Completely Sharp

[M forge]

How do you know when you have a blade as sharp as it can get, and what test do you do to check it.

Iknow there have to be lots of ideas and opinions on this. Sorry if this question has already been asked.

 

[THG]

There is no such thing as "as sharp as it can get." If you can find finer and finer sharpening materials, then you can keep going sharper and sharper.

A test for a good cutting edge for me has always been the paper-cutting test.

If you're looking to see if you have a finer edge, you might try shaving some arm hair.

If you're looking for even higher levels of sharpness, see if you can split a hanging hair.

Another high-sharpness test going around lately is push-cutting toilet paper.

 

[singularity35]

The question should probably be "is my knife as sharp as I can get it?"

 

[richard j]

here is a vid of a knife i made cutting free hanging newspaper. i could get it sharper but it would be a waste of time. http://knifetests.com/kII.html (the video is a little dark)

 

[M forge]

I guess this is a very broad question and maybe I didn't state it right. I was just wondering how folks decide when their blade is sharp enough and what they use to test the edge. I know there are lots and lots of ideas on that and wanted to hear different ways.

 

[Obsessed with Edges]

The question should probably be "is my knife as sharp as I can get it?"

I'll modify that a bit more, and ask, "as sharp as I can get it today?" As mentioned previously, there's seemingly no limit, except for our own inconsistencies in technique. Basically, the better you can control your angle, and the better you regulate pressure, the better your results will be. Keep the angle consistent, and gradually lighten pressure as the edge gets more refined. Combine that with a logical and tightly-sequenced grit chain, and you'll start to find out just how far 'sharp' can go.

Slicing fine paper, like newsprint, phonebook pages, etc., is the best 'real world' indicator of a good edge, I think. I check all my edges that way, and if they consistently slice through the paper, along the full length of the cutting edge (heel to tip), without slipping or snagging, I then find those edges to be excellent for virtually all tasks. Shaving edges are fun, but when they're that fine, they won't necessarily be all that durable. And wire edges will often shave, but will fail quickly under tougher cutting chores.

 

[M forge]

I can get them pretty sharp using my belt grinder followed by a light buff with some green chrome compound or a DMT combo stone. I use 3/4" hemp rope to test. however that is for a hunting or camping type knife. I would do something finer say for a sushi knife.

I am interested in hearing more methods and tests, let hear em guys.

 

[unit]

There are many levels of sharpness, and many ways to gauge sharpness.

Sometimes I want to turn sharpening into a hobby and see how sharp I can get it.

Other times I simply want functional sharpness that will get my jobs done for the next few days.

There is a wide range of users here...there is not even a consensus on what "sharp" means or how to define it.

 

[me2]

I've used all of the following at one point or another.

Cut paper
Fillet paper
Shave arm hair
Catch arm hair above skin
Whittle beard hair
Whittle wife's hair (from brush, she bites if I yank one out)
Catch hair on the back of my head about 1" above the skin
Cut foam peanuts
Push cut a piece of phone book paper folded vertically about 1/2 inch
Slice a light plastic grocery bag
Push cut a light plastic grocery bag
Slice a folded piece of paper from a pocket notebook standing up right on the corner of my table
Cut a paper towel roll standing on the table
Slice toilet paper (only tried once with mixed success)
Push cut toilet paper (only tried once with mixed success)

I've heard there's a german knifemaker who push cuts a grape without moving it, but have not seen it.

 

[HeavyHanded]

I've heard there's a german knifemaker who push cuts a grape without moving it, but have not seen it.

Now that's a test.

I used to use a rapid combo of 3/8" sisal rope, furniture grade foam (slice it thin and try to cut off pieces), and crosscutting fine catalog paper. What I found was that different grinds performed better at each task, but an edge that could do all three well was plenty good for just about any other job too. Most of the time I see how well it can whack a clean slice through some TP using just the belly of the blade, then see how it does push cutting the same piece of TP. I generally hold the blade raked back a bit. After that, slicing across some freehanging paper towel and of course crosscutting phonebook paper or similar. Problem is, different grinds do different things better, so anything that can do all of the above fairly well is a good edge. For highly refined edges, nothing is better than whittling hair, hanging hair test, dry shaving whiskers without irritating your face much. Even a fairly coarse edge can do all the other tests, but your face can always tell what kind of grind an edge has.

 

[pwet]

this is a fun edge test.


[video=youtube;Zk3EB5VRUQQ]http://www.youtube.com/watch?v=Zk3EB5VRUQQ&feature=youtu.be[/video]

 

[Obsessed with Edges]

I'm not sure push-cutting grapes is that hard. Thin blades with acute edges do that fairly easily. And as is the case with so many other 'tests', there is a lot of variability between grapes too (some are thicker-skinned, and the relative firmness makes a difference too). I'd thinned the edges on a couple of Opinels a while back, including taking the edge to a zero-bevel convex, and had some fun tossing grapes at the edges of these knives & splitting them. One thing I did notice, a thin convex edge is much better at this than a V-bevel. The hard shoulders on a V-bevelled edge really hang up on a grape.

 

[me2]

Like I mentioned, I haven't seen it, but I figure the blade is moved so that if the grape moved, it slides along the table top. The same maker grinds edges down to 0.01" for knives that chop bone, or so I'm told.

 

[unit]

Just keep in mind that many sharpness "tests" rely on a certain amount of technique.

Also it is good to note the speed that the blade moves in any "test".

 

[thombrogan]

The same maker grinds edges down to 0.01" for knives that chop bone, or so I'm told.

That's not from the same source who got many of us believing the same maker could harden 13C26 to RC 65-67 is it? : D

 

[me2]

Maybe. Ive never heard that before. I know the maker considers 0.02" rather thick for a chopping knife. After using a large chopper with a 0.015" edge, I tend to agree.

 

[thombrogan]

What sort of steel would hold up to chopping bones with 0.01" thick shoulders?

 

[me2]

Dunno, and dunno what kind of bones either. I know I hit some nails accidentally with a 1055 chopping blade with a thickness behind the bevel of 0.015" and, while damaged, the damage didn't extend past the cutting bevel, which was only 1/32" wide. I don't have any doubt the bone chopping can be done. I'll check around and see if I can find the steel.

 

[thombrogan]

That'd be wonderful. Thank you. It's probably going to be 13C26, but my idiot-mitten mind is thinking 4340-mod, S5, 8670-mod, 9260 and the like.

 

[foxx]

There are so many variables to sharpening. Technique is important, so is the intended use of the knife, and it's edge geometry. I can push cut or slice thin catalog paper with some of my machetes, sure it's overkill, but a good practice for me.
It's hard to shave my thin arm hair with a knife, but I know when it can do that, slice through newspaper, catalog paper, and cut wood without chipping or rolling, then I've got it sharp enough for me.