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

[Twindog]

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

How deep into the steel does that temperature effect go?

 

[HeavyHanded]

This topic is very interesting to me, not least because I have yet to find an English language copy of the oft referenced Landes study that details the methodology. Specifically how does one isolate the temp of the very apex from the temp of swarf that is in close proximity and just ground off (and been isolated from any other thermal mass that might distribute heat while being further abraded).

Just musing, it would seem to me that temps generated by hand sharpening dry or at low speed would not be an issue.

When chopping seasoned hardwood, my hatchet bit will become quite warm, hot actually. I have had it heat to the point of becoming uncomfortable to keep pressed against my forearm, yet it continued to cut fine and required no serious reconditioning when it was touched up. And not to mention the act of pulling the edge across fairly abrasive rope, triple wall cardboard etc etc etc, yet none of these lead to immediate loss of temper that is notable. The cardboard in particular can heat a blade to an easily detectable level when several feet are being rapidly cut.

I can only speculate how much heat is being generated at the apex when cutting some of these materials but must be comparable or even higher than what dry low speed grinding at moderate to light pressure and with abrasives in good shape will produce.

Note: I use the terms "musing", "speculation"

I have personally seen no evidence in my own not-so-scientific-cut tests that reasonable dry grinding causes any issues relative to wet grinding, but is also a question of what reasonable might encompass.

Edit to add:
I recently had a knife of M2 steel sharpened dry on SiC wet/dry outlast the same edge sharpened on a waterstone to similar finish. In that case I attributed the improvement to the SIC being a better choice of abrasive on that steel and so more cleanly grinding at the apex. Again, this is hardly scientific, but is consistent with my other observations.

 

[jungatheart]

Thanks OP, interesting.

 

[Cyrano]

... according to actual lab experiments with imbedded micro sensors, yeah....the temp at apex gets VERY hot, even on HAND grinding.

Can you please provide a link to the original source of these experiments? I've seen references to such work, but no pointers to original publications.

... 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 ...

I don't claim any expertise in machining or thermodynamic modelling, but I believe this question is far more complicated. A sheet of material can lose heat by means other than conduction in its own mass -- including convective cooling to the ambient air, and radiative cooling. In addition to these common mechanisms, grinding introduces other mechanisms for energy transfer away from the edge -- such as heated material being physically removed from the apex.

... 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.....

That's not in question. Basic physics tells us that, for a given amount of heat energy added, a smaller object will have a greater temperature rise than a larger object of the same material.

The question is: In the horse race between heat being added to the apex, and heat being removed from the apex, which horse wins, and by how much?

Given the number of books and scholarly articles published on thermal modelling of grinding, the answer is by no means easy for me to see.

 

[Cyrano]

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 ...

In my experiment, lacquer extended to the apex before grinding, and after. The material of concern -- the small region of steel which forms the apex after grinding -- is visibly covered with lacquer in the after-grinding photos.

Whatever material is ground off is of no concern, as that material is no longer part of the blade. Nonetheless, you'll find regions in the after-grinding photos where the lacquer is still visible on the burr, without indication of transition.

 

[scott.livesey]

How deep into the steel does that temperature effect go?

not sure. i guess you have to film the grinding with a hi def thermal camera. my point is you now have spots on your edge that are Rc30 instead of Rc60, spots that may bend or chip out and generally not be good. My personal motto when doing finishing grinding is Speed Kills. if you slow your grinder down to 300-500 fpm, less heat is generated. coolant placed on the belt tends to stay on the belt and not get flung in your face. higher grit belts last longer. i am testing a new setup for my Wilmont LB1000. 1 hp motor that runs at 850rpm spinning a 3" drive wheel, should be about 670fpm at 60Hz, 335fpm at 30hz.
scott

 

[Cyrano]

... 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 ...

I didn't see any evidence of transition in the green lacquer. To what are you referring?

If you're referring to the thin horizontal band of darker green in the upper-left-hand quadrant of the photo: That darkening is an artifact of the microscopy. In this region, you're seeing a region of lacquer where all its underlying steel has been ground away, but the lacquer itself retained some structural integrity. This die-hard region of lacquer formed a short, nearly-vertical wall. The darkening you see is the shadow cast by this short wall of lacquer.

 

[BluntCut MetalWorks]

Sparks are most likely from iron autoignition (basically oxidation at high rate), not due to kinetic to thermal interaction. So apex is not affected by fly-away sparks.

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

 

[BluntCut MetalWorks]

Cyrano, please re-run this experiment using hardened steel and high grit (says, 800 just to be sure) belt at max speed. Friction & burnish/impact have much higher coefficient of kinetic -> to -> thermal than fracture/abrading.

 

[bucketstove]

I didn't see any evidence of transition in the green lacquer. To what are you referring?If you're referring to the thin horizontal band of darker green in the upper-left-hand quadrant of the photo...

Hi again

How about to the right, and all the burr, all the non-green parts , what happened there?
Why aren't the non-green parts green, and what does that mean ?

Also what is the scale of the photos, how many pixels how many microns.
Any idea how thick the lacquer is?
Maybe you can apply the lacquer in vertical LINES?
Also dillute it cause the manufacturer says that shouldn't affect the color changing ability?
Have you tested the lacquery color change in an oven to make sure it works?

 

[me2]

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?

I have compared the same knives and found enough difference that I don't sharpen fine cutting blades on power equipment any more, except for a water cooled wheel at low rpm. If its a root cutting blade, well, that's not gonna stay sharp more than a couple cuts anyway.

 

[Bill DeShivs]

All those knife companies are doing it wrong, aren't they?

 

[unit]

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?

Question 1. Yes, I have tested edges on identical knives as you say.

Question 2. I can only speak for myself (as a former BladeSports competitor) and a handful of other BladeSports competitors. Some of us insisted on edges developed entirely with hand sharpening methods, and a few would use belt grinders for repair or reporfiling, but would finish on hand hones (by finish, I mean everything from approximately 600 grit and beyond was done by hand). Depending on how you define "Championship", I think most of the people I am discussing would qualify.

I can tell you the cuts I attended where attempts were made at world records, the knives used for those cuts were all finished by hand.

I would suggest that a VERY good edge can be honed on a knife with powered means. I would argue that the skill of the person doing the honing will be a VERY important factor regardless of honing/grinding method used. I would also suggest that many users will not notice a difference except in the more extreme cases. And I will practically guarantee that a far greater number people will claim to have the skills necessary to produce VERY good edge than the number of people that actually can

 

[me2]

Sharpeners skill is very important. An excellent edge off a belt sander with some overheating is better than a mediocre hand honed edge.

 

[samuraistuart]

All those knife companies are doing it wrong, aren't they?

A knife company that does quantity will probably always revert to powered sharpening. It's "good enough". And I can't recall how many times someone chimes in and replies how edge retention on a factory edge was not as good as it later became...after hand sharpening.

 

[samuraistuart]

Sharpeners skill is very important. An excellent edge off a belt sander with some overheating is better than a mediocre hand honed edge.

Bingo. It's not like we're talking absolutes here. Powered sharpening has it's place, and with some forethought as to what goes on during such, can be just the ticket for someone. Just saying, as I think was said previously, when it comes to knives like Japanese kitchen knives, with excellent steel, excellent heat treat, super thin geometry, hand sharpening is all I would think about doing. A garden knife, $10 Chinese folder, etc....powered sharpening is completely acceptable. There are just some situations that powered sharpening is not the best choice, and if any powered sharpening is to be done, it should be cooled down somehow, IMHO...IMS...YMMV.

 

[me2]

All those knife companies are doing it wrong, aren't they?

Let's not get ahead of ourselves. There are precious few knife companies that consistently deliver knives that are sharp in the first place, irrespective of overheating on dry power equipment. Wrong is such an absolute term. Let's say it could be better.

 

[Gaston444]

A knife company that does quantity will probably always revert to powered sharpening. It's "good enough". And I can't recall how many times someone chimes in and replies how edge retention on a factory edge was not as good as it later became...after hand sharpening.

Right on. And this is precisely what you never hear with Randalls, and Randall does make a point to point out they do the initial bevel by hand (even if a dull one).

They know the effect, and most companies just don't care, like they don't care to put on some actual sharpness.

Gaston

 

[Fred.Rowe]

I wet grind quite a few kitchen knives and have found the damage takes place along the thinnest section of the edge. While the edge still has a thick cross section the edge does not get damaged; when the edge approaches the final geometry the heat build up is almost instantaneous. This is with a spray mist being used. We are talking about a few microns depth at the very apex.
As stated above, if sparks are coming off the steel and the edge is thin, it will be annealed. The damage can usually be repaired if its not over done. I've done quite a bit of this as well.
Testing is good but it must match actual geometry and conditions found in the shop.

Fred

 

[Cyrano]

In response to suggestions from bluntcut, I repeated my experiment using real blade steel and a fine-grit belt.

This photo shows the effective magnification of my microscope at its highest setting (nominally 230X), using the calibration target provided with the scope:

At this magnification, the factory Scandi edge shows a microbevel of approximately 25 microns in width:

These photos show the temperature-sensitive lacquers applied to the factory edge, at nominal 230X magnification:

I sharpened the blade on the WSKO/BGA for 20 seconds, keeping the blade immobile, using firm pressure. I felt the blade get slightly warm, but nowhere near uncomfortably hot.

After this brief sharpening, all three areas of lacquer showed clear evidence of transition.

200 F lacquer at low magnification:

200 F lacquer at high magnification:

300 F lacquer at low magnification:

300 F lacquer at high magnification:

400 F lacquer at low magnification:

400 F lacquer at high magnification: