Drill all drills

Dave explained on 20/12/2009 :

Dunno if they are actually any good, but....

Any form of carbide will make a groove in a normal (gray or white) grinding wheel. A green wheel is needed to sharpen carbide.

You often see them demonstrated at Ideal Home exhibitions and Tool Fairs etc. - where they invariably show them drilling through hardened steel files, etc. I've got a set and they *do* work - and will go through most materials. They're quite useful in situations when you're not quite sure what you're going to encounter - like drilling above windows when you don't know whether the lintel is concrete or steel. But I wouldn't regard them as precision devices, and normally prefer the correct drill for the material - wood bit for wood, HSS for steel, SDS for concrete, etc.

Dave wibbled on Sunday 20 December 2009 17:20

What does cryogenically frozen mean when it freezes at 1495°C? Dunked in water, perhaps?

Cobalt-unobtainium alloy drill bits have been around for decades, but are a very niche market. Locksmithing is one user of them, for drilling hardplate in safes. Certainly the chap demonstrating drilling through files was at the model engineer exhibition 30 years ago.

They're less useful than you'd think. Just how often do you need to drill something that won't respond to a carbide masonry bit, or decent HSS ? I've used mine a few times (every couple of years), but always for demolishing stuff, not for fabrication. As always when drilling out broken drill stubs etc., wandering sideways into the softer metal is a huge problem.

The usual "cobalt" drills these days have a "cobalt" (sic) coating on them, giving a bluish rainbow look. They're cheap, Chinese, and made of a secret alloy of rubber and bananas. No earthly use for anything.

Andy Dingley wibbled on Sunday 20 December 2009 23:52

Yeah - I was thinking more about the claimed ability to drill various substrates. The Bosch drills do work, (limited depending on substrate, very well on others).

If I were the OP I'd buy the Bosch which has some record of usability before a random TV ad set.

Cheers

Tim

In desperation I tried these Bosch ones from screwfix to drill out a broken drill bit is a feed auger, it made no progress and then the tip disintegrated.

AJH

If you solidify metal quickly enough you can get metallic glass. (My metallurgy supervisor was working on this thirty years ago by dipping a thermally massive rotating wheel into molten metal, resulting in a ribbon of metallic glass whizzing off. I've no idea of the current state of the art.) Dropping hot cobalt into liquid nitrogen might do it, but whether cobalt glass is any use for anything, I can't say.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Jon Fairbairn saying something like:

Reminds me of dropping molten lead into a bucket of water. Weird spikey shapes.

I thought that was how they made perfectly round musket balls?

They would form and solidify during the long drop in air.

AJH

I've wondered about that in the past - surely they would be a bit "pear" shaped from their passage through air

Yes, I use them everyday. They are useful for being abe to drill through wood and masonry at the same time. They are pretty dire at wood and metal alone, taking twice as long as a normal drill, as they have a masonry style tip, so do not cut through the material, but rather just wear it away.

On masonry, they are good. Alan.

In article , geoff writes

Quick Google suggests they were also moulded.

Thinking about logistics, they would have needed to mass produce them. Wellington had 67,000 troops at waterloo, assuming they had 60 rounds each, thats around 4 million balls minimum.

It must have been a huge scale industrial production.

The Bosch are great for drilling through tiles and into masonry or wood beneath.

They are utterly useless on hard porcelain tiles.

No, you need an alloy with good surface tension, and a low viscosity fluid to fall through. This is why round shot is made by dropping through air until it's solid, rather than pouring directly into water.

"Pear" or "teardrop" shapes might be an efficient shape for streamlining (i.e. lower drag for a shape that's already that shape), which also implies it's the lowest energy shape and thus the shape a drop is encouraged to form into by the viscous drag. However at Reynold's numbers this low, there's not enough drag force (relative to surface tension) to force a drop into any shape, so surface tension predominates.

Swaged (nit-pickingly more specific than "moulding"). Not in the small sizes though. Dropped shot is cheap & simple.