When machining dense indigenous hardwoods like Pakistani Sheesham (Dalbergia sissoo), standard textbook feeds and speeds are an invitation to catastrophic tool deflection. Sheesham exhibits an interlocking, irregular grain structure with high silica content and a Janka hardness exceeding 1,600 lbf.
### The Fundamental Chip Load Theorem
Chip load ($c_z$) is the actual thickness of material sheared away by each cutting edge during a single revolution:
$c_z = \frac{\text{Feed Rate (mm/min)}}{\text{Spindle RPM} \times \text{Number of Flutes}}$
If your chip load is too thin (
lt; 0.04\text{ mm}$), the carbide cutting edge rubs against the grain instead of shearing clean chips. This friction generates localized thermal spikes exceeding $650^\circ\text{C}$, causing premature flank wear and tool glazing. Conversely, if your chip load exceeds $0.15\text{ mm}$ on a 6mm 2-flute carbide bit, the lateral cutting forces snap the flute instantly.
### The Production Sweet Spot
On our Hefei Mingda 1325 running a 5.5 kW water-cooled spindle, our production baseline for raw Sheesham slab roughing is:
- **Spindle Speed:** 18,000 RPM
- **Feed Rate:** 3,600 mm/min
- **Tooling:** 6mm 2-flute solid micro-grain tungsten carbide up-cut endmill
- **Resulting Chip Load:** $\frac{3600}{18000 \times 2} = 0.10\text{ mm}$ per tooth
- **Cut Strategy:** Climb milling with a 3.0mm maximum stepdown (axial depth of cut) and 40% stepover.
Maintaining positive chip extraction prevents heat from soaking into the spindle collet, keeping tolerances within $\pm0.02\text{ mm}$ across 12-hour production cycles.