## 1 Introduction  
Polycrystalline diamond compact (PDC) drilling bits have gradually replaced conventional roller bits in oil and natural gas exploration worldwide since their invention [1; 2]. The market share of PDC bits had reached more than 65% by 2010 and has since continued to grow [3]. PDC bits have a simple mechanical structure, usually consisting of a polycrystalline diamond layer and a hard metal substrate layer distributed on the main bit body and can be used with dozens of PDC cutters. PDC bits offer a superior performance to roller bits due to their higher penetration rate and longer service life [4]. The performance of the whole PDC bit during the drilling process is a synthetic result of many single cutters and, therefore, the study of PDC bits in a laboratory usually begins by investigating the behavior of a single cutter [5; 6]. Specially designed apparatuses have been developed in various research institutes all around the world to carry out studies on single PDC cutters [7; 8]. In order to continually improve the performance of PDC cutters and bits, great efforts have been made to understand the cutter-rock interaction and related phenomena surrounding their functionalities. In particular, considerable attention has been paid to the following: (1) the friction and heat transfer during the cutter-rock interaction, (2) optimization of the design of PDC cutter geometric parameters, including their spatial distribution, back rake angle, side rake angle, chamfer, and groove geometry [9; 10], (3) the monitoring and control of the dynamic rock-cutting process, (4) cutting-force response in the cutting process and its influence factors, and (5) wear behaviors of cutters in the drilling process [11; 12; 13]. Among all the above-mentioned areas of research, cutting-force response is the most fundamental indicator and a prerequisite for understanding the behaviors of a single cutter. Experimental and simulation studies have been conducted to explore the forces acting on a PDC cutter in various cutting conditions. An independent experimental evaluation of friction and cutting force by [PERSON] et al. revealed that impact friction increases cutting force and, therefore, reduces cutting performance [14]. When [PERSON] et al. applied the discrete element modeling method to simulate the cutter-rock interaction, they found that the cutting force on a cutter is determined by a multi-directional flow mechanism and cannot be directly used to measure cutter-rock friction [15]. In a study exploring the formation of rock fragments, [PERSON] concluded that cutting force was significantly affected by the mechanical properties of the rock, as well as the factors of rake angle and velocity [16]. [PERSON] and [PERSON] performed a series of studies on polycrystalline diamond cutting of rock, and obtained experimental results regarding a number of influencing factors on cutting force, such as cutting depth, feed rate, rake angle, and speed [4; 8; 17; 18]. The industry's need to drill superdeep wells for oil and gas exploration continues to facilitate the advancement of air drilling technology [19]. A major difference between air drilling technology and conventional drilling technology is that no drilling fluid is applied in air drilling. Increased recognition of the benefits of air drilling demands a better understanding of PDC cutter-rock interaction, especially the fundamental force response of the cutter, however, most previous studies on force response in rock cutting have focused on force measurement with drilling fluid and therefore, cannot be applied to force prediction for air drilling. In response to this need, this study aimed to investigate force response in dry conditions. Moreover, the results obtained in both dry conditions and the drilling fluid environment was compared. In our previous study, a polypropylene glycol (PPG) additive in drilling fluid was found to effectively reduce friction force between steel contacts [20].