CNC MACHINING & HELICAL CAM PREPRESS
CNC Helical Thread Milling CAM Prepress Guide
Helical thread milling allows 3-axis CNC machines and wood routers to cut large metric, UNC, and pipe threads without needing rigid tapping synchronized spindles. This guide breaks down programmed feed rate adjustments, tangent roll-in loops, and multi-pass cutting schedules.
CNC machine controllers execute linear feed rates along the tool centerpoint path (X, Y). When milling an internal thread, the toolpath circle radius is much smaller than the outer thread perimeter. If the programmer enters the linear surface feed rate directly into G-code without compensation, the actual peripheral cutting edge will move up to 3x too fast, causing tool breakage and chatter.
Internal Thread: F_center = F_tool * (D_major - D_cutter) / D_major
External Thread: F_center = F_tool * (D_major + D_cutter) / D_major
Entering the cut wall perpendicularly will cause tool deflection marks and thread pitch errors. Helical CAM trajectories utilize a 90° tangent arc entry with simultaneous Z-axis descent to gently engage the full cutting edge.