Thread: Large drill bits

Start with a smaller drill.

John

What Wimmera Jack said, using a Brad point drill bit as a pilot

30mm dont try that with hand held drill (dont ask how I know) again even a 5-7mm drill bit as starter.

In timber yeah?
You can pre drill through a 300 mm x 19 mm length of timber or steel with a drill press if possible and use that piece placed over your wanted position and clamp it down. A clamp at each end and maybe even folded sandpaper in between to help stop movement.
Then drill your hole. It’s known as a dummy plate.

A forstner or Saw tooth bit if you have it would work ok.
H.

stevenjd

Are we talking in timber or metal and what sort of drill? Twist bit, forstner bit, spade bit or brad point. I am assuming for the moment that it is a twist bit as you mention centering and the other drills are automatically self centering and do not require a pilot hole. So that is in metal as twist bits are not the best bet for timber particularly in larger sizes? If metal, Wimmera Jack and Dinosaur are on the money with a small pilot hole first. Rotate the drill backwards by hand in the drill press to aid centering on the pilot hole.

Regards
Paul

PS: Drilling a 30mm hole in steel needs a drill press and a drill vice securely clamped.

I doubt large-diameter bits would be able to drill from a centre punch mark. I doubt any manufacturer would expect anyone to attempt it.

There’s not much of a point on a bit that large. The two flutes meet in an edge (core thickness) that’s probably much larger than your centre punch mark.

Multiple holes are required for such a hole diameter. Begin tiny (eg. 3 mm or smaller) and build up in stages. It doesn’t take long to change a bit. They will last longer not having to do so much work, and it won’t tax your drill press.

Actually, when precision is required I carefully mark the position for the hole, as we do. I have an old scribe (that I accidentally stuffed up). As you can imagine, it has a very sharp point and I use it to make an intent on the surface. It’s surprisingly easy to do just by pressing on it or with the slightest tap with a hammer. The centre punch can then find that mark easily simply by feeling for it.

It's fairly common to build up the hole size from something smaller, as has been noted. However, I was looking at a video of part of the process in a steel fabrication workshop. As you know, some fairly large bolts are often used in construction. The video shows a magnetic drill press being attached to a UC and some holes being drilled.

Things to note: no centre punch mark was used, simply a chalk mark, and the cutter looks more like a milling tool than a drill bit. It was difficult to pick a perfectly in-focus shot from the video, but the cutter looks as though it has a point. It would want to have, to ensure the hole was positioned accurately.

The point is - dump the drill bit idea and consider a mill tool instead (providing the tool has a point).

Drill.JPG

The video.

That’s a brooch they use in magnetic drills.
I used to do what you recommend start small and go up in steps.
The toolmaker whose Arboga I was using asked me why.
All you need is a small hole to clear the point as you say and then drill to your finished size.
Assuming of course the machine has enough grunt and the bit is sharp, and you are using the correct coolant and speed.
H

Thanks for that.

I spotted a video using an annular cutter. Interestingly, these things cut at the circumference only and use a centre pin to help with alignment. They seem very nice.

Thanks for the tips. I was thinking on that occasion when you might be drilling a hole using say a 40mm forstner bit.
I use a pilot hole to start the Forstner bit as it stands and when you are in a bit of a hurry drilling …..10 large dia
holes Forstner bit holes it is a very slow process if you don”t have a digital readout or similar.

TIA

Originally Posted by stevenjd

Thanks for the tips. I was thinking on that occasion when you might be drilling a hole using say a 40mm forstner bit.
I use a pilot hole to start the Forstner bit as it stands and when you are in a bit of a hurry drilling …..10 large dia
holes Forstner bit holes it is a very slow process if you don”t have a digital readout or similar.

TIA

I don't understand why you would drill a pilot hole for a Forstner bit. Depending on the application you could use a spade bit which will cut faster and longer than a cheap Forstner bit... until it doesn't. 40mm is getting up into hole saw territory but we don't know the application for the depth.

+1 you don't need a pilot hole for a foster bit. the tip in the middle is what stops it wandering off.

but it sounds like you're trying to HOG out a heap of the centre material and finish it off with a forstnet bit.

at 40mm you could even look at using a router and a template if you're trying to just hog out a ton of material and not drill al the way through.



but yeah, explaing what YOU ARE ACTUALLY TRYING TO ACHIEVE would help offer more tailered solutions

Originally Posted by ErrolFlynn

Thanks for that.

I spotted a video using an annular cutter. Interestingly, these things cut at the circumference only and use a centre pin to help with alignment. They seem very nice.

that pin also helps punch the slug out after cutting.

Originally Posted by mic-d

Depending on the application you could use a spade bit which will cut faster and longer than a cheap Forstner bit... until it doesn't.

A spade bit is probably the easiest of all drill bits to sharpen. Those with a spur at each tip are a little more tricky. I try to avoid those or just file them off. I use old triangular saw files. The cheapest sets often require sharpening out of the packet, but then become quite passable.

Regards
Paul