Maximum Resistance (mOhm) for Good Spot Weld on Steel 1

[jmarkus]

What is the maximum resistance between 2 sheets of steel (for example due to rust, or residual coating) which I can expect to still provide a good weld without "cranking up" the current vs 2 clean sheets of steel (bare, clean). Is there a reference where I can find this information in more detail (if required)?

Thanks,
Jeff

 

[berkshire]

You are asking a question that is a bit like " how long is a piece of string?" the contact resistance has to be low enough to allow sufficient current to flow to heat the metal. Rust or residual coating will act as an insulator until the squeeze pressure on the tips can make a pathway through the contaminant. Once current can flow, a lot will depend on the nature of the coating,whether it explodes or melts. Rust makes a high resistance path that can cause local heating, until the metal starts to melt, and spreads the contact area over a wider portion of the electrode.
Check what Miller has to say on the subject:

http://www.millerwelds.com/pdf/Resistance.pdf

B.E.