Understanding gun ratings

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What you need to know to select your MIG gun

by Andy Monk

When it comes to welding, too much of a good thing can add up to unnecessary costs, potential downtime and lost productivity — especially if you  have too large of a MIG gun for your application.

Unfortunately, many people believe a common misconception: that you need a MIG gun rated to the highest amperage you expect to weld (e.g. a 400-amp gun for a 400-amp application). That is simply not true. In fact, a MIG gun that provides a higher amperage capacity than you need typically weighs more and may be less flexible, making it less comfortable to manuever around weld joints. Higher amperage MIG guns also cost more.

The truth is, because you spend time moving parts, tacking them and performing other pre and post-weld activities, you rarely weld continuously enough to reach the maximimum duty cycle for that MIG gun. Instead, it’s often better to choose the lightest, most flexible gun that meets your needs. For example, a MIG gun rated at 300 amps can typically weld at 400 amps and higher — for a limited amount of time — and do just as good of a job. 

Gun ratings explained
In the US, the National Electrical Manufacturers Association, or NEMA, establishes the MIG gun rating criteria. In Europe, similar standards are the responsibility of Conformité Européenne or European Conformity, also called CE. 

Under both agencies, MIG guns receive a rating that reflects the temperatures above which the handle or cable becomes uncomfortably warm. These ratings, however, do not identify the point at which the MIG gun risks damage or failure, nor do they specify the way that it needs to be labeled or marketed. For that reason, there can sometimes be significant differences in the way different MIG gun manufacturers rate their products. 

Much of the difference lies in the duty cycle of the gun. Duty cycle is the amount of arc-on time within a 10-minute period. One MIG gun manufacturer may produce a 400-amp MIG gun that is capable of welding at 100 per cent duty cycle, while another manufactures the same amperage MIG gun that can weld at only 60 per cent duty cycle. In this example, the first MIG gun would be able to weld consistently at full amperage for a 10-minute time frame, whereas the latter would only be able to weld for 6 minutes. 

Before deciding which MIG gun to purchase, it is important to review the duty cycle ratios for the product. 

How do you operate?
Based on the gun rating, you need to consider the length of time you spend welding before you make your MIG gun selection. Look at how much time you spend welding over the course of 10 minutes. You may be surprised to discover that the average arc-on time is usually less than five minutes.

Keep in mind that welding with a MIG gun rated to 300 amps would exceed its rated capacity if you were to use it at 400 amps and 100-per cent duty cycle. However, if you used that same gun to weld at 400 amps and 50 per cent duty cycle, it should work just fine. Similarly, if you had an application that required welding very thick metal at high current loads (even 500 amps or more) for a short period of time, you might be able to use a gun rated at only 300 amps. 

As a general rule, a MIG gun becomes uncomfortably hot when it exceeds its full duty cycle temperature rating. If you find yourself welding for longer on a regular basis, consider either welding at a lower duty cycle or switching to a higher rated gun. Exceeding a MIG gun’s rated temperature capacity can lead to weakened connections and power cables, and shorten its working life. 

Understanding the impact of heat
There are two types of heat that affect the handle and cable temperature on a MIG gun and also the amount of time you can weld with it: radiant heat from the arc and resistive heat from the cable. Both also factor into what rating of MIG gun you should select. 

Radiant heat reflects back to the handle from the welding arc and the base metal. It is responsible for most of the heat encountered by the MIG gun handle. Several factors affect it, including the material being welded. If you weld aluminum or stainless steel, you will find that it reflects more heat than mild steel. 

The shielding gas mixture you use, as well as the welding transfer process, can also affect radiant heat. For example, argon creates a hotter arc than pure CO2, causing a MIG gun using an argon shielding gas mixture to reach its rated temperature at a lower amperage than when welding with pure CO2. If you use a spray transfer process, you may also find that your welding application generates more heat. This process requires an 85 per cent or richer argon shielding gas mixture, along with a longer wire stick out and arc length, both of which increase the voltage in the application and the overall temperature. The result is, again, more radiant heat. 

Using a longer MIG gun neck can help minimize the impact of radiant heat on the handle by placing it further from the arc and keeping it cooler. The consumables you use can in turn affect the amount of heat that the neck absorbs. Find consumables that connect tightly and have good mass, as these absorb heat better and can help prevent the neck from carrying as much heat to the handle. 

You may also encounter resistive heat in your welding application. Resistive heat occurs by way of electrical resistance within the welding cable and is responsible for most of the heat in the cable. It occurs when the electricity generated by the power source cannot flow through the cable and cable connections. The energy of the “backed up” electricity is lost as heat. Having an adequate sized cable can minimize resistive heat; however, it cannot eliminate it entirely. A cable large enough to completely eliminate resistance would be too heavy and unwieldy to manuever. 

Air and water-cooled 
As an air-cooled MIG gun increases in amperage, the size of the cable, connections and handles also increases. Therefore, a MIG gun with a higher rated capacity almost always has greater mass. If you are an occasional welder, that weight and size increase may not bother you; however, if you weld all day, every day, it is better to find a lighter and smaller MIG gun suited to your application. In some cases, that may mean switching to a water-cooled MIG gun, which is smaller and lighter, but can also provide the same welding capacity. 

Using a lighter MIG gun can improve productivity since it is easier to maneuver for longer periods of time. Smaller MIG guns can also reduce your susceptibility to repetitive motion injuries.

When choosing your MIG gun, remember that not all products are created equal. Two MIG guns rated to 300 amps could vary widely in terms of their overall size and weight. Research your options. Look for features like a ventilated handle that permits air to flow through it and keeps it running cooler. Such features can allow a gun to be rated to a higher capacity without adding any size or weight. Finally, assess the time you spend welding, the process and shielding gas you use, and the materials you are welding. Doing so can help you select a gun that strikes the ideal balance between comfort and capacity SMT

Andy Monk is product manager for Bernard and Tregaskiss. |



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