Strange drill bits

[Staffan Hamala]

I've been cleaning a lot of drill bits that I've purchased at online auctions. I came across two strange bits that I'm wondering about.

I'm attaching a photo. At first I thought there was a piece missing, but then I've noticed that the part "missing" (the left side in the photo) seems to be superfluous. When I've tried drilling, that part is actually is outside of the hole. It seems to function as a depth stop.

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On the bit to the left, the "depth stop" is about 5 mm down. The other has the stop about 1 mm down.

Does anyone know what these were used for?

[Dan Hintz]

Looks like a custom file to me, but what do I know...

It would seem to defeat the purpose of a deep cutting edge to put a stop on it that's just past the centering point, so I kinda doubt that's what it is.

[keith micinski]

It looks to me like that would leave a counter bore if sharpened.

[Peter Evans]

Look something like centre bits, but there are a lot of small differences - and then the big differecne to the cutting edge. My first thought was they are modified centre bits - but I doubt this is the case. What were they used for?

Here is a typical centre bit.
Cheers
Peter

[Staffan Hamala]

One thought I had was that they were larger drill bits, that have been filed down. However, I don't think that's it, as the spur is perfectly lined up with the cutting edge when drilling. It's just a part of the metal that goes outside of the hole when drilling.

A few more photos..

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This is after drilling until the "depth stop" makes the cutting edge stop. It just won't go further.

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If I go on rotating the brace with more pressure, the "depth stop" finally cuts a shallow counter bore. It's not sharpened, however, so this is slow going.

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The other drill bit makes a deeper hole, but here it has stopped cutting.

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Pushing on can make this also cut a counter bore..

[Staffan Hamala]

An interesting thing about the second bit is that the center point is shaped like a spoon bit. It cuts much faster than the other bit does.

[Eric Brown]

Perhaps these bits were used to countersink a flat recess for washers?
I envision using your strange bit first followed by a though hole for the bolt.
They are curious.

Eric

[Dan Andrews]

I agree with Kieth. I think the "depth" stop is for a counterbore, and needs sharpening. Maybe George or Harry with chime in on this.

[keith micinski]

I agree with me also. Now that I see the photo that has to be to make a flat counterbore for a washer. Good call Eric.

[harry strasil]

My thought would be pre forstner bit coin bits. I think forstner bits for the brace came about around 1828 about the same time as book matches.


Jr.