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I have read this question about the shape of the jaws of callipers, but it didn't explain why there is a small notch at the end of the depth probe. You can see it on the far right of this image:

enter image description here

It could be as simple as allowing it to go into a smaller diameter hole without compromising stiffness, but on my callipers, it's still quite wide at the narrowest point. Is there another purpose?

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  • Probably for the same reason as the knife edges on the jaws. To correctly measure depth of a hole with a rounded bottom, like made with a drill bit.
    – crip659
    Feb 11 at 15:18
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    "It could be as simple as allowing it to go into a smaller diameter hole without compromising stiffness" I think that is your answer there. Though I looked at the datasheet for my Mitutoyo's and didn't see any mention of it.
    – Glen Yates
    Feb 11 at 15:21
  • Usually those tips are not that long, would only be good for very short holes.
    – crip659
    Feb 11 at 15:29
  • They are for something but I always thought that's just how you make a precision instrument that needs to meet in a corner. If you don't keep it clean your calibration will be off and it will eventually look like that anyway from filling it down so the jaws meet correctly. The same way drill press vices work (the corners never meet); if it bottoms out in the corner then it can't grab .008 sheet metal.
    – Mazura
    Feb 12 at 5:18
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    Thank you, @DanielHatton, I have accepted your suggestion and removed vernier.
    – Daniel
    Feb 14 at 10:03

1 Answer 1

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The narrowing allows you to avoid the rounding of an edge (e.g. of a lip) or the rounding of a conical hole bottom (e.g. of a drilled hole).

Or you can opt to deliberately measure the depth at the edge or bottom before any rounding.

This is also why the narrowed protrusion is offset to one side, e.g. in your picture it is aligned with the bottom of the gauge, and not centred.

A drilled hole's depth is generally the depth at full diameter, so it's measured along the wall. The blind hole is measured at its flat bottom, and any rounding (perhaps due to a fillet of grime or crud) at the bottom corner should be avoided. Important thing is to be aware of these subtleties in order to make the measurement that matters for the case at hand.

Depending on how it's held, you can aim for an offset or not and either way run it tight along the edge for a straight measurement.

enter image description here

enter image description here

Ref: https://www.stefanelli.eng.br/en/use-vernier-caliper/

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  • #TIL! I'm curious, though, for the blind hole measurement, how would you do it any other way?
    – FreeMan
    Feb 11 at 18:39
  • @FreeMan a drilled hole's depth is generally the depth at full diameter, so it's measured along the wall. The blind hole is measured at its flat bottom, and any rounding (or grime) at the bottom corner should be avoided. Important thing is to be aware of these subtleties in order to make the measurement that matters for the case at hand. Or perhaps I am eagerly writing what was already glaringly obvious to you...
    – P2000
    Feb 11 at 18:52
  • Ah! I see now. The notch in the depth rod goes over the crud making a fillet at the bottom of the hole. Nope this was not glaringly obvious to me. TYVM, +1 again, if I could.
    – FreeMan
    Feb 11 at 18:56
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    @FreeMan better than an extra +1 is the gift of a new word "a fillet is a rounding of an interior or exterior corner of a part design" which was precisely the word I was looking for...
    – P2000
    Feb 11 at 19:45
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    Well, there you go, we both learned something today!
    – FreeMan
    Feb 11 at 20:05

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