There’s a kind of magic about welding. Drop the visor, strike an arc, and seconds later what was a pile of metal parts emerges from the blinding white heat as something useful. Yes, as metal fabricators we shear, bend, saw, grind, punch, notch and polish, but welding is where the magic happens. It’s how we make light boxes for trailers, frame extensions for utility trucks, and generator cradles for RV’s.

Making quality welds takes both practice and experience. That’s despite there only being two main types of continuous weld: the fillet weld and the butt joint weld. Some fabricators will say there more, but we could make the case that they’re all forms of fillet or butt.

One plane or two?

If two surfaces are being welded in the same plane, that’s a butt weld. They are just butted-up against one another before being joined.

If two surfaces are perpendicular, (meaning there’s a ninety degree angle between them,) the weld fusing them together is a fillet. The fillet weld forms a 45 degree angle between the two pieces whereas the butt joint weld looks like a seam or bead.

Intermittent or continuous welds?

Continuous welds, whether fillet or butt, span the entire length of the join. An intermittent weld is one where the pieces appear tacked together. There could be one inch of weld – fillet or butt – then an inch or more of unwelded length before the next weld. There are pros and cons to the two methods. Read, “How To Determine Between Stitch Welding vs. Seam Welding For Your Metal Fabrication Product,” for more information.

Butt weld basics

For a butt weld two pieces of metal are brought together until almost touching. Typically there will be a gap of around 1/8” (3mm). The welder strikes an arc and feeds in filler to create a pool of molten metal. This pool is then moved along the joint with filler added continuously. The gap is there to ensure molten metal penetrates all the way through the pieces being joined. If the gap is too small there may not be enough penetration. Make it too big and you’ll get a large or heavy seam on the reverse side of the pieces.

Metal thickness influences penetration. When pieces are more than 3/16” a chamfer is usually ground on one or both top edges. This makes the gap wider and lets metal flow down the full thickness. Conversely, if the pieces are very thin it might not be necessary to have any gap at all.

An interesting special case is when you’re welding pieces of different thickness but that are in the same plane. These are overlapped to create a lap joint. Then on each side of the joint there’s a ninety degree angle between the two pieces, making this a fillet weld.

Fillet weld fundamentals

Some welders will say they make more fillet than butt welds. That’s probably because fillet welds don’t need any edge preparation, like chamfers, so it’s a faster technique.

In fillet welding the idea is to build up a triangular section weld between the two pieces. When finished the weld surface should be at 45 degrees to both parent materials with the fillet size related to their thickness. More specifically, the throat of the weld – the distance from the inside corner out to the weld surface – should be the same as the thickness of the base metal. A fillet weld smaller than this probably lacks strength while one larger has wasted time and filler, and may have put too much heat into the metal.

When the pieces being welded are thick – say 3/16” or more – the welder usually makes several passes rather than trying to deposit a lot of metal all at once.

Art and science combined

Many people can weld, but it takes practice and experience to make consistently high quality welds. Understanding the difference between fillet and butt joint welds, and when to use each is a start. There’s more to it than that but we can assure you there’s no magic involved.