# Cooling tools when sharpening.



## Rorschach (29 Aug 2020)

Watching a recent Rob Cosman video prompted this PSA.

If you are grinding HSS tools, be that lathe tools (wood or metal) or drill bits, you don't need to cool them or worry about "blueing" them. HSS doesn't lose it's temper like high carbon tool steels. If you are holding the tools in bare hands you might want to dunk them in water for comfort but if the tool is cool enough to hold, don't dunk it, no need.
Makes sharpening drill bits a lot easier when you don't keep stopping to dunk them, you won't lose your angle.


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## Inspector (29 Aug 2020)

Don't dunk a hot HSS tool in water either . The thermal shock can cause micro fractures in the edges of the metal, making them more susceptible to dulling faster. Set them aside to air cool.

Pete


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## Trevanion (29 Aug 2020)

I actually have a 10mm aluminium plate next to the grinder that I leave hot stuff on to cool off fairly quickly rather than using water.


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## Droogs (29 Aug 2020)

Trevanion said:


> I actually have a 10mm aluminium plate next to the grinder that I leave hot stuff on to cool off fairly quickly rather than using water.


that's quite ingenious trevanion, i shall copy that and attribute it to marcros


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## Rorschach (29 Aug 2020)

Thick aluminium quenching plates are often used by high end knife makers on specialist steels.


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## TheTiddles (29 Aug 2020)

That doesn’t sound quite right, I’ve managed to overheat HSS tools in the past (turning tools and router bits) sufficiently enough to turn the edge blue, it’s certainly harder to do than carbon tool steel but I think it still happens.

Did he mention which grade? There’s quite a few, that and the name “high speed” was used over 100 years ago when high speed was a lot slower than we can easily obtain these days.

I’d certainly agree that dunking hot metal into cold water is going to stress the cutting edge, I’d not do it for that reason.

Aidan


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## Rorschach (29 Aug 2020)

Blueing still occurs, the blue colour comes from oxidation on the surface of the steel at certain temperatures. What the blue does not indicate is the hardness of the steel. On carbon steel, the temperature at which blueing occurs is high enough to draw the temper and make it soft or rather softer than you want. On HSS the blue colour doesn't matter, the composition of the steel is such that drawing it's temper requires long soaks at particular temperatures. So in short HSS will still blue, but it doesn't affect the cutting edge.


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## TheTiddles (29 Aug 2020)

Ah yes, I see what he means now, don’t worry about them going blue, not that you can’t, gotcha

Aidan


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## Rorschach (29 Aug 2020)

TheTiddles said:


> Ah yes, I see what he means now, don’t worry about them going blue, not that you can’t, gotcha
> 
> Aidan



You were correct in one thing you said though, HSS doesn't blue as easily as carbon steel, the composition of the alloy means the oxidation occurs at a higher temperature so when it does go blue it is much hotter than a piece of carbon steel would be at the same colour. But since the blueing doesn't matter anyway you don't need to worry.


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## Trevanion (29 Aug 2020)

So when does it start to lose it's temper? When it's been badly ground and it's got its knickers all in a twist about it?


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## Rorschach (29 Aug 2020)

Trevanion said:


> So when does it start to lose it's temper? When it's been badly ground and it's got its knickers all in a twist about it?



 You have to tell it you are going to make it drill stainless with no lubricant at the incorrect speed for it's size.


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## --Tom-- (29 Aug 2020)

High speed steels have 2 tempering peaks the second usually giving more toughness at the expense of corrosion resistance. If you go over the second peak you will start to lose hardness, so you can overheat HSS but it takes a much higher heat to do







At the temperature steel blues you’re still more than 200 degrees C away from softening it though


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## Rorschach (30 Aug 2020)

Note that your colour chart is for carbon steel, the colour temps are different for HSS but general point holds.


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## --Tom-- (30 Aug 2020)

Rorschach said:


> Note that your colour chart is for carbon steel, the colour temps are different for HSS but general point holds.


Cheers - yeah couldn’t quickly find a hss colour chart so was lazy but it’s not far out


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## AES (6 Sep 2020)

Thanks for that Tom. Hope it's OK, I've down loaded those 2 jpegs and saved them for my own use. Very useful.


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