| With the aging of the population,the incidence of fractures is increasing.Whether fracture reduction or joint replacement,internal fixation is one of the important processes.Whether screw fixation with metal plate or intramedullary nailing is used,it is necessary to drill holes at the fracture by using a medical drill bit.However,due to the biological activity,the cutting force and temperature will cause mechanical and thermal damage to the bone tissues,and seriously affect the postoperative recovery of the patients.Moreover,bone tissue is an anisotropic brittle material,the micro-cracks are easily generated on the surface of the hole during drilling,which affects the bonding strength between the implanted screw and the bone tissue,resulting in screw loosening and causing secondary damage to the patients.Therefore,in order to reduce the generation of microcracks without causing additional damage to the bone tissues,it is a technical challenge that needs to be tackled in orthopedic surgery.The technology of low-frequency vibration assisted is to apply a certain frequency on the tool or the workpiece to realize the periodic change of the relative position,achieving the intermittent cutting process.It can effectively decrease the cutting force and cutting temperature under the condition of vibration assisted technology.Therefore,in order to reduce the tissue damage and shorten the recovery time of patients,it is of great theoretical significance and practical application value to study the mechanism of low-frequency vibration assisted bone drilling.On the basis of traditional drilling,the kinematics characteristics of axial low-frequency vibration assisted drilling was analyzed in this paper.The interaction mechanism between the drill bit and the bone tissue was revealed.The trajectory equation of cutting edge under the condition of axial low-frequency vibration assisted was constructed.The trajectory of the cutting edge under the vibration-assisted conditions was constructed.Also,the correlation between the cutting time and the feed rate,the vibration frequency and amplitude were clarified.In addition,the relationship between the average friction power and the parameters were derived.The friction experiment of the pin and disk sliding friction pair composed of bone and tool material were carried out.The influence of friction parameters on the friction coefficient and the wear rate were studied.Moreover,the effect of vibration on the friction temperature was revealed by simulation.It was showed that the friction temperature generated in the vibration assisted friction process was lower.In addition,the wear mechanisms between the bone and the tool material were clarified.It was considered that the oxidation wear,the adhesive wear and the abrasive wear were the main wear failure forms.Based on the force generation mechanism,the cutting edge of the drill bit were equivalent to the oblique cutting model and the orthogonal cutting model respectively,which were divided into a series of cutting units.The drilling force equivalent model of the cutting unit was established.Due to the anisotropic characteristics of the bone tissue,the influence of the dynamic cutting area caused by drilling angles was considered.In order to verify the correctness of the theoretical model,the drilling experiments were carried out under the condition of low-frequency vibration assisted.The effect of parameters on the drilling force was obtained through bone drilling experiments,which provide guidance for optimizing the process parameters.Compared with traditional drilling,the drilling force obtained under the condition of low-frequency vibration assistance was lower,which can effectively reduce the mechanical damage of bone tissues.Based on the cutting temperature generation mechanism,the theoretical model was established by the shear deformation zone and the friction zone.To better control the cutting temperature,the method of the drill bit pre-cooling was proposed,and the influence of the drill bit initial temperature on the heat transfer was considered.The theoretical model was verified by the single-factor drilling experiments.At the same time,the effects of process parameters on the drilling temperature were explored.The results showed that the drilling temperature obtained in the low-frequency vibration assisted was lower,and after the pre-cooling of the drill bit was the lowest.In addition,according to the response surface experimental results,the initial temperature of the drill bit was the main factor that affecting the drilling temperature.Therefore,the method of the drill bit pre-cooling under the condition of low-frequency vibration assistance has a significant effect on reducing the drilling temperature,which can reduce the thermal damage to bone cells during drilling.The quality of drilling surface was revealed.The chip formation process,the chip morphology,the surface roughness and the screw pull-out force were analyzed.In addition,under the same experimental parameters,the effects of traditional drilling and low-frequency vibration assistance on the morphology,the surface roughness and the screw pull-out force were compared.The results showed that under the condition of low-frequency vibration assistance,the chip size and the surface roughness can be reduced,and the pull-out force of the screw can be improved.Moreover,with the increase of the drilling force and the surface roughness,the screw pull-out force decreased.The orthogonal experiments were designed by design expert statistical software,and the influence of drilling parameters on the cutting force,the cutting temperature and the surface roughness were analyzed by response surface method.The variance analysis and residual analysis of the experimental results was carried out respectively,and the quadratic regression prediction model was established.The multi-objective optimization was conducted by using the genetic algorithm method,which was take the minimum drilling force,drilling temperature and surface roughness as the optimization objectives.Then,the optimal parameter combination was obtained.The drilling experiment was carried out to verify the accuracy of the method in the process of solving the optimal parameters. |
