Piet van der Horst

Piet van der Horst

In 1970 Piet made welding his trade en since then he never stopped learning about that trade. By now he is well past his retirement age, but not welding is still not an option for him. It is not just work, it is a passion.

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Gases for welding processes

Inert and active gases

Classification according to the standard In NEN-EN-ISO 14175 standard, gases are divided into main and subgroups. In this article we discuss four main groups of

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The contact tip position in relation to the gas nozzle

Introduction

During the MIG-MAG process, the contact tip can be mounted in the welding gun in different ways. This can have quite an impact on the welding result. The following story explains the different possibilities and their consequences. 

The contact tip can be fitted to the welding gun in three ways;

  • With an equal position, where the contact tip sits flush with the front of the gas nozzle.
  • With a retracted position, where the contact tip is a few millimetres inwards relative to the front of the gas nozzle. 
  • With a forwarded position, where the contact tip is a few millimetres outside the gas nozzle.

There are three ways to achieve these positions;

  • Adjusting the length of the contact tip holder.
  • Using different lengths of contact tips.
  • Using different lengths of gas nozzles.

The differences in arc behaviour with the three positions can be quite significant. To understand why this is the case, it is important to know that a MIG-MAG power source has a flat current-voltage characteristic. This means that when the stick-out, the distance between the workpiece and the contact tip, changes, the current source will try to keep the welding voltage the same and the welding current will vary. With today’s super fast power source controls, this will work just fine. 

We will now look in more detail at the different positions of the contact tip in relation to the gas nozzle. In this explanation, the stick-out change is only achieved by moving the contact tip and applies to a mechanised or robotic application. The welding gun always remains at equal distance from the material to be welded. 

Position contact tip and gas nozzle

 

  1. Stick-out for forwarded contact tip (red)
  2. Stick-out for equal contact tip (orange)
  3. Stick-out for retracted contact tip (yellow)
  4. Contact tip
  5. Gas nozzle
  6. Welding wire
  7. Workpiece

Equal position

If we assume that the position, contact tip flush with gas nozzle, is the neutral position, then in this position we have a balance between welding current (Ampere) and welding voltage (Volt). The neutral position, where the contact tip is flush with the gas nozzle, is often used for pulsed MIG-MAG welding. Welding programmes for pulsed welding with synergic machines are often made with this position. When welding by hand, the stick-out is never exactly the same, but the power source can correct these deviations very well. If the contact tip is now moved backwards or forwards, the welder must make a correction with the correction button on the machine in order to restore the balance in the welding arc. In this way, a good droplet separation will be achieved again. It is often the case that there is not enough room for correction left to also compensate the welder’s stick-out variations. It is therefore important to know with which position of the contact tip the welding programmes of synergic machines are made. 

Retracted position

If we now change the stick-out by moving the contact tip back a few mm, this balance will be disturbed. The welding voltage will remain almost the same but the welding current will be lower. With an unchanged wire feed speed, there should be less deposition, because the welding current is lower. The deposition will nevertheless remain almost the same because in the longer wire length there is a greater resistance that preheats the wire, as a result of which it generates the same deposition with less current.

Now the question arises, where can I apply this? A retracted contact tip is usually used in applications where high welding currents are used on heavy constructions. If the contact tip is positioned further away from the workpiece, it will become less hot through radiation heat from the welding arc, which means it will last longer. Also, less welding spatter will stick to the contact tip. This applies to manual, mechanised and robotic welding. 

Forwarded position

When the contact tip is placed outside the gas nozzle, the stick-out will be slightly smaller. As a result, the welding current will increase slightly, while the welding voltage remains almost the same. Because there is now less resistance in the wire, a higher current is needed to melt the wire. Another consequence of placing the contact tip in front of the gas nozzle is that the “pressure” in the welding arc increases. This means that the arc becomes somewhat more stable and thus less sensitive to arc deviations. This characteristic is often used when small A-heights have to be welded mechanically or robotically at high wire speeds. A disadvantage of the forwarded contact tip is that it becomes more sensitive to weld spatter sticking to it. Due to the high welding speed, the heating up of the contact tip will be lower rather than higher than with a contact tip that is placed back at the same wire feed speed.

This position of the contact tip does not make much sense for manual welding. The high welding speed that one wishes to achieve is usually not attainable by hand and the contact tip is more vulnerable, which increases the risk of problems.

Longer or shorter gas nozzle

A retracted or forwarded position of the contact tip can also be achieved by varying the length of the gas nozzle. This has no effect on the stick-out because you are working with a fixed gun setup. If you still want to change the stick-out, the welding gun must be moved up or down by adjusting the welding programme.

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