What bit to drill tool steel?

[Beanwood]

I was being lazy.
It was dark and raining, and I was in a hurry.
I was trying to get one more set of holes drilled before packing up for the night.

All of teh above excuses apply.

However, I was trying to dril a 4.2mm hole in some nice steel, for tapping m5 threads. It's quite bright shiny stuff - quite 'chrome' like all the way through, so I think maybe tool steel of some sort. Anyway, the drill was a little blunt, but I only had 4 more mm to go, so I lent on the handle.
Even with Rocol, it seemed to be getting a bit hot. Cue blunt drill, and shortly after a nice cherry red hot drill bit.
I let it cool, sharpened the drill - still no go. (The bit may be toast I suppose). Tried another sharp drill - nope - not going to cut it.
This makes me think I have tool steel (I hope), which I have heat tempered (Earlier than I'd hoped)

What do I need to drill this stuff in the current state please? Cobalt, Carbide or something else?

 

[Bornfree]

If you have nothing else a sharpened masonry bit will do it.

 

[Redsailor]

It sounds like stainless steel to me and you have work hardened it.

 

[Beanwood]

It sounds like stainless steel to me and you have work hardened it.

Agreed - but what to drill it with now.....

 

[Agroshield]

Agreed - but what to drill it with now.....

If it is too hard to drill with HSS, how will you tap it afterwards, no matter how you eventually make the hole?

Stainless is usually non- or very weakly magnetic, so that would be an easy test.

Carbide end mill would go through it but finding one in 4.2mm will be a challenge.

Turn it over and drill from there. The hardened layer will not be all that thick and as you are meeting it point to point, a standard bit might break through to the other side and open the doors of perception.

 

[Seadog]

Agreed - but what to drill it with now.....

And what are you going to tap it with?

 

[Redsailor]

Really sharp hss drill bit, cutting oil and drill slow speed with a lot of downforce might be worth a try.

 

[RobCox]

And what are you going to tap it with?

A quick google shows that M4 carbide taps are available.... €108!!!!

 

[Beanwood]

I might just change the design - drill a bigger hole with carbide, and tap it at M6 or M8...

It was after all only a really hot drill bit for a few seconds - so I imagine the extreme heat would be qute localised....

 

[Agroshield]

I might just change the design - drill a bigger hole with carbide...

It might be too small at 6mm, but you could bypass (go around rather than through) the hardened layer with a holesaw.

8mm might be a little small, but maybe drill 9mm or 10mm, chamfer both sides, loctite in a piece of solid material to plug the hole, peen the ends and disguise the join and then drill and tap it M5 as original.

If it is stainless, the cutting characteristics of a Fred Dibnah ratchet drill (slow speed, plenty of cutting pressure) is a good model for doing anything to it.

We do not want to see the next question: how do I remove a broken-off carbide tap from a hole?

 

[Beanwood]

OK - clearly I hadn't considered all of the implications - for the amount of work it takes to make this part, I'll just start again rather than try to rescue the current piece. (Effectively it's a 40mm long, 40mm diameter barrel with 6 holes and a couple of threads already cut - so not the end of the world - as long as I have another piece of stock.

Last question for now then - is it possible to 'un harden' it by using heat someway?? I can then use this as a low cost 'learning opportunity'

 

[Agroshield]

Last question for now then - is it possible to 'un harden' it by using heat someway?

In principle, yes. It is generally called annealing. A broad brush description would be heat up to a certain temperature, let it soak at that temperature for a certain time and then let it cool.

The specifics depend on the material: copper you can heat up to red and plunge into water; that will not work with carbon steel.

If you want to anneal a file, you heat it up until it loses magnetism and then let cool slowly (in the ashes of a wood fire or a bucket of vermiculite - cat litter (unused).

This is interesting reading on stainless, but might not be relevant to what you have:

How to Anneal Stainless Steel After Forging - Knife Steel Nerds

Stainless actually has more to benefit from forging than carbon steel. But even so, how do you heat treat it after?

knifesteelnerds.com

This is an accessible book on the subject:

01 Hardening, Tempering & Heat Treatment : Free Download, Borrow, and Streaming : Internet Archive

64 pg A comprehensive exposition of the structure of steels & effects of different heat treatments.

archive.org

 

[Kram]

A cobalt drill will be perfectly good to drill the part through the work hardening. I've not had problems with tapping afterwards. A red ring machine tap for hard metal wont have any problem, and I suspect neither would any decent machine tap as they are usually HSSE cobalt anyway.

Best not to use a old blunt HSS serial tap as the quality is much lower and their only goal in life is to snap in a hole.

The specifics depend on the material: copper you can heat up to red and plunge into water;

There is no requirement to plunge copper into water. The only benefit of that is to remove the oxide scale formed during heating. It makes zero difference to the hardness of the part.
Copper will also soften over time if its kept warm, say 200c. It does not have a magic curie point loke steel.

For basic carbon steels, you heat to the curie point of about 730c, let the temperature soak theough, and then slowely allow to cool.

Other kinds of steels can harden by slow cooling.

 

[Morrisman]

I might just change the design - drill a bigger hole with carbide, and tap it at M6 or M8...

It was after all only a really hot drill bit for a few seconds - so I imagine the extreme heat would be qute localised....

Drill it clearance size and use a nut and bolt.

I’ve drilled leaf springs before, immediately after being told by a dozen people that it’s impossible.

That was in my Pistonheads forum days, when the standard reply was negativity, no matter what the question

 

[Agroshield]

There is no requirement to plunge copper into water.

Nor did I state or imply there was. Please read the "can" in the post above as "it is possible for speed and if you do not want to burn your fingers".

 

[caprinut]

Carbide stub drill to drill it then an EDM to tap it.

 

[Beanwood]

Carbide stub drill to drill it then an EDM to tap it.

Thanks - but what's an EDM ?

 

[brightspark]

If you have nothing else a sharpened masonry bit will do it.

yea i put a proper cutting edge on them and can drill brick concrete steel and wood with them you can buy bosch multidrills thats what i use and touch them up on a green stone before they get 2 blunt

 

[caprinut]

Electric Discharge machine google it, used for making very precise small object in hard materials for industry and medicine.