Loading...

Simulation of the Chip Formation and Dynamic Analysis of Micro-Milling Process Considering Size Effects

Tajalli, Ahmad | 2014

881 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 46385 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Movahhedy, Mohammad Reza; Akbari, Javad
  7. Abstract:
  8. Micromilling is one of the promising micromanufacturing processes used for creating 3D complex micro-scale features such as those used in MEMS devices. The mechanism of chip formation in this technique is different from that in conventional machining, because the size effects due to hardening behavior of microstructures at micro-scales and also the crystallographic orientation of a workpiece texture affects the mechanical behavior where a severe plastic deformation occurs in an induced shear band in front of the tool edge. In the present thesis, micromachining process is investigated by considering these effects. In order to capture the effects of size scale during high speed and large strain deformation of the chips, dislocation-based strain gradient plasticity formulation is included into the constitutive equations of crystal plasticity-based finite element (CPFE) framework. The constitutive formulation is implemented in the commercial FE code Abaqus/Explicit by employing the interface subroutine VUMAT. The semi-implicit tangent modulus criterion is derived and employed in order to increase numerical stability of time integration during the solution. Also, ALE algorithm is utilized to model chip formation. The results reveal that the initial crystallographic orientations of the workpiece with respect to the cutting direction affect the chip morphology, cutting forces and specific cutting energy. In the second part of the thesis, the stability of the delay differential equations (DDEs), describing self-excited vibrations in a micro-milling process, is investigated by considering setup errors, size effects, and process damping phenomenon. Due to the stubby geometry of micro-tools, the shear deformation and rotary inertia effects are considered by using the strain gradient Timoshenko beam for modeling the tool. The extended Hamilton’s principle is used to derive a detailed dynamical model of the spinning micro tool in which the gyroscopic effects cause coupling of equations. Considering the actual geometry of the micro-end mill, dynamic stiffness (DS) formulations are developed to investigate the tool’s free vibration characteristics. In order to explore the stability of the process, stability lobe diagrams are depicted based on semi-discretization and Altintas-Budak methods. These diagrams delineate the considerable influence of process damping on the stability borders of the system especially at low spindle speed. Also they indicate that size effects cause higher stability for the micro-milling process
  9. Keywords:
  10. Crystal Plasticity ; Strain Gradient Theory ; Finite Element Method ; Self-Excited Vibration ; Stability Lobe ; Micromilling ; Dislocations Theory

 Digital Object List

 Bookmark

  • Pages from Ahmad Tajalli_PhD_Thesis
  • 2
  • 3