Acceptable vs. non-acceptable consumable wear - shield
So far during our series on “Acceptable vs. non-acceptable consumable wear” we’ve covered the electrode, nozzle, and swirl ring. Next up is the shield. The shield has two main purposes, two critical features, and two things that often cause it to fail.
The two main purposes of the shield are to (1) shield the nozzle from double arcing during piercing and accidental contact with the plate, and (2) direct secondary gas flow so it cools the nozzle and controls the arc for squarer cut edges.
Like the swirl ring, a shield can last a long time. The primary reason a shield will fail is from piercing too close to the plate. This is why it is so important to read your Operator’s Manual and follow the recommended pierce height settings and pierce delay times. Also, regularly test your torch height control to be sure the initial height sensing (IHS) feature is working correctly. The second reason for shield failure is something we learned not to do in our very first post of the series. Can anyone guess what it is? If you guessed, use the electrode to catastrophic failure, you are right! As explained, using an electrode to failure will destroy your entire consumable set and possibly your torch.
Moving on, the two critical features found on a shield are the main orifice and the bleed orifices. The main orifice must be perfectly centered to the main orifice in the nozzle, while the bleed orifices are critical to controlling cut edge angularity.
The shield and all of the orifices are held to very tight tolerance bands. In many cases, our tolerances are eight times narrower than a human hair. This is one reason why we only recommend the use of Hypertherm consumables. Hypertherm machinists go through intensive training and work on sophisticated machines—many engineered and manufactured specifically for Hypertherm—to ensure we not only meet but hold those tight tolerances. As an aside, we also make consumables for non-Hypertherm cutting systems that adhere to our same strict tolerance requirements.
When inspecting your shield, you want to be sure the main orifice is perfectly round with sharp edges and that the bleed orifices are not plugged by metal or other debris. The photos below show a shield that pierced too close to the plate. The photo on the left is an outside view of the shield with a close-up of the orifice, while the photo on the right shows the inside.
If you have a shield that looks like this, it may still be useable. The first thing you’ll want to do is try and remove as much of the accumulated metal as possible. If you don’t wait too long, you can often pick it off the copper. Next, look closely at the main orifice. Examine it from the both the outside and the inside. Finally, look at the bleed orifices. If the shield orifice is round, and the bleed holes are not plugged, you can keep on using it.
Here are a couple of tips that can minimize the amount of metal that sticks to your shield in the first place. First, after inspection, polish your shield with a Scotchbrite pad before reinserting it on the torch. A smooth shield, as it is when new, will accumulate less metal than one with spatter on it. Second, spray a water based (not silicone or oil based) MIG welding anti-spatter spray on the front of your shield. You’ll want to do this after the shield is reinstalled on the torch so you don’t rub off any spray when putting your torch back together.
How often do you replace your shield? Do you use either of the two tips provided above?