I was underground recently and took this shot of a drill hole in a piece of granite. I first spotted it umpteen years ago and finally I have a half decent picture of it.
Rough diameter 65mm ish as best you can estimate on a pentagonal pattern that is.
What and how is the question.
I've dismissed the 'laser cut by aliens looking for Dilithium crystals' suggestion.
And by the way, that's the last time I take anyone called Ike underground..........!

🔗Personal-Album-342-Image-86261[linkphoto]Personal-Album-342-Image-86261[/linkphoto][/link]

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I believe it is something that can happen with softish ground and poor air pressure. The drill pushes in with a relatively low rotation speed.

I believe it is something that can happen with softish ground and poor air pressure. The drill pushes in with a relatively low rotation speed.

Could be. More often results in a triangular shaped hole, but could be the same kind of thing?

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The number of sides relates to the speed of rotation, hammer effect and the rate of advance. Once the shape is established it stays that way for a while rattling around in the misshaped hole.

Funny looking hole, if the hole was fired then you wouldn't normally expect it to have survived unless it was a socket ie: back end of the hole, could be the back end of a five hole drag cut although the 65mm diam doesn't seem to tally up?

Lozz.

These are often found near the start of a hole therefore in the stemming. They are quite common in a part of Geevor driven with arm and bar machines.

I believe it is something that can happen with softish ground and poor air pressure. The drill pushes in with a relatively low rotation speed.

I was under the impression that Granite was hard?

By your photo Roy,that rock does`nt look very thick,is that so?

The number of sides relates to the speed of rotation, hammer effect and the rate of advance. Once the shape is established it stays that way for a while rattling around in the misshaped hole.

To have that shape from using a holman drill,there would be no rotation.

Imagine the bit is fitted in the hole,it could never turn.

OK I give up, obviously easier to convince people it was done by aliens :glare:

Geoff could be right, just dug out an old drill steel (2ft starter circa 1972) the chisel bit is 35mm across, the hex shank is approx 22mm, the first 150 mm of the shank running back from the the bit head is around 20 to 21mm diameter and well rounded, anyone do the maths on that ref 65mm?
Geoffs description seems to bring back memories of a 303 jackleg rig, the whole rig rotating about its axis when starting a hole with a long steel, the shank rubbing on the side of the hole at the face, the circular grinding motion of the shank causing the begining of the hole to increase in diameter, if the rig is held at one postion for a short while a semi circular excavation can result and so on and so on, hard for me to explain in words but I now know what Geoff means.
Next question is how old is the hole, this might give a clue as to wether it was hand or machine driven, if it was machine driven then we are in the right direction.

Lozz.

I initially thought along similar lines but this lump of granite is good and hard and the pattern is perfectly straight from one side to the other. I too have seen odd shaped holes from drilling eccentricities, both hand and machine.
Each one of the five holes looks like its been drilled with fine precision, ie the sides of each are clean suggesting a slow feed speed. I'm
stumped, it looks as if someone has used a normal diy drill with an 18 mil or so masonry bit, accurately drilled five holes, and then carefully taken the center out.
Great suggestions guys but none really hit the mark. As for age, likely to be post WWII.

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Interesting, maybe it was done by some past mine explorers, would it be in Cligga by any chance?

Lozz.

Lozz PM sent.
I can't see anyone being able to achieve that sort of accuracy with a hand held drill. Each of the five holes are straight, true and parallel to one another. The lump is about 9" thick and looks as if it has slabbed off from somewhere (no signs anywhere) . The other side has the same dimensions, no deviation, and no sings of breaking out on the edges suggesting to me that it has broken away from a longer hole.

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I stand by my theory, so does David Ike 😉

.........David Ike ;D

:lol: :lol: :lol: :lol: :lol:

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The more I think about this the more I think that Geoff is right, years ago we were drilling at Crofty, part way in the drill hit an elvan dyke.
I remember we got through twice as many drill steels that we would normally use for drilling an 8ft hole in pure granite, it was slow going we had to have the Jackleg at full whack, this meant that the shank would rub at the hole in the vicinity of the collar, to counter act that we used to pull down on the whole rig to get the steel in the center of the hole, if left unattended then there was plenty of time for the drill shank to grind away parts of the hole in the length near the collar due to the interaction of the shank with the drill chippings and the slow advance speed due to the elvan.
Drilling an accurate 8ft cut required a certain amount of skill as the cut holes were closely spaced, if the drill "ran in" then that would be the end of it.
As has been suggested drilling holes seperated by a next to nothing wall thickness would be more or less impossible using the standard mining kit of the day.

So my updated theory is they were drilling granite and hit elvan part way in.

Lozz.

With a five sided drill??

I can see this happening on an airleg drill and no rotation, but I cannot remember coming across a three and half inch bull steel with five edges :confused: :confused:

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We're not talking about zero rotation but a normal chisel bit flapping around so it is rotating but is off centre. This is a well known phenomenon of which I could show you numerous examples, the effect running over quite long lengths in a hole. The thing being questioned is whether this particular hole is too regular and smooth sided to have been formed in this way. I don't think so, it has already been said just how difficult it would be to create this effect deliberately.

I mentioned soft ground in my initial response, it doesn't have to be that soft just enough friability to get it started, once the drill sets off in this way it will continue on that path for quite long lengths.

It's got to be the shank grinding away the side of the hole near the collar, can't see what else it can be?
It could also happen with a reamer once the reamer is well into the hole, if it was a reamer I would expect the said grooves in the hole shown in the picture to taper somewhat. according to Roy's description the grooves are more or less parallel throughout their length.
Maybe the driller did it on purpose just out of curiosity, it's the kind of thing I would of done just to make pretty patterns to break the boredom....!
The slab could have come adrift towards the end of the drilling and slid onto the drill steel, hence the preservation.

Never seen a five chisel star bit/reamer either, normally four or six.

(Edit: I wrote this post while Geoff posted his... apologies for any confusion)
Lozz.

according to Roy's description the grooves are more or less parallel throughout their length.

I could show you a place in Geevor where such grooves run totally parallel over a greater length than the thickness of the rock in question. These holes were drilled with blacksmith sharpened drill steels with a chisel bit.

I can imagine the path the end of the drill is taking but I'm not sure I have the time or ability to draw that....