Does Sharpening with a Grinder Ruin Your Edge?

Ocelot85 said:
Good article Larrin.

I only use a belt grinder when working on a badly chipped edge or when a edge bevel needs an aggressive re-profiling, after that I move to my Tormek T-7 or my Edge Pro for final sharpening depending on the persons preferences or intended use.

I would like to get wootzblade wootzblade commentary and insight into this article, considering their expertise and research into the subject.
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Larrin's article is one of those that draw knife people from the bliss of ignorance to the discomfort of voluntary ignorance. Not everyone is willing to change his sharpening methods in view of new facts, let alone the equipment.

We take the edge overheating seriously, as we hone on slotted paper and felt wheels and strive for lasting sharpness.
Judicious sharpening with a belt grinder and honing with fine belts and paper wheels is safe provided that the blade feed rate is right.
I based my estimate for 10" paper wheels that we use, on the below study, and safe is pulling the blade across the paper wheel at a feed rate approx. 10cm per 1 second on full speed grinder/buffer, and 5cm on half-speed.

W B Rowe “Temperature case studies in grinding including an inclined heat source model.” School of Engineering, Liverpool John Moores University UK
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.895.1681&rep=rep1&type=pdf

One has to interpret the grinding speed and the workpiece feed rate in their experiments to pulling the knife blade across the belt on his belt grinder, to ensure the sub 200C (400 F) heating.
E.g. the blade feed rate not to overheat the edge on 10" paper wheels on a half-speed buffer is not the same as for your belt grinders.
For a given grinding/honing speed there is a certain safe feed rate, and if you pull the blade slower than that across your belt or wheel, the friction becomes detrimental to the edge hardness/temper.
 
Aidenag, actually my shop does have heat, I just don't keep it heated all of the time. My shop is located right behind a rental property of mine several miles from where I live. For me it's much more convenient for me to sharpen my knives where I make them, even taking my kitchen knives to the shop every once in a while isn't a big deal. As far as the initial edge on a new knife I cut the initial bevels and a 120 grit belt running really slow with water sprayed on the belt, then go to a 320 grit belt and repeat the process again very slowly. I hone the final edge on a surgical black Arkansas then final strop on green chrome rouge, never had any problems with edge retention.
 
It may not be practical, but I would be interested to see them test the grinders with a cold air gun directed at the point of contact. We've used them in certain situations machining where liquid coolant isn't desirable / feasible where I work.

https://www.vortec.com/frost-free-cold-air-guns/overview

Vortec claims air temperature as low as -30F being produced at point of contact.
 
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Larrin said:
I added some awesome microhardness measurements of edges to the article that Roman Landes sent me.
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Do you know the speed the belt was moving in his tests? Assuming a fast speed, I would love to see the same tests done at a slow speed, and then again with cooling.
 
bmilleker said:
Do you know the speed the belt was moving in his tests? Assuming a fast speed, I would love to see the same tests done at a slow speed, and then again with cooling.
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Unfortunately, no.
 
I wonder if there is a machinist / knife maker / evil genius with a large shop compressor already, who may be willing to test the cold air gun on the grinder. Nathan the Machinist Nathan the Machinist

My employer would most likely frown on me bringing my belt grinder to work and confiscating their cold air gun in the name of blade science.
 
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Rick Marchand said:
I get my bevels pretty close with my 2x72 and set the final edge with a 600 CBN plate. The next step is where I'll probably get some head scratches. I do my final polish/strop on a slotted paper wheel. The slots blow air onto the blade. You can put your bare finger on it and it won't burn. Very light pressure.

I get consistently sharper and longer lasting edges, than when I strop by hand. Unless I'm using my high grit water stones and finish on a newspaper strop.... which is nice for showing off but impractical for most situations.
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Rick, you may find useful our research on effect of paper wheels on edge retention:
http://knifegrinders.com.au/SET/Effect_of_felt_and_paper_wheel_on_edge_retention.pdf

Excerpt of RECOMMENDATIONS:
FELT
Deburring on solid felt wheel at high RPM is not recommended.
Deburring on slotted felt wheel at 2850 RPM is not recommended.
Deburring on slotted felt wheel at 1425 RPM is conditionally acceptable, provided that the contact of the edge with the wheel is less than 1 second per pass, sides are alternated with each pass, and number of passes is limited to 2 in one go.

PAPER WHEEL
Deburring on solid paper wheel at high RPM is not recommended.
Deburring on slotted paper wheel at 2850 RPM is conditionally acceptable, provided that that the contact of the edge with the wheel is less than 1 second per pass, sides are alternated with each pass, and number of passes is limited to 2 in one go.
Deburring on slotted paper wheel at 1425 RPM is recommended.

Following these recommendations ensures the edge temper is not compromised in the process of deburring.
 
Ocelot85 said:
I wonder if there is a machinist / knife maker / evil genius with a large shop compressor already, who may be willing to test the cold air gun on the grinder. Nathan the Machinist Nathan the Machinist

My employer would most likely frown on me bringing my belt grinder to work and confiscating their cold air gun in the name of blade science.
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We just use water. I think it probably has quite a bit better heat capacity / thermal conductivity than cold air.

I did some math once and figured you could raise .005" back from the edge 1000F instantaneously and get less than one degree temperature rise in the rest of the blade, proving to me a person could wreck their edge and never know it. If you're looking at 1000 F, or even 500F and the air is -30 rather than 80, I don't think that's enough. Water basically fixes the problem. It conducts heat much better than air and it absorbs heat during its phase change when it boils.

If you're grinding edges in production, in steels with low thermal conductivity and high in vanadium carbide (grinds hot) and you want to know with some confidence that every inch of every knife edge is good you want water. We figured this out years ago and made the change. It's not difficult or particularly messy once you set up for it.
 
Big Chris said:
I used to sharpen with a grinder exclusively, now every knife that leaves my hands has a hand sharpened edge.
Very slow speed with medium worn 120 and 220 grit belts before finishing on a well worn 400 and then power power stropping on a loaded felt belt.
I did this for years and never had a single edge holding complaint from customers, actually many praises for how sharp I could get the knife.

Then I started to do a lot of heat treat experimenting, edge retention testing, and experimenting with different sharpening methods.
I started to notice that after two or three sharpenings my edge retention would actually increase.

My current process is to break the edges on the grinder and finish on DMT stones.

I had great edge holding before, but now it was even better.
I tested the edges on clean and dirty cardboard and manilla rope.

When I stopped fully sharpening on a grinder I was no longer getting increased edge retention after a couple of sharpening sessions.

Most all this testing was with 10V, 3V, and M4 with 1000 degree tempering temps.

Just because something has been done one way for a long time and is known to work does not mean it it the BEST way.

The other key is to TEST, TEST, and TEST some more.
Never be satisfied that you have a sharp edge able to cut notebook or printer paper.

Also just because someone posts a lot, be it on forums or social media pages, this does not make them an expert.
Be very wary who you take advice from and take all advice with a grain of salt.
If it makes you stop and think, test it out before claiming it as TRUTH or starting to bash someone.
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Chris I like this post so much the "like button" wasn't doing it for me, I need an actual :thumbsup: here
 
I think it's worth noting there are plenty of applications where having a few thou of the edge softened a bit from powered sharpening doesn't matter much. It's possible to make a big deal out of nothing. But folks buying a knife with good edge holding that doesn't come out of the box with good edge holding are going to be disappointed. But if you make thin narrow edge knives with good edge holding in abrasion resistant steels I think you need to look into this.

Hell there was a time where a 55 HRC knife was considered "fine". Many people still feel this way. And some people buy knives 1/16" behind the edge and sharpened at 30 DPS and don't have a problem with that and the little bit of dull edge at the apex doesn't effect cutting ability. So I think this burnt edge thing is a real problem, but only for some applications.
 
Nathan the Machinist said:
We just use water. I think it probably has quite a bit better heat capacity / thermal conductivity than cold air.

I did some math once and figured you could raise .005" back from the edge 1000F instantaneously and get less than one degree temperature rise in the rest of the blade, proving to me a person could wreck their edge and never know it. If you're looking at 1000 F, or even 500F and the air is -30 rather than 80, I don't think that's enough. Water basically fixes the problem. It conducts heat much better than air and it absorbs heat during its phase change when it boils.

If you're grinding edges in production, in steels with low thermal conductivity and high in vanadium carbide (grinds hot) and you want to know with some confidence that every inch of every knife edge is good you want water. We figured this out years ago and made the change. It's not difficult or particularly messy once you set up for it.
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My Tormek T-7 is totally water cooled, so I recognize the value of water cooling. I'm also debating on eventually getting a Fogbuster system or a mist coolant system on my grinder. I was simply curious though if under controlled conditions with a moderate belt speed if the cold air gun would be enough to keep you under the 400F threshold in that small cross section for many steels. If it did it would avoid the corrosion and clean-up issues associated with running water.

Thanks for the response Nathan.
 
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When I estimated whether WorkSharp can be safe in sense of the edge overheating, the original version used in that New Zealand research simply can not. So not matter what variables you consider, the benchstone-sharpened edge will win in stability.
The advanced variable speed KO version can be safe when run at the lower speed, but actual users of it tell me that they have to use it at the high speed because at its low speed it is not as effective in steel removal, the belt may even stop rotating.

When you consider how many businesses producing sharpening equipment would have to modify in view of the facts highlighted in Larrin's article and by many trusted researchers before, no wonder that Global removes their comprehensive study from their website, no wonder we hear little of it, unless we look in professional literature that is difficult to find, and when you find it, difficult to read and interpret to a non-professional.
People like Larrin have our utmost respect for bridging the highly special professional knowledge to us.
 
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I get OCD about this topic. All post HT grinding is done with a heavy water mist (I actually change clothes, and it sucks during the winter....thank God I live in central Texas). And all sharpening is done on stones with either oil or water (depending on stone of course).

I will even ensure that any decarb layer that might happen (even with protected ATP641 during HT and any possible grinding heat) is abraded from the edge when I go to the hand rubbed finish part of my process. Edge is taken to zero by hand, then that zero edge I will cut into the stone or sandpaper to remove any possible decarb and set a nice flat to establish my edge upon.

There shouldn’t be any need to sharpen 3-4 times in order to reach good steel. I didn’t always do that....but I have the past year or so.
 
Larrin said:
"I couldn't tell the difference" is not very useful information. We have many decades of anecdotes pointing in every possible direction on the performance of steel, edge geometries, forging processes, heat treatment, etc. Without controlled, quantitative testing we will be stuck in the same cycle of mystery and misinformation.
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No kidding. :rolleyes: I DO realize that, I only do aerospace testing for a living after all. ;) You can't tell me that over decades of carrying a knife and using it daily, you don't get a good feel for how long your edge lasts? And you don't know how often you typically need to touch-up your edge, or even do a full resharpening? You couldn't be able to tell when you get 20% more use out of it? Even 10%, when your "data" set is long enough?

This may be both semi-qualitative and semi-quantitative since it's not perfectly controlled, but it gives a pretty good idea, certainly close enough that it shouldn't be completely dismissed. On a knife I carry frequently, I can tell you with near perfect certainty what day I'm going to need to sharpen it. That's not invalid data. It may not be perfect, but it's plenty good enough to get close. And how close is good enough? Sometimes it doesn't have to be perfect. If I had to resharpen every 2 weeks, and suddenly I get 2-3 extra days from the edge (~15-20%), wouldn't that be enough to notice? It certainly would! And you're saying that I wouldn't be able to notice a difference even greater than that? No way! That all being said, this isn't to take anything away from careful, methodical testing either. It has its place.

Instead of cherry-picking the non-quantitative part of my post, why not address the other questions I posed? Maybe address how the careful use of a belt grinder isn't all that bad? Try to prove to me and everyone else that using a dry belt slowly, and/or keeping the edge moving at the proper speed across the belt doesn't make a BIG difference. Because it absolutely does!!! Sure, water cooling will only make it better, but I would bet big money that proper belt sharpening on a dry belt can be done safely, without grossly ruining an edge. My qualitative "data set" leads me to believe that there's likely less than a 10% difference between hand sharpening and grinding for me. And I can get a pretty scary edge by hand!

Next time, maybe try to do something yourself for once and see? Data on a page only means so much when you don't have a real-world grasp of it. As I said, time is worth something, so maybe I'm OK with an edge that lasts 92.3657% of what a hand sharpened edge does, if it only takes me 5 minutes to apply versus 30 minutes or more when doing it by hand. It's a trade-off, like most things in life. Not every sharpening on a belt is "ruining" the edge as you are insinuating here, and I think it's very important that people know that. Proper scientific method is to give fair treatment to both sides of a problem/experiment, and I don't think you did that here.
 
A lot of knife companies over the years have gotten flak for bad heat treating, leading poor edge holding and easy edge damage. Looking back, given reports of seeing temper colors near the edge, it is more likely that overheating during sharpening was the culprit. Users complained of "chipping" but I've examined such knives and in fact they were rolled edges. The sharpening process had softened the steel at the edges.
 
What about carbides , nobody mention them ? What's happening to them during sharpening edge with belts ? Vanadium carbides are harder then ceramic as I think ? So they are tear off out of the matrix ? That would be bad for edge holding , right ?
 
SpyderPhreak said:
No kidding. :rolleyes: I DO realize that, I only do aerospace testing for a living after all. ;) You can't tell me that over decades of carrying a knife and using it daily, you don't get a good feel for how long your edge lasts? And you don't know how often you typically need to touch-up your edge, or even do a full resharpening? You couldn't be able to tell when you get 20% more use out of it? Even 10%, when your "data" set is long enough?

This may be both semi-qualitative and semi-quantitative since it's not perfectly controlled, but it gives a pretty good idea, certainly close enough that it shouldn't be completely dismissed. On a knife I carry frequently, I can tell you with near perfect certainty what day I'm going to need to sharpen it. That's not invalid data. It may not be perfect, but it's plenty good enough to get close. And how close is good enough? Sometimes it doesn't have to be perfect. If I had to resharpen every 2 weeks, and suddenly I get 2-3 extra days from the edge (~15-20%), wouldn't that be enough to notice? It certainly would! And you're saying that I wouldn't be able to notice a difference even greater than that? No way! That all being said, this isn't to take anything away from careful, methodical testing either. It has its place.

Instead of cherry-picking the non-quantitative part of my post, why not address the other questions I posed? Maybe address how the careful use of a belt grinder isn't all that bad? Try to prove to me and everyone else that using a dry belt slowly, and/or keeping the edge moving at the proper speed across the belt doesn't make a BIG difference. Because it absolutely does!!! Sure, water cooling will only make it better, but I would bet big money that proper belt sharpening on a dry belt can be done safely, without grossly ruining an edge. My qualitative "data set" leads me to believe that there's likely less than a 10% difference between hand sharpening and grinding for me. And I can get a pretty scary edge by hand!

Next time, maybe try to do something yourself for once and see? Data on a page only means so much when you don't have a real-world grasp of it. As I said, time is worth something, so maybe I'm OK with an edge that lasts 92.3657% of what a hand sharpened edge does, if it only takes me 5 minutes to apply versus 30 minutes or more when doing it by hand. It's a trade-off, like most things in life. Not every sharpening on a belt is "ruining" the edge as you are insinuating here, and I think it's very important that people know that. Proper scientific method is to give fair treatment to both sides of a problem/experiment, and I don't think you did that here.
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You're coming after Larrin for bias when he only posted the data he and others found and reported from independent third party testing. You've shown no actual data to support your belief besides relaying your own past anecdotal experiences in a non-quantifiable way since you didn't actually test anything to verify the results and record them.

You keep speaking about factors such as sharpening time which have absolutely nothing to do with the topic, the paper speaks only to one specific variable which is edge retention between different sharpening methods. All other factors that make your life more enjoyable or the process more expedient are completely and totally irrelevant to the subject.

Also you talk about the scientific method and fairness, yet your bias is as blatant as it could possibly be. If you disagree with the data set out to disprove it with the scientific method that you claim to care about. Set up a testing procedure and show us your data. Since you're a aerospace engineer I'm sure you'll design a procedure that will eliminate personal bias and show us verifiable proof of your claim. I look forward to seeing your paper on the topic.
 
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