MIG wire feeding problems: birdnesting and stutter
Feeding faults account for most of the downtime on a semi-automatic set, and almost all of them are consumables rather than the machine. Work along the path the wire takes and you will find it.
The contact tip
Start here, always. The tip carries the current into the wire through a hole that the wire is constantly rubbing. It wears oval, then the wire arcs inside it instead of sliding, and the symptoms are a stuttering arc, wandering current and eventually burnback — the wire fusing to the tip.
A tip is the cheapest part in the system and the most frequent cause of trouble. If the hole has gone from round to oval, or you can see a groove worn on one side, replace it. On heavy work a tip may last a shift; nobody has ever regretted changing one too early.
The liner
The steel spiral or PTFE tube the wire runs through inside the torch cable. Fills with swarf and dust from the wire, kinks if the torch is coiled tightly, and eventually the friction exceeds what the drive rolls can push against.
Blow it through with compressed air from the machine end. Replace it if the torch has been dropped, run over or coiled around something tight. And lay the torch out straight while welding rather than in a loop on the floor — a coiled torch cable is a liner working at a permanent disadvantage.
Two liner rules that catch people: a steel liner is for steel wire and a PTFE one is for aluminium and stainless, and the liner must be cut to the right length so it seats against the back of the contact tip. A liner cut short leaves a gap where the wire buckles.
The drive rolls
Three things to get right. The groove profile: a V groove for solid steel wire, a knurled groove for flux-cored, a U groove for aluminium. Using a V roll on soft aluminium wire flattens it. The size: rolls are stamped with the wire diameter, and a 1.0 mm roll on 0.8 mm wire slips. The pressure: enough that the wire does not slip and no more.
The spool and the brake
Too much brake and the rolls fight the spool; too little and the spool overruns when you release the trigger, throwing a loose loop that jams on the next start. Set it so the spool stops within about a quarter turn of releasing the trigger.
Also check that the wire has not slipped off the side of the spool and jammed under a turn. That produces a sudden hard stop that looks exactly like a liner blockage.
Diagnostic order
- Change the contact tip. Half the cases end here.
- Feed with the torch straight and unloaded: if it feeds smoothly now, the cable routing was the problem.
- Remove the tip and feed the wire out by hand-triggering — smooth means the liner is fine, notchy means it is not.
- Check the roll profile, size and pressure.
- Check the spool brake and that the wire is not trapped on the spool.
The consumables and their symptoms are described in more detail in MIG/MAG torch consumables.
Frequently asked questions
What is birdnesting?
The wire jams somewhere ahead of the drive rolls, the rolls keep turning, and the wire piles up between them and the inlet guide into a tangle. It is a symptom, not a fault: something downstream stopped the wire, almost always a worn contact tip or a kinked liner.
How tight should the drive roll pressure be?
Just enough that the wire does not slip. Set it by feeding into your gloved hand: the wire should stop or slip when you press against it, not deform. Over-tightening flattens the wire, which then will not pass the liner — the cure becomes the cause.
The arc stutters but the wire feeds fine.
Then it is electrical contact, not feeding. The contact tip is worn oval and the wire is arcing inside it instead of making solid contact. A tip is a consumable that costs almost nothing; change it before investigating anything else.
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