MIG/MAG torch: liner, contact tip and nozzle
A MIG torch is four consumables and a handle. Each wears at a different rate and each produces its own symptom — which makes the torch the easiest part of the machine to diagnose, once you know which part gives which.
The four parts
| Part | Job | Symptom when worn | Life |
|---|---|---|---|
| Contact tip | Carries current into the wire | Stuttering arc, wandering current, burnback | Hours to a shift |
| Gas nozzle | Shapes the gas shield | Porosity, visible spatter build-up | Days to weeks |
| Gas diffuser | Spreads the gas evenly | Turbulent shield, porosity despite good flow | Weeks |
| Liner | Guides the wire down the cable | Erratic feed, birdnesting | Months |
The contact tip
Current passes into the wire through a hole the wire is constantly rubbing. The hole wears oval, contact becomes intermittent, and the arc starts to stutter. Left long enough, the wire fuses to the tip — burnback — and the tip is finished anyway.
Buy the right size: a 1.0 mm tip for 1.0 mm wire. Aluminium is the exception, because it expands more when hot, so aluminium wire runs in a tip one size larger. And check the tip is tight — a loose tip arcs against the diffuser and destroys both.
The nozzle
Spatter builds up inside it and gradually strangles the gas flow. The result is porosity that survives every adjustment of the flowmeter, because the problem is downstream of it. Clean it with pliers and a reamer, or replace it — a nozzle with the spatter chipped out badly enough to score the bore no longer shapes the shield.
Nozzle diameter follows the current: about 12 mm up to 200 A, 15–16 mm above that. Too small a nozzle at high current gives a shield that does not cover the pool; too large a one wastes gas.
The liner
The most overlooked part, because it is invisible. It fills with dust and swarf from the wire, and as friction rises the drive rolls have to push harder until eventually the wire buckles and birdnests.
Blow it through with compressed air from the machine end when you change the spool. Replace it if the torch has been dropped or run over. Cut it to length so it seats against the back of the contact tip: a liner cut short leaves a gap where the wire has room to buckle, and that fault is maddening to find.
The cable itself
Lay the torch out straight while welding. A cable coiled on the floor makes the wire negotiate several extra bends, which is friction you have added for nothing — and it is the single most common reason a torch that fed perfectly on the bench misbehaves on the job. The rest of the feeding fault tree is in wire feeding problems.
Frequently asked questions
How often should the contact tip be changed?
When the hole is no longer round, which on heavy work can be within a shift. It is the cheapest part in the system and the most common cause of a stuttering arc, so the honest answer is: sooner than you think, and certainly before you start investigating the machine.
Does anti-spatter spray matter?
It saves the nozzle, which is the second most expensive consumable. Spray or dip the nozzle and the tip, but keep the spray away from the joint itself — silicone residue in the weld causes porosity, and that fault is hard to trace back to a can on the bench.
Steel liner or PTFE?
Steel for steel wire, PTFE for aluminium and stainless. A steel liner scrapes soft aluminium wire and fills with the shavings; PTFE is too soft to survive steel wire for long. Using the wrong one produces feeding faults that look like everything else.
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