On Design Of Twist Drill With Large Length-diameter Ratio And Deep Hole Drilling

Deep-hole processing is widely used in aerospace,automobile,firearms,petroleum,chemicals,mining and engineering machinery.Deep-hole processing conditions are very serious,in which cutting heat is not easy to dissipate and chip removal is difficult,so the cutting efficiency is low,and deep-hole drilling is considered to be one of the most difficult metal cutting problems.The ratio of hole depth to pore diameter is usually up to 12-30 in drilling crankshaft oblique oil hole,connecting rod oil hole,main oil channel hole of engine cylinder,mold water transport hole.The rigidity of the cutting tool is so poor that the performance of current deep hole drill cannot meet the requirements of low cutting force,long tool life and high production efficiency.Hence,the breakthrough in design and manufacture of deep-hole twist drill is urgent in engineering.This work aims to study the design and cutting performance of deep-hole coated twist drill with large length-diameter ratio between 10 and 30,and with inner coolant hole,by means of theoretical modeling and finite element analysis combined with cutting experiment,in order to solve the problem of poor chip removal capacity and short tool life in deep-hole drilling.First,a conical drill tip is designed and optimized.Based on the geometrical characteristic of conical flank,by defining a parameter named cutting edge offset distance,major design parameters such as the chisel edge,semi-point angle,drill point height and constructive clearance angle are established,and an explicit analytical solution to flank grinding parameters is derived after fixing axis angle.In order to avoid the flank rubbing,the auxiliary design parameter of drill tip named drag amount is put forward,and the method to solve flank grinding parameters of conical drill tip with curved cutting edge is established.By writing grinding parameters calculation program,several calculation examples for conical drill points with diameter of 8 mm and 10 mm are given.Four 3D solid models of conical drill point of different structure parameters with diameter of 8 mm are established,and all the conical drill points are manufactured successfully.The accuracy of the mathematical model of conical drill point and the explicit analytical solution to grinding parameters is verified by comparing the error between the measured value and the designed value of design parameters.Next,the forming spiral grooves with variable web thickness are optimized.The mathematical model of helical flute is built based on the geometry model of profile grinding wheels and the kinematic motions in machining helical flute.The flute cross-section equation is deduced.In light of this,the CAD of helical flute is carried out to investigate the influence of grinding wheel characteristic parameters on the flute cross-section.The principle of design helical flute section of deep-hole twist drill is analyzed from three aspects:chip capacity space,web thickness(rigidity)and land width.It is found that the tool rigidity can be ensured and the capacity space can be improved by increasing the middle arc radius R1 of the grinding wheel cross-section,moderately increasing the arc radius R2 and R3 of the grinding wheel cross-section separately corresponding to the rake face and secondary flute surface,and reducing the distance b2 and b3 from the center of each arc to the grinding wheel cross-section with a maximum diameter.Statics simulation is carried out to study the rigidity of five kinds of drills with variable web thickness,and the drill with constant web thickness in the front end and linear decreasing web thickness in the back end is optimized.Then,the deflection of the deep-hole twist drill is studied.An equivalent cantilever beam model of twist drill is established and the formulas for calculating the moment of inertia,the product of inertia and the polar moment of inertia of twist drill section are deduced.The numerical calculation and geometric analysis for obtaining the characteristic parameters of twist drill section are both described.The average moment of inertia of twist drill section is defined and the effect of twist drill length on average moment of inertia is analyzed.The results show that the average moment of inertia for longer twist drill is to half of the polar moment of inertia.Modification factors for bending and torsional deflection are proposed,and the bending and torsional deflection models of twist drill are established through the static finite element simulation and data fitting.This research can be used to predict the deflection of the deep-hole twist drill and to implement the optimal design for helical flute.After that,the optimal design scheme to the benefit of cooling is given on the basis of CFD.Six kinds of deep-hole twist drill schemes including DHS,DHD,DHL,DHM,DHO and DHG are designed considering three aspects of inner coolant hole,helical flute profile and variable web thickness,and 3D solid models and finite element mesh models of these deep-hole twist drills are established.Computational fluid dynamics simulations of the coolant used in deephole drilling are carried out,and the cooling effect of different schemes is evaluated by analyzing the coolant flow rate and the temperature on cutting edges.The simulation results show that the DHL scheme is the best selection for designing the helical flute of a deep-hole twist drill with large length-diameter ratio because of the better fluid streamline distribution,faster cooling speed in cutting edge tip and better overall cooling effect.At last,the cutting performance evaluation and application research of the deep-hole twist drill are carried out.Different cutting edge preparations are designed for newly developed Ztype deep-hole twist drill in the project,and the influence of cutting edge preparation on the chip breaking performance and the cutting force in deep-hole drilling is studied.The negative chamfer 0.04×(10°～25°)is selected and applied in the Z-type deep-hole twist drill.The contrast experiments on cutting force and tool life are carried out for six kinds of deep-hole twist drills,including X-type,Y-type,Z-type,MWS-type,ADO-type and RT100T-type.The experimental results show that the Z-type deep-hole twist drill has the lowest cutting force and the best cutting performance in machining 42CrMo and C45 steel.