CNC Laser & Plasma Lead-in, Pierce Delay & Corner Loops Prepress Guide

1. The Problem: Pierce Divots and Edge Notches

When a cutting torch or laser beam fires through solid metal or composite plate, the stationary initial dwell creates a molten blowout crater that is significantly wider than the steady-state cutting kerf:

• Plasma Jet Piercing: High-amperage plasma arc generates violent upward molten metal splatter before piercing through the plate floor. The pierce crater diameter is typically $1.5\times$ to $2.0\times$ the running kerf width.
• Laser Piercing: High-pressure oxygen or nitrogen assist gas blasts molten slag outward. Piercing directly on a part perimeter leaves an irreversible scallop or notch defect.
• Waterjet Piercing: Supersonic garnet piercing exerts immense shock pressure that can delaminate composite prepregs or crack brittle stone if pierced on the net part boundary.

2. Lead-In Geometry: Linear vs. Quarter-Arc

Choosing the correct lead-in trajectory depends on machine acceleration limits and sheet thickness:

• Quarter-Arc Lead-in (Recommended): Enters the contour along a 90° tangent circular arc. Because the motion vector transitions smoothly into the part contour with continuous derivative ($C^1$ continuity), the CNC controller maintains velocity without stopping or shuddering, yielding a flawless edge.
• Linear 45° Lead-in: Suitable for thin sheet metals ($< 2.0\text{ mm}$) and sharp corners. However, abrupt direction changes can cause servo overshoot marks.
• Straight + Arc Compound: Used on heavy plates ($> 12\text{ mm}$) where a long straight run is required to establish full beam velocity before entering the tangent arc.

3. Lead-In Length & Pierce Offset Math

Calculate minimum lead-in length $L_{\text{lead}}$ to ensure the blowout crater does not touch the finished edge:

4. Lead-Out Overburn Overlap

When the cutting head completes the 360° closed contour, the thermal lag and jet deflection cause the trailing bottom edge to remain attached by a tiny micro-tab if the cut stops exactly at the start point. Applying a $0.5 - 2.0\text{ mm}$ overburn overlap past the initial touchdown point ensures clean part drop-out without burrs.

5. Outside Corner Loops to Prevent Corner Melting

When cutting sharp 90° corners on CNC plasma or laser systems, the cutting torch must decelerate to near zero velocity to make the abrupt turn. This dwell time dumps excessive heat into the corner, rounding or burning away the sharp edge. Adding an external loop ($R = 1.5 - 3.0\text{ mm}$) allows the torch to swing out into scrap space, maintain full cutting speed, and re-enter the adjacent edge cleanly.