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Beginners' Guide to Welding, Part 2

Introducing the oxy acetylene system

by Julian Edgar

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At a glance...

  • Oxy acetylene gear
  • What you can do with it
  • Setting up
  • Safety
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This article was first published in 2007.
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Go into any workshop in the country, large or small, and you’ll almost always find an oxy welding kit. There’ll be the black bottle (oxygen) and the smaller maroon bottle (acetylene), some hoses and a handpiece. But isn’t an oxy set a bit old-fashioned? After all, these days you can buy cheap MIGs, plasma cutters – even TIG welders. So what use would have an oxy welding kit have?

The short answer is: a lot! An oxy-acetylene welding kit can do all the following:

  • fusion weld

  • braze weld

  • silver solder

  • heat metal to allow it to be bent and formed

  • heat metal to allow it to be hardened or softened

  • cut metal

That is an incredible list! Why? Well, before we get into the nitty gritty, here are some real world uses.

  • Take the first – fusion welding. In brief, that’s where you melt together metals of the same sort (eg steel), usually with the addition of small amounts of a filler rod made from the same material as the metals. Years ago, I fusion welded in this way steel intercooler plumbing. The same approach works well on exhaust pipes.

  • Brazing? Again it works well on intercooler plumbing, and I’ve recently had an oil return line from a turbo cut and then brazed to suit its new angle. Brazing doesn’t require that the metal is melted – just heated to a dull red. As a result, there’s less distortion and the process can be used on very thin metal. Brazing can also be used to join dissimilar metals – eg copper to steel.

  • Silver soldering? Again I’ve used this on a turbo conversion – in that case, on a high pressure oil supply fitting. Silver soldering (nothing like normal soft soldering!) is like brazing except it uses a rod containing silver, and is good for very close-fitting parts. It’s also very strong.

  • Whenever metals are bent, for example when making a bracket, hot bending subjects the material to lower stress than cold bending. The 10 x 4mm steel bar that needs a huge hammering in the vice when it’s cold becomes the bar easily bent with some gentle taps when it’s hot.

  • By heating steel to different temperatures (usually indicated by the colour of the material) and then quenching it in different baths (eg oil or water), steel can be hardened. Also, materials that work-harden (eg copper) can be softened by heating and allowing to slowly cool.

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  • Finally, an oxy set can be used to cut metal, including quite thick steel plate. It doesn’t give the neat edge of water-jet cutting, but the steel plate of the turbo exhaust manifold I built a few years ago was oxy cut, the edge being then cleaned-up with a file and grinder.

So an oxy-acetylene kit allows you to do nearly everything required when welding, brazing, softening/hardening or cutting of metals is needed. However, compared with other welding and cutting techniques, it’s often slower – which is a downside in production work. But for one-offs and home workshop use, the slow pace of the work allows far better user control.

For example, I have recently been brazing together some very thin wall (0.9mm) high tensile (chrome moly) steel tubing. The brazing rods being used are nickel bronze – a very strong brazing material. Normally, welding such thin wall steel tube would be very difficult – whether by MIG or TIG. (These tubes are quite a lot thinner than exhaust tube, for example.) But brazing these tubes with the oxy kit is child’s play.

Why? Well, I can braze each joint without fear of melting away the parent material, I can add or remove heat as easily as applying or removing the flame (and of course, also set the starting point by the appropriate selection of flame and tip size), and I can tack the joints and then came back later and seamlessly extend them to full welds. I can also make nuts captive by brazing them in place, and I can easily do tricky things like brazing a disc flush over the end of the tube. And, if I make a mistake, I can very easily ‘un-tack’ the braze. Now brazing isn’t as strong as MIG or TIG or fusion welding, but its versatility and ease of control make it unbeatable for my skill level in this application.

The other point this leads to is that an oxy-acetylene kit is best suited for smaller, fiddly jobs. If I built a trailer, I’d use electric welding – arc or MIG. You could certainly do it with an oxy, but you’d be there a long time. In the same way, material thicker than about 5mm is usually electrically welded. Finally, while brazing rods are available for aluminium, the success or otherwise of this depends a lot on the exact make-up of the aluminium (something normally unknown!). So an oxy kit is not normally used to weld aluminium.

OK, enough of the prelude: let’s look at the equipment.


As the name suggests, oxy acetylene welding uses two gases – acetylene and oxygen. The acetylene is the fuel and the oxygen helps achieve the very high 3100 degrees C flame temperature required.

Acetylene cylinders are filled with a porous mass which is saturated with acetone. The acetylene dissolves in the acetone much like carbon dioxide is dissolved in the liquid in a soft drink bottle: when the pressure is lowered, the acetylene bubbles out of the acetone. Acetylene cylinders are shorter than oxygen cylinders, are painted maroon (deep red) in colour, and use a left-hand (ie reversed) thread to prevent inadvertent coupling of oxygen fittings.

Oxygen cylinders are taller than acetylene cylinders. They are painted black and use a conventional right-hand thread. The oxygen is compressed to a larger degree than the acetylene and so the cylinders use heavy walls and are in turn heavy.

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Oxygen fittings should be kept completely free of grease or oil; should these contaminants come in contact with the oxygen, an explosion can occur.

Each cylinder is equipped with a shut-off valve – like a tap in your bathroom, rotate clockwise to close. Fittings connected to both types of cylinder should be specifically design for the application. For example, copper fittings should not be connected to acetylene cylinders as the copper reacts with the acetylene, creating highly explosive copper acetylide.

Each cylinder uses a pressure regulator. These are used to reduce the pressure from the massive bottle pressure to that which is suitable for use. Each regulator has two gauges. One shows the bottle pressure (so giving an indication of how much gas is left in the cylinder) and the other shows the set pressure of the gas being fed to the handpiece. Typically, full acetylene and oxygen cylinders will have pressures of 1800 kPa and 17500 kPa respectively, while the gas pressures for normal fusion welding or brazing are set at 50 kPa.

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The name given to the combination of the handpiece, control valves and welding tip is blowpipe.

The control valves on the blowpipe allow user-variation of the flow of the two gases. These controls are very important as they allow two things: (a) setting of the flame intensity, and (b) setting of the ratio of oxygen to acetylene.

The handpiece is not only the bit you hold but also contains two tubes that feed the gases to the mixer. As its name suggests, the mixer brings the two gases together. Furthermore, the mixer contains some safety devices preventing burning-back of gases through the hoses.

The welding tip is the curved nozzle through which the mixed gases pass. Tips are available in different sizes, varying both with both physical size and also the diameter of the orifice at the end. (Tips with small orifices are physically smaller overall.) Welding tips are easily swapped as required – they simply unscrew from the blowpipe.

Hoses are used to connect the regulators to the blowpipe. These hoses are colour-coded – blue for oxygen and red for acetylene. Finally, flashback arrestors are sometimes fitted to the blowpipe. These lessen the chance of the flame burning its way back towards the cylinders.

Setting Up

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Unlike arc welding that we covered in Part 1 of this series, an oxy acetylene kit has some potentially major safety issues. The bottled gases are under very high pressures, are extremely inflammable when mixed, and even when unmixed are hazardous. A hose leaking at a fitting is clearly very dangerous. (Note: hose and regulator fittings should be done up with an appropriate spanner. But don’t go mad with tightening torque – a nip-up is sufficient.)

The following process should be followed when initially setting up the gear:

  1. Ensure your hands are free of grease and oil.

  1. After ensuring that there is no source of ignition in the vicinity (including gas hot water heater pilot flames!), momentarily open the cylinder valve to blast any foreign bodies from the outlets.

  1. Make sure the regulator knob is undone (rotated anti-clockwise until loose) and then attach the regular to the cylinder. (Remember the threads are different direction, depending on the cylinder!)

  1. Open the cylinder valve slowly. The high pressure gauge will show full cylinder pressure.

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  1. Check for leakage by closing the cylinder valve and checking that the indicated pressure does not drop.

  1. Do the same with the other cylinder.

With the regulators safely on the cylinders, you have now completed the first step – you have low pressure gas available from the cylinders. Now to get that gas to the blowpipe.

  1. Connect the appropriate coloured hoses to the appropriate cylinders – blue to oxygen, red to acetylene.

  1. Purge the air from the hoses by momentarily screwing down the regulator knobs. You’ll be able to hear gas flowing from the end of the hoses. Firstly, ensure that there is no source of ignition in the vicinity.

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  1. If you are using flashback arrestors (some kits don’t include them), connect them to the ends of the hoses. The blue hose goes to the oxygen arrestor and the red hose to the acetylene arrestor.

  1. Connect the hoses to the blowpipe. If you are using an off-the-shelf kit, the hoses will be staggered in length to match the offset of the blowpipe fittings.

OK, so now you have the complete system set up and ready for working. Now for a very important test.

  1. Close the blowpipe taps and screw in the regulator knobs until each associated pressure gauge indicates 50 kPa (7 psi).

  2. Now close the bottle valves and check that the 50 kPa gauge readings don’t slowly drop. If they drop in reading, there is a leak!

  3. If there is no leak, open the bottle valves again.

  4. If there is a leak, check all fittings and hoses.

Now we’re getting close to being able to weld....

Select the appropriate welding tip and screw it into the blowpipe. This table shows how to go about selecting the tip.



Tip Size

Acetylene (kPa)

Oxygen (kPa)





























Finally, check that delivery pressure is maintained (ie 50 kPa) when the blowpipe taps are opened. Again ensure that there is no source of ignition in the vicinity. If the delivery pressure drops, open the main bottle valves further.

So that’s the welding gear set up and safe.

Shutting Down

We haven’t even started welding but it’s best if at this point we cover shutting the system down.

  1. Close the main cylinder valves

  2. Unscrew the regulator handles

  3. Open the blowpipe valves and release the gas in the hoses. Again, ensure that there is no source of ignition in the vicinity.

  4. Close the blowpipe valves

Next week: neutral, oxidising and carburising flames, and brazing.

Cylinder Hire

Gas welding kits are widely available. They contain the regulators, hoses, blowpipe, welding tips and so on. However, you’ll find, in Australia at least, that you have to hire the cylinders. So not only do you pay for the gas, you also pay to have the cylinders sitting there, perhaps doing nothing for a long time.

I found that cylinder hire costs varied a great deal: when I enquired by phone a few years ago, the cost was outrageous. But when I walked into the local BOC outlet, wearing big steel-capped work-boots and a dirty AutoSpeed shirt, and told them I had an Australian Business number (ABN), the cylinder hire costs were very reasonable....

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