Keywords: internal turning, active boring bar, actuator, finite element model, Coulomb friction force.

This paper addresses utilization of three-dimensional finite element (FE) modelling of an active boring bar for the estimation of the control path frequency response functions (FRFs), i.e. FRFs between the actuator voltage and acceleration measured by the error sensor. The three-dimensional FE model of the system "boring bar - actuator - clamping house" enables variable contact between the contact surfaces of the boring bar, clamping house and clamping screws. The control path FRF estimated for the case with no friction force in the FE model displays a good correlation with the experimental estimates. The possibility of improvement of the control path FRFs by incorporating the nonlinear friction force between the contact surfaces of the boring bar, clamping house and clamping bolts was investigated. The influence of the nonlinear friction force generated on these contact surfaces on the control path FRFs estimates examined by means of the arctangent and the bilinear model of the Coulomb friction force for a set of values of governing parameters.

The arctangent and the bilinear friction force models were first tested on the SDOF model of the active boring bar for different excitation force levels and sets of values of the relative sliding velocity and the slip threshold.

The three-dimensional finite element model of the system "boring bar - actuator - clamping house" enabling variable contact has time-varying dynamic properties. The introduction of the Coulomb friction force approximated with the arctangent model in the three-dimensional FE model of the active boring bar enabling variable contact between the clamping house and the boring bar resulted in significantly degraded control path FRF estimates for the set of relative sliding velocities compared to the case with no friction force in the FE model. On the other hand, the bilinear model of the friction force incorporated into the three-dimensional FE model of the active boring bar enabling variable contact resulted in improved control path FRF function estimates compared to the case with the arctangent friction model. The selection of slip threshold seems to influence the fundamental resonance frequencies of the system "boring bar - actuator - clamping house".

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