1.Chongqing Research Institute, China Coal Technology Engineering Group，Chongqing , China； 2.National Key Laboratory of Gas Disaster Monitoring and Emergency Response Technology,Chongqing ,China

In order to solve the problems of the casing drilling process through discarding drill bit or drilling the drill bit for through or subsequent drilling, a extractable reaming bit is designed, which is combined with small diameter full section bit to form a two-stage cutting mechanism，and the two-stage cutting mechanism is pulled out of the casing after drilling through fractured zone so that the subsequent deep hole drilling operation can continue. This paper elaborates on positioning mounting mechanism,the cutting wing and locking mechanism for cutting wing expansion，and explains the working mechanism of each mechanical structure;The paper introduces in detail the whole operation flow of the fixed installation of the extractable reaming bit,the expansion of cutting wing, reaming drilling of casing hole section, rotating into the drill pipe that extracts the bit, closing of cutting wing and extraction of two-stage cutting mechanism from the casing to casing drilling process for pulling out the bit,and describes the switching process of extractable reaming bit in the closed and expanded state; and the design and calculat of the key component spring which affects the stability and reliability of extractable reaming bit is given. The designed extractable reaming bit was tested in the coal mine,and the results of the field trial showed that the expanded cutting wing is locked firmly during cutting,and the cutting wing is closed smoothly when extracting ,and the cutting ability was strong ,and the cutting reliability was high,and the feasibility of the structure design and the casing drilling process of extractable reaming bit is verified.The extractable reaming bit can also be used in the underground sealing drilling process,which has good economic benefits.

国家科技重大专项资助项目（2016ZX05045001-007）