Help me understand high speed grinding

[David farmer]

I have a strange theoretical question.
I've been a woodworker 30 years. I know sharpening on a high speed grinder works without softening tools if you keep them relatively cool. We all do it. It's clearly true. But, I have never understood how it can be, that as an abrasive particle rips off a piece of steel so fast it glows orange as a spark, the exact point where it separates from the steel left behind remains so cool it's temper is not drawn.
Does the separating glowing steel not heat up until it is fully detached? It seem as though at the moment the steel broke free the friction would stop so it must be heating while still connected.
I just can't seem to wrap my head around it. Dose anyone feel they can explain it?

[Mike Brady]

You have stated, more thoroughly than I can, why I never grind an edge.

[Patrick Chase]

I have a strange theoretical question.
I've been a woodworker 30 years. I know sharpening on a high speed grinder works without softening tools if you keep them relatively cool. We all do it. It's clearly true. But, I have never understood how it can be, that as an abrasive particle rips off a piece of steel so fast it glows orange as a spark, the exact point where it separates from the steel left behind remains so cool it's temper is not drawn.
Does the separating glowing steel not heat up until it is fully detached? It seem as though at the moment the steel broke free the friction would stop so it must be heating while still connected.
I just can't seem to wrap my head around it. Dose anyone feel they can explain it?

That's a very complex question, and one on which opinions vary somewhat. Here's a simplified answer:

• Cutting the workpiece releases heat via mechanical deformation, just as a coat hanger heats up when you bend it back and forth.
• The shaving is deformed more (by curling etc) than the surface from which it's cut, particularly if the abrasive is sharp and cutting cleanly. It therefore absorbs more heat than the part of the workpiece from which it's cut.
• Temperature rise is simply heat divided by mass
• The sparks are formed by bits of steel that have been ripped off of the workpiece. In addition to receiving the majority of the deformation energy their mass is extremely small, which means that the temperature rise from deforming them is quite large.
• The corresponding deformed parts of the workpiece are still part of the workpiece, so their temperature rise is much smaller even though they've absorbed a fair bit of heat. One way to think of this is that the workpiece acts as a heatsink for its own deformed surface.

In reality the workpiece isn't isothermal over the timescales involved, which is why you ideally want to keep the workpiece fairly cool to the touch such that even if its surface instantaneously becomes a fair bit warmer it still won't de-temper. Also, we remove the deformed bits of the surface anyway when we hone away the scratches after grinding.

EDIT: I ignored frictional heating for simplicity. Similar logic applies there - Most of the heat goes into the material being removed, and the underlying material is heat-sinked by the workpiece and doesn't see as much temperature rise.

[Patrick Chase]

You have stated, more thoroughly than I can, why I never grind an edge.

Do you understand how jet engines work despite having turbine inlet temperatures fairly close to the melting points of their materials? If not, do you let that prevent you from flying?

Grinding works, and it doesn't detemper the steel provided you follow some simple guidelines. The fact that you don't understand why that is so doesn't change the truth of the matter.

[David farmer]

Thank's Patrick!
Your simplified answer is just my size.
As I say, I have the faith from experience, I have just found my imagination lacking to explain it.
The lager deformation of the chip makes sense to me and I can imagine an overheated layer that remains on the work piece might be so thin as to be irrelevant to our sharpening.
The required instantaneous heat dissipation into the more massive piece still strains my brain but I think I can learn acceptance.
Now it will be easier to spring for that new CBN wheel.

[Warren Mickley]

It could well be that the steel particles coming off a grinder become somewhat hotter than the steel left behind on the tool. However this does not mean that the tool does not get hot enough for the temper to be affected. That is because the tool does not have to glow and turn red or orange (like the sparks) for the hardness to be affected. The tool does not have to turn blue or grey to have been damaged; the lesser colors could easily show damaged temper and even with no colors showing the tool could be damaged.

It would be difficult to measure the heat on the surface of the tool, the depth of damage to the tool, or the hardness at the very edge when the tool is ready to use. The most effective way to determine the condition at the tip of the tool is by judging the performance of the tool. At the level that really counts for an edge, analysis becomes extremely complicated.

Frankly there seems to be a rather high correlation between those who trumpet the safety of a high speed grinder and those who find the simper carbon steels inadequate or inferior for routine work.

[Patrick Chase]

Now it will be easier to spring for that new CBN wheel.

CBN and diamond are good precisely because the points of the abrasive stay sharp and cut cleanly. That in turn makes the simplified model I gave more valid, because there isn't as much heat from deformation of the underlying surface or from friction. IIRC CBN left ~1/3 as much heat in the workpiece as AlOx in one published experiment.

[Patrick Chase]

be a rather high correlation between those who trumpet the safety of a high speed grinder and those who find the simper carbon steels inadequate or inferior for routine work.

Err, if you actually look at the tempering tables as opposed to "theorizing" you'll find that non-HSS alloy steels like PM-V11 are no more heat-tolerant than HCS. They're typically sold in harder temper, so if anything tools made of non-HSS alloy steels are *less* tolerant overall.

For example PM-V11 at its Rc62 "as-sold" hardness loses temper at 350F. O1 is typically sold at Rc59 or so, and loses temper at >500F at that hardness. Those of us who use alloy steels (with the exception of HSS) have to be much more careful when grinding, not less, so your argument is totally backwards.

I think the true causal relationship here is "people who are are open to new data and can accept that something useful might have been invented in the last 2 centuries tend to be more accepting of both nontraditional steels and high-speed grinders".

[Patrick Chase]

Err, if you actually look at the tempering tables as opposed to "theorizing" you'll find that non-HSS alloy steels like PM-V11 are no more heat-tolerant than HCS. They're typically sold in harder temper, so if anything tools made of non-HSS alloy steels are *less* tolerant overall.

For example PM-V11 at its Rc62 "as-sold" hardness loses temper at 350F. O1 is typically sold at Rc59 or so, and loses temper at >500F at that hardness. Those of us who use alloy steels (with the exception of HSS) have to be much more careful when grinding, not less, so your argument is totally backwards.

I think the true causal relationship here is "people who are are open to new data and can accept that something useful might have been invented in the last 2 centuries tend to be more accepting of both nontraditional steels and high-speed grinders".

A quick look at the tempering tables also shows why we're advised not to grind super-hard Japanese white-steel tools: White steel (HCS) at Rc64 de-tempers at 300F, so there's not a whole lot of margin there. On top of that there's very little discoloration at 300F, so it's hard to tell when you've damaged the tool (PM-V11 also has this issue).

I think it's fair to say that you should know what you're doing and have some idea how tolerant your tools are before using a grinder. Like many tools they can cause destruction when misused.

[Warren Mickley]

I never said that pmvii or any other steel did not lose temper from grinding. Please read my post again! The way to determine if damage has occurred is by judging the performance of the tool.

[Normand Leblanc]

For example PM-V11 at its Rc62 "as-sold" hardness loses temper at 350F. O1 is typically sold at Rc59 or so, and loses temper at >500F at that hardness. Those of us who use alloy steels (with the exception of HSS) have to be much more careful when grinding

I wasn't aware of that so I'll be more careful in the future, thanks for this info Patrick.

[Patrick Chase]

I wasn't aware of that so I'll be more careful in the future, thanks for this info Patrick.

To be clear, we don't know what temperature LV uses to heat-treat and whether they refrigerate, and those both change the tempering schedule: https://cartech.ides.com/ImageDispla...pered+Hardness

The 350F number I cited is for Rc62.5 (LV's stated nominal for PM-V11 tools) when the steel is heat-treated 1900F and then refrigerated before tempering. If they heat-treated at 1950F and then refrigerated then it would be 400F. Either way you get into trouble at lower temperatures than for O1 at Rc59.

Given that LV doesn't caution against grinding I think it's safe to assume that they do refrigerate (because otherwise the tool would be extremely susceptible to detempering from Rc62.5), and I also think that it's more likely than not that they harden at the higher temperature.

[Kees Heiden]

Even when the steel is superficially detempered (those last few steel crystals at the utter end of the edge), you probably hone the edge after grinding and remove them on the first swipe over the stone. Good practice is not to grind to a sharp edge anyway, and leave the last little bit for the coarse bench stones.

And even when you have an Oops moment and raise some brown or even blue colors on the edge, don't loose too much sleep about it. The steel is now somewhat softer then before, more like the French liked to make their tools. Continue sharpening and using the tool and the "damaged" area will be gone after a while.

I grind everything I have, vintage, Hock O1, some A2, Japanese, and have yet to be confronted with a useless tool after grinding. I don't recommend to use an angle grinder though to correct weird bevels on mortise chisels. I have mixed results with that technique

[Frederick Skelly]

That's a very complex question, and one on which opinions vary somewhat. Here's a simplified answer:

• Cutting the workpiece releases heat via mechanical deformation, just as a coat hanger heats up when you bend it back and forth.
• The shaving is deformed more (by curling etc) than the surface from which it's cut, particularly if the abrasive is sharp and cutting cleanly. It therefore absorbs more heat than the part of the workpiece from which it's cut.
• Temperature rise is simply heat divided by mass
• The sparks are formed by bits of steel that have been ripped off of the workpiece. In addition to receiving the majority of the deformation energy their mass is extremely small, which means that the temperature rise from deforming them is quite large.
• The corresponding deformed parts of the workpiece are still part of the workpiece, so their temperature rise is much smaller even though they've absorbed a fair bit of heat. One way to think of this is that the workpiece acts as a heatsink for its own deformed surface.

In reality the workpiece isn't isothermal over the timescales involved, which is why you ideally want to keep the workpiece fairly cool to the touch such that even if its surface instantaneously becomes a fair bit warmer it still won't de-temper. Also, we remove the deformed bits of the surface anyway when we hone away the scratches after grinding.

EDIT: I ignored frictional heating for simplicity. Similar logic applies there - Most of the heat goes into the material being removed, and the underlying material is heat-sinked by the workpiece and doesn't see as much temperature rise.

Thanks Patrick! This was helpful.
Fred

[michael langman]

I don't know if this was said yet, but,..

Grinding wheels are designed to run cool and transfer heat. They do not conduct heat well.

Metal on the other hand conducts heat well, but the heat will always travel to the point of least resistance. In this case the heat is being pushed towards the edge of the tool being sharpened.
In the shop we used to call this heat transfer.

Carbon burns well, and the slag that comes off of the part being ground is the parts of the steel that is left over from the grinding.