Design Of Bta Drill Pipe Based On The Principle Of Hydrodynamic Lubrication And The Study Of Its Self-centering And Self-rectifying Mechanism

A novel BTA(Boring and Trepanning Assoeiation)drill pipe is designed based on the principle of hydrodynamic lubrication,and the mechanism of its self-centering and self-rectifying is studied to improve the straightness and position accuracy of processed deep holes.Wedges are set on the outer cylindrical surface where exists the cylindrical drill pipe including the fixed wedge and moving wedge.Fixed wedges are installed between the drill bit and the drill pipe which could be guided by self-centering force and self-correcting force producing by oil film pressure The workpiece and the wedge are not in contact directly,supporting force generated by the wedge-shaped oil film could reduce workpiece deformation prevent the straightness difference of drill because of vibration,workpiece hardness and other factors.The slender drill rod tends to bend in the middle position,thereby reduce the machining accuracy.This paper designed a supporting device located in the drill pipe,which the moving wedges are used as the moving support points to keep the workpiece and the pile straight,which minimizes the bending degree of the drill pipe and reduces the eccentricity of the drill pipe.The oil film pressure produced by the fixed and moving wedge during the machining process stabilizes the drill rod in the correct position like a three-jaw chuck to realize the self-centering.The mathematical model is established for the tool system with the wedge body in the correct position,the oil film thickness expression in the first wedge space is derived based on the geometric relationship,and the differential equation of the oil filmpressure is integrated by the numerical integration method.Finally,oil film pressure(self-centering force)produced by the first oil wedge can be calculated based on the theory.The self-centering force changes with the arc convex radius or the eccentricity of the wedge shape.On the basis of the change law,the radius of the arcuate protrusion and the size of the eccentricity of the wedge can be selected according to the required self-centering force.If the drill rod is deflected,the resultant force of the oil film forms a corrective force,pushes the tool to the correct position to achieve self-correction.A mathematical model was established for the skewed tool system,and the oil film thickness is calculated for each of the three wedges based on the geometric relationship.The force of the each oil film and the total correction force is deduced.The research data shows that the correcting force changes with deviation angle and it varies with eccentricityFluent software was used to simulate the oil film of the wedge body for verifying the previous calculation.