Sheet metal makes it difficult to install fastener threads. They have to be cut into the metal, and thin sheets don’t give you much material to work with. Create a threaded hole in 1/8 inch steel for a standard 1/4 inch bolt and you’ll end up with about two and a half turns of thread holding it. It works fine the first time. But if you take the panel off and put it back a few times, the threads will strip out.
That’s why most enclosures, brackets, and chassis use pressed-in sheet metal fasteners instead of tapped holes. Whether those fasteners last usually comes down to three things: the size of the hole, which side you press them in from, and when they go in during the product build.
Why thin material needs pressed-in hardware
There are three ways to put usable threads in a sheet metal part, but the first two can create problems later:
- Cut threads into the metal: There isn‘t enough thickness for the threads to grip a bolt, so they wear out fast.
- Weld a nut onto the back: The heat warps a thin panel and leaves bits of melted metal that have to be ground off before the part can be painted.
- Press a fastener into the metal: The fastener gets squeezed into place cold, with no heat and no warping, and the threads are full depth no matter how thin the sheet is. This is the preferred method of hardware installation for sheet metal.
How self-clinching fasteners lock into the sheet
They work by squeezing metal instead of cutting it. The fastener sits in a plain round hole, and a press squeezes it from both sides. Metal around the hole gets pushed into a groove that runs around the body of the fastener, which is what keeps it from popping back out. A rough, textured ring on the body also keeps it from spinning in the hole.
Two things have to be true for this process to work. The fastener has to be harder than the panel, or it gets crushed instead of pushing the metal aside. And the hole has to be just barely bigger than the fastener, often by only a couple thousandths of an inch.
PEM fasteners are what most engineers write on a drawing. PEM is a brand name that has become shorthand for self-clinching hardware in general. If your drawing calls out a specific part number, the hole size, minimum sheet thickness, and hardness limit all come from that manufacturer’s catalog.
Types of sheet metal hardware and what each one does
There are multiple kinds of sheet metal hardware, and picking the correct one early saves assembly time later.
- Self-clinching nuts put usable threads in a panel that is too thin to hold threads on its own. The back side stays smooth, which matters when that side is visible.
- Self-clinching studs are threaded posts that stick out of the panel. Hang a heavy part on them and tighten the nuts down, instead of holding the part up while you line up bolt holes.
- Standoff fasteners hold two parts a set distance apart. This is how most circuit boards get mounted with a gap behind them.
- Locating pins are smooth with no threads. They line up two parts so the screws don’t have to.
- Panel fasteners are screws that stay attached to the door when you take it off, so they can’t get lost.
- Floating hardware is a nut that is made to shift slightly in its housing when holes across a large assembly don’t line up perfectly.
Design rules that decide whether the hardware holds
Most sheet metal fastener failures in the field are the result of one of these issues:
- The hole is bigger than you’d expect: It’s sized to the body of the fastener, not to the screw that goes into it.
- The panel is harder than the fastener: The fastener has to push the metal aside to set itself. If the panel is too hard, it won’t.
- The panel is too thin: Every fastener has a minimum. Below it there isn’t enough metal to grip, and the fastener becomes loose over time.
- Hardware is too close to edges and bends: Pressing pushes metal sideways. Near an edge it has nowhere to go, so the panel bulges or cracks.
Which side you press from matters
Punched holes aren’t perfectly straight. The tool has to pass all the way through the metal, so the hole comes out a little wider on the side the tool exits. The thicker the metal, the bigger the difference. Press a fastener in from the wide side and there isn’t enough metal to grip it. Press from the narrow side and there is.
When hardware is installed during the build
Hardware goes in during fabrication, before finishing. That order matters.
- Coating changes the hole: Powder coating adds thickness everywhere, including inside the hole. If you press hardware in afterward, it clinches into coating instead of steel.
- Threads need protection: Once hardware is in, the part runs through the coating line with the threads exposed. To protect them, some sheet metal fasteners come with plugs and film already in place to keep coating out of the threads. Masking them at the coater is another option.
- Metals have to match: A stainless fastener in a mild steel panel will corrode where the two touch in wet conditions. Coated fasteners can also be damaged by the cleaning chemicals used before some finishes.
A shop that handles fabrication and finishing can make sure the process happens in the correct order. When panels ship out for coating and come back, a masking note is one more thing that can get missed.
What fastener information to include in product drawings
Give your fabricator these four things and they can quote and build without a phone call:
- The exact part number: Not just a thread size. The specific part determines the hole size, how thin the panel can be, and how hard it can be.
- The hole size: Taken from the fastener manufacturer’s specifications, including how much variation is allowed.
- Which side to press from, and how thick the panel is: Marked directly on the drawing so both survive a revision.
- Finish and masking notes: Indicating which threads have to come out of the coating line clean, plus how much force the fastener has to withstand for joints that carry weight.
Partner with Highland Machine on your next hardware heavy project
Highland Machine has been manufacturing in Highland, Illinois since 1944, and our quality management system is ISO 9001:2015 certified by TÜV Rheinland of North America. Fabrication, machining, finishing, and product assembly all happen at our facility. Hardware, coating, and assembly stay on one job instead of three purchase orders.
Bring us in early. Our engineering services team reviews drawings for cost and buildability, and sheet metal fasteners are one place where that conversation pays off. Hole sizes, edge distances, press access, and masking notes are cheaper to fix on the drawing than on a production run. From there we can build a prototype, so you can check thread access and panel fit on real parts.
We fabricate, finish, and assemble custom sheet metal enclosures and components to your precise specifications. For more on what combining those steps does to cost, see our article on how sheet metal assembly services reduce your costs.
Ready to move forward? Request a quote for your sheet metal fabrication and assembly project, or contact our team at 618-654-2103 to talk through your hardware requirements.
Frequently asked questions about sheet metal hardware
What are PEM fasteners?
PEM is a PennEngineering trade name, used loosely for self-clinching hardware in general. These fasteners press permanently into a plain round hole in sheet metal. Panel material is forced into a groove on the fastener body, which holds it in place, and ridges or knurling stop it from turning.
What is the difference between self-clinching nuts, studs, and standoff fasteners?
A nut gives you threads flush with the panel. A stud gives you a threaded post you can hang parts on before the nuts go down. Standoff fasteners hold a set gap, which is how most circuit boards get mounted off a panel.
How thin can sheet metal be and still take self-clinching fasteners?
Thinner than most people expect. PennEngineering lists styles rated for metal as thin as 0.008 inch. Every part number has its own minimum, and going under it is a common reason hardware spins or pulls out months later.
Should sheet metal hardware go in before or after powder coating?
Before, in almost every case, with the threads masked for the coating line. The other way around, the fastener clinches into cured powder instead of bare steel and loses most of its holding strength.
Why do self-clinching fasteners spin after installation?
Usually the hole was too big, the panel was too thin or too hard for that part, or the press force was too low. Pressing from the wrong side does it too, since a punched hole is wider where the punch exits.
Can PEM fasteners be used in stainless steel panels?
Yes, with the right part. Carbon steel hardware is not hard enough to displace a stainless panel, so those jobs need a hardened stainless series rated for the panel you are putting it in.
Does Highland Machine install sheet metal hardware as part of fabrication?
Mechanical assembly is one of our product assembly services, and it covers joining fabricated parts with fasteners and hardware. Send drawings with your callouts and we will confirm the order of operations, the masking, and anything that needs changing.