Do I understand science behind shaving sharp knife correctly?

[damib]

Hello,
I am what would you call a beginner in world of knife sharpening although I never really had any problem with angles. I learned sharpening on gardening tools and I can bring them to quite decent edge (and I also learned to love carbon steel by sharpening them).
I noticed some chips on TOPS CAT yesterday so I decided to buy some 1200 grit sandpaper and repaired and resharpened the edge to be quite sharp. It cuts paper like butter. I would say that it's somewhere between decent and superb sharpness, but I would like to have an edge that can shave without damaging the skin with too much problem. So I went on the internet and started to learn about how to achieve this insane level of sharpness. There are some tutorials that require you too have too much equipment in my opinion, so I summed what I've found below and I will be happy if I could discuss each thing with you.

1) You can definitely produce mirror polish or near mirror polish edge with lower grits, because it has got to do more with pressure than grit, but it will still be better to work with higher grits because in terms of reproducibility. Mirror polished edge is way better speaking in terms of push cuts, so it is more useful for knives you use for cutting things like vegetables or meat.

2) You also don't need to use higher grits for achieving shaving sharp knife, because it's matter of two sides of the edge meeting in one line, so it's matter of properly making the burr more than anything.

And lastly maybe the most controversial statement.

3) You can produce more of a convex shape edge by hand sharpening so it should be more benficial to sharpen by hand than sharpening with sharpening systems or electric sharpeners.

 

[gadgetgeek]

I'll do my best to summarize since we don't need to reinvent the wheel, but you have made some assumptions that don't exactly fit when it comes to cutting hair and not skin, as compared to cutting meat/veg.

1) "mirror" isn't a valuable measure of the finest of the edge. The reason for the high grit hones is that the very leading edge needs to be as clean as possible. Something that is just not possible, no matter how light your hand is. You can make an edge that will take hair, but it will be ragged and will scrape skin.
2) The burr is bad. One of the reasons to strop, and one of the reasons why someone who can shave straight from the stones has a high level of skill is that hair has quite high tensile strength, and so it will grab and pull across the edge, removing any unsupported steel. This is at best, uncomfortable. In fact, it can be to the point that high carbide steels can be felt by experienced shavers as the carbides fail and are pulled from the matrix
3) in this case, horses for courses. A standard straight has a built in sharpening guide, and the edge is microscopically convexed by the strop. In the case of a knife, sure, but again, we are talking a very precise angle, and if using an appropriately fine hone, it will be difficult. Plus any mistake may gouge the hone, which will again, make the process a lot harder. Final edge geometry takes a second place to polish when it comes to a razor.

I'm not trying to keep you from trying, but know that reinventing the wheel, may well lead you down the same path that well over 100 years of razor making has lead to, which is large, fine, very flat hones, and selecting edges for uses. Paper-testing most straight razors will make them unshavable. It's just the nature of what you are doing based on a huge number of factors, where final edge geometry and polish get to the point where the silica in paper is enough to cause damage, whereas in a utility knife, a large portion of the cutting power is not fully reliant on the edge, but also the overall geometry, and the material being cut. Fun fact, the "Ginsu Knives" demonstration, is a flat-out magic trick. Skill, even unconscious can make a knife cut for one person, when it won't do the same for someone else. Not trying to rain on your parade, but I'm just summarizing about a decade of threads that I have read on this topic, having wondered the same myself at the beginning.

 

[damib]

I'll do my best to summarize since we don't need to reinvent the wheel, but you have made some assumptions that don't exactly fit when it comes to cutting hair and not skin, as compared to cutting meat/veg.

1) "mirror" isn't a valuable measure of the finest of the edge. The reason for the high grit hones is that the very leading edge needs to be as clean as possible. Something that is just not possible, no matter how light your hand is. You can make an edge that will take hair, but it will be ragged and will scrape skin.
2) The burr is bad. One of the reasons to strop, and one of the reasons why someone who can shave straight from the stones has a high level of skill is that hair has quite high tensile strength, and so it will grab and pull across the edge, removing any unsupported steel. This is at best, uncomfortable. In fact, it can be to the point that high carbide steels can be felt by experienced shavers as the carbides fail and are pulled from the matrix
3) in this case, horses for courses. A standard straight has a built in sharpening guide, and the edge is microscopically convexed by the strop. In the case of a knife, sure, but again, we are talking a very precise angle, and if using an appropriately fine hone, it will be difficult. Plus any mistake may gouge the hone, which will again, make the process a lot harder. Final edge geometry takes a second place to polish when it comes to a razor.

I'm not trying to keep you from trying, but know that reinventing the wheel, may well lead you down the same path that well over 100 years of razor making has lead to, which is large, fine, very flat hones, and selecting edges for uses. Paper-testing most straight razors will make them unshavable. It's just the nature of what you are doing based on a huge number of factors, where final edge geometry and polish get to the point where the silica in paper is enough to cause damage, whereas in a utility knife, a large portion of the cutting power is not fully reliant on the edge, but also the overall geometry, and the material being cut. Fun fact, the "Ginsu Knives" demonstration, is a flat-out magic trick. Skill, even unconscious can make a knife cut for one person, when it won't do the same for someone else. Not trying to rain on your parade, but I'm just summarizing about a decade of threads that I have read on this topic, having wondered the same myself at the beginning.

Thanks for responding. In the first place, I must really say that my questions may sometimes sound stupid, but I may rather ask questions than be sorry.


1) So, what you are basically saying is that mirror polish and sharpness are not correlated? That means you can polish edge of a knife with lower grits, but you won't get any added cutting performance?

2) I maybe used the incorrect words before. What I meant is that you need to make burr when sharping, but you remove it after so both sides of edge will be connected in ideally one infinitely narrow line. Or should I not try to make the burr in the first place?

3) If I understood this correctly, then what makes edge shave is the final polishing process, but main performance in a knife is from angle of sharpening? So, basically shaving sharp knife is some mythical beast that is not very practical because it will behave more like a razor?

To the last part of your response. I really am considering to go deep into the sharpening, because sharpening service I used to go for sharpening my knives from time to time probably changed the guy that is sharpening the knives and the work done is way worse that mine. At least not for now. If I make a mistake, I will learn from it and move on. You know, what works for me, works for me. First guy that seriously made me interested in knives really just didn't make nuclear science of knife sharpening. He just had his trusty old electric grinder and it also worked very well.

 

[gadgetgeek]

I think the major factor is that there is very little cross-over between razor honing and knife sharpening. Yes, you can get a knife sharp enough to shave with, but at the cost of any sort of reasonable use as a knife. I'm not about to say impossible, but at some point, you have to prioritize your uses. Worth while questions to have asked.

Mirror polish and sharpness are not directly correlated at the general use level, but once we get to shaving, we are in the micron level and in that case polish plays a big part. To the point that true razor edges look fundamentally different under an electron microscope, and the final apex really matters. At that point, even the grit shape can start to matter, though how much is up to debate. As you can see it becomes a "Yeah, but" sort of thing.

Burring with sharpening a knife is fine, you still want to prevent a wire-edge, and with the geometry of a straight razor, because the edge is so thin, a wire-edge is easy to build, but it will break off back behind the apex, leaving a dull edge (Dull for a razor, probably still quite sharp by normal standards). Whereas with a knife, an easy micro-bevel, or soft compounded strop solves that. If you do that with a straight, the final apex can easily become too obtuse for good cutting (of hair, on your face)

Largely yes, think of it sorta like chunky mud tires off road for an EDC and slicks on a drag strip for razors. Sure in some circumstances you could swap them, but you get more grip with the right tires. I will commonly be able to get most of my EDC knives to the point that they will easily take off arm-hair, and I'm nothing special, but facial hair is an entirely other story. And if I don't do a good job on my EDCs I'll end up with a very sharp, but very weak edge. Again we are really talking about getting a strong final apex. For me, I'm generally hitting what I want with an 800 grit ceramic, or DMT fine/extrafine, so max 1200, and then finally with a paddle strop and green compound. And when it comes to EDC, you'll find a final grit level that meets your needs, I'm sure you've already experienced the difference in cutting feel between a relatively harsh grit with a good edge and a finer one.

Edge is just one factor in this, overall geometry will play less or more of a part depending on what you are cutting, as you already have likely experienced. I would say, that getting good at sharpening your knives, being able to put the edge you want on them based on their uses is highly valuable. But don't chase razor levels of sharpness, because that's probably not what you actually want, and it's something of an artificial standard. Also once you get away from high carbon and get into stuff like 3V or the like, your method will change somewhat as those steels are much more abrasion resistant even at the same "hardness" so the curve always continues.

It's getting late for me, so I hope that makes sense.

 

[catspa]

… sharpening service I used to go for sharpening my knives from time to time probably changed the guy that is sharpening the knives and the work done is way worse that mine.

Sometimes the owner/operator of a sharpening service retires or dies or moves to Texas, and the person trying to carry it on takes a while to get up to speed. Learning to do your own sharpening, although it takes study and practice, frees you from depending on someone else to sharpen your stuff. Good skill to develop.

Just out of curiosity, did the sharpening service quality nosedive across the board, or just on certain knives (or certain steels)?

Parker

PS: your questions are not stupid, they are a natural part of learning. Members here have been in your situation, got good advice from somebody who knew, and solved their dilemma. It’s all circular.

Parker

 

[Lesknife]

Unless you are shaving hair/ beard or very accurate wood cutting/carving or similar materials you don’t really need a mirror/ shaving edge. If it’s for display only then that’s understandable for appearance.

I find that getting that fine of edge is more trouble to maintain and really doesn’t add edge retention when cutting tough or contaminated materials. General cutting use is usually dirty and sometimes includes other elements that are very abrasive. So after a few cuts the mirror shine is gone and scratches are more obvious. Cutting veggies depends on which ones or how clean and cutting meat is another matter.

For general cutting I prefer a finished edge around 600 grit with a bit of toothy micro edge and 1200 grit if I’m cutting meat or doing some whittling. They are easier and quicker to maintain and the edge seems to last better. If I want a still finer edge to trim a callus or other delicate stuff I use a ceramic rod and with a few swipes the toothy edge is a smoother less grabby edge.

The type of edge is going to depend on the type of usage for the desired results which in my opinion should be of the most efficient to effective ratio.

 

[3D Anvil]

A couple things to add...

1. You can't get a mirror finish with a coarse grit, no matter how gentle you are. You can get a mirrorISH bevel starting around the 3000 grit range. For a true mirror you need to go sub-micron.

2. You can have a mirror bevel that isn't sharp, because sharpness depends on having a clean, perfectly formed apex. If you run a mirrored edge across a brick, the edge bevel will still be mirrored, but the knife won't cut a damned thing.

3. You can't have a super KEEN edge that isn't shiny, because the relatively wide grooves cut by a coarse stone form the apex. If you look at a scanning electron microscopic image of a coarse-ground blade, the edge looks like a saw blade, i.e., it's a "toothy" edge. Of course, a saw is better at cutting many things than a razor, so the sort of edge you prefer may depend on your intended use. Or to put it another way -- the keenest edge isn't always the sharpest edge.

 

[damib]

Sometimes the owner/operator of a sharpening service retires or dies or moves to Texas, and the person trying to carry it on takes a while to get up to speed. Learning to do your own sharpening, although it takes study and practice, frees you from depending on someone else to sharpen your stuff. Good skill to develop.

Just out of curiosity, did the sharpening service quality nosedive across the board, or just on certain knives (or certain steels)?

Parker

PS: your questions are not stupid, they are a natural part of learning. Members here have been in your situation, got good advice from somebody who knew, and solved their dilemma. It’s all circular.

Parker

It just kinda nosedived on all steels. They used to do brilliant job, but they seem to take off too much material, damage the blade more seriously or make the sharpening quite unevenly and it doesn't matter what steel is the knife made from.

I wonder what would be best for maintaining the toothy edge? Doesn't stropping tends to get rid of it?

 

[gadgetgeek]

It's all going to come down to your uses, and your techniques. You could do most of your sharpening on the stones and then all the strop/compound is doing is abrading away the burr, which leaves you a nice apex, but with an edge that is only slightly smoother than the final stone.

 

[Obsessed with Edges]

It just kinda nosedived on all steels. They used to do brilliant job, but they seem to take off too much material, damage the blade more seriously or make the sharpening quite unevenly and it doesn't matter what steel is the knife made from.

I wonder what would be best for maintaining the toothy edge? Doesn't stropping tends to get rid of it?

Depends on what you're using for stropping. If compound is used, it's likely it'll diminish the toothy bite left from the stones. This is because most popular compounds are significantly harder than the matrix steel of the blade, at least, so you're polishing the steel at the edge to a finer finish. On the other hand, if you're stropping on a bare substrate like leather, paper, fabric (denim or linen), you can still clean up most or all of the burrs without altering the toothy finish much, if at all. The burrs and their remnants are weakened steel. So, bare stropping is usually enough to strip them away without altering the strong steel behind them. This assumes the burrs have been adequately thinned on the stones first, in order for them to be easily stripped away on a bare strop.

I prefer to maintain the toothy bite left from my stone. So, when I strop, I'll generally just use the bare, sueded side of a leather belt. OR, if doing so immediately after sharpening on stones, I'll just lay a clean sheet of paper over the stone and strop on that, with no compound.

 

[garry3]

I think the major factor is that there is very little cross-over between razor honing and knife sharpening. Yes, you can get a knife sharp enough to shave with, but at the cost of any sort of reasonable use as a knife. I'm not about to say impossible, but at some point, you have to prioritize your uses. Worth while questions to have asked.

Mirror polish and sharpness are not directly correlated at the general use level, but once we get to shaving, we are in the micron level and in that case polish plays a big part. To the point that true razor edges look fundamentally different under an electron microscope, and the final apex really matters. At that point, even the grit shape can start to matter, though how much is up to debate. As you can see it becomes a "Yeah, but" sort of thing.

Burring with sharpening a knife is fine, you still want to prevent a wire-edge, and with the geometry of a straight razor, because the edge is so thin, a wire-edge is easy to build, but it will break off back behind the apex, leaving a dull edge (Dull for a razor, probably still quite sharp by normal standards). Whereas with a knife, an easy micro-bevel, or soft compounded strop solves that. If you do that with a straight, the final apex can easily become too obtuse for good cutting (of hair, on your face)

Largely yes, think of it sorta like chunky mud tires off road for an EDC and slicks on a drag strip for razors. Sure in some circumstances you could swap them, but you get more grip with the right tires. I will commonly be able to get most of my EDC knives to the point that they will easily take off arm-hair, and I'm nothing special, but facial hair is an entirely other story. And if I don't do a good job on my EDCs I'll end up with a very sharp, but very weak edge. Again we are really talking about getting a strong final apex. For me, I'm generally hitting what I want with an 800 grit ceramic, or DMT fine/extrafine, so max 1200, and then finally with a paddle strop and green compound. And when it comes to EDC, you'll find a final grit level that meets your needs, I'm sure you've already experienced the difference in cutting feel between a relatively harsh grit with a good edge and a finer one.

Edge is just one factor in this, overall geometry will play less or more of a part depending on what you are cutting, as you already have likely experienced. I would say, that getting good at sharpening your knives, being able to put the edge you want on them based on their uses is highly valuable. But don't chase razor levels of sharpness, because that's probably not what you actually want, and it's something of an artificial standard. Also once you get away from high carbon and get into stuff like 3V or the like, your method will change somewhat as those steels are much more abrasion resistant even at the same "hardness" so the curve always continues.

It's getting late for me, so I hope that makes sense.

Yep, polish level doesn't necessarily mean sharper.

I find it interesting that man made whetstones seem to leave mirror polishes at grit levels I wouldn't expect from natural stones in the same grit range. Is this a mirror polish sells thing and the manufactures are using some finer grit to achieve a finer polish or is it that they are manufacturing stones with actually polishing in mind? Perhaps it's a result of the particular abrasive?

The shape of the grit and how it effects an edge isn't at all a controversial subject with straight razor shavers. I think everyone is aware that there is more to it than grit size. Some very sharp edges can come off of some large grit stones by the judicious use of pressure.

Here is a short write up comparing two related stones that have the same garnet but of different sizes and it's effect.

How does a Belgian Blue Whetstone compare to a Coticule? (a story about garnets) - home of the famous Belgian Coticule Whetstone

How does a Belgian Blue Whetstone compare to a Coticule? (a story about garnets)

www.coticule.be

 

[3D Anvil]

Yep, polish level doesn't necessarily mean sharper.

I find it interesting that man made whetstones seem to leave mirror polishes at grit levels I wouldn't expect from natural stones in the same grit range. Is this a mirror polish sells thing and the manufactures are using some finer grit to achieve a finer polish or is it that they are manufacturing stones with actually polishing in mind? Perhaps it's a result of the particular abrasive?

Just speculating, but it may be that the man-made stones have more consistent grit size than naturals. And of course, naturals don't really *have* grit ratings, so there's that. What I mean is that, with natural stones, a grit rating is always just an approximation. The grit could average 1 micron, but within that rating there may be stray bits that are 3-4 micron in size which would affect the polish.

 

[garry3]

Just speculating, but it may be that the man-made stones have more consistent grit size than naturals. And of course, naturals don't really *have* grit ratings, so there's that. What I mean is that, with natural stones, a grit rating is always just an approximation. The grit could average 1 micron, but within that rating there may be stray bits that are 3-4 micron in size which would affect the polish.

I think all man made stones have a grit range distribution and it varies between stones and manufactures.

 

[gadgetgeek]

Yep, polish level doesn't necessarily mean sharper.

I find it interesting that man made whetstones seem to leave mirror polishes at grit levels I wouldn't expect from natural stones in the same grit range. Is this a mirror polish sells thing and the manufactures are using some finer grit to achieve a finer polish or is it that they are manufacturing stones with actually polishing in mind? Perhaps it's a result of the particular abrasive?

The shape of the grit and how it effects an edge isn't at all a controversial subject with straight razor shavers. I think everyone is aware that there is more to it than grit size. Some very sharp edges can come off of some large grit stones by the judicious use of pressure.

Here is a short write up comparing two related stones that have the same garnet but of different sizes and it's effect.

How does a Belgian Blue Whetstone compare to a Coticule? (a story about garnets) - home of the famous Belgian Coticule Whetstone

How does a Belgian Blue Whetstone compare to a Coticule? (a story about garnets)

www.coticule.be

Yeah, a lot of it is going to have to do with exactly how the grit itself breaks down, and how the substrate reacts/abrades the steel. I've seen some interesting stuff in regards to high carbide steels with natural stones, and how that abrasion takes place. A very carbide heavy D2 is going to be very different than something like a 1095 or even an AEB-L.

I actually think that man-made stones are more "consistent" grit-wise, but because they abrade and break down differently, they just end up with differing characteristics than naturals and that can throw off how they present. All stones will have some distribution, but I would guess that naturals would have more "larger than rated" inclusions, where some man-made might be far tighter at the top end. But due to the nature of softer waterstones, both natural and man-made, you are going to end up with very different effects based on how you flush them, slurry, and the like, so it's really hard to predict someone else's outcomes. However, just a light tough on a clean surface is still only going to get whatever that surface is giving, since you have reduced the variables. And to be fair to the OP, apart from the sandpaper, I don't think he described what sort of stones he is using, so we all could be making some inappropriate assumptions.

 

[garry3]

Yeah, a lot of it is going to have to do with exactly how the grit itself breaks down, and how the substrate reacts/abrades the steel. I've seen some interesting stuff in regards to high carbide steels with natural stones, and how that abrasion takes place. A very carbide heavy D2 is going to be very different than something like a 1095 or even an AEB-L.

I actually think that man-made stones are more "consistent" grit-wise, but because they abrade and break down differently, they just end up with differing characteristics than naturals and that can throw off how they present. All stones will have some distribution, but I would guess that naturals would have more "larger than rated" inclusions, where some man-made might be far tighter at the top end. But due to the nature of softer waterstones, both natural and man-made, you are going to end up with very different effects based on how you flush them, slurry, and the like, so it's really hard to predict someone else's outcomes. However, just a light tough on a clean surface is still only going to get whatever that surface is giving, since you have reduced the variables. And to be fair to the OP, apart from the sandpaper, I don't think he described what sort of stones he is using, so we all could be making some inappropriate assumptions.

And yet it is the razor honers using man made whetstones that are limiting their passes on stones because they are seeing edges degrade and calling it "over honing". This reportabley isn't the only stone that does this just an example.

 

[gadgetgeek]

Over-honing is a thing, as to why, that I don't know. I know that about a decade ago a guy with access to a scanning electron microscope was going to try to figure it out, but I don't know what happened in that case. He had a lot of projects and it was not a high priority. As far as I can tell, and the theory that I think holds the most water is that some stones will start to burnish a wire-edge out inline with the axis of the edge, so that it starts to become very thin and brittle. Why some stone-steel combos seem more prone to it, I don't know. But I do know that it is a combo of both because that was tested by a group over at another forum, a lot of very good honemeisters tried to push several blades including some that were sent around, using their own stones and some didn't over-hone at all, and some would overhone on stones that were considered pretty safe. Composition and hardness of the steel matters. But it's one of those things that a person could put a PhD worth of researching into just to open up all the questions we don't yet know. For all I know, how the water sits on top of the stone is a major factor, or it could be a complete non-issue. Anyone who says they know, better bring data.
The main takeaway is that like in all things, don't confuse "different" for "better/worse".

 

[garry3]

Over-honing is a thing, as to why, that I don't know. I know that about a decade ago a guy with access to a scanning electron microscope was going to try to figure it out, but I don't know what happened in that case. He had a lot of projects and it was not a high priority. As far as I can tell, and the theory that I think holds the most water is that some stones will start to burnish a wire-edge out inline with the axis of the edge, so that it starts to become very thin and brittle. Why some stone-steel combos seem more prone to it, I don't know. But I do know that it is a combo of both because that was tested by a group over at another forum, a lot of very good honemeisters tried to push several blades including some that were sent around, using their own stones and some didn't over-hone at all, and some would overhone on stones that were considered pretty safe. Composition and hardness of the steel matters. But it's one of those things that a person could put a PhD worth of researching into just to open up all the questions we don't yet know. For all I know, how the water sits on top of the stone is a major factor, or it could be a complete non-issue. Anyone who says they know, better bring data.
The main takeaway is that like in all things, don't confuse "different" for "better/worse".

What stones is this happening on?

 

[gadgetgeek]

I'm going off memory, but it was basically everything 8k and up, anything you would consider a finishing hone, someone could get the right combo of stone and steel to overhone. I belive also glass backed films. Jnats, coticules, belgians, Chinese mystery stones, the works. Granted the blades were everything from very old to "modern" and as you know two blades from the same batch can be just a hair difference in hardness, and this seems to be the key, although I'm deep into speculation here.

 

[docflash]

has anyone looked at the temperature of the steel right at the apex during sharpening / steeling / honing? is that even possible?? i'm aware of the existence of work-hardening, and from what i've read recently, understand that speed and pressure of honing makes a difference - and both those would have an effect on temperature.

i wonder what would happen to a blade that was kept in a freezer for a while before sharpening / steeling / honing???

 

[damib]

has anyone looked at the temperature of the steel right at the apex during sharpening / steeling / honing? is that even possible?? i'm aware of the existence of work-hardening, and from what i've read recently, understand that speed and pressure of honing makes a difference - and both those would have an effect on temperature.

i wonder what would happen to a blade that was kept in a freezer for a while before sharpening / steeling / honing???

I think that it could be possible. I am not exactly expert in steels, but there are materials in which micro fluctuations in temperature can be somewhat significant for their properties, but I don't think that it would be the same for steel as it is typical crystalline material. And even if it was somewhat significant, there is the question if it would be of some practical use.

I also don't really know if work-hardening can make some difference as the sharpening is more about abrasion than tension. Some work hardening maybe could occur on the surface, but that would be really a topic for itself, but I don't think that it would be of any practical use also. I mean that... Well that would be for whole lecture on surfaces.

I am also not an expert in abrasion science, but lowering temperature makes materials more brittle, so I think that it may cause maybe some chipping of the steel and no benefit whatsoever. It wouldn't change composition of the steel in my opinion, maybe slightly. Look in the science of phase diagrams for the answer why. There are lots of good books focused mainly on metals.