Wire Feed Speed and Welding Current (Burn-Off)

On a constant-voltage machine there is no amperage dial. The welder sets a voltage and a wire feed speed, and the arc draws whatever current is needed to burn the wire off at exactly the rate it is being fed. If the gun moves closer and the arc shortens, the current rises, the wire melts faster, and the arc length restores itself. That self-regulation is the reason constant-voltage power sources made semi-automatic welding practical, and this equation is its arithmetic.

The melting rate has two parts. The first is proportional to current: energy delivered at the arc, at the electrode tip, per coulomb that passes. The second is proportional to current squared and to the length of energised wire between the contact tip and the arc — that is ordinary Joule heating, the stickout warming itself up on the way to the arc, and the square comes from $I^{2}R$ exactly as it does anywhere else. Lesnewich established this two-term form in the late 1950s and everything since has been a refinement of it.

The practical content is what stickout does. Pull the gun back and lengthen the electrode extension, and the wire arrives at the arc already hot, so more of it melts for the same amps — or, holding the feed speed fixed as a constant-voltage machine does, the current drops instead. Either way the deposition holds up while the arc gets weaker, and penetration falls. Welders use this deliberately to fill a gap without burning through. They get bitten by it accidentally when a poor fit-up or an awkward access forces a long stickout on a joint that needed penetration, and the resulting lack of fusion is invisible from the outside. The effect is strongest on small-diameter wires and on flux-cored and metal-cored wires, where the current runs through a thin sheath rather than a solid section and the resistance per unit length is correspondingly higher.

Measure the extension from the contact tip to the arc, which is the energised length, not from the gas nozzle, which is what the welder actually sees. Those two differ by however far the tip is recessed or extended, and that is the usual reason a procedure's stickout and a booth's stickout disagree by five millimetres. The disagreement is not cosmetic — it is a real change in the resistive term, so a real change in current, penetration and bead shape at the same dial setting.

The coefficients are unit-bound, and this is the page's one real honesty problem. This site asks for them in SI: the anode term in metres per second per ampere, the resistive term in reciprocal second-ampere-squared. Published burn-off coefficients are almost always printed in inches per minute per ampere and its companion, and a number lifted straight from such a table into this page is out by a factor of 1524. Fit your own instead, which takes two readings: run at a short stickout and a long one at the same voltage, record the feed speed and current each time, and solve the pair of equations. Coefficients belong to a specific wire diameter, alloy and shielding gas, and they are not transferable between them. The model itself is also an approximation — the resistivity of steel climbs steeply as the extension heats, so a coefficient fitted at one stickout drifts at another.