How to cut a circular notch in a rectangular tube?

In mechanical processing and pipe installation, it is frequently necessary to cut circular notches into rectangular tubing to facilitate the installation of fittings or the routing of cables and lines.
 
Since rectangular tubing features a square cross-section while the required notch is circular, the cutting process must prioritize both positional accuracy and edge neatness. First, the center point and cutting contour must be marked precisely; a compass, template, or projection tool can be used to trace the circular outline onto the tube surface, ensuring that the dimensions and positioning strictly adhere to design specifications.
 
Cutting techniques generally fall into two categories: manual and mechanical.
Manual methods—such as using an electric drill, hacksaw, or grinding wheel—are suitable for thin-walled rectangular tubing. While offering operational flexibility, these methods provide limited processing precision and typically require subsequent grinding and finishing to smooth the edges.
Mechanical methods—such as CNC milling, laser cutting, or plasma cutting—are better suited for processing thick-walled tubing or applications requiring high precision. These methods ensure a smooth cut surface and accurate dimensions while minimizing thermal deformation. Following the cut, it is typically necessary to deburr and finish the edges to prepare them for subsequent welding or the installation of fittings.
 
The applications for cutting circular notches into rectangular tubing are extensive. Common scenarios include pipe interfaces within structural steel frameworks, access holes for routing cables or lines within mechanical frames, and assembly holes required during the fabrication of furniture or equipment.