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:
That is an incredible list! Why? Well, before we get into the nitty gritty, here are some real world uses.
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.
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.
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.
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:
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.
OK, so now you have the complete system set up and ready for working. Now for a very important test.
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.
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.
We haven’t even started welding but it’s best if at this point we cover shutting the system down.
Next week: neutral, oxidising and carburising flames, and brazing.