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Harris 543MX Acetylene

Harris 543MX Acetylene FAQs

Acetylene cutting, welding and gouging parts for a medium-duty gas torch, with practical advice on choosing and using them.

Why do I need nozzles made specifically for acetylene?icon-minus-redicon-plus-red

Nozzles are designed for a particular fuel gas, and each torch should be fitted with the nozzle intended for the gas in use. Acetylene gives the hottest, most concentrated flame of the common fuel gases, which is why it is popular for short cuts, piercing and thin sheet.

A nozzle made for another fuel gas such as propane has different channel and preheat design, so do not swap them. Check the fuel gas shown in the maker's data against the gas on your torch before fitting.

What does a cutting attachment do compared with the handle it fits?icon-minus-redicon-plus-red

The handle, or shank, carries the gas from the hoses and holds the valves. A cutting attachment fits onto it and adds the extra oxygen route that cutting needs. A cutting nozzle uses oxygen in two ways: some is mixed with the fuel gas to feed the preheat flames, and a separate lever-controlled supply gives the cutting jet.

The preheat flame brings the steel up to temperature, and the cutting oxygen then burns through it. Check that the attachment, mixer and nozzle you choose are specified for your handle and for acetylene.

What working pressure should I use for acetylene?icon-minus-redicon-plus-red

Acetylene is used at low pressure on the torch side, much lower than the cylinder pressure. HSE guidance says the acetylene pressure should not exceed a low limit for most welding and cutting unless the equipment has been specifically designed for more, so do not raise it to compensate for a worn or blocked nozzle.

Set the pressures for your nozzle size from the manufacturer's data. Use only regulators, hoses and blowpipes designed and marked for acetylene and oxygen, and read the data plate and manual for anything specific to your equipment.

Why does the nozzle size affect how much acetylene I can draw from one cylinder?icon-minus-redicon-plus-red

Acetylene is dissolved in acetone inside the cylinder. If gas is drawn off too quickly, acetone can be carried into the gas, which may make the equipment malfunction, and acetylene can also condense and drip from the torch, which is a fire risk. The rate of withdrawal should therefore be restricted to the values given in the BCGA guidance.

Large nozzles and heating heads use a lot of gas, so check the maker's flow figures against your cylinder size. Your gas supplier can advise whether you need more than one cylinder connected, which has its own rules.

How can I tell if my cut is good and what should I adjust?icon-minus-redicon-plus-red

A good cut has a square edge, a smooth surface, small drag lines and no slag on the underside. Coarse drag lines or slag on the bottom edge can mean you are travelling too fast, and too little cutting oxygen pressure can also leave an uneven surface and extra slag. Too much preheat can melt the top edge, and holding the nozzle too high or too low disturbs the cut.

Adjust one thing at a time, starting with travel speed and nozzle height, and check the nozzle is clean, undamaged and the right size for the thickness. Set the pressures from the nozzle maker's data.

Which flame should I set with a welding nozzle?icon-minus-redicon-plus-red

Gas welding of steel is normally done with a neutral flame, which has equal quantities of oxygen and acetylene. Adding more oxygen gives an oxidising flame and adding more acetylene gives a carburising flame, and these are used only for particular jobs.

Welding nozzles are made in a range of sizes selected by the thickness of metal and the welding speed required, so choose the size from the maker's chart. A swan-neck copper nozzle is fitted to the blowpipe, and the right size is the one that melts the joint without overheating the work.