GMAW andFCAW contact tip burnt out question

admin

Question:  When a GMAW or FCAW contact tip gets over heated and starts to make the welding less smooth, what is actually happening to the copper the make that happen?I mean I know if the wire has turned it into a hot dog in a hallway there is less surface area of the wire getting contact, Im just looking for the over heating reasoning, like when the tip turnes a bright gold.
Reply:Originally Posted by MrWeldSoCalQuestion:  When a GMAW or FCAW contact tip gets over heated and starts to make the welding less smooth, what is actually happening to the copper the make that happen?I mean I know if the wire has turned it into a hot dog in a hallway there is less surface area of the wire getting contact, Im just looking for the over heating reasoning, like when the tip turnes a bright gold.
Reply:Generally, the fact that the electrical resistance of metallic conductors increases with temperature means that as the assembly gets "hot", voltage to the arc through the electrical stick out is lost and current flow reduced. Also, in the case of copper (contact tip) and steel (wire), both expand with increasing temperature, only they expand at different rates. Since copper expands more, the fit of the wire through the contact tip is looser when the assembly gets "hot". A loose fitting contact tip, IMHO, makes for inconsistent weld. Tips DO actually wear out (steel is much harder than copper).HTHXMT304 (school)SP125+ (home)HF 4x6 BandsawGood judgement comes from experience and much of that comes from bad judgement.
Reply:Originally Posted by wireheadGenerally, the fact that the electrical resistance of metallic conductors increases with temperature means that as the assembly gets "hot", voltage to the arc through the electrical stick out is lost and current flow reduced. Also, in the case of copper (contact tip) and steel (wire), both expand with increasing temperature, only they expand at different rates. Since copper expands more, the fit of the wire through the contact tip is looser when the assembly gets "hot". A loose fitting contact tip, IMHO, makes for inconsistent weld. Tips DO actually wear out (steel is much harder than copper).HTH
Reply:Originally Posted by wireheadGenerally, the fact that the electrical resistance of metallic conductors increases with temperature means that as the assembly gets "hot", voltage to the arc through the electrical stick out is lost and current flow reduced. HTH
Reply:Originally Posted by shovelonI never heard of a hot torch reducing current flow. Hot wire melts faster than cold wire.
Reply:Indeed higher temperature will increase resistance, and thats why I think if i tip gets over heated it runs a lot more rough, im just curious if its some certain point in temperature that it reaches and there is no turning back, the tip will not run any smoother unless you replace it.
Reply:Originally Posted by wireheadThe MIG machine tries to keep the contact tip at a constant voltage relative to ground on the opposite side of the arc. If resistance of the stickout increases with temperature, then what must the associated current do? Don't blame me, some guy who died over 150 years ago by the name of Georg Ohm who was a lot smarter than I had a bright idea. Ohm's Law says the current must decrease.
Reply:Here are some super helpful links a friend sent me, ENJOYhttp://www.thefabricator.com/article...r-contact-tipshttp://www.boulder.nist.gov/div853/P...ntact_Tube.pdfhttp://www.superiorconsumables.com/c...alloy-tips.asphttp://www.superiorconsumables.com/c...-tips.asp#tips
Reply:Learn something new every day! Thanks, here's the link to the more complete published version of #2 for the reference shelfhttp://www.nist.gov/customcf/get_pdf.cfm?pub_id=851177XMT304 (school)SP125+ (home)HF 4x6 BandsawGood judgement comes from experience and much of that comes from bad judgement.