Troubleshooting guide › Cutting problems › Gas (oxy-fuel) cutting problems
Oxy-fuel cutting is a chemical process: the preheat flame brings the steel to ignition temperature and the oxygen jet burns it away. It only works on steel that oxidises (mild and low-alloy steel), and the quality of the cut is decided by the nozzle, the pressures, the speed and the cleanliness of the plate. This page covers hand torches and portable (PUG) cutting machines on acetylene and LPG, and the safety faults that must never be ignored.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
| What you see | Cause | Change |
|---|---|---|
| Top edge melted or rounded | Speed too low, preheat flame too strong, nozzle too close | Speed up; reduce preheat; raise the nozzle to 8-10 mm |
| Heavy slag stuck hard to the bottom edge | Speed too high, cutting oxygen pressure too low, nozzle dirty | Slow down; set the pressure for the nozzle; clean the nozzle |
| Drag lines curve strongly backwards at the bottom | Speed too high | Slow down |
| Cut face gouged or grooved, irregular | Dirty or damaged nozzle, uneven travel, plate scale | Clean or replace the nozzle; clean the track; remove scale |
| Cut face not square | Torch not vertical; bevel scale not at zero | Square the torch |
| Bottom edge undercut, kerf wider at the bottom | Oxygen pressure too high for the nozzle | Reduce the cutting oxygen pressure |
| Cut does not go through; stops partway | Nozzle too small, oxygen low, speed too high, plate not preheated to bright red, dirty plate | Correct nozzle and pressure; slow down; preheat properly; clean the line of cut |
| Kerf wide, both edges rough | Nozzle too large for the plate, or worn | Smaller nozzle; replace |
| Cut starts well and degrades along the seam | Nozzle heating up and clogging; oxygen cylinder emptying; hose dragging | Clean the nozzle; check contents; support the hoses |
| Plate | Nozzle | Cutting oxygen | Acetylene | Speed |
|---|---|---|---|---|
| 3 mm | 1/32 in | 2.1 kg/cm2 | 0.15 kg/cm2 | 550-650 mm/min |
| 6 mm | 1/32 in | 2.1 kg/cm2 | 0.15 kg/cm2 | 450-550 mm/min |
| 12 mm | 3/64 in | 2.5 kg/cm2 | 0.15 kg/cm2 | 350-450 mm/min |
| 18 mm | 3/64 in | 2.5 kg/cm2 | 0.15 kg/cm2 | 300-380 mm/min |
| 25 mm | 1/16 in | 2.8 kg/cm2 | 0.15 kg/cm2 | 250-320 mm/min |
| 50 mm | 1/16 in | 3.5 kg/cm2 | 0.15 kg/cm2 | 150-220 mm/min |
| 100 mm | 3/32 in | 4.5-5 kg/cm2 | 0.2 kg/cm2 | 100-140 mm/min |
Guide values for nozzle-mix cutters; confirm with a trial cut. For bevel cutting use one size larger nozzle and about 60-70 % of the speed. For LPG use the LPG nozzle for the same thickness, raise fuel pressure to about 0.3-0.5 kg/cm2, and expect a longer preheat.
Nozzle dirty, damaged or loose; dirty seat in the torch head; gas pressure too low; nozzle too close to the plate or touching slag.
Burning inside the torch, usually from the causes above plus a leaking valve or a reverse flow of gas into the hose.
Fuel pressure low, cylinder empty, too much preheat oxygen at lighting, acetylene cylinder drawn too fast.
Regulator faulty or freezing, water in the LPG line, hose kinked.
Normal with acetylene until oxygen is added; excessive means too much fuel.
| Symptom | Likely cause | What to do |
|---|---|---|
| Motor runs, machine does not move | Drive wheel lifted or slipping, track dirty, gearbox fault | Lower all wheels onto the track; clean; have the gearbox checked |
| Speed uneven or will not adjust | Speed potentiometer faulty, dirty track or wheels, hoses dragging | Replace the potentiometer; clean track and wheels; loop and support the hoses |
| Cut wanders off line | Track not parallel (75-100 mm from the line), castor not locked, wheels not all in the guides | Reset the track; lock the castor; seat all wheels |
| Cut face angled | Bevel scale not at zero, torch holder loose | Square and tighten |
| Machine stalls on long cuts | Pushing by hand with the drive engaged damages the gearbox; cable too long or thin | Lift the drive wheel before moving by hand; use the specified cable |
| Circle cut not closing | Centre pin slipping, radius bar loose, hoses pulling | Fix the pin; tighten; loop the hoses |
| Nozzle clogs quickly on a machine | Nozzle too close; slag splashing back on thick plate | Raise the nozzle; angle slightly; clean on a schedule |
Acetylene preheats faster and pierces better, and is the choice for thin plate and hand work. LPG is cheaper per hour, safer to store and fine for machine cutting of thick plate once the preheat is established; it needs LPG nozzles and more preheat oxygen.
The nozzle is dirty and the acetylene cylinder is being drawn too fast as it empties. Clean the nozzle at lunch and change or manifold cylinders.
Yes: set the bevel angle on the torch holder, use one size larger nozzle, reduce the speed to about 60-70 %, and for bevels over 30 degrees reduce it a little more.
For plate under about 25 mm, mixed materials and speed, plasma. For plate over 25-50 mm, long straight cuts and low running cost, gas. The cost calculator compares the two per metre for your own gas, power and consumable prices.
Lamps, fault codes and the checks an operator can do outside the cabinet, for the machines this page applies to.
A portable gas cutting machine gives the one thing a hand cannot: constant speed and constant height. Nearly every cut-quality fault on a machine is a set-up fault: nozzle, pressure, speed dial, track alignment. Nozzle-mix cutters resist backfire better than injector types and run on LPG as well as acetylene.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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