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A friend of mine likes to buy old tools and smilar at car boots, and he picked up a set of bits which neither he nor his friends could identify. He showed me two of them:

mystery bits 1- a '60' and a '10' side by side

They have two tips, one mounted radially outboard of the centre tip but pointing parallel to it, offset by a gap. The gap varies through the set and the bits are numbered, I am guessing they correspond to the gap/offset distance.

Reverse side:

mystery bit 2- '60' reverse side

They look like they have (tapered) square shanks round shanks, but I will double check that with the chap who bought them.

If I was forced to guess, it looks like they are intended for scribing or cutting a circle? My non-exhaustive research didn't show up anything identical; the closest with two tips I could find was the adjustable bit on a DIY FAQ.

Update from the guy who bought them: "They have round shanks and are numbered from 10 to 64 all in a fitted box with the brass hand drill. They are made of steel which appears to be quite brittle as I broke one when I was trying to remove it from the box."

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  • I am messaging the chap† who bought the set to ask if they are square-shanked, if he knows what they are made of, and how many there are in the set. If there are other questions that would help clarify, please let me know and I will pass them on! †: this is one of those 'my friend XYZ' where there is an actual friend involved!
    – bertieb
    Nov 28, 2022 at 15:46
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    They do, in fact, look like hole cutters or circle scribes. However, the fact that there is so much metal "outboard" of the outer "cutter" makes me question that. Also, the point doesn't appear to be centered on the shank, so any attempt to spin this in a drill would scribe a small inner circle and a larger outer circle.
    – FreeMan
    Nov 28, 2022 at 15:50
  • Good eye on the 'centre' tip not actually being centred @FreeMan ! I agree, the metal outboard of the outer tip is extensive -- they actually remind me of keys somewhat, tho I may just have binged too many LPL videos -- unless that was done that way for ease of fabrication, say? Cast a 'blank', cut away the gap to size..?
    – bertieb
    Nov 28, 2022 at 15:56
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    Almost looks like fixed size scribes or marking gauges. Would be hard to use that way unless there were some sort of additional handle that your friend didn't get with the set. Do the 10 and 60 markings correlate to any known units of measure?
    – FreeMan
    Nov 28, 2022 at 15:59
  • I'll ask if he got anything else with them, but he didn't mention it at the time! Not sure on the markings, I didn't have calipers with me, but they weren't obviously metric (from a mk1 eyeball 'measure')
    – bertieb
    Nov 28, 2022 at 16:06

3 Answers 3

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Possibly thread chasing tools. i.e. not drill bits at all. The below image is of a replacement blade (probably two blades, by the look of it) for a thread-chasing tool.

[thread chasing tool blade image from https://th.misumi-ec.com/en/1

If, as it appears, the offset point is sharp and the center point is not, the non-edged point would follow a good thread to permit the edged point to cut and repair a damaged thread, offset by the thread pitch.

A problem for search is that there are many other styles of thread-chasing tools, so that was as close as I found to a similar one.

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  • Oh, interesting suggestion! I'll ask him if the outer point is sharp compared to the centre(ish) point. Are there any other features I could ask him about that would help confirm / refute your hypothesis?
    – bertieb
    Nov 28, 2022 at 16:36
  • Well, the "almost certainly not a drill" aspect can be confirmed by looking at the shape of the points end-on. From these pictures they appear far more "chisel-shaped" than drill-shaped.
    – Ecnerwal
    Nov 28, 2022 at 18:16
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They remind me of ACME thread gauges, used to check threads on a production line. Check if the angle is 29 degrees. Being brittle also makes sense as these types of gauges are typically hardened.

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  • interesting thought! any idea why they would come with a brass handle drill if they're for threads?
    – bertieb
    Dec 1, 2022 at 20:42
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My best guess is an equidistant center-punch.

  1. Strike the rod to make a mark
  2. Place the offset point into the punched divot
  3. Lift the offset point off the material so that you are resting on the rod
  4. Strike the rod to make your next mark
  5. Repeat as needed
  6. Use the brass drill to make holes

The only issue with my theory is that it doesn't hold up well to this observation:

They are made of steel which appears to be quite brittle as I broke one when I was trying to remove it from the box.

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  • interesting thought! there's definitely some physical purpose to the offset setup here, marking equidistant holes (or a set of them) could well be one
    – bertieb
    Dec 1, 2022 at 20:45

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