What are thread forming screws?
If you buy hardware for enclosures, panels or cabinets, two screws on your BOM can carry nearly the same description and do opposite things to the sheet metal.
A thread-cutting screw removes material. It has flutes ground into the tip, and those act like a tap: they cut a mating thread and produce chips doing it.
A thread-forming screw removes nothing. Its thread displaces the metal and pushes it into shape, so a thread appears without anything being cut away.
Both end up with a screw in a hole. What's different is what's left behind, how strong the joint is, and how much torque it took to get there.
The chips have to go somewhere
In general sheet metal work, chips are a nuisance you blow out. Inside an electrical enclosure they're something else. The chips are metal, they're conductive, and they're now loose inside a box that's going to have energized components, terminal blocks and busbar in it. They collect on the bottom of the panel, they sit in the bottom of a wireway, and some of them stay there through final assembly no matter how carefully the box gets cleaned.
That's the reason this choice carries more weight in your product than it does for a general fabricator. A thread-forming screw doesn't create the debris in the first place.
It isn't a reason to ban thread-cutting screws from the plant. It's a reason to know which joints have them, and to decide that deliberately rather than by whatever the last buyer ordered.
What forming buys you besides no chips
Displacing metal instead of removing it work-hardens the material around the thread. The joint holds better under pull-out load, and it resists loosening under vibration better than a cut thread, because the formed material stays pressed against the screw.
You also get no lost material. A cut thread has metal taken out of it. A formed thread has the same metal, rearranged, which is part of why the engagement is tighter.
The trade-off is the hole. A forming screw has to displace a set amount of metal to make the thread, so the pilot hole gets sized to the material and the thickness, not just to the screw. Too big and the thread never fully forms. Too small and the torque it takes to form climbs toward the torque that snaps the screw.
When cutting is the right answer
Thread-cutting screws exist because forming doesn't work everywhere.
Brittle material won't displace. Gray cast iron and brittle plastics crack rather than flow, and forming only works where the substrate is ductile enough to move, which rules out harder material. Those want a cutting screw, and one with room for the chips to go.
Zinc and aluminum die castings are softer, and a thread can be formed or cut in them. They're the material cutting screws get specified for most often.
Thickness is the other limit. The more material a thread has to be formed through, the more torque it takes, and the forming torque has to stay well under the torque that breaks the screw. That's the ceiling.
A cutting screw has a requirement of its own: the panel has to be softer than the screw. That's normally true in sheet steel and aluminum, and it's the thing to check when a print calls for a cutting screw into something hardened.
The types you'll see on a quote
Type A. The older sheet metal thread, coarse and widely spaced with a gimlet point. It forms rather than cuts. It isn't recommended for new designs any more, but it's still ordered, mostly in thin sheet and non-structural work.
Type B. Blunt point, spaced thread, incomplete entering threads. It forms rather than cuts, in thin metal, non-ferrous castings and plastics.
Type F. A machine screw thread with a blunt point, tapered ending threads and multiple cutting edges near the tip. This is a cutting screw, and it's the one to look at when you need a machine-screw thread cut into a panel.
Type 23. A cutting screw with a large triangular cut-out at the tip, which gives the chips somewhere to go. Suited to cast iron and to soft metals like zinc and aluminum castings, and to some plastics.
You'll also see trilobular thread-forming screws, which have a three-lobed cross section rather than a round one. The lobes do the forming and the flats between them reduce drive torque, which is what makes forming practical in metal at production speed.
When the line tells you the screws are the problem
Screws snapping or camming out usually means the torque needed is higher than the joint can give. On a forming screw that's a hole slightly undersized, material harder than the screw was chosen for, or stock thicker than the thread can form through. The fix is the hole or the screw type, not the driver setting.
Holes stripping is the other end. On a forming screw that's a hole slightly oversized, so the thread never fully forms and there's nothing for it to hold. On a cutting screw in thin sheet it can simply mean there wasn't enough material to cut a thread into.
Both get blamed on the fastener and both are usually the hole. Worth asking what the print calls for before you change what you buy.
What to specify
Say cutting or forming, not just the size. "Number 6 pan head sheet metal screw" is not an order. It leaves the most consequential part of the decision to whoever fills it.
Give the material and the thickness you're driving into. That's what decides whether forming is even possible, and a supplier who knows the answer will tell you when what you asked for won't work in what you're using.
Ask what hole size the screw needs. A forming screw's hole gets sized to the material and the thickness, and a hole spec that came off an old print is a common reason a line suddenly starts stripping.
Say whether the joint gets taken apart. A formed thread takes repeated removal and reinsertion, so it holds up on a door, a cover or anything a service tech will open.
And if the enclosure is going somewhere that debris is a problem, say that too, because it changes the answer on joints where either type would otherwise work.
We keep the thread-forming and thread-cutting screws we supply across Type A, Type B, Type F, Type 23 and trilobular forming, and when a build needs a head, drive or finish you can't find in a catalog, we get it run to a print. If you're not sure which one a joint needs, tell us the material and the thickness and we'll tell you.
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For help choosing between thread-forming and thread-cutting screws for your enclosure or panel build, contact us at [email protected].
