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1898 |
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Frequency Response Prediction for Rotating Tool-holder-spindle Assemblies |
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Equipment, Machines & Instruments: Analysis & Modeling |
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| Content |
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High-speed machining (HSM) is an important process for discrete part manufacturing because it can provide high material removal rates together with good surface finish and part accuracy. To realize these benefits, stability lobe diagrams, which define regions of stable cutting as a function of spindle speed and axial depth of cut, can be applied. Computation of these diagrams requires that the dynamics of the cutting system (the tool-holder-spindle-machine assembly) as reflected at the tool point be known. However, due to the diversity of tool holders and tools available to end users, it is time consuming to perform impact testing, where a modal hammer is used to excite the tool point and the vibration response is measured using an appropriate transducer (typically a low mass accelerometer), for each combination.
The difficulty in cataloging the tool point frequency response functions (FRFs) for each tool-holder-spindle-machine combination in a particular facility is compounded if the assembly dynamic response varies with spindle speed. In this case, to obtain accuracy stability predictions, it would be necessary to record the tool point FRF at discrete speeds over the full range of interest. In addition to the time required to perform these measurements, there is also the clear obstacle in measuring the FRF for fluted tools at the location of interest (i.e., the tool point).
Conventionally, the tool point FRF is measured at zero speed and is presumed to remain constant at different spindle speeds. However, researches have reported FRF shifts at higher spindle speeds, which are typically ascribed to gyroscopic effects. In this paper, we present an approach for predicting rotating FRFs by coupling measurements of the spindle-machine substructure (identified using a simple geometry standard holder and inverse receptance coupling techniques) to models of the actual tool-holder substructure. The tool-holder models are developed using Timoshenko beams including gyroscopic effects.
The experimental identification of the spindle-machine FRF at speed offers a number of challenges. These include runout, variations in the spindle thermal state, and noise. Noise minimization is particularly important because the inverse receptance coupling approach includes a finite difference computation that amplifies any existing noise in the measured FRFs. For example, we have observed that the surface roughness of the standard holder must be minimized to obtain low noise data from the capacitance probe setup applied in this research. A time-domain technique for runout removal is also presented.
Experimental results are provided for a commercially-available spindle (20,000 rpm, 16 kW) and various tool-holder combinations. Tool blanks are used for tool point prediction validation. Finally, the influence of rotating FRF variations on stability lobe diagrams is presented.
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