## 1 Introduction  
The mechanical pre-cutting method was first introduced as a tunneling technique in the United States in 1950 and applied in traffic tunnels in France in the 1970s [1]. Since then, it has been widely used in Europe. In the 1980s and 1990s, this technique was used in approximately 30 tunnels in France and Italy [2]. The mechanical pre-cutting method is economical and effective for rocks with a strength of up to 70 MPa and can be applied up to a compressive strength of 100 MPa for a short distance [1]. This technique improves the quality and stability of tunneling, reduces ground settlement, noise and vibration, and accelerates excavation [3]. This tunneling method involves a pre-cutting machine, as shown in Figure 1, that uses a cutting arm that resembles a chainsaw as a cutter. As the cutting arm moves along the pinion track, a precut is formed in the shape of the tunnel to be excavated. Recently, this machine has been developed as an excavation equipment called a tunnel widening machine (TWM), which is also used to enlarge existing tunnels [4,5].  
Similar to tunnel boring machines (TBMs) and roadheaders, pre-cutting machines can also be classified as tunnel excavation machines that use cutting tools. Cutting tools can be broadly classified into roller and drag types. The disc cutter is a roller type, while the drag type typically comprises a conical and chisel pick [6]. A linear cutting test is widely used to design the cutter head of such excavation equipment for evaluating the performance of such tools [7]. Therefore, various studies have been conducted on cutting tools using linear cutting tests to improve the excavation efficiency of excavation machines.  
Various studies have been conducted to evaluate and analyze the effects of the parameters related to the shape of the cutting tool using a linear cutting test. [PERSON] and [PERSON] [8] performed linear cutting tests on V-type disk cutters with varying diameters, edge angles, and penetration depths, and proposed a cutter force prediction model. Subsequently, various studies have confirmed that a constant cross-section (CCS)-type disc cutter is more effective with high rock-cutting efficiency and low tool wear [9; 10; 11]. [PERSON] [12] proposed a model for predicting cutting force by analyzing the theoretical and experimental values for chisel picks with rake angles of 10\({}^{\circ}\), 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\). [PERSON] et al. [13] analyzed the effects of the clearance angle, rake angle, and penetration depth on the cutting force and specific energy using five types of conical picks and proposed an empirical model accordingly. [PERSON] et al. [14] analyzed the rock chip formation mechanism by varying the rake angle of the drag type cutting tool. The literature reveals that although several studies have been conducted on the shape of disc cutters and pick cutters, there have been no comprehensive studies on the cutting tools of pre-cutting machines. In order to increase the excavation efficiency of the pre-cutting machine, research on the shape of the cutting tool is essential.  
The contribution to this study is that it can be a reference in determining the cutting tool shape of a pre-cutting machine that could improve cutting efficiency. The linear cutting test was performed to analyze the effect of the shape of the cutting tool of the pre-cutting machine. Because pre-cutting machines do not have uniformly shaped cutting tools, such as TBMs or roadheaders, the basic shape of the cutting tool is assumed. The clearance and rake angles were considered as variables in this study, and tests were performed by changing the compressive strength of the rock specimen and depth of the cut. The shape effect was analyzed by focusing on the cutting force generated by cutting rocks of different strengths. Moreover, cutting volume and specific energy were also analyzed.