Need expert guidance on rookie DIY build.

[irving2008]

Interesting requirements. Any chance of explaining the purpose of these parts, especially given the low accuracy requirement.

Some clarifications please:
1. Is the slot axial or radial to the CHS, or continuous in both e.g a spiral?
2. You say the slot is 12mm dia. Do you really mean width? The sides of the slot are perpendicular and the slot is centred axially on the workpiece's rotational axis.
3. Is this a one or two man operation? 3m of 114 x 5mm CHS weighs 23kg and is bulky.
4. How close to the end of the material does the slot have to be machined?

Given the length of the parts I would consider buying an off-the-shelf mill with required rigidity and throat capacity then building a custom material transport. I'd envisage a series of v-rollered supports for the bulk of the workpiece, some form of hollow hydraulic chuck to clamp/rotate the workpiece, fixed to the table of the mill for fine X movement and a feeder mechanism to give the bulk/rapid X movement.