I managed to get 0.4mm off the sag, better than nothing, will use the technique like those who do not machine the profile or base. Simply will use filler gauge - industrial sheet in the middle to compensate - do not want to have tension on the bearing block while is sliding from middle towards end.
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After welding so many frames came up with solution - there is not substitute for slow welding with min heat. Mean the welding bed have to be short and fully cooled before we weld again near by.
I put all theory to the bin with current setting to metal thickness and speed wire on MIG.
I use Clarke MIG TEC230 Turbo which is capable to weld up to 7 mm on max setting (switch pos.6)
My profile is 2-2.5mm with plates 5mm and I use pos.3 - current/power and wire speed min as possible to stop splatter but fast enough to get good constant sound (not like a shooting gun)