Rivet nut versus weld nut: the joint choice for thin sheet

An M6 property class 8.8 bolt torqued to 10 N·m pulls about 8.2 kN of preload into the sheet. A steel rivet nut in 2 mm CRCA holds about 3.5 kN before the sleeve pulls through. The thread does not fail. The sheet does, and it fails while the operator is still tightening.

That gap is what the rivet nut versus weld nut decision is really about. Both give you a thread in thin sheet. They differ in how much load the sheet can take around that thread, in whether the joint can survive the powder oven, and in whether anyone can repair it at the customer's site without a welder.

The numbers below come from the standard preload calculation and from published insert capacities, not from a catalogue preference. Sheet thickness and access decide most of the choice before cost is considered.

What load does the bolt actually put into the sheet?

A tightened bolt is a spring. The preload it develops is set by torque, and the joint has to carry that preload plus the service load. Use the short-form torque relation T = K × F × d with K = 0.2 for as-received threads, and F at 70% of the bolt's proof load.

For class 8.8 at 580 MPa proof:

• M6, stress area 20.1 mm²: proof load 11.7 kN, working preload 8.2 kN, torque 9.8 N·m.
• M8, stress area 36.6 mm²: proof load 21.2 kN, working preload 14.8 kN, torque 23.7 N·m.
• M10, stress area 58.0 mm²: proof load 33.6 kN, working preload 23.5 kN, torque 47.0 N·m.

For A2-70 stainless at 450 MPa proof, the M6 numbers drop to 9.05 kN proof, 6.3 kN working preload and 7.6 N·m. That is still more than a rivet nut in 2 mm sheet will hold.

This is the calculation that decides the joint. If the drawing says M8 class 8.8 at 24 N·m, the sheet has to carry 14.8 kN at each fastener, and no insert set into 1.5 mm steel does that. Either the bolt gets smaller, the torque gets lower, the sheet gets thicker, or the load path stops going through the sheet.

Rivet nut versus weld nut: which joint carries the load?

The table compares the three insert types against the same sheet, the same thread and the same question: what does the joint hold, and what does it cost to make and keep.

The pull-out figures are for a single fastener in 2 mm CRCA at the lower end of the range and 1.5 mm at the upper end, and they fall roughly with thickness. Treat them as a design envelope and confirm with a first-article pull test on the actual sheet, because the number that matters is the one your part produces.

Where each joint wins

Weld nut: the strongest joint and the least flexible

A projection weld nut collapses three small projections against the sheet in a resistance welder, typically 6–10 kA for two cycles. The weld area is small, maybe 35–40 mm² across three nuggets, and the joint fails either by tearing the sheet around the flange or by pulling the nuggets out. In 1.5 mm CRCA both happen around 8 kN, and the washer face spreads the clamp load well.

The cost is in flexibility. The nut goes on before pretreatment, so it goes through the oven, so the thread needs a plug. The nut also leaves a heat mark on the reverse face, which is visible on a 1.0 mm painted panel and invisible on 2.0 mm. And once the panel is coated and shipped, a damaged weld nut means a weld repair at the customer's site, which most assemblers will not do.

Positioning is the other constraint. A flange nut with a locating spigot sits in its hole within about ±0.3 mm; without the spigot the welder's fixture sets the position, and ±0.5 mm is realistic. On a panel that mates with a machined part, that tolerance belongs in the drawing stack-up.

Rivet nut: one side, after the oven

A blind rivet nut collapses a sleeve behind the sheet and clamps the sheet between the sleeve and the flange. The M6 body needs a 9.0 mm hole, which is well outside the ISO 273 medium clearance of 6.6 mm for an M6 bolt, and the tool needs a straight pull of 15–20 kN.

Its weakness is bearing area. The sleeve bears on a ring of sheet a few millimetres wide, and 1.5 mm CRCA yields there at about 3 kN. That is enough for a canopy access door, a rack side panel or a cover held by six fasteners, and not enough for a lifting point.

Its strength is sequence. The nut is installed after coating, from one side, with a hand tool. No masking, no thread chasing, no oven, no press. For a powder-coated enclosure, that is often worth more than the 4 kN the weld nut would add.

Clinch nut: press-installed, no heat, no oven

A clinch nut is pressed into the sheet, displacing metal into an undercut. There is no heat, no spatter and no oven exposure, and the joint is tighter to the face than a rivet nut. Pull-out runs 4–6 kN for M6 in 1.5 mm sheet and torque-out 12–18 N·m, which sits between the other two.

The constraints are access and marking. The press needs both faces and a flat area around the hole, and the die leaves a ring mark on the coated face if the nut is set after coating. Set it before coating and the thread fills, which brings the masking problem back. In practice, a clinch nut is best on bare or plated parts assembled later, or on a face that will not be seen.

How does the coating order decide the joint?

Powder coating at 180–200 °C is the operation that changes the answer. Three sequences are possible, and each has a cost:

Weld, then coat, then mask. The nut is welded before pretreatment. A silicone plug rated above the cure temperature goes in before the booth and comes out before packing. Any film that still reaches the thread is chased with a tap, which removes the coating from the thread and leaves a chip to clean. Budget 2–4 s for the plug and 4–6 s for chasing.

Coat, then rivet. The panel is coated and cured, then the rivet nut is set through the coated hole. The setting tool bears on the flange, so the film around the hole is compressed but not broken if the head is flanged. This is the cleanest sequence for a visible panel.

Coat, then clinch. Possible, but the die ring shows and the displaced metal exposes bare steel at the undercut, which is a corrosion site on a coated panel. On an outdoor enclosure, that is a repaint point within two years.

The rivnut and blind nut guide covers hole size and grip range in more detail, and the welded enclosure guide covers the weld side of the same sequence.

What does each option cost in process time?

Hardware price is the smallest part of the difference. A weld nut and a rivet nut differ by a few rupees; the setting method differs by tens of seconds.

• Rivet nut: about 6 s per fastener with a hand tool, including loading and a pull check.
• Clinch nut: about 11 s, including handling to the press and a visual check.
• Weld nut: about 25 s, including loading, the weld cycle, cooling and masking.

On a panel with eight fasteners, that is 48 s against 88 s against 200 s. At 5,000 panels a year the difference between the rivet nut and the weld nut is 211 hours of station time, which is a machine and an operator, not a consumable.

Against that, the weld nut buys roughly twice the pull-out and no field-replacement procedure. The decision is which of those two the assembly needs. If the joint is a cover held by M5 screws, the rivet nut is correct and cheaper. If the joint carries a counterweight, a motor or a lifting eye, the weld nut is correct and the coating line has to accommodate it.

What happens when the joint is repaired in the field?

Serviceability rarely appears on a drawing and always appears in a warranty claim.

A spun or stripped rivet nut is drilled out in two to three minutes and replaced with a new one using the same hand tool. A technician can do it in a lift shaft with a cordless drill and a setting tool, which is why lift and genset assemblies use them for access panels.

A weld nut cannot be replaced without a welder and a fire permit. Where the panel is accessible, the repair is a bolt and a loose nut on the reverse, which needs access that the original design did not have. That is the real cost of the weld nut: not the weld, the future.

A clinch nut sits in between. It can be drilled and re-clinched in a workshop, but not on site, and the new joint needs the press.

What does Ganesh run for threaded inserts?

The insert choice is made at DFM, because it decides the route. Weld nuts are set before pretreatment on the manual spot and SPM stations, then the assembly goes through seven-tank pretreatment and the powder booth with the threads plugged. Clinch nuts need press access, which the 20–300 T presses and the in-house tool room can support, including a fixture that holds the panel flat around the hole. Rivet nuts are fitted after coating with a hand tool and are not a press operation at all.

That is also the limit. We do not set inserts into a closed box section where the tool cannot reach, we do not weld a nut to aluminium, and we do not apply a thread-locking compound as part of the build unless the drawing calls for it. Where the joint calls for a captive fastener on a blind face with high load, the design needs a different joint, not a different insert.

The design and CAD hub covers the drawing side of joint design, and the clinching guide covers the rivet-free alternative for panel-to-panel joints. The stamping capability page lists the press range and the in-house tool room that a clinch or weld fixture is built in.

What to put on the drawing

• Thread size, pitch and class, for example M6 × 1.0 6H, and the standard the nut is bought to, ISO 21670 for a flanged weld nut.
• Nut property class and the class of the mating bolt, from ISO 898-2, so the thread and the bolt are matched.
• Hole diameter and tolerance for the specific insert, not the ISO 273 bolt clearance.
• Position tolerance for the nut centre and the face it sits on, with the keep-out around it for the setting tool.
• Grip range for a rivet nut, or sheet thickness range for a clinch nut, because both are thickness-specific.
• Assembly torque and the preload it produces, checked against the first-article pull-out.
• Coating sequence and masking instruction, written as a process note rather than left to the shop.
• First-article pull-out and torque-out values, with the sample size and the acceptance limit.

When none of the three is right

A single M6 insert in 1.5 mm sheet cannot carry a 10 kN service load, and adding more inserts around the same hole does not help because the sheet tears between them. That joint needs a backing plate, a welded boss, a thicker local pad or a through-bolt, and the drawing should show which.

The same applies below 0.8 mm. A rivet nut will hold, a weld nut will burn through, and a clinch nut will distort the panel. If the design needs a thread in 0.6 mm sheet, fold the edge, add a doubler or move the fastener to a thicker part of the assembly.

Send the drawing with the sheet thickness, the fastener callout and the service load to request a quote. The DFM review checks the insert against the press, the welder and the coating line before the quotation is issued.