Sheet metal joints are found in every sheet metal assembly. From ducts to frames to skins, these sheet metal joints are extremely common but generally unknown to the common viewer. Sheet metal fabrication techniques create these special joints to enable the proper function of the higher-level assemblies in which they reside.

From the outside, you may only see a seam, bump, or nothing at all. Some common design choices for sheet metal joints are based on usage and seam characteristics. Things like material, pressure, thickness, and application come into the design consideration.

Today we will look at 5 common types of fabricated sheet metal joints that are extremely common across the industry.

Lap Joints (Plain and Flush)

As the name suggests, lap joints are two edges that are overlapped in a specific manner.

Plain lap sheet metal joint

Plain lap joint

A plain lap joint requires no fabrication and can be as simple as two edges offset simply by their thickness and some length along the edge.

Flush Lap Sheet Metal Joint

Flush lap joint

A flush lap joint creates a seam with one edge being offset to slide under the other piece of material, with the face of each piece being on the same plane.

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They can be joined in a wide range of methods like spot welding, solid welding, rivets, and soldering.

Standing Seam Joint

Another common type of sheet metal joint is the standing seam.

Standing Seam Sheet Metal Joint

Standing seam joint

The standing seam requires both edges to be formed. The first form is a flange bent at a 90-degree angle.

The second edge is a 90-degree bend with a hem formed over. The hem fits over the flange and can be joined with rivets or weld.

It is important to note that the standing seam protrusion must face the inside of the sheet metal fabrication that is being assembled.

Standing seams can be used in larger applications like plenums and skins.

Grooved Seam Joint

The grooved seam sheet metal joint is a curious combination of a flush lap and open hems.

Groove Seam Joint

Groove seam joint

In a grooved seam, the edges of both pieces are formed into an open hem. It is reminiscent of two hands locking fingers together.

Sometimes known as lock seams, they can speed the assembly process by positioning themselves easily in one axis. Assemblers can quickly join the seams and a welder can clamp and weld into position with ease.

Corner Joints (Raw and Flange)

Raw and Flange Corner Sheet Metal Joint

Raw and flange corner joint

The raw and flange corner joint is a type of sheet metal joint that is very similar to a plain lap.

It is used at corners and consists of a flat piece of stock joined to a 90-degree bend that creates the corner form.

The flat piece (raw) is overlapped across the corner bend (flange) then joined by spot weld, rivet, or solder. Tack or solid welding is suggested for heavy gauge materials.

Flange and Flange Corner Sheet Metal Joint

Flange and flange corner joint

The flange and flange corner joint, however, is created in the same manner, but both pieces are formed to flange and overlaid.

This sheet metal joint is good for flush outside corners and heavier gages.

Double Corner Seam Joint

Double Corner Seam Joint

Double corner seam

The double corner seam is some kind of mix between a corner seam and a grooved seam.

Constructed with two open seams like a grooved seam, the double seam has a 90-degree angle to form a corner.

It can be used for compound curves and sometimes requires special tooling to clasp it tightly.


Insight Guide 3 - Selecting a Sheet Metal and Welding FabricatorThese 5 sheet metal joints are just a taste of the many dozens of joints that are available in sheet metal fabrication.

Keep in mind, to create full sheet metal fabrications and assemblies, you’ll need welding, joining, bending, and forming operations.

For more information about sheet metal assemblies, download our 8-question checklist to ask yourself before selecting a sheet metal and welding fabricator for your project.

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