Investigation of conical bit rotation in full scale cutting tests

Open Access
Author:
Kim, Eunhye
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 03, 2010
Committee Members:
  • Jamal Rostami, Dissertation Advisor
  • Jamal Rostami, Committee Chair
  • R Larry Grayson, Committee Member
  • Derek Elsworth, Committee Member
  • Francesco Costanzo, Committee Member
  • Antonio Nieto, Committee Member
  • Yaw D Yeboah, Committee Member
Keywords:
  • Finite Element Analysis
  • Full scale cutting test
  • Rock fragmentation
  • Conical bit rotation
Abstract:
Conical bits are very common in the excavation of soft to medium rock in many mining and construction applications and are used on machines such as roadheaders, continuous miners, drum shearers, and road milling machines. Rotation of the conical bits in its block can extend bit life through a process of even wear on the tip that allows the bit to maintain its tip shape and work more efficiently for an extended period of time. Frequently, this fact does not hold true in actual conical bit application. Grinding on one side of the bit, breaking the bit tip, and severe irregular deformations of conical bits are found when inspecting bits on the cutterhead of mechanical excavators. Strikingly, little is known about causations of bit rotation. Reliable measurement of bit rotation has not been made prior to this study. Therefore the effectiveness of bit rotation and its impact on bit life has been in doubt. To investigate the conical bit rotation phenomenon and parameters affecting this process, a new device was designed and fabricated to measure rotation of a bit in bit holder. For a given conical pick, the main controlling parameters for bit rotation are considered to be skew angle, cut spacing, depth of penetration, and travel speed. To measure the impact of these parameters on bit rotation, full-scale cutting tests were performed at the Kennametal rock cutting lab in Latrobe, PA. This includes linear and rotary cutting tests with single and multiple bits to examine the rotation mechanism of conical bits. Linear cutting tests involve full scale cutting of rock samples along preset lines to measure normal, drag, and side forces and bit rotation while changing the cutting geometry (including skew angle, spacing, and depth of cut). A skew plate was used to change the skew angle for testing. Two different bits, AM-470 from the mining tools and RZ-24 pick from the construction tools were selected for testing. Results of the linear cutting test showed no systematic bit rotation. Some sporadic rotation on the RZ-24 conical tool was primarily due to the contact of the bit body with the ridges formed at higher spacing, something that should be prevented in operational conditions. As such, bit rotation did not show any trends with the primary cutting geometry parameters. A series of analytical and numerical solutions were performed to evaluate the configuration of forces at the bit tip that can cause bit rotation under various loading conditions. This analysis focused on calculation of the required lateral forces for rotation of the bit under certain axial loading of the bit which causes friction between bit shoulder and bit block. This friction prevents bit rotation. The results of the analysis supported the linear cutting test results and observations. Subsequently, rotary cutting tests on a drum laced with a single bit were performed. And bit rotation was measured using the instrumented bits at different skew angles, cutting geometry, and cutting speeds. The results of rotary cutting test show that limited bit rotation occurs as the bit enters and exits the rock (cutting surface). Among the cutting parameters, the skew angle has a significant effect on the bit rotation. However, higher bit rotations were observed at lower skew angles from zero to six degree. The use of higher skew angles (i.e.-12̊ or 12̊) reduced bit rotation and increased cutting forces and specific energy of cutting. Higher bit rotation also coincided with narrow spacing. Intriguingly, the direction of the bit rotation follows the face that contacts the rock surface first. Thus, positive rotation (CW) was observed with positive skew angles. Similarly, negative rotation (CCW) was measured at negative skew angle. Positive skew angle in this case was considered cutting towards the lower cutting surface (or the adjacent line that is previously cut) Rotary cutting tests using the drum laced with multiple bits were also performed. This drum was fitted with four (4) bits at two (2) bits per line at equal circumferential spacing of 90 degrees. A higher bit rotations was observed at a lower spacing and skew angle seems to impact bit rotation. The limited data available to date are inconclusive on the final impact of the skew angle on bit rotation and optimum skew angle for maximum rotation of the bit One of the main conclusions of the rotary cutting tests is that the bit rotation occurred at small increment at each revolution during the point of entry and exit of the bit into the cut. At this stage, several mechanisms have been proposed to improve bit rotation. The effectiveness of these solutions requires confirmation by additional full scale cutting tests.