Oxyfuel Cut Degrades Midway Through a Plate
You are burning 3/4 in. carbon steel on a track torch with a #2 (1/16 in.) two-piece tip, 40 psig cutting oxygen, 7 psig acetylene, neutral preheat, 6 ipm travel. The first 4 ft of cut is good. Over the next 6 ft the kerf widens, the drag line runs about 4° off vertical, the bottom edge carries a heavy adherent dross bead that takes a chisel to remove, and the top edge starts to round over. The plate is dry and mill-scaled on both faces. The shop is at 35°F and the oxygen manifold feeding the regulator has been drawn down to 300 psig. A #3 tip and a spare 3/4 in. hose are in the gang box.
1. Rank the probable causes and state what test separates them. Why is "the cut got hot" not an explanation?
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Symptoms, read separately
- Widening kerf plus top-edge rounding points at cutting-oxygen flow, not speed. A starved cut loses its jet shape and burns sideways.
- Heavy, adherent, rounded bottom dross is classic low-oxygen or slow-speed dross. Fast-speed dross is usually fine and needle-like and knocks off easily.
- 4° drag is the one symptom that points to travel speed being slightly high.
Ranking
- Oxygen flow starvation - 3/4 in. plate on a #2 tip with a manifold pulled down to 300 psig is marginal on flow even though the regulator shows 40 psig. Regulator pressure is not flow.
- Tip size too small for the thickness at that speed.
- Travel speed slightly high (the drag).
- Surface condition - least likely, the plate is dry and the first 4 ft cut clean on the same scale.
Separating test
Make a 12 in. scrap cut at half the travel speed without touching gas settings. If the dross persists and the top edge still rounds, the problem is gas/tip, not speed. Then repeat with the #3 tip on a fully charged manifold. "It got hot" explains nothing because the first 4 ft cut clean at the same heat input - the change is in flow or speed, and those are measurable.
2. The operator proposes turning the cutting-oxygen regulator up from 40 to 60 psig. Evaluate that move and give the correct correction.
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Why turning up the regulator is the wrong first move
- With the supply manifold at 300 psig and falling, the regulator may already be wide open in effect; raising the setpoint adds almost nothing until the supply recovers.
- Even with good supply, an oversized jet on a #2 tip makes the kerf wider and rougher, worsens the top-edge wash, and wastes oxygen.
- Pressure is not the same as flow. A restricted or kinked 1/4 in. hose, a freezing regulator at 35°F, or a partially open manifold valve all show correct gauge pressure and starve the tip.
Correct correction
- Check the whole oxygen path: manifold valve full open, no kink, correct hose size, regulator not frosted.
- If flow is still short at 3/4 in., fit the #3 tip and run the manufacturer's pressures and speed for 3/4 in. - a larger tip needs more flow, so this only works after the supply is restored.
- Re-set travel speed from the tip chart and re-check drag on a scrap cut.
- Leave the fuel gas where the chart puts it; never run acetylene above 15 psig.
3. The part is re-cut and must hold square within 1/16 in. through the thickness. List the specific changes you make and how you confirm them before cutting the part.
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Changes
- Pierce or enter off the part and lead in, so the pierce blowback does not round the top edge of the finished edge.
- Use the #3 tip with the maker's cutting-oxygen pressure and the matching preheat setting for 3/4 in.
- Set travel speed from the chart for that tip and thickness; verify by a scrap cut with the drag line vertical or leaning slightly rearward.
- Set the torch perpendicular in both planes. A torch that leans in the travel direction produces a bevel that no speed adjustment fixes.
- Support the plate so the kerf does not close behind the torch and pinch the cut.
Confirmation before cutting the part
Run a 12 in. cut on drop of the same heat and thickness. Check: minimal easily-removed dross, top edge sharp, drag near vertical, kerf width within a tip-size range, and the cut face square with a machinist square at both ends. If it does not pass, change one variable at a time - speed first, then pressure - not all at once.