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

[Toothy Wolf]

So I have gone back to the drawing board and tried out a few different things, watched a bunch of videos and read a bunch of old threads here and other places. Came up with I’ve been stropping at to high of an angle and rounding over the apex. Not a lot, but enough. I lowered my stropping angle to less then the angle I was sharpening at. I did this after the 3000 Chosera and got an edge that was tree topping sharp and fell threw the skin of my tomatoes! I really don’t see the need to go to a higher Grit stone with these results. Not sure I could, or would need to get a better preforming edge then this. Thanks for the advise.

 

[kreisler]

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.

I quickly coded a visualization in 3D (since the word "perpendicular" only makes unambiguous sense in 2D, i.e. in plane geometry), hope this helps, also for future reference (just remember/search <kreisler perpendicular> and you'll find this post in the search hits). The 6 pics show the identical object, just from 6 (slightly) different viewing angles to help you understand what you're seeing:

Graphics3D[
 {
  Sphere[{0, 0, 0}]
  , Polygon[{{1, 0, 0}, {1, 1, 1}, {0, 0, 1}}]
  , Polygon[{{1, 0, 0}, {1, 0, 1}, {0, 0, 1}}]
  }
 ]

The 3D-visualization is self-explanatory, it shows two "blades", one blade cutting at an exact 90.00000°-angle ("perpendicular"), the other blade cutting at a non-90° angle. And i am claiming that it is way way harder for a sharp edge to cut (by the weight of the knife) at 90° than at non-90°. Neroknives doesn't mention the perpendicularity and he uses strawberries instead of tomato skin but he does cut at perpendicular angle and with the weight of the knife and shows the principle of sharpness which i've been talking about. He is showing two knives, and to me there is no doubt (unfortunately he doesn't show it) that both knives could cut his strawberries at a non-90° angle (with the weight of the knife)!
Personally, i am scared of testing my freshly sharpened knife at 90° angle on a tomato because it is easy to FAIL the test and it'd make me feel bad about myself haha.

Here the imho most important/relevant video on sharpness testing. I never go for shaving hair or hair whittling, but for 90° tomato angle as he (basically) does:

 

[Realysm42]

So I have gone back to the drawing board and tried out a few different things, watched a bunch of videos and read a bunch of old threads here and other places. Came up with I’ve been stropping at to high of an angle and rounding over the apex. Not a lot, but enough. I lowered my stropping angle to less then the angle I was sharpening at. I did this after the 3000 Chosera and got an edge that was tree topping sharp and fell threw the skin of my tomatoes! I really don’t see the need to go to a higher Grit stone with these results. Not sure I could, or would need to get a better preforming edge then this. Thanks for the advise.

This is interesting (and confusing) idea to me.

If you lower the angle, won't you miss the apex you just created?

Not saying you're wrong (sounds like you're getting good results with it!) I'm just curious to understand.

 

[Toothy Wolf]

When I lowered my stoping angle, I got the resuts I was looking for. I use the rough side of a bench strop. The best I can figure, I rounded over the apex slightly. Now I strop on a polishing stone and leave the leather out of it entirely. Been getting consistent results that I’m very pleased with. Or I’m entirely wrong about the whole thing, which would be a complete surprise to me.

 

[Obsessed with Edges]

When I lowered my stoping angle, I got the resuts I was looking for. I use the rough side of a bench strop. The best I can figure, I rounded over the apex slightly. Now I strop on a polishing stone and leave the leather out of it entirely. Been getting consistent results that I’m very pleased with. Or I’m entirely wrong about the whole thing, which would be a complete surprise to me.

You're on the right track now.

On compressible stropping material like leather, the compression of the material allows it to wrap around or roll over the apex. And more so, if the stropping pressure is more heavy. So, to compensate for that, holding the angle lower than would be used for sharpening on stones is the key. That is what will protect the apex from rounding over.

Don't know if you've seen it yet, but the sticky thread linked below is a great reference on the concept of maintaining the appropriate angle and pressure on a leather strop, with perfect visual aids.

Stropping: angle plus pressure

A lot of people have noticed that stropping can make an edge duller as well as sharper. Some say you need a lot of pressure, some say you need no more than the weight of the knife. Actually, you need to get the right combination of angle and pressure. I tried to capture this relationship...

www.bladeforums.com

And on a hard strop like the polishing stone, the lack of compression in the surface is what protects the apex. A strop of hard wood also works very well in this regard, and also would be a litte better at preventing the formation of more burrs, as can happen on a stone.

 

[Toothy Wolf]

You're on the right track now.

And on a hard strop like the polishing stone, the lack of compression in the surface is what protects the apex. A strop of hard wood also works very well in this regard, and also would be a litte better at preventing the formation of more burrs, as can happen on a stone.

Thanks Obsessed. That’s what I’ve been learning. I’ve got some blocks of MDF I’m going to coat with various sizes of Diamond spray/paste. I’ve read these work great for woodworking tools, so I figured why not knives. I really am happy with what I’m getting now. But I figured just for fun some rainy day.

 

[cudgee]

Thanks Obsessed. That’s what I’ve been learning. I’ve got some blocks of MDF I’m going to coat with various sizes of Diamond spray/paste. I’ve read these work great for woodworking tools, so I figured why not knives. I really am happy with what I’m getting now. But I figured just for fun some rainy day.

Never hurts to experiment and try different methods, one of the best ways to gain knowledge and information.

 

[FK]

I agree it is due to burr formation.
Cutting arm hair is not a very good indication of burr free apex, the burr will cut hair but if it is folded somewhat,,, it will slide off hard skin in fruits. If we chase a fine grit stone progression then strop too much, we will draw out a very small fine burr that is hard to see without a microscope.
The apex is formed with the coarse/med stone and work after that while chasing a polished edge will ALWAYS form a burr which must be removed to max efficiency at the apex. How we remove the burr is key to a fine/polished edge cutting efficiency.

For burr removal on high alloy steels,,,,, try rock hard felt with 1 micron diamond spray. This will efficiently remove the burr down to the clean apex. I draw the blade at an angle to the normal stropping/sharpening stroke direction but not enough to cut the felt strop.
Next a few very light passes on sub micron diamond on hard thin leather to polish the apex.,, I like 0.25 micron.

A polished edge will easily cut tomato skins if it does not have a folded burr. It will also last much longer wrt a coarse edge.

Regards,
FK

 

[WC53]

The dreaded tomatoes! I was trying to work with a relative today on her sharpening for maters….. did not go well.

Can anyone recommend a simple YouTube video on sharpening that I can forward to her..lol… she has a dmt coarse/EF. Knives are a global, Mac pro and a W Ikon. If it’s on YouTube it will be true.. My take on her technique is not apexing and burr removal.

Thanks!

 

[kreisler]

And i am claiming that it is way way harder for a sharp edge to cut (by the weight of the knife) at 90° than at non-90°.

I am observing yet another difference, interesting:
A tomato have a north and south pole. It is harder to cut the skin (at 90° angle of attack) from north to south, i.e. on a meridian (circle of longitude), than (at 90° angle of attack) from east to west, i.e. on a circle of latitude.
This observation was particularly true with such oval cherry tomatoes:

If your knife can cut such a cherry tomato in any direction (always at 90° angle of attack!) equally with ease, then kudos to you! With my lazy sharpening efforts i content myself with 90°-cutting from east to west; that's pretty sharp already

 

[Triple_D]

I'm a year late fo the party, but I found this video quite informative about how to remove a burr from a knife edge. It works for me.

 

[cudgee]

I am observing yet another difference, interesting:
A tomato have a north and south pole. It is harder to cut the skin (at 90° angle of attack) from north to south, i.e. on a meridian (circle of longitude), than (at 90° angle of attack) from east to west, i.e. on a circle of latitude.
This observation was particularly true with such oval cherry tomatoes:

If your knife can cut such a cherry tomato in any direction (always at 90° angle of attack!) equally with ease, then kudos to you! With my lazy sharpening efforts i content myself with 90°-cutting from east to west; that's pretty sharp already

Just tried the North to South method, though not on oval cherry tomato's, don't have any. But this sliced through them with just the weight of the knife. Is this what you mean.???

 

[kreisler]

Is this what you mean.???

yes! And there is no doubt that super sharp knives have no trouble to pass the cut test.
Btw the point is not to produce slices of tomato but to let the edge sink in 1mm (by the weight of the blade), i.e. just enough to prove to oneself that the edge is super sharp and could sink in at any spot along a circle of longitude. It's best to start this test near the belly of the oval tomato (equator), and not directly at the north pole. When you cut into the tomato flesh like that (scoring, 1mm depth), you produce two opposite 90° edges (correct!), but when you produce parallel slices of a tomato (incorrect!), only 1 slice will have 90° edges ((your left tomato part doesn't seem to have a 90° edge, ah never mind)).
Again, for the correct tomato cut test, one does not produce parallel slices; instead, one litters the skin surface with tens of tiny cuts (cutting length = 7mm), all at 90° angle of attack, and in all possible cutting directions, not only along circles of latitude, which was the point of my previous post. And by doing so (littering), one needs to be dead honest/strict with oneself and realize what the edge manages to do on the tomato skin and what not, compared with a brand-new Gilette razor blade. Cutting at 45°/135° angle of attack, or cutting full parallel slices would be "cheating".

Anyway, it could well be true that small oval cherry tomatoes have a different cutting resistance than a normal big spherical tomato. Check the video at 03:31, the blade fails to cut along the meridian (at 90° angle of attack); and for his other cuts he is cheating, namely by not cutting at 90° angle of attack!

Good example of how the cutting test should not be performed (she cuts the green pepper at a very shallow angle instead of at 90° degrees):

 

[cudgee]

Btw the point is not to produce slices of tomato but to let the edge sink in 1mm (by the weight of the blade)

But i was making lunch.