how long

[wheresmejumper]

with mig are you aiming to do the full length of the run in one go?
the longest ive done consistently is about 12 - 15 inches,mainly due to the size of the scrap pieces ive scrounged for learning.
but i need to join two plates for my bench top along its length which is 4 feet of 4mm ms.the thought alone is a bit daunting.

 

[cumbriasteve]

Personally I would do it in several runs working at opposite ends between runs to hopefully reduce distortion.

 

[wheresmejumper]

what length should i be going for each time?

 

[ChrisJohnsSculpture]

It really depends on the job. There are some applications where a single run is desirable, especially as MIG can be prone to cold starts (ie poor penetration at the start of the weld), particularly where you are after a completely sealed joint eg for tanks or pipe work.

On the other hand in structural stuff where distortion is an issue its often better to do evenly spaces 'stitches' and fill in afterwards...your bench top probably falls into this category.

Also for long welds it can be helpful to do a lot of tacks and grid them flush before you do the full seam.

If you want the best possible weld quality it can help to grind back into the previous weld bead to get some overlap onto completely clean metal.

For butt welding a bench top together I would suggest tacking maybe every 100mm, then grind the tacks flush and do runs between each tack alternating sides and ends.

Probably the biggest issue will be distortion as the welds cool so try to work as quickly and evenly as possible and keep checking that the whole thing stays reasonably flat. Bevelling the plate should reduce the amount of current you need to get decent penetration and help to mitigate any potential distortion.