Hi Max! A 3-axis CNC machine is capable of moving the cutting tool along the X, Y, and Z axes, which allows it to machine many curved surfaces. But it has limitations in more complex curvatures that require cutter presentation at various angles or need continuous rotation to keep surfaces piecewise continuous and smooth. The big difference with a 4-axis machine is it can rotate the workpiece around an additional axis, typically the A-axis, which expands the range of cutter presentations that are available.

Examples that necessitate a 4th axis are helical or spiral cuts. These require a continuous, helical motion around the workpiece. A 3-axis machine would need multiple setups or specialized tooling and there’d be a mismatch between operations, while a 4-axis can rotate the part continuously while cutting, achieving better accuracy and surface continuity.

Parts with complex undercuts or angled features, curved surfaces that extend beneath other features or at steep angles. A 4-axis machine reduces the need for repositioning and allows access to areas a 3-axis machine can’t reach. Some repositioning might still be needed though. Contour milling around cylindrical or irregular parts needs constant curvature around multiple faces – typical of cams or turbine blades. The 4-axis machine can rotate the part to maintain tool path continuity on these spline-like curved surfaces, delivering finishes and higher precision.