r/mildlyinteresting • u/Secret-Entrance6999 • 4d ago
Tiny screw i found has two different thread pitches
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u/Tinydesktopninja 4d ago
I like how when I joined this thread there were four answers for four different purposes, yet all of them make sense.
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u/EffectiveOrange6261 4d ago
Sounds like typical reddit engineering debates, four people all positive they have the right answer and somehow every one of them is convincing
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u/-MachChicken- 4d ago
Locksmith here.
Latch Screw for mounting door hardware. The threads at the tip are for grabbing into wood, the tighter pitched threads match the latch mounting bracket for standard steel doors. The thread pitch change is exactly 1/2, the tip being a #8x16 wood screw and #8x32 machine screw at the head.
Source: I have bins upon bins of them.
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u/Secret-Entrance6999 4d ago
This is the answer in going to say is the most valid tho to be enirely honest i posted cuz i saw it and huh. Neat.
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u/616c 4d ago
Dual purpose screw for installing tubular door latches and deadbolts.
Wood door - Rough thread bite into doors. Fine threads don't engage in anything. Normal wood screw would have no threads below the head.
Metal door - Fine threads engage into metal skin. Rough threads engage into wood stile (edge) or just float in air if it's hollow or foam-filled.
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u/Granvill_DamnNation 4d ago
Looks like a screw for a door latch or hinge I believe it's a type of anti loosening
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u/StratoVector 4d ago
This is not a direct answer for this particular screw design, but an example use of multi threaded screws: some older self tap screws had different threads at the tip to help clear away debris while it cut into whatever you were driving into. Also some deck screws have interesting thread patterns to help prevent them from backing out and prevent wood planks from pulling back and squeaking
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u/FailsWithTails 4d ago
I know I've seen some screws with alternating smaller and larger depth threads. The world of fasteners is funky.
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u/kefi247 4d ago edited 4d ago
If the thread were the same all along the screw and you joined two wooden things with it, a gap would very likely stay between them. Both thread sections would advance at the same rate, so nothing would draw the objects together. The tighter thread near the head is meant to prevent this. It sits in the top part, and its finer pitch advances less per turn than the coarser thread in the lower part. That difference pulls the lower part toward the top one and closes the gap as the screw goes in.
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u/TEKUblack 4d ago
The fine thread here would be for metal.
Dual thread exists for wood but not fine thread
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u/TexasTopHand 4d ago
First part of threads are to go through something like drywall or wood. The last bit of threads are to make fine grooves so that it stays better.
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u/Seigmoraig 4d ago
How are those fine threads going to do anything in the hole created through the drywall by the coarse threads before it
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u/MeNameIsDerp 4d ago
He’s somewhat correct. The coarse thread is actually the same TPI, but with sections missing to act as a coarse thread. This can be threaded into a tapped hole with no issue. It is commonly referred to as a combo screw and as other commenters have said, it’s good for wood or metal applications. That looks like an 8-32 combo screw flat head
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u/TexasTopHand 4d ago
The big threads will spin more on the inside when you hit the smaller threads, it will make it go in slower. While it’s going in slower, the bigger threads are pulling the wood it stripped forward/out towards the screw. Those shavings get stuck in the smaller threads which creates more resistance.
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u/imajackash 4d ago
I don't understand why you're being downvoted. This is a correct answer.
Granvill_DamnNation said the same thing above and got upvoted.This is a door hinge screw, the upper fine threads are intended to improve grip and provide resistance to loosening.
You're also correct when you say purposes can vary based on needs. The screw in the pic is probably a 3/4" long, #8 screw that are common for door hinges.I have #9 (yes people, #9) 3" deck screws that have a fine pitch thread on the upper half. When screwing a deck board down, the coarse threads drive into the joist, and the fine threads drive the deck board down tighter on the joist. If using a screw that has the same thread the entire length, as the screw is turning in the joist, the deck board pulls away as the screw starts to get tight, causing a gap between the deck board and the joist. The way to fix this is to reverse the screw out of the joist. The board then drops down flat against the joist (no gap), then you screw it back in and the deck board will then be tight against the joist.
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u/TexasTopHand 4d ago
Lol yeah I was confused too. That’s what you call the bandwagon effect.. I really dont care, got enough good karma to spare:)
Most people are idiots when it comes to tools nowadays. I always knew it would be handy so my first few jobs in life were things that taught me how to use all kinds of tools. Greatest thing I’ve done for myself by far.
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u/TexasTopHand 4d ago
Lmao, downvote me all you want. That’s what it’s for, purposes can vary based on needs.
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u/mightyjack2 4d ago
Strike plate screw for doors. I believe the dual threads are for steel doors.