Tab and slot for sheet metal: joints that hold for welding

A frame welded from six laser-cut panels comes back 1.6 mm out of square across the diagonal, and every tab was drawn exactly to the sheet thickness. The joint looked right in CAD because the slot and the tab were both 2.00 mm. On the machine, the laser took 0.2 mm of kerf out of the slot walls and none off the tab, so each joint assembled with a 0.2 mm gap, the welder filled it with wire, and the shrinkage pulled the frame as it cooled.

Tab and slot for sheet metal is a locating method, not a fastening method. It holds parts square long enough to tack, and it does that job only when the clearance is small enough for the weld to bridge and large enough for the parts to go together without a hammer. Both of those numbers are computable, and neither is "the thickness of the sheet".

How much clearance does a tab and slot joint need?

Total joint gap should be 0.1 to 0.2 mm for hand assembly and no more than 0.3 mm for any joint that will be welded. That is the total gap: slot width minus tab thickness, summed across both sides of the tab.

The trap is that a laser-cut slot is wider than the drawing. The beam follows the slot contour and removes kerf from both walls, so a slot drawn at 2.00 mm measures about 2.20 mm on 2 mm CRCA with a 0.2 mm kerf. The tab, cut as an external contour, loses kerf outside the profile and finishes within 0.02 mm of nominal. Net gap: 0.2 mm at 2 mm thickness, which sits at the top of the hand-assembly range before any position error is added.

Two corrections follow. Draw the slot 0.05 to 0.25 mm narrower than the tab width, and check the kerf at the thickness you are using, because kerf grows with thickness:

• 0.8 to 1.5 mm CRCA: kerf about 0.15 mm. Draw the slot at t − 0.05 mm.
• 2 to 3 mm CRCA: kerf about 0.20 mm. Draw the slot at t − 0.10 mm.
• 4 to 6 mm mild steel: kerf about 0.30 to 0.40 mm. Draw the slot at t − 0.25 mm.
• Punched slots: no kerf correction, because the die is sized to the finished slot. Set the slot at t + 0.1 mm.

Stainless behaves slightly differently. The kerf is narrower at the same power because the material absorbs less energy, and the cut edge work-hardens, so the tab does not deform to fill a gap the way mild steel does. On 304 we work to 0.15 mm total gap and radius the tab root before anything else.

Tab and slot dimensions that survive cutting and welding

The table is for joints where the tab passes through a slot in a mating sheet of the same or similar thickness. Values assume CRCA or mild steel; the Ganesh column records what the shop will query.

Tab and slot locating logic

A tab and slot joint locates in the plane of the mating sheet, and it locates only as well as the number of tabs allows. This is the same 3-2-1 logic a fixture uses, applied to the part itself.

One tab locates nothing

A single tab through a single slot fixes position in two directions but allows rotation about the tab. It is useful only where the part is also clamped by something else, such as a hem or a second panel. Drawings with one tab per joint and no other locator are the ones that arrive at the welder looking like a parallelogram.

Two tabs on one edge still allow the panel to hinge

Two tabs on the same edge stop the panel rotating in the plane of the sheet, but the panel can still hinge about the line through the two tabs. With 0.2 mm of clearance in each slot and the tabs 60 mm apart, the pair allows about atan(0.4 / 60) = 0.38 degrees of out-of-plane rotation. At 500 mm from the tab line that is a 3.3 mm lift at the far edge, which the welder closes with a clamp and which the frame remembers after cooling. Move one tab to an adjacent edge and the hinge disappears.

The rule we use: two tabs on one edge plus one tab on an adjacent edge, or two tabs plus a folded return that lands against a mating face.

Three points and a weld order

With three locators, the panel is fully positioned and the tacks decide the rest. Tack the tabs first, then the seams, and alternate sides. A frame tacked along one seam from end to end pulls as it cools and closes the opposite joint, which then needs force to bring home. Alternating tacks over 150 mm centres keeps the heat balanced and the diagonals within about 1 mm per metre.

When clearance is not enough: a fixture

Tab and slot gets a joint to roughly ±0.4 mm: ±0.2 mm of joint clearance plus ±0.2 mm of cut position. Where the assembly tolerance is ±0.5 mm overall, that is adequate, and the welded assembly tolerance guide shows how the rest of the stack fits around it. Where it is ±0.2 mm, no amount of tab geometry substitutes for a fixture that clamps the part on its datums, which is why we build welding fixtures in the same tool room as the press tools.

How strong is a tab?

Tab strength is worth calculating because it decides whether the tab can be the only connection or whether it needs a weld to back it up.

Take a tab 10 mm wide and 2 mm thick, with a root radius of 1 mm, in CRCA at 450 MPa tensile. Tear-out happens across two shear planes, one on each side of the tab:

Shear area = 2 × 10 mm × 2 mm = 40 mm²

Shear strength at 0.6 × tensile = 270 MPa

Tear-out load = 40 × 270 = 10,800 N, about 10.8 kN

That is the load to pull the tab out of the slot on a part where the sheet around the slot is thick enough to resist. In practice the failure is not tear-out. It is the tab bending. The same tab, 8 mm long, loaded at its tip as a cantilever, reaches yield at:

F = σ_y × b × t² / (6 × L) = 250 × 10 × 4 / (6 × 8) = 208 N

Two hundred newtons is a knock with a mallet. That number explains most tab damage at goods-in: the tab is strong in the direction the design cares about and weak in the direction the handler pushes it. If the tab has to survive handling, shorten it, widen it, or add a folded lip.

The weld is usually the weaker link, which is the opposite of what most drawings assume. A 3 mm fillet down each side of the same 10 mm tab gives 20 mm of weld length. Throat thickness is 0.7 × 3 = 2.1 mm, and at a conservative 230 MPa shear strength for mild steel weld metal the capacity is 2.1 × 20 × 230 = 9,660 N, or about 9.7 kN. Tab tear-out was 10.8 kN, so the two are within 10% of each other. That balance is worth aiming for deliberately: a 5 mm fillet would give 16 kN of weld on a tab that tears out at 10.8 kN, which is wire spent on a joint that cannot use it.

Bending governs before either of them. A tab that is pushed sideways during transport fails at 208 N, and the joint that was designed for 10 kN comes back as a warranty claim on a panel that will not sit flat. Designing for the handling load is not over-engineering; it is designing for the load the part actually sees.

What Ganesh runs

Tabs and slots are cut on the 12 kW fibre laser with the 40 ft × 16 ft bed, which means a 3,000 mm frame member and its tabs come off in one setup, with the kerf recorded per material and thickness so the slot allowance can be checked before the first part. Press tools take over where volume justifies them: the 20 T to 300 T presses and 250 T punching line blank and pierce tabbed parts, and the in-house tool room builds the dies and the welding fixtures that go with them.

Welding runs on three CLOOS robotic MIG cells and more than 30 manual MIG, MAG, spot and SPM stations. Robotic cells need the wider 0.2 to 0.3 mm clearance because the torch does not feel the joint, while a manual welder can work to 0.15 mm by easing the tab home. After welding, parts go through seven-tank pretreatment and powder coating in the 18 ft × 14 ft booth, with DFT and cross-hatch checks; a slot left open in a coated frame is also a drainage path, which is worth remembering for anything that goes outdoors.

The welded assemblies capability page lists the cells and the fixture route, and the design and CAD hub collects the tolerance and flat pattern rules that sit alongside this one.

What to put on the drawing

1. Tab width, length and root radius as separate dimensions, not as one profile.
2. The slot width with the kerf allowance stated, or a note that the slot is to finished size for punching.
3. Which member carries the tab, and on which face it protrudes.
4. The locating scheme: mark the datum tabs, typically the two that set the edge and one that sets rotation.
5. Weld symbol and size on the tab, per ISO 2553, with a note on whether the slot is plug-welded or the tab is fillet-welded at the face.
6. Diagonal tolerance for the finished frame, in mm, measured across corners.
7. Coating mask on any slot that must stay open for a fastener or a cable.

When tab and slot is the wrong joint

Do not use it on sheet thinner than 1.2 mm if the tab carries anything but its own weight; a 1.0 mm tab folds under the clamp force. Do not use it on a visible face where the slot outline will telegraph through a powder coat, which it does at 80 µm over a 0.2 mm gap. Do not use it where the assembly has to come apart, because the weld is what holds it.

It is also the wrong answer for a joint that has to be watertight. The slot is a through-thickness path, and a fillet on one face does not seal the other. For those, a flanged lap joint with a continuous seam is the design, and the welding fixture design guide covers how to hold it square.

We cut and weld tab and slot joints every week, but we will say at review when a joint needs a fixture rather than a tab. Send both members, the thickness, the weld symbol and the diagonal tolerance, and the clearance can be checked against the kerf before anyone cuts metal. Use request a quote for that review, or read the slot design guide for the width and end radius limits that apply to every slot, tab-related or not.