Thursday, December 8, 2022

How to select the correct torch for a welding process?

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Whatever the application of welding that we are going to use, the selection of the torch or pistol Proper treatment is essential not only for the technique itself, but also for the best results. In the moment of choose a torchIn addition to factors related to the welding process itself, the user must take into account the total cost of the torch and its expected life, including consumables, as well as personal preferences. Therefore, a correct selection saves time and money in the long run.

In previous articles of Of Machines and Tools we detail the main characteristics and the composition of a blowtorch typical for autogenous welding and related processes. In this article we are going to dedicate ourselves especially to the torches used for gas-protected electric arc welding.

The design of this type of torch is a bit more complex than that of oxyacetylene torches. For starters, these torches carry a consumable (MIG/MAG welding) or non-consumable (TIG welding, plasma welding or PAW) electrode inside. They also have components to transmit the current to the electrode and to direct the shielding gas. On the other hand, the large amount of heat generated in the torch during welding (especially in automated systems) must be dissipated in some practical way and therefore these torches are provided with a cooling system.

Although each arc welding process requires a different torch, we can focus the selection based on the common characteristics that the torches of all these processes have, which we will deal with next.

Amperage

The first thing we should keep in mind when select a welding torch by gas protected arc is the type of material to be welded, as well as its thickness. These factors will give us an idea of ​​the amperage necessary to carry out the adequate welding. Therefore, we have to choose a gun with an amperage that matches the needs of the application and is related to the capacity of the power source to be used.

Do not make the mistake of choosing a torch with a higher amperage than is necessary for your welding, as it could be too heavy and cause unnecessary fatigue, or it could be too large, making it difficult to access in confined places.

This means that, for example, we should not look for a 500 amp gun if we only need 350 amps for the application.

Work cycle

Once the amperage is determined, we can establish the required duty cycle of the torch, that is, the time that the torch can work continuously in a 10-minute cycle.

If we are going to use the torch in semi-automatic applications, a duty cycle of 60% is the most common and should be sufficient. Instead, for a robotic weldingSome manufacturers offer torches rated at 100% duty cycle. One factor related to the duty cycle of the torch is the type of shielding gas used. Semi-automatic guns specified for a 60% duty cycle allow for the use of carbon dioxide as a shielding gas. However, when argon-containing mixtures are introduced, the amperage of those torches at a given duty cycle is reduced.

Cooling system

The cooling of the torches used for arc welding can be of two types: by air or by water.

The air cooled torches they are traversed by a relatively cool flow of air and/or shielding gas. They are heavy and inflexible, but simple in design and easier to assemble, operate and maintain, so they are also less expensive, although they are limited to a maximum welding current below 200 amps. Because they do not use hoses or tanks, they are easily portable and are recommended for use in outdoor applications. In the following figure we see a schematic representation of a TIG welding circuit with an air-cooled gun.

Schematic representation of a TIG Welding Circuit with an air-cooled gun

The water cooled torches they are fed with a continuous flow of water, air and protective gas circulating in an external system. This flow enters through an inlet hose, circulates through the torch and exits through an outlet hose. These torches are especially suitable for welding currents that require more than 200 amps. Its use is more recommended in fixed installations such as a workshop, where the water tank can be located and the hoses can be handled more comfortably. The figure below shows a TIG welding circuit with a water-cooled gun.

TIG welding circuit with a water-cooled gun

TIG welding circuit with a water-cooled gun

In short, the water cooled torches they have the ability to run cooler and are lighter in weight, but the cost can be more than twice that of an equivalent air-cooled torch with the same duty cycle.

Consumable Characteristics

Assessing the quality of consumables—parts that degrade over time due to heat, spatter, and wear during normal welding operations—directly affects the maintenance of your chosen torch. In fact, the increased durability of these parts will lower the cost of the tool in the long run.

A good start, for example, is to carefully examine the contact point design, that is, the pieces that, in addition to helping lead electrode to the weld puddle, they are responsible for transmitting current to the electrode to start the arc. Tips with a conical or flattened profile are very suitable, which increases the cross-section in the place where heat is normally concentrated. This technology reduces melting, prolonging tip life.

Contact Tips for MIG Torches

Contact Tips for MIG Torches

Then let us consider the contact point mass. Remember that, within reason, the larger the tip, the more heat it can withstand for long periods of time, giving better heat deflection and longer life. Therefore, let’s try to choose contact tips with higher mass, for example, those that contain up to 40% more mass.

Let us also take into account the type of alloy used in the contact tip, as it affects both heat resistance and wear resistance. For example, when the electrode is fed through the tip, the hole can become elongated or distorted over time if the material does not resist heat or wear well, leading to poor electrical contact and welding problems resulting from poor welding. wandering arc, discontinuities due to cave outs and other inconveniences. Although many contact tips are made from different types of copper, some manufacturers offer designs that contain harder materials. These harder materials are more resistant to heat and to elongation or tip wear.

Like the contact tips, the nozzle or nozzle It is an important part to obtain a good welding quality. The main function of the nozzle is to direct the shielding gas to the weld. For that reason, we must select a high-quality nozzle that is capable of providing consistent gas shielding and is resistant to damage such as dents and scratches.

Welding Torch - Various Nozzles

Welding Torch – Various Nozzles

The nozzles come in small and large sizes, and in various sizes of inside diameter. Ultimately, the best option for any application is to use the largest nozzle possible that allows access to the joint. In this way we ensure a greater gaseous coverage. If the joint is of restricted access, we must look for a small and conical nozzle that allows us to place the contact tip in the vicinity of the weld puddle. If you have a high amperage application that requires a high gas flow rate, select a large diameter nozzle as it provides the best shielding gas coverage.

Personal preferences

Select Welding Torch

Select Welding Torch

Since the welder use and hold the torch in your hands all day, it is important that you consider your own comfort levels and preferences when select a torch, since these factors can affect the quality and welding efficiency.

For example, some welders may prefer a curved handle like the one in the figure above for a MIG gun, which is more ergonomic and adapts to the shape of the hand. Sometimes these handles are light in weight, reducing instances of fatigue. In turn, others may prefer a straight shank, like the one we see below, which presents a pencil torch for water-cooled plasma welding.

Welding Torch - Pencil Type

Welding Torch – Pencil Type

This shape offers more grip positions and has a smaller diameter that some users find more comfortable than a curved style.

In the case of MIG torches, we can also consider finding a neck style specific to our application. Many manufacturers offer fixed, rotating and flexible necks in various lengths and angles that allow easier access to joints. Regardless of the style of neck we choose, however, we should opt for one with a good shell capable of providing protection against damage that could lead to short circuits or premature failure.

Everything we have just described concerns the general choice of torches for gas shielded arc welding. However, as we already mentioned, the torches for the MIG/MAG process, for plasma and for TIG differ from each other in other details that we will consider in the future.

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