Measuring the maximum temperature of an edge during belt sanding: initial results

[Cyrano]

In this thread on the CRK forum, I was warned that sharpening using a belt sander (like my WSKO/BGA) could easily ruin the heat treatment of a blade. It was claimed this could occur even though the blade did not feel hot to the touch, because of increased local heating at the edge apex.

In researching this question on these forums, I found many opinions, explanations, arguments, anecdotal evidence, and hearsay -- including repeated claims that an edge can easily reach 1000 F ! -- but I didn't find any credible experimental results.

To get a measurement of the actual maximum temperature reached at a blade edge during belt sanding, I performed the following experiment:

I created a test blade from a 2mm-thick bar of steel. I used the WSKO/BGA, with its coarsest belt, to create a convex apexed edge along most of the length of the bar.

I marked the edge of the bar with temperature-indicating lacquers, which undergo an irreversible physical change upon reaching a specified temperature.

This photo shows the lacquer as-applied, at low magnification. The purple lacquer has a transition point at 200 F, the red at 300 F, and the green at 400 F:

These photos show the lacquer as-applied, at high magnification, at the edge apex:

I then subjected the blade to aggressive grinding on the WSKO/BGA:

• I used the coarsest belt.
• I used the maximum belt speed.
• I pressed the blade into the belt with much more force than I typically use.
• I did not move the blade laterally to grind the entire edge, but instead kept the blade fixed in one position, to maximize heat build-up in the area marked by the lacquer.
• I held the blade in place, under pressure, immobile, until the blade became too hot to hold, which was approximately 60 seconds.

This aggressive grinding removed quite a bit of steel, and left a large burr:

The 200 F lacquer showed obvious transition, but there was no evidence of transition in the 300 F or 400 F lacquers:

From these results I conclude the steel remaining in the blade edge did not reach or exceed 300 F.

 

[Bobby3326]

For standard sharpening I'll dip the blade in cool water every now and then(more often for a thinner knife) just as a preventive. If I'm reprofiling and aggressively removing steel with the course or extra course belts I dip more frequently, I try not to let it get hot to the touch but it happens sometimes.

That being said I've never experienced ill effects from excessive heating . Even on cheap knives with bottom of the barrel steel I haven't had any issues, and those blades tend to heat up fairly quick. I wouldn't worry about it too much unless you begin seeing the steel change color, that'd be an indicator your edge is getting to hot.

Btw I'm also using the blade grinding attachment.


Fyi your pix aren't working, it says they're content protected by owner

 

[BluntCut MetalWorks]

Wow, good idea :thumbup:

Fine belt 220+ grit generates much more heat, especially a dull high grit belt. Please re-test using higher grit belts. Looking forward to seeing your results.

 

[Bill DeShivs]

And thinner, convex blades will heat up faster. I have sharpened a lot of carbon steel on a 2 X 72 belt grinder. You CAN overheat the edge if you don't know what you are doing.
As far as the tiny Worksharp or a 1 X 30 sander, it would be difficult to do-but I'm sure some of you could manage!
Carbon steel turns very visible colors when heated. Those colors correspond to definite temperatures.
Stainless steels would not be so affected.

 

[bucketstove]

... The 200 F lacquer showed obvious transition, but there was no evidence of transition in the 300 F or 400 F lacquers: From these results I conclude the steel remaining in the blade edge did not reach or exceed 300 F.

Hi,

the pictures aren't showing

The way I understand burned edges, its the apex that reaches +300F, the smallest piece of steel, the part that you're grinding off where the lacquer doesn't remain

If you dip the blade in water after grinding, is there steam? If there is steam that would mean it got overheated

 

[kelbro]

Can't see your pics... It displays 'content protected by owner'.

 

[Ernie1980]

Interesting idea, just fix your pictures so we can see the results:thumbup:

 

[BluntCut MetalWorks]

..
The way I understand burned edges, its the apex that reaches +300F, the smallest piece of steel, the part that you're grinding off where the lacquer doesn't remain

Examine the other side of the bevel.

If you dip the blade in water after grinding, is there steam? If there is steam that would mean it got overheated

Rate of evaporation is proportional to temperature gradient between air & water on the blade. So seeing some small amt of steam when blade bevel grinding is normal. However, for sharpening, the heated volume (sharpening bevel) supposed to be small and if steam is visible, well temperature could reached past the edge tempered temperature.

Note - for hss and other high alloy blades w/ secondary temper temperature, 'high' means exceed ~1000+F.

 

[Bobby3326]

Hi,

the pictures aren't showing

The way I understand burned edges, its the apex that reaches +300F, the smallest piece of steel, the part that you're grinding off where the lacquer doesn't remain

If you dip the blade in water after grinding, is there steam? If there is steam that would mean it got overheated

I never let mine get hot enough for there to be stream, I dip mine in water every couple of passes or if I start feeling it get warm.

To grind aimlessly assuming you're not nearing damaging temps would be foolish. Play it safe and give it dunk now and then, especially on thinner blades.

Also as mentioned the finer belts do tend to generate slightly more heat, especially if trying to remove more steel than the grit can handle.

 

[Twindog]

Awesome post. Awesome photos. Cool stuff.

 

[BluntCut MetalWorks]

OP - if possible, please test against a hardened steel (says a file knife or some hard stainless steel knife, just to be sure ) instead of mild welding steel (homedepot).

 

[WuGod]

Great test and thank you. But please address some of the issues presented. You the man.

 

[Bill DeShivs]

Steam is not an indicator of overheating. Water boils at 212 degrees.

 

[marthinus]

Here are some comments from R. Landes. Author of Messerklingen und Stahl and is a metallurgist and knife maker. I hold his work and approach to knife making in high regard.

Hi guys,

sorry for delay but i was out of office.

I read on another forums that Roman Landes said at ASHOKAN that he has found that sharpening a knife on a dry stone can induce unwanted tempering and lowering of the hardness in the vicinity of the edge and more so with abrasive belts.

Yes that is true.

Grinding generates energy (heat) and every step of sharpening is grinding even the strop.
This heat can be sucked away by the right coolant such as water.
If the grinding action lacks the coolant, the heat goes mostly into the torn out part and the body (blade/edge) it was torn off.
It becomes obvious to see the induced energy when you see the sparks fly (Burning steel!!).

Depending on how hard you go over the piece the more energy is induced the hotter it gets, thats basic physics.
Than the guys come and say but I can do it so sensitive that the edge will not suffer and I'm dipping the blade each run into cold water....
Well that is a nice effort, but when it comes down to the very edge this tiny fraction is overheated faster, than the eye can see or the wrinkled fingers can feel.
Unfortunately the edge becomes thinner the close you come to the very edge/point means generated heat will get jammed in the tip.
In addition to that tempering colors that would visually proof this are ground away immediately when they appear.
and Stainless steels need a higher temperature to generate tempering colors and longer time to build them up.
Nevertheless one can do metallurgical examination that can proof the issue testing micro hardness
There are some old german study's that examined this issue in the very detail.

And as you probably know Robert, every metallurgical sample preparation works the way, to avoid heat creation whist grinding.

I had a book dedicated to general grinding methods, in this book i found a test application.
A normal steel block apx. 2"x2"x4" that had a large number of highly sensitive thermocouples integrated in the surface.
The block was slit dry by hand over a 1000grit grinding paper.
The peak temps measured, walked up to 2000°C for split seconds in the very surface (some microns).
Of course the block did not melt since the volume fraction of induced heat was to tiny to affect such a large solid piece of steel.
But the effect was there and proofen.
In a edge we just talk about some microns of material, here the effect is solid an clear.
Every manufacturer of razorblades knows this and does excessive cooling whilst grinding and polishing edges, that need to hold an super sharp edge for very long.
It seems just some the magic makers out of the custom knife scene think, the physical principles like this, do not apply to them....

That being said. A few proper sharpenings with coolant etc can remove damaged steel. But keep in mind. We on the forum are a few between a thousand. Most people (speaking about the masses out there) will barely tell a difference in their edge retention because they dont really pay any attention to it. Nor do they really care.

 

[Twindog]

Bluntcut made some good obervations -- the test would be better if the OP used actual hardened steel and finer grits where heat buildup will more likely be a factor.

Still, the OP ground the living heck out of that steel and didn't appear to get much heat.

If grinding an edge off a belt or stone grinder is a problem, then virtually every factory knife and most custom knives are going to have faulty edges.

I've been experimenting with a small belt grinder, and I haven't seen any heat build up that would even approach a level of concern, although I'm using techniques that are designed to avoid heat build up. And, of course, I have no way to measure heat at the very apex.

I also had a large 3V chopper resharpened on a belt by a custom builder, and that edge has stood the test of time.

I'd never argue steel with Marthinus -- much less Landes -- but my sense is that you'd almost have to try to overheat an edge with a belt grinder for there to be a problem.

Lately, I've been reprofiling with a belt grinder and then using a Wicked Edge for finishing and refining the new profile. I can't tell any difference between that approach and my old system, where I did all reprofiling with a flat stone.

 

[Gaston444]

That being said. A few proper sharpenings with coolant etc can remove damaged steel. But keep in mind. We on the forum are a few between a thousand. Most people (speaking about the masses out there) will barely tell a difference in their edge retention because they dont really pay any attention to it. Nor do they really care.

If you sharpen a big knife at 12-14 degrees per side and use that to chop into Maple, the difference in "solidity" of the final few overheated microns is really, really obvious (Chopping wood with a thin power-finished edge is not at all, it seems to me, the difficult to observe effect of normal gradual wear from slicing soft materials): The power-finished edge easily creates wire edges, easily chips and even crumbles... It is not subtle at all, but requires high energy impact use at very thin angles.

The effect is very shallow: The effect is obvious when re-sharpening without power tools just once or twice to an even thinner angle: Say from 14 degrees with observed crumbling (after power tool finishing) to something like 12 degrees (by hand) and no observed crumbling, the fragility of the apex seems to immediately go away despite the thinner edge. This is not scientific of course, but it is reasonable to suppose that the overall edge strength is deeply dependent on the cohesion of the last microns at the apex, at least for impact work on wood.

Gaston

 

[samuraistuart]

I appreciate your approach, OP, to the subject at hand, but I just don't think that temp laquers have any where NEAR the ability to detect the temp increase AT the apex of an edge. We are talking the VERY apex of an edge that, according to actual lab experiments with imbedded micro sensors, yeah....the temp at apex gets VERY hot, even on HAND grinding. That is why guys who are super adept at sharpening will tell you the best edge retention comes with hand sharpening...lubed.

It is the case that you can sharpen a blade and not perceive that you are overheating an apex, because the heat is extremely localized at the apex, and does not have the time to move back into the mass of the blade.

To me it is very easy to see that something that is a micron or two or three or four in cross section can EASILY have heat transferred to it, enough heat to do damage to a temper.....NOT the entire edge bevel....but the VERY apex.

I used to employ powered grinding. One of the knives we used constantly was the sheep's foot single blade from Swiss Army. We loved that style for tearing down boxes. We had a paper wheel system, and the edges we put on using that system did not last as long as edges we sharpened on a stone. My belief is that the very apex was being overheated a bit. Not enough to feel at all in the blade mass itself.

And it is always good to remember, not all knives are tempered at 300°F-400°F. Some of the higher alloy stuff is tempered much higher, 900°F and above. Not that it matters much, when you're reaching 2000°C instantly.

Just to stress, the heat Roman is talking about is at the apex of an edge, and extremely precise technology is needed to gauge it, and it's not like we're talking about ruining the whole temper of an edge. Just that very apex.

 

[HeavyHanded]

Very clever testing!

The presence of green lacquer even on the burr remnants is pretty convincing. While it appears the temp came close enough to cause some transition, this is all steel that would be removed in the progression anyway.

In the course of doing regrinds I have gotten steel hot enough to make a bit of noise and feel a slight bit of vibration when it was dipped in water, I have never seen steam come off.

I guess one test that might even be a necessary precursor would be apply some lacquer to a very thin piece of steel or even Aluminum foil, and pass over an alcohol flame of some other of a known temp. Then see how much time it takes to effect a change or if it is indeed instantaneous.

Thanks for thinking of and doing this.

 

[Twindog]

If the effect of power grinding is at only the very tip of the apex, the lacquer method won't work because it will be ground off.

Does anyone know whether power sharpening vs hand sharpening has been tested in two identical knives, with the chopping/cutting ability of the edge compared afterwards?

Do the Blade Championship contestants hand sharpen their knives for a competitive advantage?

 

[scott.livesey]

there is a very simple observation to make. did you make sparks? if you made sparks, somewhere in the area you were grinding had localized temperatures greater than 1300F. if you are using a basic carbon steel, 1084, 52100, or O1, you now have a spot that has been annealed.
scott