In MIG (GMAW) welding the contact-tip-to-work distance quietly controls current, penetration, and arc stability. Because the wire sticking out past the tip carries current resistively, changing that length preheats the wire differently and shifts how the constant-voltage power supply self-regulates. This tool quantifies the resistive voltage drop and the resulting current change, and warns when the settings invite burnback.
How it works
The electrode extension is a resistor whose resistance depends on wire cross-section:
R_per_mm = rho_hot / area area = pi x (d/2)^2
V_drop = current x R_per_mm x change_in_CTWDA thinner wire has a smaller area, so higher resistance and a bigger voltage swing per millimetre of CTWD. On a constant-voltage machine the wire also self-regulates: more stickout means more I-squared-R preheat, so the same wire-feed speed melts with less current:
new_current ~ current x (1 - k x deltaCTWD / CTWD0)That falling current is why pulling the gun back softens penetration even though the dial has not moved.
Typical CTWD ranges by transfer mode
The correct stickout range is not arbitrary — it is set by the transfer mode, which is in turn determined by wire diameter, current, and voltage:
| Transfer mode | Typical CTWD | Current range | Notes |
|---|---|---|---|
| Short-circuit (dip) | 6 – 13 mm | Low | Used for thin metals and out-of-position work |
| Globular | 13 – 19 mm | Medium | Transition zone; coarser, more spatter |
| Spray transfer | 19 – 25 mm | High | Smooth arc, good penetration, flat/horizontal only |
| Pulsed spray | 13 – 19 mm | Variable | Wire-feed set for spray; current pulses control fusion |
What happens when CTWD drifts
On a constant-voltage machine, the supply holds the arc voltage roughly constant. When stickout increases:
- More wire is heated resistively (I²R preheating) before the arc.
- The wire melts off more readily, so the same feed speed delivers the same burn-off rate with less arc current.
- Lower current = less arc force = shallower, wider bead with reduced penetration.
When stickout decreases, the reverse happens: less preheating, the wire takes more arc energy to melt, current self-regulates upward, penetration increases, and burnback risk rises if stickout drops too low for the wire-feed speed.
Tips and notes
- Hold a consistent CTWD. The single most reliable way to get repeatable beads is to maintain a constant gun angle and travel speed. Even a 5 mm drift in stickout produces a visible change in bead width.
- Too short a stickout at high current invites burnback and spatter at the contact tip. If the wire is fusing to the tip frequently, increase stickout slightly or reduce wire-feed speed.
- If you must reach into a deep joint and stickout grows unavoidably, expect penetration to drop. Compensate by increasing wire-feed speed or bumping voltage.
- This is a first-order resistive model. Real behaviour also depends on shielding gas composition, wire chemistry, electrode polarity (DCEP vs DCEN), and the machine’s voltage slope. Use this tool to understand the direction and approximate magnitude of the effect, then verify with a test pass.