How would you sharpen X?

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Troopah_Knives said:
This is absolutely a real phenomenon…
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Oh yes for sure I know it’s real. I am very curious though about how the specifics of technique affects this. Things like how many grits, how many passes, how high in grit did they go, what kind of pressure was used, time spent on the belt, speed of belt, was there any water cooling at all or time taken between passes, etc, etc.

For example, Workaharps instructional videos spend a good bit of time on the belt and tell you for example to move slowly across the belts (I think they say an inch a second or something) and it’s just pass, pass, pass, pass, pretty quickly, So could different technique affect that?

I would love to see all how all those little details come into play, and if it’s possible to have no negative effects, or if it’s simply a matter all powered abrasives are going to be destructive period. I imagine the answer is yes, but is it possible to minimize that destructiveness to an acceptable level or not?

I do have a couple knives I use that I have sharpened this way for personal testing and I haven’t been able to notice a major difference in the day to day but it’s not exactly scientific evidence and Im always happy to be proven wrong.

Anyhooo,
 
Seems Larrin wrote this on the subject:

https://knifesteelnerds.com/2019/04/08/does-sharpening-with-a-grinder-ruin-your-edge/
I seem to remember that the Knife Deburring book went a little further and indicated some threshold belt speeds before which there was no discernible overtempering or whatever, but I’m sure there are a ton of variables there too. Like @crockerculinary, I’d be curious to know if it’s really a significant factor when I put a knife on the belt, given that I’m running a 2x72 on lower speeds, moving the knife quickly, and dipping it into a bucket on every pass. Drops in edge retention after sharpening dry on a 1x30 fixed speed Harbor Freight machine are interesting, but don’t necessarily apply.
 
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Troopah, I can't thank you enough, really, for your comment on belt sanded hardness reduction on blade edges.

I spent my whole professional life designing medical plastic products and then making the molds. Never, ever in my whole life did I see a belt sander in a tool shop. You saw it in fab shops working on mild steel and such, but never on anything that had to stay hard.

I try to indicate to my customers and potential customers that I've seen over a half dozen common problems with belt sanded edges at the "Farmer's Market" including soft edges, but few people seem to really understand. I suspect some people think I am just trying to earn more money. I get it as they've never worked with steel & blades.

I'm still trying to figure a great way to explain the issues better to knife owners.
 
On an experimental level, I recall I did one time try to "rough" a chamfer on a blade for a fixture to cut heating wire.

I put my index finger on the back side of the blade as I used a belt sander or grinding wheel to put the basic angle on.

With fairly lite pressure on the blade, it took less than a second or two max to start to burn my finger. That was about .06" from the other side of the blade. Hence, I would assume the metal in contact with the wheel was hot enough to get softened.

All the grinding we did on the surface grinder HAD to have high flow cutting fluid on it non-stop. If you did not do that, you got burn marks everywhere.
 
BoSharpens said:
On an experimental level, I recall I did one time try to "rough" a chamfer on a blade for a fixture to cut heating wire.

I put my index finger on the back side of the blade as I used a belt sander or grinding wheel to put the basic angle on.

With fairly lite pressure on the blade, it took less than a second or two max to start to burn my finger. That was about .06" from the other side of the blade. Hence, I would assume the metal in contact with the wheel was hot enough to get softened.

All the grinding we did on the surface grinder HAD to have high flow cutting fluid on it non-stop. If you did not do that, you got burn marks everywhere.
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This isn't really evidence against belt sanders, though. The fact that you at some point overheated an edge on a belt doesn't mean that others are always doing it.

When I do an edge on the belt, it's always cold to my fingers and there are never burn marks, since the blade is coated with water and the speed is low. The discussion above suggests that the precautions I take might not be sufficient to avoid overheating the very apex. But they don't say that the apex is definitely overheated. The only thing that you can say with certainty is that some people* overheat knives with belts, which is obviously true.

That said, the discussion above makes me want to be extra-cautious with heat generation.
 
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ian said:
This isn't really evidence against belt sanders, though. The fact that you at some point overheated an edge on a belt doesn't mean that others are always doing it.

When I do an edge on the belt, it's always cold to my fingers and there are never burn marks, since the blade is coated with water and the speed is low. The discussion above suggests that the precautions I take might not be sufficient to avoid overheating the very apex. But they don't say that the apex is definitely overheated. The only thing that you can say with certainty is that some people* overheat knives with belts, which is obviously true.

That said, the discussion above makes me want to be extra-cautious with heat generation.
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Another user above noted using a belt sander set up to run lower speeds. That is likely what it takes to have controled blade work.

El-cheapo belt sanders don't have speed adjustments, however.

When people bring me knives to sharpen which have been belt sanded carelessly, I see lots of uneven edge angles & wavy edge profiles with occassional scratches on the side of the blade. It is just FAST careless work.
 
BoSharpens said:
When people bring me knives to sharpen which have been belt sanded carelessly, I see lots of uneven edge angles & wavy edge profiles with occassional scratches on the side of the blade. It is just FAST careless work.
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Absolutely. Before I got into all of this I had my knives sharpened a couple times by someone with a belt in a truck. They never came back much sharper than before (I didn't really know what to expect) and the bevels were crazy bad. 🤦‍♂️
 
captaincaed said:
I think I just figured out why they put Co in VG-10. They expect people to give it to a grinder to sharpen it, and they want it to hold hot-hardness a bit better. Maybe?
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I hadn't thought of that. It would certainly help! Although I think the original reason was for putting different coatings on it which often happens in the 500f range.
 
That kind of speed is so unthinkable, it hurtz.

Edit: Bad pun aside, I just realized I have no idea what 90 hz means. So this is like 5400 rpm.... which I guess is kinda fast? But idk. I'm not smart enough to know what rpm I'm working with. I just set things to slow or medium or fast. Maybe I should work this out at some point. 😃
 
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Depends on use - how has the edge failed - if rolled and chipped it needs the bevel thickening, if just blunted then can drop the angle a bit more. Arbitrary angles assume arbitrary use.

I’m gentle on my knives, so they’re thin behind the edge and cut with little resistance. I gave a knife like that to someone else who’s rougher on their knives and it was soon chipped and rolled. Thickened the edge up for them and was fine. We both know how to use a knife and use it our way- with the right bevel we’re both happy. I feel I lose performance with a thicker edge for no gain, they need a thicker edge for it to survive
 
Very nice spreadsheet! The study done with the color-changing lacquer was very interesting and the spreadsheet is a great collection of data.

The author made two key errors.

1. In regards to VG-10 and general interpretation of any steel that a manufacturer says "displays secondary hardening". Although it can be coated at up to 450c kitchen knives simply aren't tempered in that range and this could easily cause a loss in hardness of 6rc points so I would not recommend using a belt grinder to sharpen it unless you can keep temperatures below whatever the factories temper is.a

2. Many steels were not developed for knives and heat treatment can diverge significantly from the recommendations of a manufacturer. For example, it should be noted that many knife makers don't temper in the range recommended by the manufacturer as a better balance of hardness and toughness is usually achieved through a cryo treatment and low-temperature temper. So I still would not recommend sharpening steels like k390 on a belt as many makers use a 350-400f temper unless you know the tempering temperature used on the knife.

Basically to best apply the data in the color-changing lacquer study, unless you know the tempering temperature of the knife you are sharpening assume that any temp over 300f will soften your edge.

Finally as noted by the person who wrote the post highlighted "there might be a problem in the tests performed and that the very edge might have too little mass to transfer enough heat into the lacquer to make it translucent". So we still, unfortunately, cannot rule out significantly higher temperatures at the very edge of the knife. In fact, we can infer that in the original testing done by knifesteelnerds a higher temperature was achieved because the very edge of the blade actually showed an increase in hardness which could have been caused by secondary hardening (~1000f) or actual hardening of the edge (1400f+). With more extensive testing maybe we can get a better idea of under what circumstances that extreme heating occurs.
 
I think the belt grinder ruins edge argument is strange. Seems like everyone believes whole heartedly that each steel can be heat treated 1000 different ways for 1000 different results. But using a belt sander ruins your edge in all circumstances. My belt sander can go so slow that I could probably have a knife on it for 3 minutes without burning the edge. Or so fast that it can turn a 1.5”x3 foot (38mm x 1m) bar of steel into dust in one minute.
 
@Dominick Maone Im sure it is possible not to overheat your edge on a belt grinder it's more a question of how hard it is not to. It's rather hard to empirically test if you have overheated your edge as you need expensive equipment. With the short time scales, we are talking about the edge won't change colors. I have yet to see a single empirically study where a knife was sharpened on a belt grinder and didn't show any negative effects but once again I'm sure if you go slow enough it's possible. The real question is how slow, how much coolant, and what abrasives. Without more study, we won't know for sure!
 
Troopah_Knives said:
Basically to best apply the data in the color-changing lacquer study, unless you know the tempering temperature of the knife you are sharpening assume that any temp over 300f will soften your edge.
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this is all amazing and I’m so glad to see people are trying to get real data on this, but I am ADDaf and not very metallurgically edumacated AND charts and data make my head swim. So is this a good general takeaway? As long as we don’t hit 300f degrees, no damage will occur to the steels hardness?
 
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@crockerculinary I'd say it say it's not a super helpful conclusion as that is exactly what we knew before looking that that big spreadsheet. We do know from the study linked in that Reddit post that it is easy to heat the blade above that temperature on a macro scale (no way to know how much worse if at all, it is at the very apex). In many cases, the edges were heated above 325(the min temp they looked at). On their belt grinder examples, they found 1-2 seconds of light pressure depending on the knife did overheat the edge.

Basically, due to issues with measuring the heating at the very apex, the above studies don't tell us much other than 1-2 seconds of light pressure on 220grit belts will overheat your edge. We still don't know the important info of how little time do you have to do to not risk overheating the edge, and if constant water cooling can eliminate that.
 
Yea, still nothing conclusive or strongly suggestive that obviously applies to the way belts are usually used. Cool that people are trying tho.

I think the microhardness measurements that Larrin cited are still the most compelling data, but they also leave a lot of questions open.

The takeaway is still just “Be cautious about heat generation. It can be a problem even if the blade doesn’t get hot to the touch.”
 
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