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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?

Wrong.     

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. This not only looks sloppy, but if you're trying to glue the joint together, there won't be enough contact and 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 drill bits. 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 screw (not including the threads). Since the bit is smaller, you can drill it right through the clearance hole in the top.

Alternatively, you can begin by drilling the pilot hole through both pieces, then ream out just the clearance hole in the top piece only with the larger bit. This is a great method if everything is super secure and clamped 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 you're 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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Moshe antebi on Apr 14, 2019:

Well guys, the simple way to do it, is to make the first pilot screw the screw almost to the and, than unscrew till you see the to pieces are joined and screw again firmly.


Fred Wright on Apr 09, 2019:

Damn brilliant


SRS on Mar 30, 2019:

Get a Kreg kit, all your problems are solved


Anonymous on Mar 27, 2019:

I could be wrong but it looks like you board isn’t cut straight. so the joint would never be flush, no matter the joinery technique.
And this day and age a countersink bit is about $5-10. So that defeats all the head scratching over the size bit to use. But good article on complicating things...!


Kerri Slayden on Mar 20, 2019:

Thanks so much for that!!! Just getting into woodworking and really enjoying it! So much to learn tho- and tired of being "taught" how to do something only to find out it's a tutorial on how to use their "gotta have" product! Thank you so much!


Gemma Bruce on Mar 08, 2019:

thanks you so much. i've screwed up so many time, having to back the screw and clamping everything . This is gonna help a lot.


Leo T on Mar 06, 2019:

I think the best joint is where the “clearance” hole is larger than the pilot hole but still slightly less than the thread diameter. It will draw up tight but has more holding power than just the head bearing on top. If the surface under the head decays the joint will loosen and shear capacity is also firmer


Enver Boyar on Feb 17, 2019:

Gayet güzel ve faydalı bilgiler verdiniz... Teşekkürler...


Geoffrey Carter on Jan 23, 2019:

the most drill important thing is to drill a countersink hole first so the top wood dose not split.a very thin bit always helps guide the screw into the second section .


George on Jan 22, 2019:

I beg to differ very slightly from your instructions. Yes—it's true that you should use two sizes of drill bits. However for the best accuracy in positioning first drill thru both pieces with the smaller pilot bit. Then enlarge the top piece with the larger clearance bit. This ensures greater concentricity.


George on Jan 22, 2019:

I beg to differ very slightly from your instructions. Yes—it's true that you should use two sizes of drill bits. However for the best accuracy in positioning first drill thru both pieces with the smaller pilot bit. Then enlarge the top piece with the larger clearance bit. This ensures greater concentricity.


Michael J McDonough on Jan 22, 2019:

Use pocket holes


reginald lonsdale on Jan 07, 2019:

people who make wood screws make different length shanks, which is the difference between the head and thread,take deck screws for example.


Russ on Jan 03, 2019:

Every time I have attached 2 pieces of wood using a screw, I have used fastener screws where the top bear the head is unthreaded. If this unthreaded section is at least as long as the width of the top piece of wood (as it should be) you will never have to drill any “clearance holes” or give any extra thought to it at all.


Willy R. on Dec 31, 2018:

Good tip. Worth following up on it. I feel such a dumb dumb after all the pilot holes I've drilled as well.
Good to point out why the unthreaded part of the screw is there in the first place.


Willy R. on Dec 31, 2018:

Good tip. Worth following up on it. I feel such a dumb dumb after all the pilot holes I've drilled as well.
Good to point out why the unthreaded part of the screw is there in the first place.


Nicholas Adams on Dec 30, 2018:

Thank you for the article but all the honors go to Bob Brenneck who gave the right way to do to avoid a jacked screw, and have a precision joint.


KFC on Dec 22, 2018:

i never thought about the non threaded screw but now i do.... thanks. I'll have to change out the screws i used which caused the jacking of my fence gate frame.


KFC on Dec 22, 2018:

i never thought about the unthreaded screw but now i do.... thanks. I'll have to change out the screws i used which caused the jacking of my fence gate frame.


Bob Brenneck on Dec 01, 2018:

50-60 years ago, my Dad taught me the dynamics of this joinery lesson. Drill a pilot hole the proper size for the screw end to be embedded into second piece. It should allow for the threads to get a bite into the wood (at least half the distance of the screw entering the bottom piece) as it is installed. Then drill a second hole through the top piece only. Sizing of the second or top hole should be one that the screw passes through easily, but snuggly. When drilling occurs the threads of the screw on the bottom piece will draw the top piece into it tightly.