End Grain Drilling

Hi, I am planning to build a garden pergola. I want to drill 17mm END GRAIN holes into the 4"x4" posts and I need to buy a suitable 17mm drill. The holes will be about 200mm long so I plan to use a hex extension bar if necessary. What is the best type of bit to drill end grain; flat, auger, twist? Many thanks for any advice.

Personally I'd probably use a hand auger, because I have some. The tricky thing is that any of the ones with a conical leadscrew in the middle won't work as well as they might across the grain.

You could just use a flat ("spade") bit with a powered drill, but it's worth taking the time to make sure the cutting edges are as well honed as possible - all the cheap ones I've bought have been fairly not-sharp.

The same applies to a Forstner bit: get a decent one, and you might also find that makers that specialise in timber-framing tools will sell long ones. Fisch and Colt both make excellent drill bits for that field, and might go as large as 17mm.

If you use a power tool, and especially if the wood is wet, stop and clear the chips frequently and don't let the bit overheat. Once the harder cutting edge overheats, it will blunt quickly and ruin your day.

So... a Google search brought up this AI overview and puts forward an auger bit...slow to medium speed...

AI Overview

Drilling 17mm holes, 200mm deep into 4x4 posts requires a specialized, long-reach bit, specifically a 17mm (or 11/16") Auger Bit designed for wood. Due to the depth and diameter, an auger bit is essential to clear sawdust efficiently and prevent overheating.

Required Tools
Drill: A high-torque, 1/2" corded drill or a robust, heavy-duty brushless impact driver/drill.
Bit: 17mm x 200mm+ Auger Bit (17x230mm is standard, often with a hexagonal shaft).
Guide/Jig: A wooden guide block (made with a scrap piece and a drill press) or a portable drilling guide to ensure the hole is straight.
Support: Clamps to secure the 4x4 post.

Step-by-Step Instructions
Prep the Post: Clamp the 4x4 post securely to a work surface. Mark the exact center of your hole with a pencil or punch.
Create a Guide (Optional but Recommended): To ensure a perfectly straight, 200mm deep hole, take a 50mm-thick piece of scrap wood and drill a 17mm hole through it on a drill press. Clamp this "guide block" onto your 4x4 post, aligning the holes. This prevents the bit from drifting over the 200mm depth.
Start the Hole: Place the pilot screw point of the auger bit on your mark and start drilling at a slow to medium speed.
Drill and Clear Chips: As you drill, regularly back the bit out (every 20–30mm of depth) to clear the accumulated wood shavings (chips). If you don't clear the chips, the bit will jam, heat up, and potentially break or burn the wood.
Achieve Depth: Continue drilling until you reach 200mm.
(Alternative Method (Two-Sided): If you are going all the way through, drill halfway (100mm) from one side, then mark and drill from the opposite side to meet in the middle, ensuring a straight passage. )

Pro Tips for Success
Prevent Burnout: If the wood starts smoking, you are moving too fast or the bit is too hot. Let the bit cool down and clean the flutes.
Check Alignment: Use a speed square against the drill bit to verify you are perpendicular to the post.

Starting Technique: If the bit wanders, use the drill in reverse for a few seconds to create a starter indent.

Lubrication: Applying a small amount of beeswax or paraffin wax to the auger bit can reduce friction during deep drilling.

I would use a spade bit.

I'II third the spade bit.

Well yes I agree but perhaps a 5mm hole as deep as you can get it 100mm? Otherwise the spade will tend to wander.
Would it matter?

At some point in time I will need to do the same, drill end grain posts (sizes yet to decide), to build a cover from a wall across to the garden 'flower' bed for a covered area - leading to the workshop as I'm fed up of all this wet weather. The hole sizes for mine will be 12mm because I've some 12mm Stainless steel bar and corresponding holes in the brick boundary wall of the area/flower bed.

I'm wondering why 200mm into the posts as I'd considered 100mm, possibly 150mm max, into the post

Alignment might become an issue. I built this for end-grain boring, but it might be overkill for what you want.
S

You can get long series drill bits I have a 10mm 300mm one.

Pete

I used Star-M auger bits to drill holes in my shed posts. Made in Japan. Worked great. I first drilled a guide bushing in side grain in a wood block using a drill press. Then clamped that to the post bottom. You need that or the auger screw will follow any weakness in the end grain. After getting the hole started I removed the guide block since these augers self center with their sides. This one was 21 mm diameter.

I'm sure other techniques work, too.

Eric the Viking said:

Personally I'd probably use a hand auger, because I have some. The tricky thing is that any of the ones with a conical leadscrew in the middle won't work as well as they might across the grain.

You could just use a flat ("spade") bit with a powered drill, but it's worth taking the time to make sure the cutting edges are as well honed as possible - all the cheap ones I've bought have been fairly not-sharp.

The same applies to a Forstner bit: get a decent one, and you might also find that makers that specialise in timber-framing tools will sell long ones. Fisch and Colt both make excellent drill bits for that field, and might go as large as 17mm.

If you use a power tool, and especially if the wood is wet, stop and clear the chips frequently and don't let the bit overheat. Once the harder cutting edge overheats, it will blunt quickly and ruin your day.

Click to expand...

KeithV said:

Hi, I am planning to build a garden pergola. I want to drill 17mm END GRAIN holes into the 4"x4" posts and I need to buy a suitable 17mm drill. The holes will be about 200mm long so I plan to use a hex extension bar if necessary. What is the best type of bit to drill end grain; flat, auger, twist? Many thanks for any advice.

Click to expand...

Excellent. Many thanks to all the experts for such helpful advice.

The Hovering Pergola - Many thanks to all the Woodhaven experts who gave valued recommendations. Since the end grain drilling had to be accurate I used the long pilot hole suggestion. I made a simple jig using a straight section of steel angle and a wood block which had an accurately measured centre hole. I started a very short pilot hole at centre of the post and clamped the jig in position. By moving the jig progressively along the pilot hole was perfectly aligned. I used a 5mm x 300mm long pilot drill because it would be stiff and then I used a new 13mm 'flat' drill as recommended. See the image of the Bosch 'almost flat drill' which had a screw tip. The drill was very keen to pull hard into the end grain and it took some control to 'hold it back'. The result was exactly what I needed and the 4"x4" posts stood vertically when landed on the stud bar. I used a 3mm thick 3"x3" steel washer plate under the post to spread the load. Thanks again for the valued guidance.

Brill, glad to be of service.