Plain answers on choosing, setting up and looking after machine-mounted oxy-fuel cutting torches, tips, adaptors and bevelling heads.
What is a machine cutting torch and how does it differ from a hand torch?
A machine cutting torch does the same job as a hand torch: a fuel gas and oxygen flame heats the steel, then a jet of pure oxygen burns through it. The difference is that it is clamped to a carriage, track or cutting machine, so the machine moves it along the cut instead of the operator.
That suits long, straight, bevelled or repeated cuts where a steady speed and torch height give a cleaner edge. Machine torches are usually made as a body with a tube or barrel that holds the tip, and they often use tips designed for machine work. Always set up and run them to the maker's instructions for your machine.
Should I use acetylene or propane with a machine torch?
Both are used for oxy-fuel cutting, but the torch and the tips must be made for the fuel you choose. Acetylene gives the hottest flame and the shortest time to start a cut, and it is often preferred for bevel work. Propane burns cooler, so piercing is slower, but it can tolerate a larger gap between nozzle and plate and is widely used for longer cuts on plate.
Propylene is a third option with its own oxygen requirements. Do not change fuel gas on a torch unless the maker says it is designed for it, because regulators, hoses, safety devices and tips are all fuel-specific.
How do I choose the right cutting tip?
Tip choice depends on the fuel gas, whether the torch is used by hand or on a machine, and the thickness of steel you cut. The oxygen bore is sized to the material thickness, and the maker publishes a tip chart showing the matching tip, gas pressures and cutting speed. Use that chart rather than guessing.
Machine work sometimes uses high-speed tips with a different bore shape, which are only suitable if your torch and gas supply are designed for them. Construction varies: acetylene tips are usually one-piece, while two-piece designs are common for machine cutting. Tips are made for a particular fuel gas and are not interchangeable between fuels. Check the data plate or manual for the tip type your torch takes.
What flashback protection should a machine cutting torch have?
Flashback arresters should be fitted to both the oxygen and fuel gas hoses near the regulators, and, for long hoses, at the torch end as well. The blowpipe should also be fitted with spring-loaded non-return valves.
A non-return valve only reduces the chance of a flashback by stopping gas flowing the wrong way; it does not stop a flame once one has started. A flashback arrester contains a flame-stopping element. None of these devices replaces safe working practice, so purge the hoses, use the correct pressures and tip size, and keep the equipment in good condition. Replace any arrester that has been through a flashback and check it with your supplier.
What causes a backfire and what should I do if one happens?
A backfire is when the flame burns back into the torch, often with a sharp bang. It can happen when the torch is too close to the work or the nozzle is partly blocked. Other common causes are an overheated tip, a loose tip, dirt on the tip seat or gas pressures that do not suit the tip.
If one happens, shut off the torch valves in the order given in the maker's or your employer's procedure, shut off both cylinder valves, cool the blowpipe with water if necessary, and then check for damage, particularly at the nozzle. A backfire is often a sign of a fault, so do not keep relighting without finding the cause.
How do I know the cutting speed is right?
On a machine, you set the travel speed, so it is worth getting right. The aim is the highest speed that still gives an acceptable edge. Too fast and the oxygen jet trails behind, leaving coarse drag lines and slag on the bottom edge, and the cut may fail to go right through. Too slow and the top edge can melt back and the cut gets wider and rougher.
Start from the speed in the tip maker's chart, then adjust by looking at the cut face. Steady torch height, clean plate and a clean, undamaged tip all help. Preheat settings that are too high can also melt the top edge.