Sheet Metal Seams, Hems and Lock Joints

A sheet metal part is only as good as the place where two edges meet. Sheet is too thin to weld easily in many cases, and on duct, roofing and enclosures a folded seam can be faster, stronger and cheaper than a weld. But every seam takes metal to form, and if you forget to add it to your blank, the part comes out short. Knowing the seams, the allowance each one needs, and the way each one fails separates a fabricator from someone who only runs the shear.

What You Will Be Able to Do

• Choose a seam or edge based on strength, sealing and how the part goes together.
• Add the right allowance for hems, wire edges, grooved seams and Pittsburgh locks.
• Lay out a duct section with lock seams.
• Size a blind rivet and space it correctly.
• Find the cause of a seam that leaks, opens or will not close.

Choosing a Joint

Ask four questions when you choose:

1. Does it have to seal? Against air, water or food?
2. What load does it carry? A seam in a guard is different from one in a tank.
3. Can you reach both sides? A blind rivet or a Pittsburgh lock works from one side, and a spot weld does not.
4. How does it go together? On a job site, a lock you can close with a hammer may beat anything that needs a machine.

The print or the project standard may decide this for you. Duct work, for example, is commonly built to standards published by SMACNA, the trade association for sheet metal contractors, and the job specification names what applies. Your own shop's practice has to fit inside that.

Allowances

An allowance is the extra metal you add to a blank so that you can form an edge or seam. These hand layout rules come from US Navy sheet metal training material. W is the width of the finished hem, or the width of the lock.

For a Pittsburgh lock, the matching flanged edge must be narrower than W, so that it fits fully into the pocket. For a 1/4 inch lock, a 3/16 inch flange works.

Two warnings about those rules:

• They are hand layout rules. Press brake work on tight tolerances takes off a small deduction for a closed hem, about 0.43 x the thickness. On light duct that is under 0.02 inch, and the hand rule is fine. On a precise panel, use the deduction or a test.
• Machines set their own allowance. A roll former makes the seam its rolls are cut for. Many common Pittsburgh machines need about 1 inch for the pocket edge. Read the machine's manual, or run a sample and measure it. The machine's number beats the table.

Worked Example: A Duct in Two Halves

You need a straight rectangular duct, 12 by 8 inches, made from two L-shaped halves. Each half has a pocket on one edge and a flanged edge on the other, so that the two halves lock together at opposite corners.

Using the hand rule, with a 1/4 inch lock:

• Side dimensions: 12 + 8 = 20 inches
• Pocket allowance: 11/16 inch
• Flanged edge: 3/16 inch
• Blank width: 20 + 11/16 + 3/16 = 20-7/8 inches

Using a roll former that needs 1 inch for the pocket:

• Blank width: 20 + 1 + 3/16 = 21-3/16 inches

The difference is 5/16 inch per piece. If you lay out by one rule and form by the other, the duct comes out the wrong size. Find out how the seam will be formed before you set the shear.

The light gauges used for duct have a small bend deduction, so a one-bend half is close enough for most duct. On heavier sheet, or where the fit is tight, take the deduction off too.

Forming a Grooved Seam

A grooved seam joins two flat edges so that they lie nearly flat. It is common on flat panels and round pipe.

1. Add the allowance. With a 1/4 inch lock, add 3/8 inch to each edge.
2. Fold both edges. Fold each edge the same width, one up and one down, so that they can hook together.
3. Hook the edges. Slide them together and pull the sheets apart until the hooks engage.
4. Flatten it. Tap the joint down with a mallet, working from both ends toward the middle.
5. Lock it. Run a hand groover along the seam to set the offset, or use a grooving machine.

Working from both ends keeps the seam from walking or bunching up at one end. Use a mallet on the seam itself, because a steel hammer can mark and stretch the metal.

Closing a Pittsburgh Lock

The Pittsburgh lock is the usual corner seam on rectangular duct, because you can form it on a machine and close it by hand on site.

1. Check the sizes. Confirm that the flanged edge fits into the pocket without binding.
2. Add sealant if the job calls for it. Put it in before you close the seam, because it cannot get in later.
3. Seat the flange. Slide it fully into the pocket along the whole length.
4. Hammer over the lip. Work along the seam, starting at one end, and fold the pocket lip tight against the flange.
5. Check the corner. Look down the duct to see that the corner stayed square.

If the flange will not seat, do not force it. A flange that is too wide or a pocket that has been crushed will not close properly, and the seam may pop open under pressure.

Standing Seams and Cleats

A standing seam has upturned edges that are folded together, so the joint stands up from the sheet. It sheds water and adds stiffness, which is why it is common on metal roofs and large duct. The exact dimensions depend on the system, so use the manufacturer's details.

Cleats are separate strips that join two duct sections. An S cleat slides over two edges, and a drive cleat is driven along the side and folded over at the ends. They let you assemble long duct runs quickly, and the sealing comes from the sealant, not from the metal fit.

Blind Rivets

Blind, or pop, rivets go in from one side. Use these rules of thumb as a starting point. The edge and spacing minimums come from military riveting manuals, so the print or the rivet maker's data overrides them.

Worked Example: Sizing a Rivet

You are joining two 16-gauge (0.060 inch) sheets.

• Diameter: 3 x 0.060 = 0.180 inch, so use 3/16 inch (0.1875 inch).
• Grip: the stack is 0.120 inch, so pick a rivet whose listed grip range includes it.
• Edge distance: 2 x 3/16 = 3/8 inch minimum
• Spacing: 3 x 3/16 = 9/16 inch minimum

Drill the hole size the rivet maker lists. Deburr both sheets and clamp them tight before you pull the rivet, because a burr or a gap holds the sheets apart and leaves a loose joint.

Do not mix metals carelessly. Two different metals in contact with moisture can corrode. As a general rule, keep the rivet and the sheet in the same metal family. If you have to mix them, the small part should not be the one that corrodes. A stainless rivet in aluminum sheet is usually far better than an aluminum rivet in stainless sheet.

Spot Welds

Resistance spot welding fuses overlapping sheets with current and pressure. The flange has to be wide enough for the electrodes, and both sheets must be clean and pressed together.

Test the setup with a peel test. Weld two scrap strips and peel them apart. A good spot weld tears a round button of metal out of one sheet. If the sheets simply separate at the weld, the nugget is too small or the weld is cold.

Warning: Spot welding or fusion welding galvanized sheet burns off zinc and makes zinc oxide fume, which can cause metal fume fever. Use local exhaust at the weld, and use respiratory protection if your employer requires it. Keep your head out of the plume.

Sealing and Stiffening

Sealing. Sealants go in as the seam is assembled, on clean, dry metal. Use the product your job specifies, and follow its cure time. On duct, leakage limits may apply, so the specification may require a test.

Stiffening. A joint can also add strength. A hem stiffens a raw edge and removes the sharp edge from a customer-facing panel. A wire edge adds even more strength to an open rim. A standing seam stiffens a wide panel. Large flat duct panels are often given a slight cross break, a shallow X pressed into the sheet, so that the panel does not drum.

When a Seam Goes Wrong

Warning: Roll formers and lock formers have nip points that can pull in fingers and gloves. Keep your hands clear of the rolls, feed short pieces with a pusher or as the maker's manual says, and lock out the machine before you clear a jam or change rolls.

Common Mistakes

• Forgetting the allowance. The part comes out short by the width of the seam.
• Using the hand rule on a machine seam. The rolls decide the allowance.
• Forcing a flange into a pocket. You damage the seam and it may open later.
• Adding sealant after closing. It only sits on the outside.
• Using an oversized rivet. It can tear out or distort thin sheet.
• Handling raw seam edges bare-handed. Folded edges still cut, so wear cut-resistant gloves.

Check Your Understanding

You are making a 1/2 inch double hem along a panel edge. How much metal do you add?

Twice the hem width, so 1 inch. If this is precise brake work, check it with a sample.

A grooved seam with a 3/8 inch lock joins two panels. How much do you add to each edge?

The total is 3 x 3/8 = 1-1/8 inches, and half goes on each edge, so 9/16 inch per edge.

Your duct blanks came out short after you moved from hand forming to a roll former. What is the likely reason?

The roll former needs its own allowance, which can be larger than the hand rule. Check the machine's manual or measure a sample, then fix the blank size.

You are joining two 0.048 inch sheets with blind rivets. What diameter is a good start, and how far from the edge should it sit?

3 x 0.048 = 0.144 inch, so a 5/32 inch rivet is a good start. Keep the center at least 5/16 inch from the edge, which is 2 times the diameter.

Key Takeaways

• Choose a joint by sealing, load, access and how the part goes together.
• Every seam needs metal, so add the allowance to your blank.
• Hand rules and machine allowances differ, and the machine's number wins.
• Add sealant during assembly, not after.
• Size rivets to the sheet, and keep them clear of the edge.
• Prove a spot weld with a peel test, and control zinc fume on galvanized sheet.