Why are sheet rock screws typically coarse threaded? And, are coarse threaded screws better at self-tapping and cutting through “substrate” materials than fine-threaded? I assume coarse threads acts more like an auger than fine threads?
Close threaded screws are usually machine screws that are built to tightly fasten thinner, tougher materials together. Courser threaded screws are used to bore into thicker, softer materials. In the case of sheetrock/drywall, finely threaded screws will essentially grind the gypsum into dust and effectively strip out the hole on the first fastening.
While we’re here: self tapping screws may have either cutting or displacing threads.
And it is possible to buy fine-thread self-tapping screws, – but nobody does, because (being used in tough materials), they are so hard to turn (require high torque to insert). That means they have to be made out of even harder metal, which is seldom a good or cheap idea.
In woodworking, I use both course and fine threaded screws. As mentioned, course for softwood because they provide a better hold and fine for hardwood for the same reason with a slight benefit of preventing the wood from splitting.
I will just point out here that drywall screw threads are designed to fasten into wood or metal studs, not drywall. The head of the screw is designed to grip (or at least not tear) the paper surface when sunk properly.
There are threaded drywall anchors with very coarse thread designed to grip drywall.
IME, fine-threaded screws are designed to screw into solid holes (usually metal or plastic) which themselves have pre-formed threaded sides for the screws to slide into. Coarse screws are designed for smooth holes where the threads themselves cut into the material outside the hole.
RIght, the head of the screw, the paper, and compressed gypsum are holding the drywall against the studs. With steel studs finer threaded sheet metal screws are used.
Yes, fine threads are meant to be used in pre-threaded material, or very soft material to be self threading. Coarse threading may be used to cut threads as you say, such as with drywall screws and other wood screws, in those cases the coarseness means wide flanks between high angle threads on the shank. In other cases a coarse thread means not only wider symmetric thread spacing but a closer ‘fit’, meaning the screw and mating threads are machined to a much closer match for security. This is typical of bolts and screws used in cars such as the bolts that hold down seats that can’t loosen from vibration, and they’ll typically be made of stronger material as well.
Self-tapping coarse screws are designed for smooth holes where the threads themselves cut or displace the material outside the hole. Some self-tappers have cutting threads. Some self-tappers have threads which push the material out of the way.
Sheet-metal screws are coarse without being self-tapping.
That’s right. But a finely threaded screw will make many more turns into the material and will do more damage to softer materials. Fine threads screwed into a wood stud would also create more damage to the fibers and have an overall weaker bite.
For me, as a carpenter, fine threads really are for bolts/machine screws. I can see an application in very hard wood, but it is easier for me to use the existing supply of coarse thread screws and pilot the holes the appropriate size for that thread in the rare occasion I am fastening hardwood with screws.
Modern wood screws tend to be quite coarse with features like serrated starting threads, variable diameter shafts, counter sink ramps. Proper drive heads like Robertson or Pozidrive instead of Phillips (good for when you actually want the driver to cam out) are so important too. Some post on this board a while ago stated that old screws were better and I have no idea what perspective they were coming from. Maybe quality of alloy?
I can’t imagine they were ever better overall. There will always be cheap versions of anything for sale, and maybe more now than ever, but screws have never been made before to such fine tolerances or with better alloys.
Not screws specifically, but I once was questioned why I was specifying 317 stainless steel at greater cost for a certain application when the old piece of equipment was 316 stainless and had lasted almost 20 years. I explained that 316 SS wasn’t as good as it used to be, because they made it better.
The long explanation is the 316 is between 2% and 3% molybdenum, which is what gives it corrosion resistance. 317 will be between 3% and 4% molybdenum which is better for corrosion. Back in the day 316 SS would be right there around 2.5% molybdenum because manufacturing methods weren’t so precise and they’d try to split the range so as not to go off spec. Now, they’ve improved on their techniques to the point that they can reliably churn out 316 stainless at 2.01% molybdenum, never go off grade, and save a lot of money on molybdenum. The downside is that applications that will work with 2.5% molybdenum will not work with 2.0%.
Note that screws that are used to attach things to the ground - like these Amazon.com are very coarse indeed.
But that 2.5% was sometimes 2.1% due to the lack of consistency you mentioned. How could you be confident that you were always getting >2.5% with the old stuff?
You couldn’t be sure, it was a total crapshoot, but because they weren’t good at making it, they’d shoot for a higher standard so they wouldn’t have to downgrade it. Putting an extra couple tenths of a percent of molybdenum in was cheaper than losing a whole batch.
I’d do the same thing, I ran the bleach plant in a paper mill, and we’d have grades where we guaranteed a certain brightness, and you’d always lean to the high side, because spending an extra dollar worth of chlorine dioxide to make it was cheaper than having to sell the final product for $100 a ton less.