Force-penetration curves of a button bit generated during percussive rock drilling

Percussion rock drills have been widely used in mine development and civil engineering. During percussive rock drilling, a hammer in the rock drill collides with a shank rod several thousand times per minute, and elastic waves generated by the collisions propagate in the shank rod, rods, and a bit. Cemented carbide buttons, which are embedded in the bit, are penetrated into rock by the elastic waves. The penetration process of a bit into rock is represented by the relation between the force applied to the bit and the penetration depth, which is called a force-penetration curve. To estimate drilling rate and efficiency and to reproduce the drilling processes in computer simulation, it is indispensable to obtain appropriate force-penetration curves for the button bit. In this study, a new test apparatus was developed to conduct a single blow penetration (SBP) test and a consecutive blow penetration (CBP) test using an actual rock drill. The CBP test was conducted under conditions close to actual percussive rock drilling. Force-penetration curves for a button bit were obtained from the SBP and CBP tests with a block of granite using the correction method with a single blow test without the rock block. The results showed that the drilling efficiency and the linearity of the force-penetration curves in the loading phase are higher in the CBP test than in the SBP test, which indicates that the degree of contact between the bit and rock is higher in the CBP test under a thrust force and rotation than in the SBP test. In the CBP test, the force was represented by power functions of the penetration in the loading and unloading phases. On the basis of these results, the modeling method of force-penetration curves for the button bit was proposed for the simulation of percussive rock drilling.