| With the rapid development of smart manufacturing and robotic technology,orthopedic surgical robots have attracted widespread attention because they can provide reasonable surgical planning,precise surgical positioning and operation in orthopedic surgery.The orthopedic robot for bone cutting methods mainly includes drilling,milling,sawing,and grinding and so on.Cutting parameters directly determines the success rate of orthopedic surgery.Due to the bone property on brittleness and low thermal conductivity,it easily cause bone cracks and thermal damage,and then cause the surgical failure or postoperative complications,delaying surgical recovery,and bringing the patient much longer pain.In addition to the doctor’s operational skill,the choice of tools and process parameters directly determines the success rate of orthopedic surgery.Therefore,this study takes cortical bone as the research object.It mainly deals with cracks and surface quality problems in planar machining,such as milling and planning,and thermal damage caused by excessively high temperatures in hole machining.The study analyze these problems deeply to reveal the material removal mechanism and drilling temperature control technology on cortical bone.It can provide theoretical and experimental basis for the design of surgical tools and the selection of process parameters in orthopedic surgical robots.The main research contents of this article are as follows:(1)Based on the special microstructure and anisotropy of the cortical bone,the orthogonal cutting force is analyzed and the chip deformations are predicted in bone osteon cutting angles 0°,60°,90° and 120° to reveal the mechanism of the material removal.Due to the brittleness property on cortical bone,the material removal methods of the cortical bone in a small cutting depth is insensitive to the bone osteon cutting angle.While the material removal methods of the cortical bone in a large cutting depth has a significant effect on the bone osteon cutting angle.When the bone osteon cutting angle is 0°,60°,90° and 120°,the material removal methods are stripping cutting,shearing cutting,fracture cutting and fragmentation cutting,respectively.Studying on the bone material removal mechanism provides a theoretical basis for the design of tool parameters,and helps orthopedic surgical robots to choose reasonable cutting parameters.(2)The material removal methods on the cortical bone are analyzed in different bone osteon cutting angles and the models on the fracture toughness are established in large depths of cut and bone osteon cutting angles.Chip deformations from the orthogonal cutting experiments in different bone osteon cutting angles are studied,and simultaneously the chip deformation and the cutting force in the orthogonal cutting experiment are analyzed.The law on the depth of cut and bone osteon cutting angle effects on the chip deformation during the bone orthogonal cutting is as follows: in a small cutting depth,the bone osteon cutting angle has little effect on the chip morphology,and the chip shows a continuous shape.In a large cutting depth,the bone osteon cutting angle has a significant effect on the chip morphology.When the bone osteon cutting angle is 0°,60°,90° and 120°,the chip morphology shows continuous curl morphology,continuous saw-tooth morphology,particles morphology and powder morphology,respectively.According to force data from the orthogonal experiments,the fracture toughness values of cortical bone under different he bone osteon cutting angle are calculated.This research not only provides theoretical foundation for tool design,but also provides a basis for orthopedic surgeons to choose a reasonable tool.(3)A novel type of notch hollow drill bit that can be used to control the drilling temperature is designed and manufactured.The rake angle and rear angle models are established and a circular arc center coordinate model of the drainage groove in drill bit is established.According to the circular arc center coordinate model,a new type of notch hollow drill bit is accurately manufactured.Compared with the original K-wire bit in cortical bone drilling experiments,the new notch hollow drill can significantly reduce the drilling thrust,torque and drilling temperature.Therefore,this design of the notch hollow drill can reduce the drilling temperature in orthopedic surgery and shorten the recovery time of patients.(4)The characteristics of heat generation and transfer during cortical bone drilling are analyzed.Based on the inverse heat transfer method,the heat flux density model of cortical bone is established.Through comparing temperature from the experiment and simulation to calculate the heat flux density inversely,the finite element model of drill for simulating chip removal is established.Heat flux density values of the two drill bits is obtained without cooling.If the temperature in bone tissue is above 50℃,thermal damage in bone tissue will be generated.Based on this characteristics of bone tissue,the thermal damage region of K-wire bit and new type notch hollow drill after drilling is analyzed.The new type of notched hollow drill bit significantly reduces the thermal damage of bone tissue after drilling.This new type of notch hollow drill bit is used in orthopedic surgery,which will reduce the risk of thermal injury of bone tissue and surrounding tissues.(5)The cooling system of hand drill in orthopedic surgery is designed.The connector and fixed tail cover between the drill bit and the cooling pipe is designed and manufactured.Drilling experiments on cortical bone without cooling,air-cooling and saline cooling were conducted.The results show that air-cooling can significantly reduce the bone temperature after drilling,while saline cooling has no significant effect on temperature reduction.Based on the calculation method of heat transfer method,a model of heat flux density and convection coefficient during the notched hollow drill was established.Through simulation and experimental data to inverse iterative calculations,the heat flux density and convection coefficient values during drilling in air cooling and saline cooling were obtained.Through simulation experiments,the thermal damage region was calculated during bone drilling process with air cooling and saline cooling.The thermal damage of saline cooling is about twice that of air cooling.Therefore,it can significantly reduces thermal injury in orthopedic surgery with air-cooling. |
