Welding 5-8mm steel box-section tubes

[Hulksmash]

To start I would go with a stick welder (arc welding)

For the price point - say £150 to £200 you can weld all types and thicknesses of steel. a mig to weld over 5mm you are looking at some serious investment + the bottled gas ££££

You can often buy a TIG torch for many of the good Arc welders and do TIG at a later date - but you will need gas ( Argon or Argon Helium mix - this can be used on both steel and aluminium)

Many cheap MIG machines claim they can weld 5mm+ thick but they are really poor welds as the metal does not pool enough to create good adhesion.

You don't need gas to use stick welding

The welding electropdes (sticks) are the cheapest consumable. Normally depending on the thickness of the steel usually determines the thickness of the rod - however people can have preferences - i.e. I like using 2.5 or 3.2mm for most welds unless less than 1.5mm thickness

Stick welding can weld through rust paint ect (however - all steel should be cleaned prior to welding otherwise you will get foriegn objects and porosity that weaken the weld) - but it is the most forgiving

Go watch the videos @ weld.com and follow some of the instructions

There are things like DC+ or DC- (which way the torch and the return is plugged in)
How many amps to use for different thicknesses
Types of weld angles and joints
Type of electrodes (sticks) i.e in UK ppl mainly use 6013 however in the states they use 7018 - they differ as to how much penetration and what position you can weld.

hope this gives you something to look at :o)

Mike