## 2 Structure Design of the Modular Drill Bit  
A modular bit to enable switching between coring drilling and non-coring drilling without lifting the whole set of drilling tools was designed according to the structure of wireline coring systems, as in Figure 1. This drill bit includes an external bit (1), two kinds of internal bits (2) and (5), an isolating casing (3), and a torque ring (4). The external bit (1) can be connected to an outer tube of a conventional wireline coring system using threads, whereby speed and torque for cutting rocks can be obtained from the drill pipes. The isolating casing (3) and the torsion ring (4) are installed inside the external bit (1). On the isolating casing, several radial holes (b) are opened, where drilling fluid flows from the clearance between the external bit and the internal bit to nozzles on the external bit. There are two group of splines on the outside and inside of the torque ring (c, d), which can mesh with the splines inside the external drill bit (e) and splines outside the internal bit (a), respectively. Through the above splines, the rotation speed and torque of the external bit can be transmitted to the internal bit, ensuring synchronous rotation between the two bits and enabling effective rock cutting. The two kinds of internal bits are a coring bit (2) and a non-coring bit (5), which can be connected to the inner tube assembly of a wireline coring system. To switch from coring drilling to non-coring drilling just requires pulling out the inner tube assembly and changing the coring bit to the non-coring one without lifting the whole set of drilling tools from the borehole. Conversely, the same steps can be taken to change the non-coring bit to the coring bit. In this paper, several modular bits designed for drilling boreholes with a diameter of 216 mm are described. The external drill bit with a hollow structure has an outside diameter of 216 mm and a minimum inside diameter of 105 mm at the bottom. Six nozzles (f) are arranged on the external drill bit, through which drilling fluid can flow to the borehole bottom. The internal coring drill bit is a hollow impregnated diamond bit which can obtain rock cores with a diameter of 58 mm. Meanwhile, the internal non-coring bit is also equipped with several nozzles (h) on its bottom, used for drilling fluid circulation and hydraulic rock fragmentation. Figure 1: _Cont._  
As Figure 1c,d show, the drilling fluid circulating process is different for the coring bit and non-coring bit. When coring drilling, drilling fluid flows into the clearance between the external bit and internal bit. Then, most of it reaches the bottom of boreholes through radial holes on the isolating casing and nozzles on the external bit, while the remainder of the drilling fluid flows to the borehole bottom through the clearance between the isolating casing and the internal coring bit. Finally, the above two streams of the fluid meet at the borehole bottom and, subsequently, return along the annular clearance between the external bit and borehole wall. During non-coring drilling, a small amount of drilling fluid flows to the borehole bottom via the clearance between the external bit and internal bit, while most of the drilling fluid flows to borehole bottom via the central channel of the internal non-coring bit and is jetted out from the its bottom nozzles. Then, the drilling fluid converges at the borehole bottom and flows upward. Figure 1: Construction of the large diameter modular bit: (**a**) 3D model of the external bit; (**b**) 3D model of the coring and non-coring internal bits; (**c**) internal structure of the coring drill bit; (**d**) internal structure of the non-coring drill bit.