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This is a drill bit I got in a kit that included brad point bits, masonry bits and regular drill bits, all without instructions or labels.

At first, I thought the ridges were meant to make screw threads like a tap bit, but the ridges don't extend beyond the approx 1/4" diameter of the screw.

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What kind of drill bit is this? What's it for?

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That's sometimes called a "drill-saw" bit used for drilling a hole and then enlarging the hole into a slot (or other shape) by applying sideways pressure.

I have a couple of cheap ones and didn't find them very effective in softwood. To make a long slot (1ft or 300mm) I had to drill lots of adjacent holes conventionally and then used this type of bit to join the holes. If I had to do it again, I'd consider buying a cheap router for the job. A hand jab-saw/pad-saw/keyhole-saw might be quicker and might be a similar amount of effort.

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  • Also known as a "saw bit".
    – isherwood
    Feb 5, 2019 at 15:12
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The last picture here shows it. I have never personally used one though.

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  • Link-only answers aren't considered valuable on SE because they go extinct and leave nothing of value in the answer. At least mention the "saw bit" part here.
    – isherwood
    Feb 5, 2019 at 15:10

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