Effect of apex offset inconsistency on hole straightness deviation in deep hole gun drilling of Inconel 718

Abstract

Abstract Hole straightness deviation is the critical performance indicator in deep hole gun drilling process, particularly in manufacturing of corrosion resistant alloys like Inconel 718 with high yield strength. Poor understanding on the root causes of straightness deviation and difficulty to reduce it has dramatically increased the complexity and the manufacturing cost of this manufacturing process. Due to the fast tool wear in machining of Inconel, gun drills have to be changed frequently after a very short drilling distance. However, it has been found that the gun drill tool geometry cannot be precisely controlled, as there is a lack of appropriate gun drill regrinding and measurement system. In this study, a customized gun drill regrinding system integrated with an in-situ gun drill measurement system is designed and developed to obtain high-quality gun drills with precisely controlled tool geometry. The effect of apex offset variation on the hole straightness deviation is studied through 12 unsupported gun drilling experiments at thin-wall positions of an Inconel 718 workpiece with 160 ksi yield strength. 4 types of apex offset variation conditions (consistent, reciprocating, decreasing and increasing) with 3 times of repetition tests are conducted, and the measurement results have shown that consistent apex offset leads to the smallest hole straightness deviation among all the conditions. Through a series of FEM analysis and further measurement of internal cylindricity of machined holes, it has been concluded that unbalanced cutting force components applied on the previous gun drilled hole due to the inconsistent apex offset will cause the new formed hole to be deviated toward thin wall, due to the relatively larger material deformation at the thin-wall side than at the thick-wall side.