Abstract

This paper will discuss the process of rock cutting with conical bits with emphasis on bit rotation. This includes discussion of importance of bit rotation and its impact on bit life. The paper will review the rock cutting process and related parameters, objectives of the current study, and the status of full scale cutting tests being performed in Kennametal Inc, rock cutting laboratory in Latrobe PA. The testing will include different bit types cutting parameters, and skew angles to identify most influential factors on bit-rock interaction pertinent to bit rotation. A series of full scale linear cutting test are underway and the preliminary results will be discussed in this paper. These tests will be followed by rotary cutting test where the instrumented bits are mounted on a drum for full scale testing and monitoring of bit rotation in rotary cutting tests.

INTRODUCTION

Various machines and tools are used in rock excavation, but the use of conical tools has become increasingly common in rock excavation for variety of applications. Conical cutters can efficiently cut rocks up to 100 MPa (15ksi), and have become the tool of choice, especially on partial face machines. The main advantage of these bits are the “self sharpening” or more realistically stated “even wear” effect, which allows the bit to maintain its profile and thus, its efficiency for a while longer than the radial or drag bits.

There are many studies conducted on rock cutting mechanism using drag type cutters and conical bits. Evans [1] and Nashimatsu [2, 3] and Goktan [4] focused their studies on theoretical cutting mechanism on wedge or conical bit. From their research works, generally two main failure theories have been considered in the excavation process, one is tensile failure and the other is shear failure.

While there are many arguments and some evidence backing each theory, it seems like the actual breakage phenomenon is a combination of both. Often, tensile failure is attributed to the brittle and shear failure is applied to the ductile behavior. Goktan (1990) suggested that in high strength material shear failure mode is more suitable than tensile failure. Along with studies of failure mechanism, impact of different parameters such as cut geometry (depth of cut, spacing), cutting speed, attack angle, bit tip geometry, and rake angle, on cutting forces have also been studied [5, 6, 7, 8, 9].

A part of study of the rock cutting phenomenon is estimation of cutting forces as well as Specific Energy (SE). SE is the energy used in excavation of unit volume of rock (or unit weight of excavated rock). Specific energy is a function of rock type, bit type and geometry, and cutting geometry and efficient cutting refers to the scenario where minimum amount of energy is used to excavate rock. To improve cutting efficiency, one can control cutting geometry (use optimum spacing to penetration ratio) and use appropriate tools, which offer better performance over the service life of the tool.

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