G'day Chris here, and welcome back to Clickspring.
The existence of an Ancient style of drill bit
is strongly implied in the wreckage of the Antikythera Mechanism.
And its fabrication would have likely hinged on a crucial process:
The ability to take a malleable wrought iron,
and transform it into something hard enough to cut bronze.
I've covered the process in detail in a separate video,
but essentially it involves packing an animal based charcoal and salt around the soft iron,
sealing it in clay, and then heating it up to a red heat.
I'm using a low carbon mild steel as a substitute for Ancient wrought iron.
Its soft and ductile.
And it has insufficient carbon to be quench hardened to any meaningful degree.
So its a good modern equivalent, in that metallurgically, it has essentially the same properties.
As to the shaping of this sort of thin stock into suitable diameters for drill bits,
I'm going to cover that in detail in a later video on lathe technology.
But there are some great examples of long slender turning
to be found within the wreckage of the mechanism itself.
There's a shaft that extended from the lunar phase gearing
up to the phase display on the front dial that's roughly 4mm in diameter.
And there are numerous other pins and shafts, some less than a mm in diameter.
I'm using a modern furnace for convenience,
but a traditional charcoal furnace is quite capable of generating the required temperature.
Once at a red heat, the process of carburising the iron is purely a time exercise.
The longer the pack stays in the heat, the deeper the carbon penetration.
If it stays in long enough, in this case about 30 minutes,
the carbon makes it all of the way through the rods,
allowing them to be through hardened upon quenching.
Now of course to be accurate, a drill bit needs to be quite straight.
So I reheated the rods, rolled them straight on a flat surface, and then requenched.
The once soft metal is now very hard,
but also extremely brittle, and so easily fractures under a light bending load.
So to be of use as a cutting tool, it needs to be tempered,
in this case to a light straw color.
And for me to be able to see that color,
the black oxide layer needs to removed.
And I should point out that I've made the rods quite a bit longer than necessary
to provide a convenient handle during the shaping process.
But in use, the bits can be quite short.
I can see this end of the rod is a good straight section,
so that'll become the actual bit, and the excess will be removed at the end of the process.
A gentle heat raises the temperature of the rod until a pale straw color
just reaches what will become the cutting end.
The material is now in a tempered state.
Still very hard, but tough rather than brittle.
Now you'll have noticed at the start of the video
that the bit I'm making is very different to a modern twist drill.
Its often referred to as a pivot drill, or more commonly as a spade drill.
And it predates the invention of the twist drill by many hundreds,
and possibly, thousands of years.
Despite no longer being widely used, it still has a number of attributes to recommend it.
The first being that its very easy to make.
Because the primary surfaces can all be formed by hand, on flat abrasive stones.
The cutting edges can be shaped for unidirectional cutting,
or as in the case that I'm showing onscreen, for single direction cutting.
It can be further relieved at the sides if necessary,
or left as is for a more rigid cross section.
And all of this can be completed by eye in minutes,
with no need for measurements or specialty tools.
Its a remarkably simple, and yet highly effective cutting tool.
And once the operational length has been decided,
it can be permanently set by scoring the metal with a file,
and then snapping off the excess material.
I've tested this type of drill bit extensively in a hand powered drill,
that I made for the construction of the Antikythera Mechanism,
so be sure to check out that video.
But as a reference point, here's how it performs in modern powered tools.
In the lathe, it can be loaded into a tailstock chuck,
and then brought to bear on the work as per normal.
But because of its rigidity, if brought gently into initial contact,
unlike a twist drill, it can often operate as its own spotting drill.
In its rigid cross section form,
this sort of drill bit will often perform much like a traditional D bit.
Delivering a very straight deep hole.
Now there is of course an obvious drawback to the drill bit design:
The need to repeatedly withdraw it from the work to clear the chips.
But for this small negative, it does come with some excellent benefits.
Aside from being easy to make, it can be made in any size that you might need.
Secondly, the absence of flutes means that the cross section of the bit is solid.
So its comparatively rigid, and therefore quite precise.
Third, the bits really do excel at cutting brass and bronze,
but they're also good for a wide range of other materials.
Like wood, plastic and light alloy.
And if treated carefully they'll even comfortably drill mild steel.
And finally, they're dead easy to sharpen.
As soon as the cutting edges start to dull,
a quick hit on the stones will bring the m back to original condition.
Now you won't want to throw out your twist drills any time soon,
but its a great option to keep in mind for those rare occasions
when you really do need a custom drill bit.
Thanks for watching, I'll see you later.
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Thanks again for watching, I'll catch you on the next video.