Hardware selection is where enclosure designs get quietly expensive. The drawing says M6, the buyer orders rivet nuts because they are cheap and quick to fit, and eighteen months later a technician finds one spinning in a stripped hole on a cabinet bolted to a concrete pad in a coastal plant. Nobody specified a load rating, a thickness limit or an installation method. Clinch nuts, rivet nuts and weld studs carry load in different ways, tolerate different sheet thicknesses, and are installed with different tooling in a different sequence.
Start with the joint, not the catalogue
Four questions about the joint eliminate most of the options before you compare any hardware.
- Will it be opened again? A cover removed every quarter for filter changes is a different problem from a busbar joint closed once in the factory. Repeated disassembly needs a real thread in a real insert.
- Can you reach both faces? Press-fit inserts and conventional bolts need access to both sides; blind rivet nuts exist for closed sections, welded boxes and tubes.
- Is the part coated after the hardware goes on? Weld studs must go on bare steel before coating, press-fit inserts survive a powder coat line if the threads are masked, and rivet nuts are usually fitted after coating, which adds handling.
- What is the governing load? Push-out, torque-out and pull-through are three different limits, and the bolts engineers worry about are rarely the ones that fail.
Four hardware families and what each one relies on
The practical difference is not price per piece; it is how the load travels from the bolt into the sheet, and therefore what fails first when the joint is abused.
| Method | How it holds | Sheet thickness | Where it bites you |
|---|---|---|---|
| Self-clinching (press-fit) nut | Pressed into a punched hole; displaced sheet flows into an undercut groove, so the sheet itself locks the insert | 0.8-4.0 mm | Needs press access to both faces; an oversized hole can halve the torque-out; hard stainless sheet needs special inserts |
| Rivet nut (blind insert) | Deformed from one side with a setting tool, expanding against the back face of the sheet | 0.5-6.0 mm | One-sided access only; lower torque-out in thin sheet; spins if the hole is loose; not sealed as standard |
| Weld stud / projection weld nut | Fused to the sheet by stud or projection welding, giving a metallurgical joint | 1.0 mm and up | Needs bare, clean steel; galvanised and aluminium parts are difficult; must be welded before painting; distortion |
Two conclusions follow. First, a clinch nut is a cold-forming joint, not a fastener with a flange: its strength comes from the sheet material pushed into the undercut, so hole tolerance and sheet hardness are the mechanism, not details. Second, a rivet nut is a one-sided access solution; where both faces are reachable, a clinch nut is usually better in thin sheet, and a weld stud is better in thick sheet.
Sheet thickness and hardness set the hard limits
Every family has a window of sheet thickness it works in, and hardness matters as much as thickness. Self-clinching inserts rely on the sheet being softer than the insert. In annealed cold-rolled steel that is easy; in 300-series stainless the sheet work-hardens and resists the flow that locks the insert, which is why hardened stainless inserts exist as a separate product family and why a standard stainless rivet nut in hard sheet spins well below its catalogue rating.
| Sheet material and thickness | Recommended hardware | Avoid or use with care |
|---|---|---|
| Cold-rolled steel, 0.8-1.5 mm | Self-clinching nut, steel, trivalent zinc plated, M3-M5 | Thread-forming screws on parts opened often |
| 300-series stainless, 1.0-3.0 mm | Hardened stainless self-clinching nuts, or welded studs | Standard stainless rivet nuts in hard sheet |
| Aluminium, 1.5-4.0 mm | Rivet nuts, or self-clinching nuts rated for aluminium | Welded studs; steel hardware in wet service without isolation |
| Galvanised or pre-painted steel | Rivet nuts, or self-clinching nuts fitted before coating | Projection weld studs: coating stops a sound weld |
Three rules are worth memorising. Hole tolerance is the number one cause of failed inserts: press-fit inserts want the punched hole within roughly plus or minus 0.1 mm, and a hole 0.2 mm oversize can halve torque-out capacity. Extruded holes beat plain punched holes for thread-forming screws, because the extrusion adds thread engagement without adding sheet thickness. And do not use press-fit inserts below 0.8 mm sheet; there is not enough material to displace into the groove. Material choice follows the same logic: keep the fastener at least as noble as the sheet, or isolate the joint.
The three ratings that decide the spec
Catalogues publish several numbers and engineers routinely read the wrong one. Sort them by how the joint actually fails.
- Push-out is the axial load that drives the insert out of the sheet. It governs brackets loaded in tension, studs used as mounting feet, and hardware on vibrating panels. Expect a clinch nut to give roughly 1.5 to 2 times the push-out of a rivet nut of the same thread size in 1.5 mm cold-rolled steel.
- Torque-out is the rotational load that spins the insert, and it is loaded by every tightening and loosening operation. Rivet nuts are weakest here on thin sheet, and this is the most common field failure we are asked to diagnose.
- Strip-out governs when the sheet itself is the thread. In 1.2 mm sheet a formed M4 thread holds far less than a real M4 nut, and its capacity drops with each re-tightening.
Specify an installation torque below the torque-out rating with a real margin, typically 50 to 60 percent of the rated value, because the rating assumes a perfect hole and a perfect installation. Field installation is neither.
Sealing: when the fastener sits inside the gasket line
On an enclosure rated IP65 or better, the gasket line defines the seal, not the cabinet wall. Any fastener that penetrates the wall inside the gasket line is a potential leak path, and a standard clinch nut or rivet nut is not a seal: water travels along the thread, sits under the flange, and appears as a drip on the inside of the door years later.
There are only three honest solutions. Move the hardware outside the gasket line, for example onto an external bracket or flange, which is the cheapest and most reliable option. Use a sealed insert with a bonded or O-ring seal under the head, installed to the correct squeeze; these exist as standard parts in both clinch and rivet nut form. Or use a closed-end blind insert where the sheet is accessible from one side only, so there is no open path through the wall. Two related details matter as much: through-bolts need a bonded sealing washer, not a plain flat washer, and outdoor cabinets should have a drain path or a membrane vent rather than breathing through their own threads.
Shop floor reality: tooling, access and direction
A design that is correct on paper still fails if it cannot be assembled. Two constraints decide the method more often than the load calculation does.
- Press access. Self-clinching inserts need a squeeze press with parallel access to both faces. A deep channel, a double-walled door or a return flange can make the press physically unable to reach the hole, so check reach before you specify the insert.
- Installation direction. Inserts are fitted from the side where the die clearance is favourable, with the burr pressed away from the seating face. Specify which face the insert is installed from, or you will get a mix of orientations within one batch.
Specifying hardware on a drawing
A complete callout takes three lines and prevents most disputes. State the hardware by standard and thread size, for example a self-clinching nut to a recognised standard in M5 with a specified plating. State the sheet thickness range it is qualified for, so the fabricator can flag a mismatch before cutting metal. State the installation torque and the acceptance test, such as a torque-out check on the first article at 80 percent of the rated value plus a visual check that the insert is fully seated and flush. Where the joint carries a lifting or safety function, ask for a first article record of hole size, insert part number and torque-out result.
Frequently asked questions
Clinch nut or rivet nut: which one should I specify?
If both faces are reachable and the sheet is 0.8 to 4.0 mm, use a self-clinching nut: higher torque-out and push-out, and it will not loosen. Use a rivet nut when only one face is accessible, when hardware must be fitted after painting, or when assembly happens on site with hand tools. For thin and hard sheet, clinch wins; for closed sections, rivet nuts are the only practical option.
Can I use stainless clinch nuts in stainless sheet?
Not the standard ones. Self-clinching works because the sheet is softer than the insert and flows into the undercut. In 300-series stainless the sheet work-hardens and resists that flow, so standard inserts either do not seat fully or hang loose. Use inserts made specifically for hard sheet, or switch to a projection weld nut.
Why do my rivet nuts spin?
Almost always hole size or sheet hardness. A rivet nut in a hole more than about 0.15 mm oversize never develops full contact with the sheet, so the first serious tightening torque spins it. Check the punched hole with a pin gauge, set the tool stroke for the actual sheet thickness, and remember that a rivet nut in 2 mm stainless is always weaker than the same nut in 2 mm cold-rolled steel.
Do I need sealed hardware for an IP65 enclosure?
Only for fasteners that pass through the wall inside the gasket line; hardware outside it does not affect the rating. If a fastener must sit inside the sealed area, use a sealed insert with an undercut O-ring or a closed-end blind insert, and never rely on the threads to seal.
Weld studs or press-fit inserts for a permanent joint?
For permanent or heavily loaded joints in steel above 2 mm, a projection weld nut welded before coating gives the strongest, lowest-cost joint at volume. Choose press-fit inserts when welding distortion on thin or cosmetic panels is unacceptable, or when parts are coated before hardware is fitted.
What should go into the technical agreement with the fabricator?
Hardware standard and thread size, plating, qualified sheet thickness, the face the insert is installed from, installation torque, seal type for sealed parts, and an acceptance test with a first article record. That list is short, but it removes the ambiguity behind most insert failures in the field.
Xingyuan Electric (Suzhou) Co., Ltd. is a sheet metal enclosure and electrical assembly manufacturer in Lili Town, Wujiang District, Suzhou, China. The factory runs a 52-person team in a 5,000+ square metre production base with more than 20 laser cutting, CNC bending and welding machines, quotes 7-15 day lead times on custom work, and builds to drawings or supplied samples. Insert selection, hole sizing and first article torque checks are handled by our process team. Send your drawing or sample for an engineering review and quotation.