Thread: If you're going to break something, make them wonder how you did it

What brand of drill was this?

Regards
Keith

No idea, the bit is completely unmarked - not even the size. This was at many years ago at trade school, so I'm going to assume it was Chinesium of some sort

Do you know how it happened?

From the picture it looks like the twist was machined too deeply and the cutting edge caught on a hole drilled perpendicular to the hole being drilled.

But then it may have been a drill bit intended very rapid drilling in wood. And then the bit was used on steel.

I do know how it happened 'cos I did it.

It was a sharpening exercise at school. I ground it as I had been shown at work, put it in the drill press and basically as soon as it touched the timber it went ping.

The drill fracture shows it's made of an alloy that is too brittle, or it was too easily hardened to brittleness.

Not uncommon with cheap Chinese tools, they're not made from "proper" tool steel with the necessary strengthening and toughening ingredients such as chrome, vanadium, molydenum, cobalt, manganese and nickel.

A common "tool" steel used by Chinese manufacturers is S2 steel. S2 contains less than 0.5% Vanadium, 0.4% Molybdenum, and 0.4% Manganese - with no Chrome, Nickel or Cobalt in the alloy.
As a result, S2 steel is cheap, and does the job 90% of the time. It's the other 10%, which comprises the gut-busting work and heavy use, is where the S2 steel is found wanting.

The best tool steel I've found is the Bondhus Protanium steel - a secret alloy mix which Bondhus invented and patented, and which they guard possessively. The Protanium steel has 20% more tensile strength than any other alloy tool steel.

Grades of Steel

My favorite drill is the American Huot, 8% Cobalt steel drill. Cobalt gives incredible toughness and resistance to deformation, when heat buildup in the drill starts to affect its temper. Naturally, it always pays to use a lubricant when drilling steel.

Originally Posted by rrich

From the picture it looks like the twist was machined too deeply

On closer inspection, I think you might be on to something there. At the shank end, the web is just over 2mm, but at the cutting end it's down to less than 1mm (eyeballing it on a 1/2mm graduated ruler it looks to be 0.7-0.8mm).

The internet says that web thinning is a normal thing to make cutting easier, but on a 10mm bit the web should be around 13% (numbers vary slightly, depending on which article you read) of the diameter so they've ground it some 40% under size.

Originally Posted by elanjacobs

I do know how it happened 'cos I did it.

It was a sharpening exercise at school. I ground it as I had been shown at work, put it in the drill press and basically as soon as it touched the timber it went ping.

So I am guessing you ground a "Split point" or "pilot tip" and somehow managed to grind a sharp notch right into the thinnest part of the web.... Then yoink!!! Split right in half...

Originally Posted by truckjohn

So I am guessing you ground a "Split point" or "pilot tip" and somehow managed to grind a sharp notch right into the thinnest part of the web.... Then yoink!!! Split right in half...

Nope, just a plain twist grind

Elan, please don't carry guilt for this the rest of your life. My first glance at the image suggested that the spiral was machined way to deeply leaving an undersized web, and your dimensions provided support this.If the drill received a 'normal' heat with the narrow web, the temperatures in the web region would not be 'normal' during the heat cycle or quench cycle as the material volume is not normal. This would likely leave the drill with a very brittle web section, more susceptible to cracking and fatigue.

Then you state that you reground the drill to fairly standard twist geometry using techniques that you were taught at work, which should have been successful. However, if you needed to regrind the drill, it was most likely because it had previously been used and abused by others enough to need a touch up. That use and abuse most likely initiated a micro crack into the web, which than travelled through the web when you went to use the bit.

In the production process, it is common to do all of the machining, heat treatment etc, then do a final inspection on the item before stamping or labelling it. The reported lack of branding and size info on the shank suggests that it might have failed and been rejected at the final inspection, been classed as waste but diverted without official sanction and placed on the market by someone effectively seeking beer money post inspection, along with the rest of that production batch.

Elan, they had those drill bits when I was at TAFE. Pretty sure they came from Suttons and were all rejects freely donated to the schools for us to learn how to sharpen. If they were still around when you went there, that would mean there was ~500 apprentices chucking them around, dropping them, changing standard twist drills into brad points and back again etc etc. The poor bits have had a tough life