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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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Andrew on May 10, 2020:

Very good tip. I usually do this, especially with hardwoods, and always with oak. I liked the tip about first drilling the pilot through both pieces to help with alignment after the clearance hole and glue and are ready for final connection. I also found with outdoor projects using redwood, cedar or treated woods- ie 2x4s and larger-that the pilot in the second piece and a slightly larger than screw diameter clearance hole will pull the connection and glue joint even tighter and better for exterior use.


Bobby on Apr 18, 2020:

Just as an extra bit of information that I picked up along the way: Shanks are often not threaded as a means of controlling friction heat. Ever taken a screw out of a piece of lumber, only to drop it immediately due to it being so hot? That's taking a screw out with little resistance.

Driving a screw into lumber generates a tremendous amount of heat and by leaving shank non-threaded, it is allowed to spin freely, dissipating some of that energy. Overheated screws are prone to snapping off. The other side of the heat issue is that of expansion. Metal expands when heated, so as you drive the screw deep into its' position, it expands along the way. As it cools off again, the metal contracts and you end up with a slightly loose fastener.

Great article. I enjoyed the read! Hope I've been able to share something new for you as well. Take care!


Asa Bender, PE on Mar 12, 2020:

This thread is a primary return while trying to look up a screw connection... I am posting to warn people because of that. If your just doing finish carpentry and don't care about strength, ignore this. This is a crippled joint based on the mechanical fastener... be careful how you use this. I would only use this to hold together a glue joint while it hardens, and then ignore the existance of the screw, similar to use of a clamp? The glue is a rigid enough connection that it will fail before the inherently loose screw donates any shear resistance (lateral). A firm contact is key, but the actual withdrawl value of the threads per inch are over twice as strong (per INCH) as the pullthrough strength of the head of a standard #10 screw. Unless your putting on a washer, your bringing the tensile separation resistance of that screw from 100% to about 30% on a 1" board (23% on a 1.5" board). You will also dramatically decrease lateral resistance (i would use a big fat zero for your connectors in shear with this installation). If it's just screws here, racking the boards will bend the screws inside the fully bored sheath in the side fastened element and rip the bottom grains in prying, everything will quickly lossen as you rock the connection, you'll be able to rip off the board at about 1/3 normal force etc. Any framer caught doing this on a project i had oversight or design responsibility on would be repremanded and probably wind up with either a pink slip or under the thumb of the master framer until further notice. HUD and FHA have laboratory testing on these sorts of connections showing them to be worthless.


Patrick on Mar 07, 2020:

i learned something new today too !
Someone else posted that it's common sense. I disagree.
Common sense is not so common anyway


Butch on Mar 01, 2020:

I'm guilty as well. Thank you! Now I know better and I'm glad I found this info.


Gary L Hammond on Feb 10, 2020:

Really that many people with common sense do not see this?


Tushar on Jan 31, 2020:

Wonderfully explained!!! Thank you!


Clay on Oct 03, 2019:

FYI

Little Tip, might not know about~for those special jobs~

Good woodworking~


ken on Sep 02, 2019:

good advice Ernie!!


Viv on Sep 01, 2019:

Thank you so much! This totally makes sense.


Ernie Wieber on Aug 21, 2019:

Great Observations.
Some screws come with threads that are meant to strip out the wood in the first layer to allow the head to pull the two pieced together.

Personally, if I have to drill clearance holes I do them last because the smaller pilot hole perfectly aligns the two boards better.

Also depending on the situation, I will drill the clearance hole just shy of going all the way through the top board, again It will align the two boards perfectly and help sink the head of the screw.

If you want the two boards super tight I will drill a small countersink on the bottom of the top board, that allows for some distortion as the screw digs into the bottom layer and pulls it up ever so slightly. The last method also works well with steel.


Sid on Aug 04, 2019:

What if we used a self drilling screws instead


Charles Stone on Jun 27, 2019:

Excellent dissertation.
I just did it wrong on a project, now you tell me!!!


Charles Stone on Jun 27, 2019:

Excellent dissertation.
I just did it wrong on a project, now you tell me!!!


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.