13

I needed to attach two 2x4 as T-joint and was looking to buy self-drilling wood screws to do that. In HomeDepot, I found a box of screws labeled as 'self-cutting' and bought it. But, when I opened the box and looked at the tip of the screws, I found that they had gaps on the threads (like this one)

screw

rather than having a drill bit. The threads are tapered on the tip but remained all the way upto the tip.

Can they self-drill? Or, do I need pre-drilling? In HomeDepot, I found that there were many self-drilling screws for metal sheet (for example, metal-to-metal or wood-to-metal) but coundn't find any for wood-to-wood attachment. Can I use those metal sheet self-drilling screws for wood-to-wood connection?

  • All the answers are great. Just wanted to say that can be called a type-17 point and give room for chips to accumulate thereby making it more effective. I would also agree that using machine threads for wood is ill advised. – Matt Oct 21 '15 at 20:19
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Those are screws that are made to drill their own hole. The cutout bit at the tip is specifically made to bite into the wood more effectively.

One tip with screws like this: a lot depends on the amount of pressure you're going to be able to apply in the actual screwing-in. If this is something where you can directly press on the driver, then typically you should have no problem with these kind of screws without pre-drilling, as long as you're not screwing a very difficult hardwood. Pine or similar will be no problem.

But, on the other hand, if you're screwing these in a weird position where you can't easily apply pressure on the driver, say directly overhead where gravity is fighting you, or a tight fit where you have to use a 90 degree angle bit, you're much better off pre-drilling first if you want your screw to go in straight.

I would not use metal screws with wood. Not only are the threads made for metal and not wood, but that gap in the threads towards the top of the screw that you find in wood screws makes the screw hold much better in wood applications.

  • Making the screw go in more easily is only part of the purpose of pre-drilling holes. The other part is to avoid splitting the wood. A tip like the one pictured, while not a "drill bit" is sufficient to cut the wood fibers as it goes past, which reduced the outward pressure on the wood and helps prevent splitting. – Perkins Oct 22 '15 at 15:47
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Can they self-drill?

Yes, the screw pictured above is indeed self-drilling. The cutout at the tip is what performs the drilling action. The relief in the screw shaft makes an edge which acts like a rotary cutter.

Or, do I need pre-drilling?

In pine, with enough edge distance for the screw, most likely not. However, pre-drilling a screw is never a bad idea.

Can I use those metal sheet self-drilling screws for wood-to-wood connection?

You can, but metal screws are generally of a finer thread type than wood screws. Since the grain of steel is much, much tighter and the metal is denser than wood, the threads are more closely spaced to give a better grip.

In wood, too-closely spaced threads to not given enough room between the threads to properly bite into the wood grain, leading to a weak screw-to-wood interface.

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Can they self-drill? Or, do I need pre-drilling?

They screws are designed to drill a hole as they go so there is no need to pre-drill.

Those gaps that you describe are actually a slot cut through the first few threads (where the screw is tapered) that creates a "one-time" drill bit that creates threaded hole. They work well in softwood, but can be problematic in hardwood. Using them sounds like an excellent way to join your 2x4's.

Can I use those metal sheet self-drilling screws for wood-to-wood connection?

You can, but you would be better off using the screws that you asked about in your first question.

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I'm going to disagree a bit with the previous answers and say, "It depends on why you pre-drill."

Pre-drilling accomplished three tasks:

  • it makes it easier to 'start' the screw, increasing its 'bite'.
  • it removes material in the space that is to be taken up by the shaft of the screw, thus lowering the odds of splitting the wood.
  • it guides the direction of the screw through the material (Thanks Matt!)

While this screw's tip will help the screw "bite", it will not remove any wood that would be removed by drilling, thus you increase your chances of splitting the work. As Matt points out, this screw also will likely not benefit from the directional guidance provided by pre-drilling.

  • That's my thought. if you're attaching near the middle of a board, these ate probably finel something like this is often used in "drill guns" for decking. Near the end or edge of a board, where support is less and splitting is more likely.... more risk. Driving slowly, so stress has time to distribute itself, maybe. And the cutting tip does help. – keshlam Oct 21 '15 at 23:16
  • predrilling accomplishes another task. It guides the screws direction as well. – Matt Oct 22 '15 at 1:00
  • @Matt: Great point! I've added that to the list. – JS. Oct 22 '15 at 1:05
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Normal "wood screws" are what you describe as self drilling. The bite on the end is not necessary.

But, they don't drill per se. They compress the wood, like a nail does.

A fresh (not aged) building stud can have a "wood screw" (up to size #8) sunk without having to drill first. The screws are designed for that.

Drilling first is better and can let you do more. If you are mounting onto studs in your house, which are not "fresh", you risk:

  • getting stuck
  • splitting
  • hitting too far from the center

Drilling the wall, even if not to full depth of the eventual screw, takes a core sample so you know you hit the stud and eases the passage of the screw.

The screw you show has a hex head that will stand above the joint. The normal "deck screw" has a bugle head that will sink itself flush.

http://www.homedepot.com/b/Lumber-Composites-Decking-Deck-Hardware-Deck-Screws/N-5yc1vZbqmj

enter image description here

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As others have said, these are self-drilling. However, so long as you're using softer woods and have plenty of wood around your screw I find ordinary screws do fine at self-drilling if you're using a good power screwdriver. The only time I've found a need to pre-drill was redwood for a planter. Even then the screws would self-drill, but just too many couldn't take the force and snapped instead.

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