Proper hole size in sheet metal for spot welding?

[340challconvert]

Getting ready to drill the holes in the trunk floor to be spot welded to the frame rails.
I could spot eye it; what size drill bit for this task?
Thanks

 

[gzig5]

Somewhere between 1/4"-5/16" should work. I think the pneumatic flange/punch tool I have does 5/16".

 

[aussiemark]

I found if the hole is too small you don't get enough penetration into the frame rail and as it's cooling it will release. Get the 2 pieces as close together as possible with clamps, vice grips, sheet metal screws or even dent it if you have to. Cut the mig wire flush with the nozzle every time before you start, I like to start the weld on the frame rail in the centre of the hole then move out and join the new panel to it.

 

[Adam]

I agree with at least a 1/4 inch. Any smaller and the arc can hit the side of the hole, resulting in poor penetration. I also like to have about 3/8 of mig wire sticking out so I can see exactly where it is...

Tip: lay the trunk pan in place and mark the edges of the frame rails from the bottom, with a sharpie or something. Then pull the pan back out and drill the holes in the pan from the bottom. This way you will not miss the frame rails, and the holes will not have any burrs keeping the pan from laying flat. Don’t forget the gas tank supports. Good luck!

 

[quapman]

I just did a bunch and had good luck with 5/16" I use a #4 center drill.

 

[budascuda]

340, count the factory spot welds.
You need as many stud welds as factory and preferably in or very near the same place as factory welds. (for proper collapse and crumpling of the structure in case of an impact), God forbid!
5/16 hole is my call.

Also, I don't usually give this tip to anybody, but for you....
Since the duration of welding a stud weld is mere second(s), the workpiece may not be ready to accept a sudden and huge temperature change to produce a weld puddle that can melt all 3 pcs. involved in the weld, therefore producing a week and brittle fusing between the mig wire and its surrounding area. In the cases where a good fusing is required but the work piece or the ambient temperature is too low (its cold outside) it is recommended to slightly warm up the work piece,
(a propane torch is more than enough, even a heat gun)Just so the surface of the metal is not cold to the touch.
In a sheetmetal application, you don't want so much heat that it would distort the metal, just slightly warm to the touch not HOT.
You'll be amazed how something so little could make such a difference.
Also, the area of the weld has to look shiny,
that is clean!

Good luck

 

[340challconvert]

Thank you all for the thoughts; some good tips from everybody. Appreciate the responses.

 

[WayTooShort]

Very useful info, I will be needing it soon too. Thank all of you very much.