Drillability of Percussion Bits in Three Types of Rock

Abstract

We reported the rock drillability of rotary-percussion drilling using a roller cone bit in a previous paper. In the present paper, investigation of the rock drillability was extended to percussion drilling using two types of percussion bits. Rocks used for this investigation were granite and two types of andesite having uniaxial compressive strengths of 131 to 214 MPa. Drilling tests were conducted at inlet oil pressures of the percussion drill of 9.5 to 13.5 MPa and at rotary speeds of 25 to 175 rpm. The main results obtained from the tests are as follows:
1. The maximum penetration rate and the minimum specific energy were obtained at the inlet oil pressure of 13.5 MPa, independent of rock and bit types.
2. With respect to the penetration rate and specific energy, the appropriate rotary speed was judged to be about 75 rpm in the high-speed bit and about 100 to 150 rpm in the button bit. In addition, the rotary energy was larger than the percussion energy at the appropriate rotary speed.
3. When rocks were drilled effectively, the effect of the percussion energy and that of the rotary energy on the penetration rate were almost the same.
4. The minimum value of specific energy in percussion drilling using the high-speed bit was correlated very roughly with the uniaxial compressive strength of the rock being drilled.
5. The minimum specific energy of percussion drilling was larger than that of rotary drilling, independent of rock and bit types.