Hand grinding tool bits

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earlgo
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Hand grinding tool bits

Post by earlgo » Thu Jul 26, 2018 9:02 am

This is probably a trick all you machinists already know, but I tripped on it the other day. I was grinding a boring bit from a 5/16" square HSS blank and as you know it gets hot really fast. I happened to lay it on the mill table to cool off and it lit in a puddle of oil. It was cold in a very few seconds. This is probably less of a shock than dipping the hot bit in water. (I imagine one could also use any relatively large flat steel surface if your mill table is across the room.)
Also the cooling trick is much faster than waving the hot bit around in the air and surely sped up the grinding process.
Just a fun thing.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

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tornitore45
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Re: Hand grinding tool bits

Post by tornitore45 » Thu Jul 26, 2018 12:50 pm

My way
Either I dip it in water often, when is uncomfortably hot to hold, in that case the shock is not likely to generate microcracks.
OR if I have a lot to grind I put the bit in a holder (I have one for each size) and grind away, counting on the fact that HSS will not anneal even at red heat.
Mauro Gaetano
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SteveM
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Re: Hand grinding tool bits

Post by SteveM » Tue Jul 31, 2018 7:22 am

Placing it on aluminum will probably draw the heat out faster than on cast iron.

Steve

Magicniner
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Re: Hand grinding tool bits

Post by Magicniner » Thu Aug 02, 2018 10:47 am

The pedestal grinder in the first workshop where I did any tool grinding had a built in water pot, I can't remember but I think it was part of the stand.

John Hasler
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Re: Hand grinding tool bits

Post by John Hasler » Thu Aug 02, 2018 11:38 am

The last .0005" of a sharp edge could reach annealing temperature if the tool is red hot farther back. It would cool to red hot almost instantly when you lifted it away from the wheel and so might not be noticeable. Even if it doesn't anneal getting the tip of the tool red hot could induce enough stress to cause microcracking upon cooling.

Honing might suffice to remove the potentially annealed and/or microcracked area.

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tornitore45
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Re: Hand grinding tool bits

Post by tornitore45 » Fri Aug 03, 2018 6:55 am

John, good point. However I would not reach that point even when holding the bit with a "handle". Still have to lift, turn and look how the facet progress. If it gets to a blue color I will not dip it.
Mauro Gaetano
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John Hasler
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Re: Hand grinding tool bits

Post by John Hasler » Fri Aug 03, 2018 7:33 am

I don't think that the microcracking I speculated about would depend on the rate of cooling.

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Re: Hand grinding tool bits

Post by Harold_V » Fri Aug 03, 2018 3:35 pm

John Hasler wrote:
Fri Aug 03, 2018 7:33 am
I don't think that the microcracking I speculated about would depend on the rate of cooling.
It does. Heated, even to redness, if you allow HSS to air cool, it shouldn't experience that phenomenon. It also won't lose hardness, at least not enough to be detected. Rate of cooling is everything with HSS---which will easily withstand being heated to redness without losing hardness.

If I've missed something here, I'd appreciate being corrected.

H
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pete
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Re: Hand grinding tool bits

Post by pete » Mon Aug 13, 2018 6:34 pm

A Youtube poster with the name of James Kilroy proved that red heat on HSS by heating the tool with a O/A torch has no measurable effect on it. Many say that micro cracking isn't a problem with water cooling. However my hunch is there's something to it if the tool edge or tip does get hot enough before it's water dipped since I can't think of one single time when quickly thermal cycling anything is really good for it. I'm not comfortable thinking HSS can be treated any way you want while sharpening it and there's no effect. For the little it's worth my less than scientific tests seems to show at least some indications that not water cooling while sharpening "may" give me a bit more durable edge. And for all I know it's total hogwash, but I have read about leaving the tool alone after grinding for even a couple of weeks before a final hone will give a sharper and longer lasting edge. Maybe one day I'll give that a try. I can't think of any logical reasons that would be true though. I think I know enough to understand that there's a lot more going on at a microscopic level with cutting tools than we think, so who knows that might even have some truth to it?

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Re: Hand grinding tool bits

Post by John Hasler » Mon Aug 13, 2018 6:45 pm

I think that the problem with water cooling is that the last thou or so of the sharp edge, having a high surface to volume ratio, will cool much more quickly than the body. This means that it will try to shrink more than the body, creating thermal stress that could cause cracking.

Letting it set for weeks is hogwash.
Last edited by John Hasler on Mon Aug 13, 2018 6:47 pm, edited 1 time in total.

pete
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Re: Hand grinding tool bits

Post by pete » Mon Aug 13, 2018 7:09 pm

:-) I think your correct about both John.

Harold_V
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Re: Hand grinding tool bits

Post by Harold_V » Tue Aug 14, 2018 3:24 am

Not being a metallurgist, I can't address the idea of letting HSS sit for a couple weeks, but such activity is known to exist. For example, aluminum that is capable of being hardened and that has been solution annealed will harden with time. It's also known that time relaxes internal stresses in materials of many stripes.

Materials that age harden do so by the means of crystal growth within the material. Such material is known as precipitation hardening, an example being 17-4 PH stainless. There are others. I don't know that HSS has that ability. About the only thing that comes to mind in allowing it to age would be the relaxation of stresses, which may or may not be present.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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