In my experience, Geoff is correct. That hole was probably made with a four edge bit that steps round giving a larger five sided hole, same as a two edge gives a three sided hole.
Nothing to do with mining but there is a Swiss engineering gadget that does similar for forming hexagons and other unusual shaped holes in metal. It works on the same principle.

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....

Hello Geoff, did you ever see the long large circular chunk of granite down on 15 near 15 incline level station, it was circular, like a column, it looked like a load of holes were drilled very closely together on a fixed circle of around 18" diameter, I can't be the only one that saw it, I remember asking Bob Orchard at the time of how and why but I can't really remember what he said...! very accurate drilling though, from memory I would estimate the thickness of the wall material between the holes to be about an inch, the whole thing looked like a fluted corinthian column, I last saw it near where the Queens "buggie" was.
So to recap the above: imagine drawing a circle on the rock face then using a standard diameter chisel or star bit to drill a series of very closely spaced holes all centered on the circumferance so there is a complete ring of holes on the circumference then lightly popping the whole thing with a small controlled blast and then pulling the resultant granite lump out of the hole, that's what it looked like, I definately remember it and not dreaming it...

Lozz.

Hi Lozz, yes you did see that. A man access was needed at the top of the incline but there was too much sensitive equipment installed so no blasting. Nigel Barnes drilled that out, hung it off a tirfor to support it as the last bits were broken free. A very clever piece of work.

Hi Lozz, yes you did see that. A man access was needed at the top of the incline but there was too much sensitive equipment installed so no blasting. Nigel Barnes drilled that out, hung it off a tirfor to support it as the last bits were broken free. A very clever piece of work.

Brill, thanks Geoff, Nigel Barnes now rings a bell, it was indeed a very clever piece of work, initially while trying to recall the said lump I had thoughts of it being drilled off by the track header for some reason.
I don't think we have ever met but I remember Dave Wright talking about you when I worked at Geevor.

Thanks for that.

Lozz.

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]

OK, But 5 ??? can't be got from 2, 4, or 6 pionted drills.??

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I might be wrong in all this but I am assuming it's the "wandering" shank that has caused the effect shown and not the bit as such?

Lozz.

Terry, it can!
Think how three comes from two.
Triangular holes are quite common in Derbyshire.
A two pointed chisel tip pivots to make three points, same only different with five from four.

I might be wrong in all this but I am assuming it's the "wandering" shank that has caused the effect shown and not the bit as such?

Lozz.

Most shanks I have come across are Hexagonal, the angles don't match, don't matter how much it "wanders" it won't make a pentagonal hole as regular as that.

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Terry, it can!
Think how three comes from two.
Triangular holes are quite common in Derbyshire.
A two pointed chisel tip pivots to make three points, same only different with five from four.

Hi Les, When you get a triangular hole from a two point (180 degree) drill, one side of the steel will get twice the hammer, no where near the uniformity of this photo.

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I might be wrong in all this but I am assuming it's the "wandering" shank that has caused the effect shown and not the bit as such?

Lozz.

Most shanks I have come across are Hexagonal, the angles don't match, don't matter how much it "wanders" it won't make a pantagonal hole as regular as that.

Yes the shanks when new are hex, and yes the shank can, it can make semi circular grooves in the hole if it is wandering.
For semi circular grooves the maximum number of grooves attainable would be dictated by the bit circumferance divide by the shank diameter, I have in my posession an old hex shanked standard single chisel bit rock drill circa 1972 fro South Crofty, the tungsten insert measures 35 mm cross, it's circumferance is 110 mm, the hex shank measures 22mm across the flats, 110 over 22 equals 5, this means that in theory that the hole can accomodate 5 semicircular grooves made by shank abrasion centered around the holes initial circumferance made by the bit.
I think the question now is to establish wether the grooves were formed by the above method or by the the bit itself kicking around in the hole, Geoffs explanation in softer granite would hold true, in tombstone granite it might be more difficult, however not impossible if the bit was biased eg: a chunk of the tungsten tip had broken away from one side of center.
I'm running out of ideas now, good thread though by Roy.

Lozz.

Best I can suggest is an offset 3 point star drill, hand hammer driven.

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Terry, it can!
Think how three comes from two.
Triangular holes are quite common in Derbyshire.
A two pointed chisel tip pivots to make three points, same only different with five from four.

Hi Les, When you get a triangular hole from a two point (180 degree) drill, one side of the steel will get twice the hammer, no where near the uniformity of this photo.

I've seen some very uniform looking triangular holes in Welsh metal mines - they resemble near-perfect equilateral triangles but with rounded corners. There was correspondence on the subject in the Welsh Mines Society newsletter about 15 years ago but at that time the effects of the number of chisel faces on the bit was neglected in favour of speculation that the circular shank of the hand drill steels had flopped around in the hole. From posts up-thread its clear that the mechanism is rather different - even though the above posts were concerned with compressed air drilling and hexagonal shank steels rather than hand drilling and circular shank steels.

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.

This gets my vote.

There is a very similar shot hole in Wheal Frementor - a five pointed star, although the Frementor hole does spiral slightly. The hole is in granite.

This foxed me for years. Terry Worthington explained it to my satisfaction: "rose bit" and low air pressure.

I can see me having to make another trip u/g to take some definitive measurements and some better photos.
I've seen triangular holes before, and even with a twist drill bit drilling in steel if the bit is ground a little off centre.
Nevertheless, it remains a good talking point if nothing else.

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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 can confirm this is the case, except that its more likely in hard ground, soft ground is unlikely to hold the shape like that. there is no rotation and the five point bit just beats its way in.
note this is from querying holes like it at CSM test mine with one of the old Holmans boys (a certain Mr Edwards...) who worked there for Holmans!

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 can confirm this is the case, except that its more likely in hard ground, soft ground is unlikely to hold the shape like that. there is no rotation and the five point bit just beats its way in.
note this is from querying holes like it at CSM test mine with one of the old Holmans boys (a certain Mr Edwards...) who worked there for Holmans!

That is the only possible outcome,any rotation would knock them corners off.

This effect was created with a chisel bit that was rotating, the number of points will relate to where the drill hammered relative to the rotation position and coincident subsequent blows in the same position. With a combination of low air and softish ground the effect may begin but then is maintained even if the ground hardens. Consider the large diameter of the overall shape and see how a smaller bit can move around within it. When one edge of the bit touches the widest outer part of the shape the other side touches the smallest diameter portion.

I will try to get underground and take some photos, that being a place machine drilled without tungsten carbide, quite likely round drill steel.

This link provides some thoughts, there is a summary if you scroll down to the bottom of the page.


http://www.stonestructures.org/html/star-holes.html 

Lozz.

And for some other comments.....

http://www.grahamhancock.com/phorum/read.php?f=1&i=238131&t=237831&v=f 

Lozz.

Anything to do with Graham Hancock will be utter self-deluding bollocks.