Knife Can Shave but....

[peanutaxis]

Hi,

Why is it that the kitchen knife I just sharpened (1200 grit diamond followed by strop with paste) will shave my arm hair easily but I can run my thumb directly down the blade without it cutting my skin?

Thanks,
p

 

[UncleBoots]

Hi,

Why is it that the kitchen knife I just sharpened (1200 grit diamond followed by strop with paste) will shave my arm hair easily but I can run my thumb directly down the blade without it cutting my skin?

Thanks,
p

Does it cut into your thumb if you move your angle of pressure off to one side or the other? If so, what's shaving your hair is a burr/wire edge that's not straight out of the apex. I ran into this just a couple of days ago, so I kept on with the deburring.

 

[Obsessed with Edges]

Chances are there's a burr (or 'wire edge') on the apex. See if the edge will easily cut anything else, like paper, etc. A burr can shave hair easily, but will deflect away or roll over when attempting to cut anything tougher - the edge will seem dull in doing so. If the burr is turned to one side, you might also notice it shaves from one side of the blade with the burr turned down into the skin, but won't if the blade is turned over with the burr turned away from the skin.

 

[peanutaxis]

Does it cut into your thumb if you move your angle of pressure off to one side or the other? If so, what's shaving your hair is a burr/wire edge that's not straight out of the apex. I ran into this just a couple of days ago, so I kept on with the deburring.

Hi. No it doesn't. It really doesn't feel like there is a burr.

 

[peanutaxis]

Chances are there's a burr (or 'wire edge') on the apex. See if the edge will easily cut anything else, like paper, etc. A burr can shave hair easily, but will deflect away or roll over when attempting to cut anything tougher - the edge will seem dull in doing so. If the burr is turned to one side, you might also notice it shaves from one side of the blade with the burr turned down into the skin, but won't if the blade is turned over with the burr turned away from the skin.

Hi. It will easily slice paper all the way down the blade. It cuts the green part of a leek extremely well.
Just did heaps more stropping so confident there's no burr. No change.

 

[Obsessed with Edges]

It's also possible the edge is more polished than is best for it, depending on what compound you used for stropping it. With the stainless used in most kitchen cutlery, they usually work best when they still have some toothy bite for slicing - the same sort of bite you'd definitely feel in your fingertips. If polished too far, they'll lose aggression for slicing tasks, but can still rely on the thinness of the edge geometry for simple cutting tasks like in paper, or in cutting most fruits & vegetables, which doesn't require a very keen apex to do (except for tomatos, which won't slice easily with a rounded/blunt apex). A lot of factory edges on kitchen knives rely on that specifically, as they can still work fairly well for what they were designed to do, but also won't be sharp enough to risk much injury to the fingertips in kitchen use. I used to notice the same tendency when feeling the edge with my fingertips, if I polished them too much. It tends to round over the apex somewhat, if not done very carefully over a hard stropping substrate.

An edge that's thin, highly polished and acutely keen at the apex will definitely cut the skin if not careful. The difference will be in whether the cut is felt or not, as the high-polished edge can cut you so cleanly, you might not even feel it. So, if the edge isn't cutting your skin at all, but doing other things well enough, it's likely somewhat overpolished or slightly rounded at the apex.

If it were me in the same circumstance, I'd just sharpen on the 1200 diamond and do as much deburring on the hone as possible, and skip stropping with the compound. Then test the edge to see how it cuts in the kitchen tasks you normally give it. Whether it shaves or not isn't necessarily important for any cutting task except shaving.

 

[SCTorrero]

Hi,

Why is it that the kitchen knife I just sharpened (1200 grit diamond followed by strop with paste) will shave my arm hair easily but I can run my thumb directly down the blade without it cutting my skin?

Thanks,
p

Sounds like a burr, unless you are an armadillo. Seriously though, I am having a hard time understanding the physics at work for a knife that can easily cut paper and shave hair, but can't cut skin. Perhaps it doesn't "feel sharp" (maybe if there is a high polish, as mentioned), but that is not the same as "not actually sharp." Some good advice above. I hope you figure it out.

 

[elevator man]

When you say you did heaps more stropping, how so?

I have found much more success in a final edge, easily whittling hanging fine arm hair, by stropping on a compressible substrate with a moderate amount of pressure to be sure the strop material wraps around the apex to fully deburr.

I found for me a less compressible substrate like balsa wood combined with tons of very low pressure stropping did nothing to remove the burr and by my guess made it worse. Literally would be a mirror polished edge you could take cool photos of, see no burr, and it would fail first serious thing I cut.