What type of drill bit should I use for pilot holes?

Question

What type of drill bit should I use for creating pilot holes?

Should I use a brad point bit for creating pilot holes or a different type of bit?

Does it matter what drill bit I should use or is diameter of the drill bit the only important thing?

Answer 1

Drilling pilot holes is needed when the screws are large or it is on finished wood.
For example a #6 or #8 wood screw into a stud I probably would not drill a pilot into fur but oak studs that I have found in old Victorian era homes the screw would snap off if no pilot was drilled. The same is true with larger 1/4” lag bolts and screws, but I do start using pilot holes for studs at 1/4”.
To answer your question.
How do I size the pilot?
I hold the drill behind the screw and if I see the drill is wider than the body (not the threads) it is too large. I only want the pilot to take the amount out that is solid as this will reduce splitting. I do the same on finish work but wood screws the last 1/3 of the screw is not threaded. I want that pilot the size of the body where it is threaded.
This is a rule of thumb, sometimes on very soft wood a smaller drill is appropriate. Where the wood is hard like 100 year old oak, a larger bit will be needed. The last part is, if it is structural smaller is usually best but decorative larger again. This is to provide a pin without splitting.

I have been sizing wood screws like this since my dad taught me this over 50 years ago and it has served me well.

Answer 2

You won't want to use an auger bit, or a spade bit or a plug cutter. You won't want a spoon bit, a hole saw, or a glass/tile bit. You won't want a Forstner bit, a masonry bit or a step bit.

That pretty much leaves your standard twist bit or brad point bits. If sized correctly, either will work just fine.

As Ed noted in his answer, you want the bit to take out wood just where the solid portion of the body of the screw is, leaving wood for the threads themselves to cut into the wood. If you hold the screw up in front of the bit and you can see bit between the threads, it's too big. If you hold the bit in front of the screw and you can see the central shaft of the screw, it's too small.

I'm sure there are charts that will tell you exactly what size bit you need for each size screw, but it takes too long to go find 'em.

Answer 3

Agree with the comments made. Don't drill out too much. You want some 'meat' left for the screw to create a thread and grip.

If you want to get scientific you could use a tap & drill chart even though this is really for tapping holes in metals, rather than screwing into wood.

Nevertheless, the principle is the same. Leave enough behind to form the thread.

https://www.aimsindustrial.com.au/sutton-tapping-drill-wall-chart-free

Answer 4

A screw has a major diameter and a minor diameter

In metal it's simple there's very little wiggle room since metal doesn't stretch easily. The pilot hold must be just slightly larger than the minor diameter. There are drill sizes specifically mated to tap sizes.

In wood, sometimes when the screw is small in proportion to the wood, and the wood is soft and not prone to cracking, you don't even need a pilot hole. Sometimes you need just a little more give, so you drill a pilot hole quite a bit smaller than the minor diameter to make driving the screw easier but still get a lot of hold. For this purpose any drill bit will do.

Brad points have a sharp little tip and spurs on the sides

They work well in wood because they make a nice smooth hole and the point makes it easy to position the bit right where you want it with little tendency to walk - to move from that spot when you start the drill spinning.

Making cabinetry or furniture, you often have a piece of wood you've invested a LOT of time in before you assemble things with screws - you REALLY don't want it to crack. And if it's a hard wood, it's far more prone to cracking than soft wood. And you're probably using true wood screws, which have a tapered shank to hold better. There are special tools made for the purpose that have a tapered bit and even a cutter at the top to remove some wood so the tapered head will sit flush:

These bits have to be set up for the size and length of the screw.

When you're assembling two pieces with wood it's even a little more complicated: you'll want a pilot hole in the top piece that lets the screw pass through freely, and a tapered countersink; and a pilot hole in the bottom piece sized just right for the tapered wood screw.