I have a little project coming up, and will need to do a little bit of 'spot welding'. In other words, drilling a hole, putting the other bit of steel behind, then MIG welding through the hole. This isn't structural - it's just to hold the two together permanently, leaving the outside smooth (after grinding).

The only problem is - I haven't done this before and have no ideal what size hole I need to drill!

What size is recommended, or what's typical?

You can get shrouds for MIG welders to help imitate spot welds



https://www.craigmo reonline.co.uk/draper-38453-2-x-mig-welding-torch-spot-weld-shrouds-for-draper-welders-except-stk-no-43952

So basically a plug weld.
I have done this a few times and normal used 8mm but you could go lower maybe 6 just depends on your skills with a welder but it never ends up with too much to grind.

Drilling through sheet metal can often result in a distorted hole if you use too large a Ø drill bit (and probably too much weld as well).

Thinking of when my Dad's been restoring some cars he's hired a spot welder before for doing floor sections.

Worth speaking to a local welding supplier to see what's available?

Having drilled out a lot of spot welds recently, I drilled them out with a 5mm bit. So I suspect I'd start with a 4mm drill and plug that with a lot of power. Any larger and you have to effectively seam weld the hole and then wiggle the thang in the middle to fill it.



[Edited on 28/6/21 by coyoteboy]

Somewhere I have a hole punch for plug welds. That is 6mm.
Have a look on te mig welding forum, it is a very good technique, plenty strong enough when done correctly. Power up to almost burn through, pull the trigger and a little wiggle works for me. I have done many hundreds over the years on old Triumphs.

Keep them small so the hole edge is local and the heat stays low. I thick I also spec around 6mm when I design them in.

Like others have said, 6mm or 1/4" is okay for thin bodywork. Make sure surfaces as as clean as possible. If you have space, use a small clamp at left and right of your weld so it doesn't lift.
Better to practice on scrap pieces before going onto the workpiece if you haven't done it before.

I wouldn't have room to fit a spot welder in - it's only 76mm diam tube! I plan to put a flat steel baffle across the tube, spot welded to the partially folded edges of the baffle. The tube and the baffle are both 1.5mm thickness. I only plan to do 6 or 8 plug welds, so buying a special shroud is a bit excessive. It doesn't have to be pretty as it'll get ground to match the tube surface - I only want it to be oil-tight at atmospheric pressure.

6mm hole diam sounds fair...

I will tell everyone what I'm up to sometime - but only if it works out OK. If it doesn't you won't hear another word!

Plug welds are an approved manufacturer repair method for modern cars where spot welds used to be. Nothing at all wrong with using them.

I've got a combined joggler/hole punch. 6 mm holes for plug welds