I've always been told that round shot holes are from mechanical drills and triangular ones from hand held drills. Whilst we were down Blakeney stone mine last night we saw loads of triangular holes but we couldn't figure out why they were triangular.

Do humans naturally turn a drill bit through 120 degrees when drilling? If it comes from the shape of the bit the surely you would have to remove the drill fully from the hole in order to turn it?

It's probably obvious but.......

---

It's not that clear cut - round shotholes that are hand-drilled are commonplace, but I've also seen some triangular shotholes that are definitely early 20th century and drilled with a compressed air drill. AFAIK the triangular shape arises when rather than turning the drill on its centre, you swing it on a corner.

---

I am possibly completely off the ball but triangular holes seem to me to be likely to be from a machine drilling.
The 120 degree turn is quite a lot of twist for a person to put on and, more importantly you would have to do it incredibly consistently. So it seems likely to be a product of machine drilling with a 120 degree advance and a chisel bit.
I wonder if this was a something that was an issue with early rock drills as there aren't very many triangular holes. I would think that it could make getting the charge right a bit awkward.

---

I had always understood that it was to do with how much the 'bit' can jump about in the hole as it's struck. Triangular profile holes are more common when hand drilled due to the non-uniform and jiggly nature of the human arm/hand/bit combination.
Mechanically drilled triangular holes are generated by the same principal, with a slack chuck or wobbly operator leading to the bit jumping about more and cutting the hole into the reuleaux triangle type shape.

I don't believe the triangle shape itself is a product of the turning of the bit through 120deg. It is unlikely that a turning round bar will produce a slotted hole (2 corners) as that requires regular 180deg turns which a drill should not do and requires movement on effectively 1 dimension only. 4 corner, or square holes probably are not geometrically stable as the bit is not able to find the corner easily each time due to shallow angle and are more likely to wear the hole round again but be over sized. I would hypothesise that the rounded corner triangle is the only possible shape (other than round) that a freely moving round bit with a chisel ground head can produce when using a machine or operator that avoids striking at 180deg for each subsequent blow.

Although as I have no background in either drilling or mining I may be wildly off.

---

We thought that a human twisted drill would be more random so the hole would be rounder.

---

This has come up before. Haven't re-read the thread so can't say whether it is helpful. :)

https://www.aditnow.co.uk/community/viewtopic.aspx?t=6183 

Jumper....

http://cmhs.org.uk/wp-content/uploads/2012/10/Drills-In-The-Cleveland-Mines.pdf 

Lozz.

I've machine-drilled long shot holes that have turned out quite triangle-shaped in cross section.

Probably a side effect of a slightly bent drill-steel causing it to oscillate in time with the hammer blows. I think when I've seen it happen it was when using a - shaped head, I don't recall a + head ever doing it.

I've not drilled many holes by hand but those I have turned out pretty round.

---

It's quite easy to drill triangular holes in steel if your drill bit has been sharpened off-centre or is bent.
I always suspected the same is true in stone.

Chris.

We got a fit younger guy to try hand-drilling into an ironstone seam but it wasn't successful because the flat cutting bit was central, not at one side. Never managed to get back with a proper bit where the cutting edge is at one side. I must say that the hand-drill in question was specially made - it wasn't an original.

In an accessible Cleveland ironstone mine with workings dating from the 1870s most of the shot holes are triangular in section but one particular passage has holes which are circular and of larger diameter. According to a sale document the mine had one hand rotary drill and dozens of hand jumper drills.

Incidentally, if you thrust a jumper into a hole and twist it, you turn it 60 degrees (or thereabouts).

---

Another old post.


https://www.aditnow.co.uk/community/viewtopic.aspx?p=118634&txtSearch=oil+pump&lblnWhere=all&lblnMatch=exact#msg118634 

Another old topic.

Posted in error.

We got a fit younger guy to try hand-drilling into an ironstone seam but it wasn't successful because the flat cutting bit was central, not at one side. Never managed to get back with a proper bit where the cutting edge is at one side. I must say that the hand-drill in question was specially made - it wasn't an original.

In an accessible Cleveland ironstone mine with workings dating from the 1870s most of the shot holes are triangular in section but one particular passage has holes which are circular and of larger diameter. According to a sale document the mine had one hand rotary drill and dozens of hand jumper drills.

Incidentally, if you thrust a jumper into a hole and twist it, you turn it 60 degrees (or thereabouts).

Just a purely trigonometrical observation - Simon is the first to point out that to supposedly produce a triangular hole you only need to turn the drill 60 degrees, not 120.