Electro-Discharge Grinding (EDG) PCD Tooling Prepress Guide

1. Principles of Electro-Discharge Grinding for PCD & CBN

Polycrystalline diamond (PCD) inserts represent the industry standard for high-speed machining of highly abrasive non-ferrous alloys, hypereutectic aluminum-silicon alloys (Al-Si 12%+), carbon fiber reinforced polymers (CFRP), and technical green ceramics. Because synthetic diamond crystals are extreme electrical insulators, conventional electrical discharge machining relies on melting the interstitial cobalt catalyst network (6%–12% volume fraction).

In Electro-Discharge Grinding (EDG) and Wire Electro-Discharge Grinding (WEDG), a rotating cylindrical electrode wheel (copper-tungsten CuW or high-density fine-grain graphite) operates without mechanical contact against the PCD blank. High-frequency microsecond electrical pulses erode the cobalt binder, causing diamond grains to dislodge under the hydrodynamic shearing action of the dielectric fluid.

2. Mathematical Formulations & Spark Gap Physics

Accurate CNC toolpath generation requires calculating the exact electrical overcut between the electrode wheel periphery and the finished cutting edge profile:

• Discharge Pulse Energy ($W_e$):

  W_e = \int_0^{T_{on}} u(t) \cdot i(t) \, dt \approx \eta \cdot V_{gap} \cdot I_p \cdot T_{on}

  where $V_{gap}$ is the average discharge gap voltage ($20 - 45\text{ V}$), $I_p$ is peak current ($2 - 25\text{ A}$), and $\eta \approx 0.70 - 0.85$ represents discharge circuit efficiency.
• Dielectric Overcut Spark Gap ($S_g$):

  S_g = K_{edg} \cdot V_{open}^{0.60} \cdot I_p^{0.30} \cdot T_{on}^{0.20}

  where $K_{edg}$ is an empirical constant dependent on dielectric oil hydrocarbon resistivity and flushing velocity ($0.10 - 0.15$).
• Cobalt Binder Leaching Depth ($h_{leach}$): Thermal diffusion of intense spark heat into the substrate causes preferential sub-surface cobalt depletion:

  h_{leach} \approx 2 \sqrt{\alpha_{Co} \cdot T_{on}} + 0.35 \cdot d_{grain}

  where $\alpha_{Co}$ is the thermal diffusivity of cobalt and $d_{grain}$ is PCD diamond crystal grit diameter ($2 - 25\text{ µm}$).

3. Vector Prepress & CAM Tangent Arc Toolpath Rules

Unlike conventional CNC abrasive grinding where wheel geometry is maintained mechanically, EDG involves continuous non-uniform wheel erosion. To ensure insert contour accuracy within $\pm 1.5\text{ µm}$:

1. Offset Layering: The programmed G-code toolpath centerline must offset the insert profile by: Offset_R = R_{wheel}(t) + S_g(V, I_p, T_{on}) + \Delta_{finishing}
2. Wheel Dressing Compensation: Modern 5-axis EDG machines incorporate continuous in-process electrical truing. Vector DXF inputs should include parametric wheel radius wear variables at every profile corner node.
3. Corner Radii & G02/G03 Arc Tangency: Sharp cutting insert nose radii ($R_\epsilon = 0.2 - 1.2\text{ mm}$) must be filleted with continuous curvature ($G_2$-continuous Bézier or bi-arc approximations) to avoid spark concentration burning at apex vertices.
4. Roughing vs Finishing Regimes: Prepress toolpath layers should be split into 3 distinct passes: Roughing ($I_p = 15\text{ A}, T_{on} = 25\text{ µs}$), Semi-finishing ($I_p = 5\text{ A}, T_{on} = 8\text{ µs}$), and Spark-out Polishing ($I_p = 1.0\text{ A}, T_{on} = 1.5\text{ µs}$) yielding edge sharpness $R_n < 3\text{ µm}$.