A 4-1/2in or 5in angle grinder will need to be used with metal cutting discs, and it won't be the fastest way of doing it, but if you go for a larger angle grinder you'll be working freehand as the cheapest stands for them I've seen are £40+. Also a larger angle grinder is not the best device to use for cleaning-up welds - too big and unwieldy. I've used a small angle grinder and a stand for cutting steel kitchen cabinet support rail for a while now and I've just bought the LIDL stand to use in the van - woeks fine with the rail I cut (2.5mm steel) albeit a little slowly.
I assume the "mitre saw" you are talking about is a hand one, not a powered one. Either way the blades are generally only available for non-ferrous metals. Cutting steel requires a much slower gear-driven saw with a clamp for the material, preferably with coolant spray, like this:
An alternative might be just to mark the cuts with a black marking pen and just cut freehand with the angle grinder, or to buy some metal cutting (wavy set, bimetallic) blades to go in a jigsaw - if you have one.
Another alternative is to go to your local hire shop. HSS do the TCT metal mitre saws like scrits picture or ones with cut off discs for around £35 a day.
A 4 1/2" grinder in a holder will not have a large enough blade to cut through 50mm box section. You will also need to use the grinder to chamfer all the ends before welding as well as cleaning up afterards.
If you do go the angle grinder route then get the very thin cut off discs, they are quicker & cleaner than the ones that are about 3mm thick. You should also use grinding discs for cleaning up not cut off discs as there is a risk of then exploding because they are not designed for cutting on the flat side, only the edge.
I got the bottom and top plywood boards today from wickes, didnt get to b&q, I also got a £15 angle grinder from argos, Its only gonna be used for the finishing the edges and welds, I am thinking that getting it cut by the suppliers would be the best, It cant be that much to get them cut, I will try to bargain the supplier down if its a lot, othereise I will take my business elsewhere if they are miles to expensive, simple as that !
I would be happy to pay £20 or so to get it cut from them, I dont know if they would do it for free or not, because I am after all buying about £65 worth or steel.
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