Box section size question

[Ubique]

It may be a simple question - and I'm probably missing something....
I need to make an adjustable frame, the 'core' will be 25mm x 25mm x 3mm box, I need something to fit over it, as a close fit - the nearest I can see is 30mm x 30mm x 2.5mm, which would be an interference fit or 30mm x 30mm x 2mm which would be a bit sloppy. The larger section would be bearing the load, approx 400kg across three points, 100mm long. 2mm would fit, 2.5mm is acceptable but tight, there is no 3mm option that I can see. It needs to be a good fit to prevent any wobble which would cause alignment issues.
Thanks.

 

[Agroshield]

Square tubing will have a weld seam inside that you have to deal with.

The proper stuff is called 'telescoping tubing'.

For 100mm long, you can tune the inside with a power file.

Tack weld a shim inside to adjust the fit. The shim could also be non-ferrous or plastic but alternative fixing methods are then required.

 

[not done it yet]

If only 100mm long, filing/grinding 50mm from each end to clean the inside should not be too arduous. The inner can easily be squared/thinned on the milling machine - or a belt sander if pushed - or even filed by hand.

 

[500e]

Not tried either mind

 

[Ubique]

...

The proper stuff is called 'telescoping tubing'.

...

I've never heard of that - thanks!

Any shims would need to be ferrous as the loading would probably deform plastic, plenty of ideas to think about now.

 

[8ob]

You can get a box inside a box combo, we use it for pull out drawbars.