Mar 16, 2017

I've Been Drilling Pilot Holes Wrong My Entire Life. Here's How I Learned to Correct My Technique.

So, ever since I learned to use an electric drill, I've followed this rule: when joining two pieces of wood, you drill an appropriately sized pilot hole completely through the top, and down into the second. This guides the screw, and the two pieces are held together when the screw's threads grab the wood and lock everything into place. The pilot hole's size is determined by the inner diameter of the screw's body, minus the threads. Right?


In fact, doing it this way can compromise the strength of the joint. In this approach, the threads insert themselves into the first piece, locking its position in place. Even worse, the screw can hold the two pieces apart from each other, resulting in a jacked, or bridged, screw. Which looks sloppy, and if you're trying to glue the joint together, there won't be enough pressure to allow the glue to bond the two pieces. 

My problem was a lack of understanding about how a screw joint actually works. 

It's not the threads biting into both pieces that secured everything in place. Rather, the strength of a screw joint comes from the threads pulling the screw through the bottom piece and securing the top from the pressure against the screw's head. The threads are irrelevant in the top piece; only the head matters. Think of it like a nut and bolt: the bottom piece of wood acts like the nut, drawing whatever is sandwiched between the hardware's head and the "nut" flush via the threads. 

 So, in order to permit both pieces to touch fully and allow the head to seat, the threads shouldn't dig into the wood fibers of the top piece at all.

DID YOU KNOW THIS?!?! Perhaps I'm the only one, but I suspect this is a  common misconception. So, how do you drill the appropriately sized screw holes in both pieces with one drill bit? You don't.

You use two. Well, you could use a stepped drill bit, with two diameters, or a tapered bit, but neither really gets you exactly where you need to be for truly strong joinery. If this is old news to you, then you are a better woodworker than I. If not, here's how you properly drill a pilot hole. 


Begin by understanding this: the hole drilled through the top piece of wood isn't a pilot hole at all — it's a clearance hole. This hole completely clears the material, allowing the screw to pass through, without cutting into the wood. 


In order to do this, you first drill a hole with a bit that matches the outer diameter of the screw's threads, countersinking or counterboring where appropriate. 


Then, drill a pilot hole in the bottom piece to accept the screw's threads. This bit should match the inside shank of the drill bit (not including the threads). Since the bit is smaller, you can drill it right through the clearance hole in the top.

Alternatively, if your project is small enough that you can clamp it together, begin by drilling the pilot hole through both pieces, then ream out just the clearance hole in the top piece with the larger bit. Only do this if everything is super secure so your parts don't become misaligned. 


In fact, screw manufacturers know you probably won't do this each time. It's why they leave that initial bit of screw unthreaded at the top of the screw under the head.

Is it an extra step? Yes. Is it necessary when doing rough construction or banging something together in your garage? No, of course not. But when you're working on a fine project your proud of, and especially if you're using glue, it's worth the extra couple of minutes to make the strongest joinery possible and keep everything flush and clean. In fact, the clearance hole is more important than the pilot hole, so if you're only going to drill one hole and you're sure the wood won't split, you can save time and alignment hassle by skipping the pilot hole altogether. 

Now that I get how this works, I feel kinda silly for not having done this on the thousands of pilot holes I've drill over the past twenty years. But, at least I know now. 



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Ian Anderson on Nov 16, 2017:

Definitely! I made up the table below to help folks understand this... https://handycrowd.com/wp-content/uploads/2014/05/screws-explained-from-handycrowd.com_.pdf

Les Tolson on Nov 07, 2017:

Hello my way is to drill a small hole all the way through both pieces then drill the first hole as you described. The first hole stops the wood splitting or you can put a bit of grease on the screw witch makes the screw go in better.


HWC on Jun 04, 2017:

Just take your phillips bit and drive it into your top piece about 1/2 - 1/3 the boards thickness. This will make an ample clearance hole. Its important for the 2 pieces to be held together tightly to prevent "jacking?" If you dont and even have spent half an hour courting this joint with holes youll get a bad joint. This lesson is most relevant to lag bolts.

Rey on Jun 01, 2017:

Thanks for the details about this error correction. This is another good reason to have three drills so you don't have to keep changing drill bits. One drills the pilot hole; the second drills the clearance hole; and the third has the screwdriver bit to put the pieces together.

Jerry on Mar 24, 2017:

I am a complete novice here but as I read this article my thoughts were to use the smaller bit as suggested by HUB KNUDGE. I've had this issue before and it never dawned on me what caused it. This is a great article and I am glad I came across it. I appreciate these responses as well.

Hub Knudge on Mar 22, 2017:

I'd drill both holes at once with the smaller drill bit sized for the screw, and then after you've located the precise position of the clearance hole, just enlarge it with a larger drill bit. Then you can be assured the pieces will align perfectly, since they were drilled at the same time.

Perry F. Townsend Sr on Mar 22, 2017:

Thanks for sharing this. I did not know this also. But I will start doing it this way from now on.