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Author: Subject: Imitation spot welding
David Jenkins

posted on 28/6/21 at 02:03 PM Reply With Quote
Imitation spot welding

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?





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nick205

posted on 28/6/21 at 03:00 PM Reply With Quote
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

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watsonpj

posted on 28/6/21 at 03:03 PM Reply With Quote
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.

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nick205

posted on 28/6/21 at 03:27 PM Reply With Quote
Drilling through sheet metal can often result in a distorted hole if you use too large a 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?

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coyoteboy

posted on 28/6/21 at 03:34 PM Reply With Quote
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]





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cliftyhanger

posted on 28/6/21 at 03:37 PM Reply With Quote
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.

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bi22le

posted on 28/6/21 at 04:26 PM Reply With Quote
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.





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harmchar

posted on 28/6/21 at 04:29 PM Reply With Quote
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.

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David Jenkins

posted on 28/6/21 at 05:41 PM Reply With Quote
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...

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Mr Whippy

posted on 28/6/21 at 06:05 PM Reply With Quote
Plug welds are an approved manufacturer repair method for modern cars where spot welds used to be. Nothing at all wrong with using them.
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JeffHs

posted on 29/6/21 at 04:09 PM Reply With Quote
I've got a combined joggler/hole punch. 6 mm holes for plug welds
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