What is Scratch-Start TIG Ignition technique?
Scratch start is a very basic and simple technique to start the arc in TIG welding, usually found on low cost inverter welding machine those are designed for Stick (MMA) welding. Scratch start in TIG is similar to scratching a match on the matchbox to ignite it. After turning the shielding gas on, the arc is established by scratching the tungsten electrode on the work piece as shown in below figure.
As we can see, the TIG tungsten electrode is scratched on the work to start the arc and must be rapidly lifted off the job to avoid it sticking on the job, but not lifted too high to prevent extinguishing the welding arc. This technique is not a very welder-friendly process and requires quite a lot of practice to master it. So, a PRO WELDER work. The Scratch-start technique is most easy to produce tungsten inclusion in the welds, which is easily recognized as a white spot on a radiographic film.
What is the Lift-Arc TIG Ignition technique?
Lift arc techniques might look similar to scratch method, but not same in their working. As in scratch arc starting, lift arc starting also uses a tungsten electrode to make contact with the base metal. But the difference is that the electrode is brought down very quickly and lightly, and then quickly lifted to “draw” up the arc.
With Lift-Arc Ignition feature, to start the arc the tungsten electrode is touched on the work and lifted off quickly. The start-up current supplied by the machine is very low and hence the tungsten hardly sticks to the work and the tungsten electrode sharpened tip is not damaged, like it can in scratch start. The tungsten is then easily lifted off the job. The machine senses that the contact is broken and begins supplying full welding current.
Lift-arc is user friendly and very controllable. It can, however, challenge manufacturers as the electronic processes must be carefully calibrated to achieve good results. If the startup current is not minimized it will cause the tungsten to stick excessively to the job.
Scratch-Start Vs Lift-Arc
It is important to be aware of the difference between the ‘Lift-Arc’ ignition system and the ‘Scratch-Start’ ignition system.
Often DC inverter welding machines will be advertised as having TIG capability when they are a ‘Scratch-Start’ machine only.
Be warned! Scratch-Start is not a user-friendly process: the tungsten must be very quickly touched on the job and can very easily stick (which means the tungsten electrode must be removed and re-sharpened each time this occurs).
In comparison ‘Lift-Arc’ is very user-friendly and very controllable.
What is HF TIG Ignition technique?
HF ignition (high frequency ignition) allows the welder to position the tungsten electrode near the work, and simply press the torch trigger to start the arc without touching the job with tungsten electrode. So, basically, arc is started without any contact as shown in below figure.
HF is mainly found on professional or dedicated TIG welding power sources, such as AC/DC TIG welders or higher end DC TIG machines. On these machines, often the controls will allow the welder to choose between Lift-Arc or HF start.