Sharpness

JGarrison Knives

JGarrison Knives

Joined
May 28, 2005
Messages
5,081
I'm interested in how you guys rate sharpness. Is cutting the hair off your arm sharp enough? Is cutting paper a better test of sharpness? Are there other tests you guys you use to see how sharp your knives are when your finished making one?
 
I measure sharpness by testing the knife on what it was designed to cut. For example, if it's a chef's knife I will test it on product. If the knife is a heavy duty bushcraft knife/camp knife, I will test it out on outdoor uses. Measuring sharpness through the shaving of arm hair or push cutting paper is nice but not always the most accurate unless the knife is specifically going to be used for that purpose.
 
For very fine edges and sharpness, I cut a free hanging hair and also whittle a hair multiple times in the same spot, making a feather stick, if you will, out of the hair. Shaving arm hair is a good test, because even a 220 grit edge will shave hair if done properly and taken to the apex fully.

-Xander
 
Cutting paper is a great test to double check if you have a burr free edge that has come to apex. I test everything on paper as a basic standard.

Will the edge stick to the side of a Bic lighter?

Shave hairs with no pressure?

Free hanging paper cuts?

Those are all fun tests for sharpness and edge geometry.

Try cutting a tomato, some cardboard, rope, cheese and foam. That will tell you a lot.
 
I've been farting around with the "Three finger edge test" which is basically where you see how much pressure you can apply on the edge with your fingers before your brain says "stop you dummy, you're about to get cut!". Works best when you've got other knives to compare to or have done it a lot.
 
I've found that a knife that shaves hair doesn't have to cut well.
The thickness of the material directly behind the edge also influences the cutting.
 
For most knives my standard is shaving my arm (or leg when the arm hair runs out). For knives that I want to be extra sharp, I push-cut receipts.
Obviously sharpness isn't the only factor that affects cutting performance. I would describe sharpness as the "terminal geometry" of the knife, but obviously there are other aspects of geometry that have to be taken into account. I assumed that the OP had these aspects worked out and was only interested in testing sharpness, as opposed to other factors.

- Chris

ETA: Just a funny story about testing sharpness. A 12-year-old son of a guy I used to know once showed me a POS fantasy knife that he had recently acquired and was very excited about. He told me how sharp it was and performed a paper slice test to show me. It mangled the paper more than it cut. It was only able to make it through the paper because the edge was so rough and badly nicked that it acted more like a saw. Upon examining it, I could find no discernible edge at all.
 
With the blades I've been making I will finish to about 2000 grit stone with 0.012-0.004 behind cutting or edge shoulders and roughly 17 dps.
As for sharpness I take the blade to my head of hair and if the blade sticks it will shave. I also like to slice phone book paper as a secondary to make sure there is no dull spots or burrs. I like a bit of tooth on my edge so it bites and knives with good cutting geometry will cut a lot of things even when dull because there is less drag.
 
Thickness does play a part. I also will mention another test many of you already know about, the skin cutting test. Was putting an edge on a knife this evening and almost cut the tip of my finger off. Not to bad but about a quarter inch of the tip is hanging on by a thread. Took a pic but its blurry, just looks like a blister when it not bleeding.
 
I like to use phone book paper, newsprint, the old shaving test, and tissue paper depending what the knife is for. If its a chopper, it gets paper, shaving, and then a chopping test. I also agree that geometry makes a world of difference in the tests. I have a hatchet that will shave easily but isn't much good for slicing tomatoes.
 
I like to fold up the edge of a piece of phonebook paper maybe 1/4" or so and see if I can push the edge straight down into it.
 
I cut paper towel sheets like you cut sheets of printer paper. Hold paper towel sheet in left hand, and start cutting off strips with the right hand. If it can do that, then I know it's past shaving sharp.
 
Here's a video I just came across from the man himself, Bob Kramer, about sharpness. Good guide for sharpness. [video=youtube;xRPrswhMdAc]https://www.youtube.com/watch?feature=player_embedded&v=xRPrswhMdAc[/video]

And here's a guide on how to hone your knife

[video=youtube;MUdrRE7W0b4]https://www.youtube.com/watch?feature=player_embedded&v=MUdrRE7W0b4#![/video]

And finally how to stone your knife

[video=youtube;lUbkPdkUDuo]https://www.youtube.com/watch?v=lUbkPdkUDuo[/video]
 
Last edited:
You must log in or register to reply here.
Back
Top