It can shave, but not slice a tomato????

[Toothy Wolf]

I’ve been using Chosera and Shapton glass stones on a variety of knives and I loose the ability to slice a tomato at some point each and every time. 3000 on the Chosera is my last good slice (I can and have gone to 10,000 with the hair leaping off my arm). And 2000 on the the Shapton as well ( I have no stones between 2000 and 6000 right now. Most knives are kitchen cutlery which is why I found this tomato issue. Now I understand the toothy and polished finish, and that this plays a large roll in it. My question is weather or not this is the place in my progression that I should expect this to happen, or is my technique flawed. Thanks in advance for the help. The people here have been great.

 

[Bill3152]

I use dmts. My slicers are.one.and.done on a 325 dmt. My chopper I refine to 1200 dmt. Cuts.like.a.champion. I've used higher grits and didn't find it necessary.

 

[WhitleyStu]

This was discussed a few weeks ago concerning the skin on a tomato being different than an orange or apple for example. It does help to keep a knife with a slight "toothy" edge for some soft items.

 

[Obsessed with Edges]

That's likely a burr, if it shaves but doesn't slice a tomato.

Fine burrs will shave easily, but will immediately fold on something more cut-resistant like a tomato's skin, or even in cutting paper. Once the burr folds, the edge effectively becomes blunt.

Either a very polished edge, or a toothy one, can slice a tomato. But it has to be a clean, stable and fully-apexed edge, free of burrs, to do it consistently.

If the kitchen cutlery is in mainstream stainless steel typically seen in these, it'll usually be fairly ductile and/or soft. Such steels are prone to producing very thin, fine burrs on high-grit finishing stones, like ceramics, for example. So the possibility of a very 'bendy' & fine burr is more likely with kitchen knives like this.

Pics below, of a Victorinox paring knife's edge I sharpened with some heavy grinding on a medium Spyderco ceramic, lubricated with oil. It's an extreme example, and the burr is BIG (wide) but very thin, and it held onto the edge tenaciously, even after I'd tried to strop it away on a cork strop with green compound (can see bits of that in the 3rd pic, on the blade). Such a burr can fold back & forth for quite awhile without coming off and will drive you crazy with erratic edge performance, if you don't know it's there. At high magnification, it looks just like crumpled foil - which is why these are sometimes called 'foil edges'.

 

[Ourorboros]

Of course Obsessed with Edges covered it.
I'll just add that my knives on .5 micron stones cut tomatoes very cleanly.

 

[tinfoil hat timmy]

maybe not toothy enough? maybe tomato too ripe?

 

[eveled]

It helps me to think of all knives as being serated. Some are obvious, some are very fine. I stop at coarse sharp or “toothy@ and I’m happy.

probably out of my league here. My advice is don’t overthink it.

 

[bgentry]

Without more info it's hard to say what's going on. If it was a burr, then it would not shave *after* failing to slice things like a tomato, but you seem to imply that it can fail to slice a tomato, and then immediately shave arm hair. That might be an overly polished edge, but that seems unlikely. It's possible to have a somewhat obtuse edge that shaves, which is weird.

Pretend that your edge is very wide, like 5mm (1/4") or so. Now if you make nice perfect 90 degree shoulders on that edge, it will be totally blunt straight on. But when you hold it at an angle on your arm that super sharp 90 degree edge will shave hair off the arm. I think this can happen on a small scale.

Again, I'm just guessing. More info would help for sure. For example, does the edge reflect light when looked at *straight* down on the edge under a strong light? Try this with a REALLY dull blade first and see if you can catch a reflection. Then try it with your problem blade and see if you can catch a reflection on that edge.

Brian.

 

[Toothy Wolf]

Without more info it's hard to say what's going on. If it was a burr, then it would not shave *after* failing to slice things like a tomato, but you seem to imply that it can fail to slice a tomato, and then immediately shave arm hair. That might be an overly polished edge, but that seems unlikely. It's possible to have a somewhat obtuse edge that shaves, which is weird.

Pretend that your edge is very wide, like 5mm (1/4") or so. Now if you make nice perfect 90 degree shoulders on that edge, it will be totally blunt straight on. But when you hold it at an angle on your arm that super sharp 90 degree edge will shave hair off the arm. I think this can happen on a small scale.

Again, I'm just guessing. More info would help for sure. For example, does the edge reflect light when looked at *straight* down on the edge under a strong light? Try this with a REALLY dull blade first and see if you can catch a reflection. Then try it with your problem blade and see if you can catch a reflection on that edge.

Brian.

I don’t think it is a burr. The edge is still shaving sharp after use. It’s not dulling after use. Cuts everything great except stuff with a skin, like tomatoes. I’ve striped on leather, both loaded and bare. I have used the reflected light and the edge is clean. I can feel the edge loosing “teeth” through the progression, three finger test. Same result with a cheap block set of stainless steel and the steel on my higher end Yaxell knife. Or Atleast I think it’s a higher end knife. Or maybe I just can’t deburr properly at higher grits. Other people here seem to be able to. So I guess it’s back to the grind stone and practice more. Thanks everyone for the insights.

 

[kreisler]

i have no insights, can't sleep, random contribution, long time no see.

the other day i sharpened 2 chef knives after 6 months of not sharpening anything. used 204M only , knowing that a cooking session would destroy further efforts (204F etc) within a day. deburred with degussit ruby and cleaned with loaded wood strop (not leather!). total 100mins for 2 knives. too long but time was flying. passed tomato skin test easily (and phone book paper). looks like one doesn't unlearn sharpening skills after a 6 months break!
i never do newspaper test or toilet paper test .. because after a cooking session that kind of sharpness will be gone anyway.

no shaptons or choseras for me. 204M is very efficient and effortless for me in the long run. saving energy. very sustainable. my goto stone, especially for kitchen stuff. Maybe one day i'll afford a Sieger Long Life (=Westmark =Degussit) ruby rod.

btw when i watch sharpening youtubes, i admire the videos most when the least amount of steps are used or the least amount of time. i manage to get away with few steps (204M ruby wood) but I'm not happy with my timings.

a sharpening exam would be graded for:
1. sharpness result (tests, BESS) 50%
2. sharpening time 20%
3. steel consumption 20%
4. efforts involved (steps, sweat, energy) 10%

i would give myself an A- , thanks. More i doht look for in this activity, even if i could get to A (or maybe even A+ ?). i REALLY know when\where to stop and take a step back and reconsider what I'm doing and for which reasons, not just with sharpening efforts but in general with anything in RL. things have to make sense. healthy attitude imho.

 

[bgentry]

the other day i sharpened 2 chef knives after 6 months of not sharpening anything. used 204M only ,

total 100mins for 2 knives. too long but time was flying. passed tomato skin test easily (and phone book paper).

The Spyderco medium (204M) is a fantastic stone. It cuts essentially any steel because it has synthetic sapphires in it. The ceramic matrix it's made of does a really great job burnishing and abrading the steel to leave a finish that's fine enough, but not too fine. I often finish knives with the 204M.

That being said, it's WAY TOO SLOW. It cuts at the speed it should for what it is. But it has a hard time removing metal and gets clogged easily. If I were sharpening very dull blades, I would start with a much more coarse stone. Maybe a 100 to 120 grit stone like a coarse Crystolon. I normally start with the DMT XXC, but I realize that this stone is not common and is somewhat expensive. The coarse Crystolon is much more affordable and it's easy to find. You could jump from the 120 grit stone right to the 204M. Or you could do something in between if you care about the looks of the blade.

Or you can join the dark side and get a powered sharpening system. It's very difficult for me to sharpen a dull kitchen knife using only hand stones now. Because I know I can knock it out in a fraction of the time with my belt sander. I'm also more consistent with a belt sander because I never get fatigued. So I can always be sharp and attentive and keep my lines where I want them. With a hand stone, I get tired and then I get sloppy and my lines aren't nearly as good.

Just some thoughts. Happy Friday!

Brian.

 

[Obsessed with Edges]

The pics I posted earlier, of the burr created on the medium Spyderco, were the proof of what I'd long suspected of the behavior of these stainless steels on ceramic hones in particular.

I agree, for grinding jobs, the medium Spyderco is a slow stone to work with. I wasn't going for speed there, but instead looking to prove if my long-held assumption about the behavior of these steels on these stones was accurate. Between the relatively fine grit, the density and extreme hardness of the stone and the loading of the stone (even with oil, but much worse without it), burrs like these get very thin and just keep getting wider, never quite reaching a point where they'll be cut away by the stone, because the steel is so ductile. At a smaller scale, without deliberately making the burr so obviously wide, it demonstrates that a very thin and almost indistiguishable burr can form and move back & forth on the apex, unless deliberate measures are taken to reduce it and remove it. The absolute proof of the burr's removal is when the edge will BOTH shave and do everything else consistently, without all that instability and erratic behavior. As the burr flops around, side to side, it'll create issues in cutting, like those described in the OP. Even bent sideways, it'll still shave from the side to which it leans. But if the edge isn't aligned straight, it'll get in the way of cutting tomatoes and such.

My mantra is 'The cutting never lies'. If an edge is strong, keen and clean along it's full length, it'll cut anything with consistency and repeatability. If it's not able to cut consistently, and especially if it's shaving but not doing other things well, the edge is either unstable due to weak burrs, not completely apexed along it's full length, or just not strong enough to support itself at the chosen geometry (meaning it's unstable, burrs or not).

 

[Artyh]

Leonard Lee in his book on sharpening recommends using a 1000 grit water stone for tomato and other veggies. I have a number of water stones, but don’t use them much. I find that a 1200 grit DMT works well as a final stone for many kitchen knives, especially those for veggies. If you are happy with the results from a 2000 grit Shapton glass stone, then why go to any additional trouble with other stones?

 

[Bigfattyt]

I used to strop my knives a lot on a leather strop loaded with chromium oxide.

I found, after stopping, they cleanly, easily shave in either direction. Slice paper like a champ....but didn't easily bite into soft tomatoes. If I went back and refreshed the edge on a diamond stone or ceramic...poof...slice slice....

I'm pretty anal retentive about making sure my edge does not have a bur on it. I can get a knife to shave cleanly in both directions and still have a bur...if not careful about the bur.

I've found that I have to be very cautious about strop use. Light/no pressure, careful with angle, to keep the strop from rounding the apex and actually increasing the angle at the very apex (like a polished mini micro bevel).

I tend to go back to Spyderco medium, then light passes on the Spyderco fine.... often I skip the strop.

 

[kreisler]

I've found that I have to be very cautious about strop use. Light/no pressure, careful with angle, to keep the strop from rounding the apex and actually increasing the angle at the very apex (like a polished mini micro bevel).

That's wahht i appreciate about my unusual stropping method: compound on raw wood, no leather. No caution needed, and i actually strop with notable pressure. The edge goes from 204M finish directly to mirror polish finish within a few strokes. C R A Z E E E. Very fast cutting. Well, similar to the effective fast cutting of a powered stropping wheel i guess. With the difference that you have more control over the metal removal. Main difference to conventional stropping: the knife is held still, fixed in space. It is the wood which moves in space, like the bow of a violinist, with the violin being the knife. This way one has absolute control over the stropping angle. It is basically 'Stropping For Dummies' lol.

I was just commenting. The topic of this thread is cutting tomato skin.

The most challenging way of cutting tomato skin is when the blade is absolutely perpendicular to the skin surface, also called "normal to the surface", i.e. you're not trying to slice into the skin "at an angle" (other than 90.000°):

If your knife can cut into tomato skin at 90.000° without force (just by the weight of the blade), then oooh it's time to stop all further efforts. That's scary sharp. Wrongly stropped edges can cut at 30, 45, 60 degrees just fine, but not at 90 degrees. On youtube, you'll see most presenters cut into tomato skin at 30-60 degrees. That's easy.

 

[wade7575]

I don't have any problems cutting a tomato with a polished edge at all if sharpened properly,with a polished edge and cutting a tomato you need to slide the knife ahead a bit first to get the skin to cut then the knife will slide right on threw it.

I often sharpen mine as high as 8k with a Suehiro G8 stone and that edge works great on everything thing for me.

 

[Alberta Ed]

A slightly coarse edge works best to slice soft fruits like tomatoes. I rarely go above DMT red/600 grit with tool steels and super steels. DMT green/1200 grit works for the very fine edge needed for wood working tools in high carbon steel.

 

[Craig James]

You’ll be here all day debating the pros of polished vs toothy edges for the humble tomato. For the purposes of your post however a highly polished edge is more than capable of very cleanly slicing even the ripest tomato

 

[Obsessed with Edges]

I've found that cross-sectional edge geometry at or below ~ 25° inclusive starts to render the finish almost moot, when slicing a tomato. The thinner the edge geometry gets, the less the finish (toothy or polished) will matter, so long as the edge is fully apexed and clean.

Thicker/wider edges will handle it better with a toothy finish, however. If a typical kitchen knife at factory geometry isn't handling a tomato well, the quick & dirty way to fix it is to make sure it's fully apexed & clean of burrs with a few passes on a fairly coarse stone, or on an aggressive, grooved kitchen steel which leaves an edge similar to what a file would do.

 

[sodak]

I'll put in my bid here for high hardness steels. I have always found the softer steels usually found in cheaper knives (but not always) to be much more challenging to sharpen then a good quality steel in a high hardness (62+ HRC).

I don't waste a lot of time chasing burrs, which for me, is one of the more frustrating things about sharpening.