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Spot Welder sizing for power

JP Machining

Stainless
Joined
Jul 15, 2006
Location
Wisconsin
Maybe a dumb question, but..

I'm looking for a spot welder to do say 2 sheets of 16ga together, maybe some weld nuts that sort of stuff. From what I gather about power requirements for input power I figure I can feed a 30 kva welder pretty easy. If I find a deal on say a 50kva machine is it ok to under power them and just know to not use the high end of the power settings, or even lock it out somehow? Thinking, hopefully in a few years I'd be in a bigger shop where feeding proper amps wouldn't be an issue, but right now I only got so much to spare and these things are amp hogs, for short bursts.

Have only ever played with the Hobo Freight hand held ones, and tired of burning them out cuz people dont understand duty cycles etc, time to upgrade to something with controls.
 
A 10kva to 20kva is a good size for most work.. I recall a 20kva draws around 90 amps on 240v. I spent most of today welding on a 20kva and I'm on the lowest setting with a .15 second weld roughly 9 cycles.. and I was still having some trouble with melting too deep for the section. Welding a .70" to .03".

For aluminum I think 30kva is minimum. There are some new machines that use square waves to get better welds with less but they are kinda expensive.

The big thing with spot welders is to watercool the tips to reduce melting. And if you can find a watercooled secondary you can pretty much weld all day.

I buy spotwelders whenever I find them local, I always ask for any spare tips. I got a whole box of tips for $20 last time because I bothered to ask.
 
I'd be curious if anyone knows any special magic to get reliable weld nut welding. We have a 16KVa spot welder and always struggled to get the right power for weld nuts in the #4 to 1/4" size range into 16 or 18ga. They either don't weld, or you overcook it and damage the threads. I suspect we didn't have enough power and going longer at lower power ends up overheating the entire nut before the projections weld properly. We switched to PEM nuts for most applications, which are more expensive but we're prototyping so it's not the end of the world.
 








 
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