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

I've sharpened thousands of blades on belt grinders and know as fact that the edges can be ruined pretty easily. I do a lot of blade grinding after hardening and with some years under my belt I know what pressure I can apply without damage to the edge or the thinner portions of the blade. I test all edges after grinding for how ductile they are and whether they chip or roll. This will tell you quickly the condition of the edge. No paint on colors, just years of acquired knowledge to come to a conclusion.

IMHO there are too many variables involved to make a declarative statement on whether an edge can survive a belt grinder. I think Bill is in this camp as well. There is just no substitute for decades of experience.

Testing is great and should be carried out by those knowledgeable in the field. People thought the earth was flat at one time; where would we be without having doubts?

Fred
 
bucketstove said:
Ok, but why is this difference important for the purposes of this discussion?
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Simply for readers who might be a little confused as to why a small amount of heat might change the temper on the edge of a blade. As was noted the discussion is about a very small amount of metal (small heat capacity) and it does not take much heat to change the temperature by a large amount. It is virtually impossible to say how much heat an object has if all you know is the temperature.
 
A short version my guess - apply to belt speed around 3000 FPM (or ~0.24m/s) slightly worn SiC 320+grit belt using resting/light-fingers pressure with edge trailing. Here is a rough threshold of expose/grind time where 0.5um of apex could exceed 200C.

For hardened steels - time in second for dynamic positioning (move blade along):

1.5 s: Simple < 0.8%C
1.3 s: Simple between 0.9%C and 1%C
1.0 s: Simple > 1.05%C
0.4-0.5 s: High alloy < 90%Fe

Time for static positioning will be shorter...
 
A long version dwell into kinetic -> thermal interaction (yep, friction + impact + abrasion), then into thermal conductivity of the blade material/steel. Grind direction plays a role whether important or not is 'depends'. Wet-grind effectiveness is also a 'depends' because water/fluid is ~60+ times less thermal conductive compare to steel.

edit: https://en.wikipedia.org/wiki/Heat_transfer
 
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*warning* thoughts -> low coherency -> text below!

The crucial 1um edge thick(base) bevel faces meet at the apex. This little wedge-oid is very small and surrounded 2 large sides by air. This geometry is isolated/shaped, e.g. 30*inclusive = 1/6 of directional plane. Therefore increase the distance due to bevel surface heating(or can look at as having lower temperature differences apex to thicker section), of which further reduce the conductive heat transfer. Thus easily having a heat up situation, especially relative to heat generated from powered sharpening.

When heat generated is faster than cooling, temperature rises. Note - cooling rate depends on temperature differences and distance. In sharpening, the amount of heat generated mostly due to kinetic (from moving objects - abrasives+swarf+blade) transfer to the blade. This transfer mechanism is a combination of key variables: rub/friction+impact+abrasion.

Fully populated surface of high grit (smaller abrasive) has more contact points with the blade than coarser/low grit. e.g. 1um has 4 times number of contact points than 2um grit. More contact = less pressure = less plough due to more carbide shielding the matrix = likely to generate more heat. Glazed/clogged abrasive surface has same affective as higher grit with lower affective hardness = rub more = more heat. Worse of all is very dull/rounded abrasive = lot of burnishing actions = fast heating.

...
 
bluntcut said:
Cyrano - Outstanding followup, thank you :thumbup:

I am a pretend-bonafide NASA re-entry module heat shielder(resume: my thick skin), let me cook up some cool air... hahahaha
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BC, you make good points in the above. There's a lot that goes into it when talking about abrasive size etc. The size not only determines the unit pressure (relative), but the amount of space for swarf between the abrasives, and depth of cut/size of chip. It is the size of the abrasive itself that regulates spacing/footprint of the mineral, although most maybe do not think of it this way. As the mineral size changes, the changes in unit pressure with the same amount of force are staggering.

Many thanks to Cyrano for doing this. It has the potential to define in better terms the risk factors. Heck, a small amount of this lacquer would be a real nice thing to have around for those that also have high powered magnification available. One could dial in best practices with a fair amount of confidence.
 
Nothing like some good internet pedantry.

Testing every assumption is what moves knowledge forward, and forums are a great place to share results that others can replicate, or refute.

With all due respect, decades of experience can often cloud one's perspective, as there maybe a dozen factors that are so in-grained to one's methods that they are no longer thought about. This can reduce one's ability to share knowledge as it becomes a literal case of having "forgotten more than I will ever know" the expert has not exactly forgotten that info, rather its just such an innate part of their thought process, they do not see it anymore. The other part of the problem with being an expert is that as one gains depth of expertise, they necessarily narrow their focus. This can happen consciously or unconsciously. Its why one can easily find very smart people saying very dumb things that are outside of their field of experience. This can also happen when one works under certain conditions for a long time. The person gets very used to their environment, and that adds to their knowledge or skill. Remove them from that, and they can struggle.

All this boils down to, it can be harder for experts to share info, because they often struggle to articulate the basic knowledge that builds a common ground between the expert and the novice. This frustrates both parties and leads nowhere.

I'm not saying that any of this applies to anyone here, just that these are common thought traps to fall into, and they limit our abilities to share information and to learn from each other.
 
Fred.Rowe said:
I've sharpened thousands of blades on belt grinders and know as fact that the edges can be ruined pretty easily. I do a lot of blade grinding after hardening and with some years under my belt I know what pressure I can apply without damage to the edge or the thinner portions of the blade. I test all edges after grinding for how ductile they are and whether they chip or roll. This will tell you quickly the condition of the edge. No paint on colors, just years of acquired knowledge to come to a conclusion.

IMHO there are too many variables involved to make a declarative statement on whether an edge can survive a belt grinder. I think Bill is in this camp as well. There is just no substitute for decades of experience.

Testing is great and should be carried out by those knowledgeable in the field. People thought the earth was flat at one time; where would we be without having doubts?

Fred
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Very interesting and well said. My impression is that variations in the final bevel temper condition dwarfs any considerations of steel type... Fortunately if there is something wrong it usually isn't very deep.

Gaston
 
ToddS said:
But the iceberg has more heat than the pot of boiling water.......
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:) yes, and its temperature is lower, so it doesn't melt. Heavier object, more energy than lighter object.
What would happen if you poured that one pot of boiling water onto a million tonne iceberg? Whole iceberg doesn't melt right :)

Consider a "firesteel" , its sparks burn at 3000 celsius, thats hotter than a pot of boiling water,
but the pot of water , even frozen, has more energy because its 1000 times (at least) heavier than sparks

Like Cyrano showed with the lacquer thermometer,
the temperature that changes the steel was reached at the apex (and beyond)
the tiny apex was damaged / burned / heat treated (again)
its going to take a few sharpenings to remove that damage
I guess more with Cyranos mora
than the usual 1-3 sharpenings reported for factory dry power sharpened blades

The definition of "heat" and "temperature" doesn't change what happens to the apex without coolant.

In verhoevens book "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel"
he explains that temperature is used as the guide for heat treatment.

I found on hypefreeblades where Roman Landes talks about measuring the damage from dry grinding
Re: Heating The Very Edge Whilst Sharpening
and there is a link to this image
Example of hardness drop, using a power grinder for sharpening without cooling!

Look at the first dot/data point in the graph and on the edge picture,
its is 0.05 mm or 50 microns away from apex
and it experienced 5 hardness points drop.
The apex is only about 1 microns and easy to damage in dry grinding.
It takes 0.3 mm or 300 microns away from apex to reach good steel / undamaged/original hardness steel That is about three full sharpenings.
 
bucketstove said:
I found on hypefreeblades where Roman Landes talks about measuring the damage from dry grinding
Re: Heating The Very Edge Whilst Sharpening
and there is a link to this image
Example of hardness drop, using a power grinder for sharpening without cooling!

Look at the first dot/data point in the graph and on the edge picture,
its is 0.05 mm or 50 microns away from apex
and it experienced 5 hardness points drop.
The apex is only about 1 microns and easy to damage in dry grinding.
It takes 0.3 mm or 300 microns away from apex to reach good steel / undamaged/original hardness steel That is about three full sharpenings.
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Read further in the thread and think about what Landes is proffering:

Roman Landes said:
The pic is taken from a professional gardening scissor.
Maybe a billon blades a year.
It is ground with a 220 grit trizack by hand

Just a little story asside,

one of our knife fellas in Germany was sharpening a standard kitchen chefs knife Solingen on a V rod ceramic stick pair.
By what ever circumstances, a little particle of metal bounced off whilst sharpening, form the blade....
He got it right into the eye, causing a great irritation, so he had to see the doctor.
The doctor removed the particle later on with the comment.
Please wear safety glasses next time you use the power grinder.
He encountered why the doctor thinks he used a power grinder.
An the docs answer was: he figured this, since the particle was burned into the surface of the eye.
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Regarding the shears-experiment:
The shears will be basic carbon steel, very little alloying elements that can improve hot hardness and reduce damage from such grinding.
There is no detail about the amount of pressure/time used to sharpen the shears - no detail of how much material needed to be removed - only a chart of some hardness measurements, and those measurements are essentially the same (i.e. undamaged steel) back 100 microns from the apex - the damaged steel is all within the first 100 microns, not the first 300. Their hardness tester gives readings between 650 & 675 for everything behind that first 100 microns. It would be helpful to know more about how the results were achieved since Landes is suggesting that crok-stik sharpening can result in burnt edges.... *shrug*

Shears.png
 
chiral.grolim said:
Read further in the thread and think about what Landes is proffering:



Regarding the shears-experiment:
The shears will be basic carbon steel, very little alloying elements that can improve hot hardness and reduce damage from such grinding.
There is no detail about the amount of pressure/time used to sharpen the shears - no detail of how much material needed to be removed - only a chart of some hardness measurements, and those measurements are essentially the same (i.e. undamaged steel) back 100 microns from the apex - the damaged steel is all within the first 100 microns, not the first 300. Their hardness tester gives readings between 650 & 675 for everything behind that first 100 microns. It would be helpful to know more about how the results were achieved since Landes is suggesting that crok-stik sharpening can result in burnt edges.... *shrug*

Shears.png
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Agreed and I for one appreciate constructive criticism &#128522;
 
bucketstove said:
Hi :)
Is that hard to believe?



Its constructive? In what way?
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It's constructive in that he is asking what the controls were and implying that without knowing the variables we do not really know much more than when we started.

For instance. One can (and people do, based off of landes research) argue that wet grinding (which I do, I have converter my 2x72 belt girder to be cooled with a coolant during grinding) will not damage the edge at all but nowhere that I've seen, and I've looked, has this topic been researched or discussed (i.e. Wet grinding vs dry grinding, with the same fresh grit belt and speed, etc with micro thermocouples at the apex and what difference is truly made with wet grinding vs dry) . Who is to say that wet grinding at x speed won't damage the temper and if so what pressures were used, how long did the knife stay on one spot on the belt, how dull were the abrasives, etc etc.
 
razor-edge-knives said:
It's constructive in that he is asking what the controls were and implying that without knowing the variables we do not really know much more than when we started.
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:) How much did we know before starting?

Ok, here is another link I've found thanks to your quoting it before hypefreeblades forums • View topic - Unwanted Tempering Whilst Sharpening
with bibliography

Thats about burning a few microns in hand grinding dry. Since edge is only about a micron, and damage is measurable ... my takeaway :) use water (at least) in hand grinding as well

razor-edge-knives said:
people argue, based off of landes research argue that wet grinding will not damage the edge at all ... Who is to say that wet grinding at x speed won't damage the temper..
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:D That is kind of a different question ... if the question is important maybe a simple test of edge retention ?

But I don't think the question is too important, unless coolant consumption is eating into your profits ... then it might be a good idea to pay for some micrographs ...

Roman Landes seems open to questions/discussion on hypefreeblades, so might be a good idea to ask directly for details.
 
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