Sensitivity Analysis and optimization of geometrical parameters over temperature and force in automatic drilling of cortical bones

Bone drilling is very common in the field of biomedical engineering as it is very prevalent in orthopedic surgeries and breakage treatment, dentistry and bone biopsy. It possesses an utmost importance because machined bone is really complicated from the material point of the view. Using automatic drills and also robotic assisted CNC machines has been made a real breakthrough in bone drilling process. Bone drilling is a delicate process; exerting high forces to the bone tissue causes breakage or at least crack initiation in the bone. In some cases tool may break in the bone. Additionally, if drilling temperature rises higher thangenerally believed ۴۷ oC, it can cause thermal necrosis phenomenon which leads cell death of the bone tissue. There are studies focusing on bone geometry optimization in order to lower down the process temperature and force. In this study considering helix angle and point angle as most influential factors, the evolution of process force and temperature was studied at the same time. First using design of experiments and response surface method, imposed force and produced temperature were modeled and the second order linear regression equation governing the model was derived. Then sobol sensitivity analysis method was used to quantitatively measure the effect of each input parameter over the process temperature and force. Finally, by optimization of the governing model, the optimum combination of the input parameters to yield the desirable process force and temperature was presented. It was found that, within the range of parametersstudied, the minimum process force imposed to the bone occurs with point angle of ۹۰ and helix angle of ۱۸. Moreover the minimum process temperature occurs at point angle of ۹۰ and helix angle of ۲۷. Considering multi objective optimization of process force and temperature, a drilling tool with point angle of ۹۰ andhelix angle of ۲۵.۱۷ was suggested for real operations. Considering the simultaneous effect of all parameters, helix angle and point angle are the most effective ones respectively.

Keywords:

machining , bone drilling , force , temperature , Sobol sensitivity analysis , design of experiment , biomedical engineering