Effect of Cutting Tool Properties and Depth of Cut in Rock Cutting: An Experimental Study

Abstract

The current paper is designed to investigate the effect of worn (blunt) polycrystalline diamond compact cutter properties on both the contact stress ($\sigma$) and friction coefficient (μ) mobilized at the wear flat–rock interface at different inclination angles of the wear flat surface and at a wide range of depths of cut. An extensive and comprehensive set of cutting experiments is carried out on two sedimentary rocks (one limestone and one sandstone) using a state-of-the-art rock cutting equipment (Wombat) and various blunt cutters. Experiments with blunt cutters are characterized by different wear flat inclination angles ($\beta$), different wear flat surface roughness ($R_{a}$), different wear flat material, and different cutting tool velocities (${\varvec{v}}$) were conducted. The experimental results show that both the contact stress and friction coefficient are predominantly affected by the wear flat roughness at all inclination angles of the wear flat; however, the cutting tool velocity has a negligible influence on both the contact stress and friction coefficient. Further investigations suggest that the contact stress is greatly affected by the depth of cut within the plastic regime of frictional contact while the contact stress is insensitive to the depth of cut within the elastic regime.

Appendices

Appendix 1: E-S diagram

Combining Eqs. 1, 2, and 5, the tangential component of the total force reads:

$$\begin{aligned} F_{s}= \varepsilon A_{c}+\mu F_{fn} \end{aligned}$$

(17)

Replacing $F_{fn}$ with $F_{n}-F_{cn}$ and applying Eq. 2, Eq. 17 becomes:

$$\begin{aligned} F_{s}= (1-\zeta \mu )\varepsilon A_{c}+\mu F_{n} \end{aligned}$$

(18)

Originally introduced by Detournay and Defourny (1992), another geometrical representation of Eq. 17 can be deduced by dividing both sides of Eq. 18 by the cutting area ($A_{c}= \omega \times d$) which yields:

$$\begin{aligned} E=E_{0}+ \mu S \end{aligned}$$

(19)

where E and S are the specific energy and drilling strength, respectively, and are given by:

$$E= \frac{F_{s}}{\omega d}$$

(20)

$$S= \frac{F_{n}}{\omega d}$$

(21)

and $E_{0}$ is a constant defined as:

$$\begin{aligned} E_{0}=(1-\mu \zeta )\varepsilon \end{aligned}$$

(22)

Equation 19 actually offers a simple graphical snapshot of a set of experiments, where results of tests performed with a sharp cutter map on the same cutting point defining the intrinsic specific energy, and results of test carried out with a blunt cutter lay on the friction line. A synthetic E-S diagram is shown in Fig. 25. The “cutting point” is the point at the intersection of the friction line with the cutting line, which satisfies the linear relation of Eq. 19. The cutting point corresponds to the response of the sharp cutter, which is given by:

$$E= \varepsilon$$

(23)

$$S= \zeta \varepsilon$$

(24)

The specific energy (E) for a blunt cutter also reads:

$$\begin{aligned} E= \varepsilon \left( 1+\frac{\mu \ \ell }{d}\right) \end{aligned}$$

(25)

As can be seen in Fig. 25, the “cutting line” is the line passing through the origin and its gradient to the S-axis is $\zeta ^{-1}$. Likewise, Eq. 19 is represented by a line with a slope $\mu$ whose intercept with E-axis would be $E_{0}$. This line is dubbed the “friction line.” All admissible states of the response of a blunt cutter lie on the friction line above and to the right of the cutting point.

Fig. 25

Schematic of E-S diagram

Appendix 2: Photographs of the Cutter Holder in Order to Change the Wear Flat Roughness

See Figs. 26 and 27.

Fig. 26

The cutter holder to change the wear flat roughness

Fig. 27

Cutter seat of the holder