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Mine Exploration Forum

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Author Shotholes
derrickman

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Joined: 18/02/2009

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Shotholes
Posted: 11/11/2011 09:54:13
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there is also a relationship between the rotation speed and frequency of blows which may, under certain circumstances, result in an approximately triangular excavation due to the blow falling at the same point each time.

However this tends to be eroded by the progressive action of drilling and doesn't appear in the final hole apart from sometimes being visible in the socket which remains after blasting - a trimmer hole may well leave a visible drill scar on the remaining face, but this will be part-circular in section.

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AR

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Location: Knot far from Knotlow in the middle of the Peak District

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Shotholes
Posted: 11/11/2011 10:43:45
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Yorkshireman wrote:



The DEMAG Handbook from 1952 mentions that triangular holes are created primarily when using chisel bits (?) with a single cutting edge as these have only little lateral guidance.

This applies to both manual drilling and the use of modern compressed air equipment.

In contrast, above all core drills (?) create clean and round shot holes.
Unquote

Not too sure about my translation of Meisselbohre = chisel bit and Kronenbohrer = core drill


That might explain the shothole I saw on Wednesday, as it was in a section of passage that had clearly been driven using compressed air drills and high explosive. It is curious that the others were round, but perhaps a badly-sharpened bit with unequal angles might behave like the single-edge bit described?

Otherwise, it just might be possible that the drill unit had broken down and the miners took the bit out and did a hole by hand using it, but I think that's highly unlikely.

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I am currently out of the office on leave and travelling through time but will respond to your message when I return last week.
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Buckhill

Joined: 08/04/2008

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Shotholes
Posted: 11/11/2011 16:49:08
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derrickman wrote:

Hand-drilled shot-holes are often triangular-ish because the steel is not rotating when the blow is struck, but rotated between blows.



Rock drills also do not rotate the bit during strike, there is a splined twist bar which turns the rod during the back stroke of the hammer only.

The piece of Coniston slate shown earlier appears to have the hole bored "cross-bate" so seems unlikely to have been formed triangular for cutting reasons. The Lakeland slate beds in general have two cleavage directions. "Bate", the main cleavage - i.e. the face of a slate - usually runs about parallel to strike and is usually highly inclined. The other - "cut" - is at right angles to this and near vertical, producing a coarse, but predictable, line of fracture. To work off a blast holes are drilled on line of cut and normal to bate. A faster drilling rate is made in this direction. It was often observed that a hole started slightly off normal would bend towards the correct line as it progressed - in the same manner that (unguided of course) rotary bored holes tend to turn towards normal to bedding in sedimentary strata.

Sometimes a "bate hole" would be drilled - horizontally across the back of the work in line with bate. These were slower to drill and only used if the cut holes weren't cleanly breaking the blast on bate. It was important to get the cut holes as near as possible on the line of cut so that they fractured in a straight continuous line. It is doubtful if triangular holes would have produced this if they weren't on cut whatever way the corners were pointing. That produced two breaks, width and depth, as seen when viewing the face, but the third, height, was not possible with explosives. That was only achieved by channelling or cutting with wire saws.
IP: 86.136.16.46 Edited: 11/11/2011 16:51:09 by Buckhill
peterrivington

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Shotholes
Posted: 13/11/2011 18:35:47
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Bloody Hell! I set out to write a mildly amusing piece asking why we spend so much time looking at shotholes. Many thanks for the information.

Here is a different method of making holes as described on an aural history tape. Tom Quirk was a fitter and turner apprentice at Roanhead from 1933 until 1937 when he left to finish his apprenticeship at Vickers.

AT Oh, is this a bull? Right, I have heard of those.
TQ Well, you used to get a piece of rectangular bar roughly about a foot long, get a hole about an inch from the end, or a bit more, and then take it to the blacksmiths and they used to draw it out under the steam hammer. This would be all one piece and when they were in this semi-hard ore where they didn’t need to drill they used to use this. They used to drive it in, somebody would hit it with a hammer and somebody would turn it and then just put a light charge in.
AT I see, that was preparatory to firing?
TQ Yes, instead of drilling.
AT Right, I see, and when it was drawn out what sort of length was it overall?....3Ft?
TQ 3 to 4 Ft
PT So they do a pattern of those instead of drilling? Presumably because it was a lot quicker
TQ Yes
AT They would fire off many holes at once then would they? And they would do a whole face?
TQ They would do a pattern with so many at the top, and so many in the middle and at the bottom......so that you got an archway torn through.



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peterrivington

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Shotholes
Posted: 04/03/2016 22:21:55
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(click image to open full size image in new window)
I came across this pentagonal example in a block of slate on Askam pier. There is a triangular hole on the left.
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