CNC Router Vacuum Hold-Down, Tab Retention & Onion-Skinning Guide

1. The Physics of Vacuum Clamping

Vacuum hold-down clamping force is purely a function of differential atmospheric pressure multiplied by part contact area and coefficient of friction:

A $100\text{ mm} \times 100\text{ mm}$ square part ($0.01\text{ m}^2$) under a 70 kPa vacuum bed with an MDF spoilboard ($mu \approx 0.35$) produces only $\sim 245\text{ N}$ ($25\text{ kgf}$) of holding force. A $\frac{1}{4}\"$ carbide compression spiral cutter can easily impart $150 - 300\text{ N}$ of lateral cutting thrust, causing immediate part ejection or ruinous edge chatter.

2. 3D Ramped Tabs vs. 2D Rectangular Tabs

Traditional 2D square tabs force the CNC router spindle to stop linear XY travel, retract Z, step forward, and plunge back down. This dwell causes friction burn marks, tool deflection witness lines, and controller stutter.

• 3D Triangular Ramped Tabs: The Z-axis smoothly rises at a $30^\circ - 45^\circ$ slope while maintaining full XY linear velocity, creating a smooth triangular bridge that is easily severed with a trim router or flush-cut chisel.

3. The Two-Stage Onion-Skinning Strategy

Onion-skinning is the gold-standard method for nesting small or intricate parts:

1. Pass 1 (Roughing): Machine all nested perimeters leaving a thin $0.5 - 0.8\text{ mm}$ membrane (the "onion skin") at the bottom of the sheet. Because the sheet remains 100% physically intact and sealed, full vacuum clamping pressure is preserved across the entire table.
2. Pass 2 (Final Cut): High-speed perimeter pass cuts through the $0.5\text{ mm}$ skin with minimal lateral cutting force ($le 5\text{ N}$), cleanly severing each part without shifting.