How to control cut perpendicularity on round tube ART-198

What exactly should be called perpendicularity?

For a round-tube end, inspection often determines how perpendicular the actual cut plane is to the part's longitudinal axis. If the end is tilted, the shortest and longest generatrices have different lengths. During joining, this creates a wedge-shaped gap or changes the assembly angle.

For a contour in the side wall, the logic differs. Every surface point has a local normal. The ideal edge orientation may be specified relative to that normal, relative to the tube axis or deliberately at an angle for welding. A round surface continuously changes direction, so one global plane does not describe the entire contour.

The drawing must distinguish end squareness, cut-face angularity, hole size at the external surface, size in a particular datum plane and a specified bevel. If these requirements are not separated, the supplier and acceptance inspector may measure different things and both consider their result correct.

The datum matters more than an attractive number

A real tube's longitudinal axis does not always coincide with the chuck axis or a line constructed between its two ends. Ovality, bow, weld seam, a local dent and clamping alter the actual geometry. If the end is measured from an arbitrary external area, the result includes more than cutting error.

For a short part, the axis can be established using a calibrated mandrel, several cross-sections or an agreed measurement procedure. For a long part, describe the section used to establish the axis and the support method. The part must not be forcibly straightened so that the measurement fixture conceals a deviation that will appear during assembly.

Record the datum, reference-section length, support positions, temperature, condition after unloading and time before measurement in the acceptance plan. The same end may give different readings on V-blocks, in a chuck and in an assembly fixture. What is needed is a reproducible condition, not an abstract truth.

Why the beam axis is not the surface normal

On flat sheet, a vertical beam and the sheet normal may be practically parallel. On a tube, the cutting point moves over a curved surface, and the system's kinematics must maintain the required toolpath orientation. The available range of head positions in a 2D or 2.5D configuration differs from full 3D-head tilt.

ART-173 covers the choice of 2D, 2.5D and 3D architecture. The practical consequence here is that one system's angled-cutting capability cannot be transferred to another. Even with 3D axes, the real envelope is limited by collisions, standoff, focus, nozzle geometry, access to the internal contour and process stability.