sensing, digital processing, and solid-state actuationcan be quite effective to improve cutting performanceof flexible milling tools.
Figure 12. Measured Stability Limits of Uncontrolled and ControlledPartial-Immersion Cuts (3600 RPM, c=0.1 mm/tooth)
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Figure 13. Photograph showing Cut Surface with Chatter Control Off (right), and Chatter Control On (left)
REFERENCES
1. Merrit, H.E., “Theory of Self-Excited Machine-Tool Chatter, Contribution to Machine-Toolchatter, Research--1”, Journal of Engineering forIndustry, Transactions for the ASME, pp. 447-453, November 1965
2. Tobias, S.A., Machine-Tool Vibration, Blackie& Son Limited, 1965
3. Tlusty, J., Ismail, F., “Special Aspects of Chatterin Milling”, Journal of Vibration, Acoustics,Stress, and Reliability in Design, Transactions ofASME, vol. 105, pp. 24-32, January 1983
4 . Shi, H.M., Tobias, S.A., “Theory of FiniteAmplitude Machine Tool Instability”,International Journal of Machine Tool DesignRes., vol. 24, No.1. pp. 45-69, 1984
5 . N. Shankar, et al., “A smart spindle unit foractive chatter suppression of a milling machine,Part I: Fabrication and Assembly,” Proc. SPIESmart Structures & Materials Symposium 1998,Vol. 3326, J. Sater, ed., March 1998.
6. J.P. Lauffer, et al., “Milling Machine for the 21stCentury - Goals, Approach, Characterization andModeling”, Proc. SPIE Smart Structures &Materials Symposium 1996, Vol. 2721, R.Crowe, ed., February 1996.
7. J. Dohner, et al., “Active Chatter Control in aMilling Machine”, Proc. SPIE Smart Structures& Materials Symposium 1997, Vol. 3044, J.Sater, ed., March 1997.
8 . J. Dohner et al., “Mitigation of ChatterInstabilities in Milling by Active StructuralControl,” Proc. SPIE Smart Structures andMaterials Symposium 2002, Vol. 4698, A.McGowan, ed., to appear.
