Contact dynamics identification of holder extension assembly via inverse structural modification method

Ayyıldız, Hanife
Chatter is a critical problem in machining operations and can be eliminated using Stability Lobe Diagrams. To determine the stability diagrams, tool point FRF of the assembly is required, and contact dynamics at the holder-extension and extension-tool interfaces directly affect the tool point FRFs. Thus, modeling and identifying the contact dynamics are crucial in the machine tool models for accurately predicting tool point FRFs. This thesis uses the Inverse Structural Modification Method (ISMM) to identify contact dynamics at the holder-extension interface in tool-extension-holder assemblies. The method, first applied to bolted beam connections, is extended to machine tool dynamics using a quasi-static model with a 2x2 complex stiffness matrix. Validation studies involve numerical data, simulated experimental data, and actual experimental results. First, the full receptance matrix of contact dynamics at the interfaces is obtained in numerical studies using finite element (FE) simulations. The study demonstrates the sensitivity of the ISMM to noise using polluted numerical data sets. Simulated experimental studies approximate unmeasured rotational FRFs using the Finite Difference Technique (FDT). The effects of the Finite Difference Technique (FDT) on identification results are also examined. The results show that second-order FDT does not yield more accurate results than first-order FDT at points near contact dynamics interfaces, comparing FDT results with FE simulation results. Since the contact dynamics faces are inaccessible, the System Equivalent Model Mixing (SEMM) method is employed as an expansion technique. However, due to numerical issues in the calculations of the SEMM method, it does not provide reliable and accurate results. Additionally, the study investigates the impact of the order of FDT on SEMM results, finding that numerical errors increase when rotational FRFs near the contact dynamics interface are approximated using second-order FDT. The SEMM method's limitations are addressed by selecting application points closest to contact interfaces on the holder and extension. These points are then used to validate the ISMM through simulated experiments, and the effects of the initially assumed complex stiffness matrix on identification results are investigated using three different models. Finally, the experimental results from impact testing of the free-free holder-extension assembly are utilized to apply the ISSM. By using these identified results to predict the modes and FRFs of the assembly, it is observed that the identified results align well with the experimental modes and FRFs. The ISMM can accurately predict the first two natural frequencies of the HE assembly, with only a one percent error, through the identified stiffness matrices.
Citation Formats
H. Ayyıldız, “Contact dynamics identification of holder extension assembly via inverse structural modification method,” M.S. - Master of Science, Middle East Technical University, 2024.