What Is 6mm Fillet Weld?

If you’re exploring the possibilities of having a career in arc welding, you may want to take fillet welds a little more seriously. Fillet welded joints may make 80% of all arc welding joints, but its terms are somewhat complicated. What exactly is a 6mm fillet weld?

The 6mm fillet weld is the default value for the size of the fillet if the designer doesn’t specify any. It’s crucial to note that this value is different from the minimum and maximum fillet weld sizes, which are 3mm and 0.7t respectively.

In this article, you’ll learn all the technicalities relating to fillet weld and why you should learn about them. You’ll also know what a 6mm fillet weld means exactly, and why it may not be as important as you think it is.

What Is a Fillet Weld?

A 6mm fillet weld refers to a specific aspect of a tool in fillet welding. Therefore, a thorough knowledge of fillet welding is required to understand what a 6mm fillet weld means, and what you can use it for.

Fillet welding is the process of joining two metals together in a perpendicular position, leaving a T-shaped result. This term doesn’t refer to a specific process in welding, it only describes the result, regardless of how you came about it.

Many procedures can be used to attain fillet welding. Some of the most common ones include tungsten inert gas welding (TIG welding), gas metal arc welding, and shielded metal arc welding.

Most weld types have different ways to measure the size of the weld. For butt weld, it’s the throat size or throat thickness. However, the size of a fillet weld is usually denoted by its leg length, and the most common leg length of fillet welds happens to be 3mm.

Most of the time, both of the legs in a fillet weld will be of the same size. In the rare exceptions when there is any disparity, the thinner of the two legs will serve as the size of the fillet weld.

Apart from the leg of the weld, there are some other important values that every welder should note in a fillet weld like the throat size, for example. While the throat size doesn’t signify the weld size, it also exists in fillet welds, and you can calculate it, just like you can in butt welds.

In the next few sections, you’ll learn how to calculate the leg length in a fillet weld job to determine the size. Also, you’ll learn how to use the value of the leg length to find the throat size, and how to reverse the throat size to get the weld size if that’s what you’re after.

What’s the Minimum Size of a Fillet Weld?

It’s usually very difficult to measure the precise size of the fillet weld due to its high temperature during the weld. However, some standards determine the minimum and maximum sizes of a fillet weld, which will hopefully explain why 6mm fillet welds are a popular phenomenon.

If a fillet weld size is unknown, the rule-of-thumb value across the industry is 6mm, which falls somewhere around the center of the common weld sizes. This value considers that full fusion may not be achieved, which acknowledges two thumb passes at the very least.

If you’ve been hearing 6mm fillet weld so much that it stuck, it’s mostly because you’ve seen it being used as the value of the leg length a little too frequently. Since the leg length is also the size of the weld, the commonest fillet size you’ll be coming across is 6mm or ¼ inches.

What’s the Maximum Size of a Fillet Weld?

While the minimum size isn’t very uncommon, you’ll rarely see or use the maximum value in any fillet weld. The reason for this is simple: the limit only exists to prevent your weld from messing up your tasks; it isn’t a very practical value for everyday fillet welds.

The maximum fillet size isn’t uniform for all welds. Instead, it is the value of the product of the base metal thickness and 0.7. If the base metal thickness for both of the fillet legs isn’t uniform, you should always use the thinner value as the base thickness.

As hinted above, the maximum size is only to check if there has been an error in your measurement or weld. It’s not a value you’ll likely be working within an average fillet welding environment.

What Is 6mm Fillet Weld?

Here, the difference between the length size and the throat thickness is already pretty clear. Also, you may already know the minimum and maximum sizes for a fillet weld, and neither is 6mm. If that’s the case, where does the 6mm fillet weld trend originate?

The minimum value for the leg length of a fillet weld may be 3mm, but the practical minimum is closer to 6mm. This is based on the value of the rule-of-thumb leg length that most people use when the actual fillet weld size is unavailable.

In rare cases where the designer doesn’t specify the fillet weld size, it’s recommended to default to 6mm. The 6mm field weld size assumes that two passes have been deposited at least and that the first run doesn’t have full fusion, which is a pretty common outcome in fillet welds.

How to Calculate Fillet Weld Size

The only way to calculate the leg length of a fillet weld per se is by measuring it. However, this is very impractical, since the conditions of the fillet weld don’t make provisions for measurements during the weld.

However, you can derive the average weld size using the value of the throat thickness or the base metal thickness. If you already know the base metal thickness, the fillet weld size will be the product of 0.7 and the thickness.

It’s important to note that the thinner of both base metal thicknesses will be the default value if you have a fillet weld with legs that aren’t uniform. 

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