CNC Knife CAM & Signmaking Prepress
CNC Drag Knife & Diamond Scribe Prepress Guide
Drag knives (such as Donek D-series) and spring-loaded diamond scribes allow standard 3-axis CNC routers to cut vinyl, cardstock, leather, and gaskets without a dedicated tangential knife C-axis motor. This guide explains caster offset lag kinematics, corner loop generation, and vector optimization.
1. The Physics of Caster Blade Lag
A drag knife blade tip does not sit at the spindle center; it trails behind by a precise distance called the Caster Blade Offset ($O_{ ext{blade}}$). When the CNC changes direction, the knife tip will round off the corner unless the CAM software inserts an external swivel loop arc to pivot the blade on its tip.
The Corner Swivel Loop Trajectory
- The tool cuts past the corner by distance $O_{ ext{blade}}$.
- The CNC executes an external circular arc around the corner point with radius $R_{ ext{pivot}} \ge 1.2 imes O_{ ext{blade}}$.
- The blade pivots smoothly onto the new tangent vector without gouging or tearing the stock.
2. Diamond Drag Scribing on Metals & Glass
Spring-loaded diamond drag bits (such as 90° or 120° industrial diamond points) engrave anodized aluminum, brass, stainless steel, and glass without running the spindle motor:
- Zero Spindle RPM: Spindle must be turned OFF to prevent destroying the diamond crystal.
- Spring Preload Calibration: Set Z-depth plunge $1.5\, ext{mm} - 3.0\, ext{mm}$ below surface touch-off to compress the internal spring ($400 - 1500\, ext{grams}$ downforce).
- Centerline Vector Art: Diamond drag engraving requires single-line centerline vectors rather than outline contours.
3. Vector Prepress Requirements for Drag Toolpaths
- Decimate Micro-Segments: Eliminate short chord segments ($< 0.1\, ext{mm}$) that cause rapid direction flipping and blade flutter.
- Close All Vector Loops: Open vectors cause the drag knife to fail to release the final corner.
- Join Continuous Polylines: Ensure contiguous contours are joined into single uninterrupted paths to minimize Z-retracts.
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