Thin Slicer, cannot cut tomatoes/skin

R

Rider675

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Feb 15, 2012
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I have a custom fixed blade made from 1/8 1080+ stock, with a thin flat grind kitchen knife style. The relief edge originally was so small you could hardly see it. The knife would not initiate a cut into a tomato, but after piercing the skin with the tip it would glide through it like water. Cuts potatoes impressively. I figured it wasn't getting sharp as it didn't feel sharp, even though it's very thin. I reprofiled a couple times on the sharpmaker in an effort to create a bigger relief edge (15-20 angle). It did, and seemed to help consistency when sharpening as I could feel the edge much easier. It's still very small, but I feel like im holding a consistent angle now. The knife still wont cut a tomato to save it's life, and feels dull (no "bite" despite the thinner edge and small increase in sharpness. Also, ive been going down to ultra fine rods and stropping on canvas and then leather with 1 micron paste.
 
Rider675, Try a few swipes over a Norton Fine india stone, it will put micro serrations on the edge to give it some bite. John
 
It sounds like your knife is dull, but you seem to be doing something to make it sharp.

It might be that the knife is so thin the edge is getting rolled. I'm wondering how the burr feels as you're sharpening. Try looking at the edge with a magnifier (high powered, preferrably) to determine the condition.
 
You are either rounding the edge with fine abrasives or not establishing a edge to begin with.
 
Don't have any bench stones unfortunately. The knife has a mirror polish, and is sharp enough I doubt it has rolled. I'll see if I can find some magnification. I don't think I get much burr on the sharpmaker.
 
knifenut1013 said:
You are either rounding the edge with fine abrasives or not establishing a edge to begin with.
Click to expand...
Ive spent 1-2 hours reprofiling each time, so maybe im still not holding consistent angles? So does this mean the knife was ground correctly?
Another thought, is that maybe im not using enough pressure with fine and uf stones.
 
I'm still struggling with sharp but coarse edges versus sharp *and* polished edges. Really polished edges seem to slide off of harder materials. Plastic is kind of amazing in this respect. Take an edge that will pop hairs off of your arm effortlessly and then try cutting a zip tie with it. It tends to slide off of the hard slick surface of the plastic.

Tomato skin is kind of similar, but not nearly as hard. You need to either pierce it, or have a more coarse edge that "bites".

My thought here is that you have one of two things going on:

1. Your knife is very sharp and very polished. It should pop hairs off of your arm and slice cleanly and easily through phonebook paper. If so, the knife is just too polished for working with food like a tomato. Use a more coarse stone to finish instead.
2. You actually don't have the edge sharp.

So the question is, how does that edge perform with other tests like I've outlined above?

Brian.
 
If you are willing to try...

Take that knife in its current state, do 5 strokes per side at 10microns(~1K) switch side every stroke, deburr if any. Perform 10 strokes at 1micron, strop on plain leather. Maybe this slightly scallop toothy edge will bite the tomatoes skin.

I am suspecting that your current edge is too fine/smooth.

edit: bgentry's nice post got in while I slow poke at the keyboard.
 
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Tomatos and grapes are very unique, in that they really demand a very good/excellent apex on a knife edge. A generally-thin blade profile, by itself, won't help much without the apex being very pure. The apex is sort of the 'toe in the door' that will enable the rest of the blade's profile to do it's job. I have a kitchen knife with a blade that's likely less than 1/16" thick, and it won't cut the skin on a tomato to save it's own life. On the other hand, with some of my EDC pocketknives with thicker blades and much sharper edges, they go through tomatos like warm butter. Adding some 'bite' to a relatively thick edge angle, with a coarser hone, can help. But, with all edges, especially at very high polish, the apex is what makes a huge difference.

I'm certain, in this case, the edge is either rounded/blunt, or there's a rolled wire edge on the blade. That will make all the difference with a tomato.
 
Rider675,

I know bluntcut, bgentry, and Obsessed with Edges are all probably more knowledgeable than I am - I've seen their posts on different threads - but I still wanted to give this problem a shot based on my experience. Their suggestions, in a sense, are to create a type of micro-serrations on your blade. I agree this approach should work; we've all seen so called tomato knives, which are generally cheap and thin serrated blades.

I understand where bgentry's point #1 is coming from, but since your blade is very thin, if it is sharp it should slice the tomato. A razor is very smooth but it will also slice any tomato, even an almost rotten one (they're harder to cut).

You wrote "maybe im not using enough pressure with fine and uf stones." But in my experience, it's always better to use a technique that applies firm strokes with light pressure. In other words, more strokes with less pressure. This makes it easier to apply the pressure evenly throughout the stroke.

You also mentioned spending 1-2 hours reprofiling each time. That's a lot of time, even on a Sharpmaker, which I don't think is intended for that task. I hope you're cleaning the rods to restore the surface.

There are some very good threads on sharpening you can check out. I can also suggest reading the article in the link below, which gives you some theory and practice suggestions, but unfortunately is based on using stones, and you have a Spyderco Sharpmaker. However, most of the practical suggestions can still apply.
http://www.caseyspm.com/Knives.html

The Sharpmaker is great but it's hard to apply firm, but light, even pressure as you work the blade's length down the rods with only one hand holding the knife. Try using one hand on the handle and a guiding hand or finger(s) on the front of the blade, to assure consistent pressure. However, I think this might not be your problem.

I will read on to see what other posts suggest, and the results you get, because I'm intrigued.
 
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Just to be clear, I wasn't recommending micro-serrations as the solution. I mentioned it can help, but in the context of adding some short-term 'bite' to an otherwise less-than-perfect edge profile (slightly rounded/blunt, or a very thick edge angle). This can usually be done with just a light pass or two on a coarse or fine diamond hone (as is often done on kitchen knives with a diamond 'steel'). Just enough to add those 'teeth'. But that's not necessarily the best long-term solution, if the edge is otherwise poorly-ground or too thick. The quick 'teeth' will go away pretty fast, leaving it dull again.

My post was primarily aimed at emphasizing the integrity of the edge/apex profile itself. In other words, a very clean & pure apex, regardless of how coarse or polished the finish might be, micro-serrations or not. For what it's worth, my own edges are often finished to 2000+ grit (mirror or nearly so), and they are wonderful slicers of tomatos and much more. An edge doesn't have to be coarse to work well in this application.
 
Bg: The knife isn't shave sharp, but slices paper like my endura which is hair popping sharp. The endura feels like it wants to bite you, this one just feels silky smooth. My other knives bite into tomatoes very easy.

Bluntcut: One issue with the way im getting these angles on the sharpmaker, is that im raising one side of the base so that I can still keep the knife perpendicular. This means I can only sharpen one side at a time, which I know is defiantly a contributing factor. I sharpened it traditionally (alternating sides) at 30 degrees before and it was much worse, so that's not the main problem.

Obsessed: I went ahead and stropped to remove any burr, and it's now catching the odd hair but doesn't feel any better.

Rj: With my other knives when I start to draw the tomato skin bursts. With this I can saw on it with as much pressure as I like, and it fails to leave a scratch. Yes im keeping the rods clean, and love this knife so I wanted to make sure the edge was established. I also didn't realize carbon steels aren't that hard to sharpen.

Thanks everyone.
 
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Rider675 said:
Bg: The knife isn't shave sharp, but slices paper like my endura which is hair popping sharp. The endura feels like it wants to bite you, this one just feels silky smooth. My other knives bite into tomatoes very easy.
Click to expand...

I'm no sharpening guru, but I'd like to think I've learned something. Your description, to me, sounds like a knife that is NOT fully apexed. By that, I mean that the two edge bevels have not met in the middle. The brown/gray sharpmaker stone is roughly 600 grit, and is one of the first stones I had that I could reliably make shave sharp with pretty much every knife. If the medium sharpmaker isn't making it shave sharp, I think the only possibilities are either an edge that is not apexed, or a knife that can't support a thin edge (poor or no heat treat). Again, I'm not an expert, I'm just speaking from my experiences.

Have you raised a burr, FULL length on one side of the blade? Have you then changed sides and raised a burr FULL length on the other side? Then removed the burr? If not, it's likely that, even though you've spent a crazy amount of time sharpening, that you have NOT apexed the edge.

As I seem to keep repeating recently, you need a more coarse stone. A Norton Crystolon (Silicon Carbide) stone is a great bargain and cuts pretty much any steel you can find.

For me, the more coarse the stone I work with, the better I do. Getting immediate feedback and having to do less strokes to remove metal seem to work really well for me. The DMT XXC is a revelation for me. Fast results. Consistent results. Consistent bevels. ...and I always gets a full length burr as long as I'm patient.

I'll be interested to hear what you figure out about your blade.

Brian.
 
what the hell is the problem here ? the guy has knife that won't shave and wont cut tomato skin and we are running in circles around micro serrations etc ...


the knife is just dull. its edge geometry helps when cutting less tricky material but it's still dull. go back to the grey rods until you got a burr on both sides, light pressure, then get rid of it an polish carefully.

i've cut tons ( actualy) of tomatoes with a 10k edge stropped to 0,5 mic, micro serration for tomatoes is just a myth, if it doesnt cut it's just dull.
 
Rider675,

I'm really intrigued by this problem you're having. It's seems you know what you're doing, from your comments in the posts and your responses to the suggestions in the posts. And it also seems you've also given this a lot of thought and plenty of effort, so you should have gotten reasonable results by now, since you have no problems with your other knives. If you can get your Endura to slice tomatoes, you should be able to achieve the same or similar results with this custom blade.

Could it be that heat treatment on the blade has produced and edge too hard to be sharpened properly?

Could it be that you're only working the relief and not affecting the primary edge, but can't tell because the blade is so thin?

If you've managed to get something with high magnification, like a jeweler's loupe perhaps, it would be interesting to see the effect on the primary edge as you start sharpening with the coarse rod and transition to the finer ones. Also try the old marker trick, coloring the edge so you can tell where the rods are touching the blade and affecting the edge.

You mentioned your relief is at a 15-20 angle. On a very thin flat-ground blade perhaps you might consider going down to 13 or so, or even doing a chisel grind (although that's not feasible in this case because you've already worked the bevel on both sides).

Please keep us posted on your progress or lack thereof.

Note: I'd mentioned that there are good threads on sharpening in the forums, and knifenut1013, who posted earlier on this thread, has three good ones, well worth a read.

The first sharpening : http://www.bladeforums.com/forums/showthread.php/732635-The-first-sharpening
The Burr : http://www.bladeforums.com/forums/showthread.php/697747-The-burr
How to make a strop : http://www.bladeforums.com/forums/showthread.php/640503-Make-a-leather-strop
 
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bgentry said:
(...)Your description, to me, sounds like a knife that is NOT fully apexed. By that, I mean that the two edge bevels have not met in the middle. (...)
Brian.
Click to expand...

That's what it sounds like to me, also. The one really positive indicator of the apex, is when the burr begins to form, and the edge definitely won't feel 'smooth' at that point. The burr leaves it at least somewhat ragged-feeling, and it'll have some 'bite' of it's own. If the burr's there, it'll at least snag or attempt to 'bite' at whatever contacts it. If the edge actually feels smooth, it's either not fully apexed, or it's been rounded/smoothed off by inconsistent angle control. Variability in angle is very easy to do with long time spent on the Sharpmaker's relatively smooth rods; the longer one has to keep at it, the more fatigue will begin to degrade the results.

A coarser hone, as suggested above, will raise a burr on carbon steel quickly. Once it's there, the character of the edge will change noticeably, and the burr can then be cleaned up on the Sharpmaker (go very light & careful), or with stropping, or both.
 
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knifenut1013 said:
You are either rounding the edge with fine abrasives or not establishing a edge to begin with.
Click to expand...

Have not read all the answers, but not establishing an edge to begin with is what I thought. Spend more time on a coarser stone or whatever you are using, then progress to finer. I just got done re-profiling a Stretch to 15 degrees, and got to my ultra fine ceramic and it would not shave when I was done. Looked very closely and could still see the factory grind marks at the edge. I had not got all the way down to the edge, and I could have spent all day on the ceramic and never got sharp. I went back to coarse and established a true full edge then got back to the ceramic. It shaves now.
 
Rider675 said:
... The knife would not initiate a cut into a tomato, but after piercing the skin with the tip it would glide through it like water...
Click to expand...

From OP post#1 - many of us read this as 'sharp'. So it was the baseline where suggestions followed.

My own tests in push (with ~2mm forward motion then down to board) cutting tomatoes (costco - vine ripe with 1 wk wrinkle) skin, where all knives refined thru 0.1micron polydiamond, then plain leather, edge inspected with 20x loupe & 100x usb microscope.

Passed: zdp, vg-10, skd, sg2, m4, blue#1, some other unknown carbons

Failed:
1) Kumagoro Hammer Finish 240mm gyuto v2 steel(similar to white#2) ~16* degree inclusive. It has no problem slice thru pretty much anything, including horrendously gooey tough beef tendon. I can slice/dice tomatoes fine but not as fun push cutting as my blue#1 knife.

2) Opinel #9 carbon reprofiled to zero-grind - sharp on everything but failed to initiate cut thru tomatoes skin, nor it shaves well.

3) Some old German WWII straight razor (marketing as: magnetic steel) - smooth shave sharp but the tomato skin seems wrapped around the edge.

Added scallop teeth geometry to 1) & 2) they both easily make surgical incision to tomatoes. Skipped 3) - didn't want to mess up my in-law antique razor.

********
On other hand, good to re-examine the basic dull/rounded/burr/wire/etc...
 
^^ I'll do the same test with my Dovo razor, though i'll be using garden grown cherry tomatoes which should be easier to cut.
Looks like it's back to the medium stones tomorrow, it needs sharpening anyway. I'll defiantly verify what angle im at, as before was a guess. That's the one thing that has given it bite so far, by making it easier to sharpen and of course the obvious. Not sure how you test the heat treat, but it's done a lot of cooking and it holds an edge longer than expected. 1084+ supposedly has added vanadium and something, im guessing that's why.
 
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Sounds like you're not apexing the edge. If you are not raising a burr on BOTH sides of the edge you still have some work to do. Basically, that edge needs to be reprofiled before your sharpmaker will be overly effective. As you're finding out, reprofiling on the shaprmaker can be a huge pain in the butt and be very time consuming and frustrating. I recommend you pick up a dmt diasharp benchstone for reprofiling work. You can pick up a 2x6" double sided (coarse/fine) diasharp bench stone for under $50 and it will save you a TON of future headache.
 
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