Why do Knives cut better in a slahing manner?

N

Nebulae

Joined
Aug 24, 2009
Messages
1,260
So I was cutting a watermelon just now and I noticed that I cant cut it very well at all with just pressure (I'm talking about cutting rings), but I can do much better with a slicing motion. So why do knives cut better with a forward/backward motion? I mean almost every knife, if not all of them cut better with a back and forth motion than with just pressure. Is it because no matter how sharp it is the edge will be slightly "toothy"? Or is it because Kinetic friction is less than static friction. (especially on smooth wet surfaces kinetic friction can sometimes be only fractions of static friction)

I think its both, but which one is more important? On the watermelon I bet its the friction.
 
Imagine you are in a wheel chair. Now, in front of you is a platform that is about three feet tall. How are we going to go about getting up there?

We could simply exert enough force to push the wheelchair and yourself straight up in the air, and then a bit over (basically throwing you) onto the platform.

Or, we could place a ramp, and gradually move up towards the platform.

A knife blade is, essentially, a wedge. A long wedge.
 
The answer has several parts, some of which you alluded to in you post. One thing you did not mention is that when slicing ("slashing" in your post) the effective edge bevel is a lower angle because it is a shearing cut.

This is really hard to explain without visual aids, but imagine a knife with a 30 degree bevel angle, blown up to super size. Cut the knife at 90 degrees to reveal a cross-sectional view. You are looking at a 30 degree wedge. Now cut that blade again, but this time at a 45 degree angle to the edge. Looking at the resulting wedge from a vantage perpendicular to the surface you just cut, the wedge has a narrower angle, about 26.5 degrees.

As you cut the blade at increasingly steep angles approaching parallel to the edge, the included angle of the resulting wedge approaches zero. This is what happens when we slice something. Add the effect of micro-serrations and it is easy to see whey slicing is so much more powerful than push-cutting.
 
sorry bill I dont think I get it let me think on it for a while. I think if I am reading this right, I see how the inside angle on the wedge approaches 0, but I dont see how that is related to slicing as opposed to pushing.

Wait I think I get it. since your moving backward and forward we are concerned about the angle of attack in two different directions. it makes since, but I cant fully grasp the Idea enough to know its true. How do you know this is true?
 
Hi Nebulae, some thoughts to share.

The point of contact is moving, constantly changing the point of contact on the blade with the material being cut.

The angle of attack is at a partial angle to the point of contact rather than 90 degrees as in a "push cut".

Push cuts are more sensitive to edge geometry.

sal
 
This is driving me nuts, what you say makes since, and I think it must be true but I want to Know it.
 
Nebulae said:
This is driving me nuts, what you say makes since, and I think it must be true but I want to Know it.
Click to expand...

You set this up for the scene in Caddy Shack with Chevy Chase.....

You must be the blade Nubuale, be the blade.....
 
Nebulae said:
Is it because no matter how sharp it is the edge will be slightly "toothy"?
Click to expand...

I believe that's part of the answer - as I unnderstand it just about every blade will be "toothy" under high magnification so when slicing, you are also sawing to a certain extent - but the very fine nature of the teeth will ensure minimal effort is required and the action still feels smooth
 
Antonio is correct. The metal grains form serrations on every knife. As you cut they roll over which makes for a dull knife. When a butcher strops his knife he is re-aligning those grains.
 
dont discount the friction though try this little push cut vs rapid slicing on Cheese. I think youl find that if you can keep the blade moving fairly fast it will be easier
 
Hey Nebulae,

As CJ said, you must "be the knife" or even more, "be the edge at the point of contact".

Understanding the intricacies of of a knife can be challenging. Much of it took a long time to sink into my thick skull, especially chemistry.

If your attention is the minds eye, put your attention right on the point of contact where the "cut" is taking place. (even make a sketch if that helps).

A push cut is a pushing a sharp, wide point through the material. While slicing, the point is moving, the angle is more favorable and there is a certain amount of abrading being done, all at the same time.

sal
 
oh ok I think I really get it now (I've been thinking-I know with what right) since when you draw back an forth it could be thought of as slicing at a very very small angle, thus it compounds the existing edge angle.
 
I think of it this way:
At a microscopic level, the edge of the blade is rough like a saw. Saws cut best by moving against the surface they are trying to cut.

To continue the saw simile, when you push cut you are essentially using a wedge or an axe to force the material apart. It takes a lot more pushing force to do that than to use a saw.
 
Added comment:
The simile about saw edges also explains why a coarser final sharpening sometimes is better than an ultimate polished edge. The ultimate polished edge is better for push cutting, but the coarser finish is better for slicing.
 
If you swing the knife and hit the watermelon instead of just push cutting then it cuts surprisingly easy. Too easy in fact, now I have 2 chopping boards instead of one:
IMG_8405.JPG

I didn't think I was swinging my BK-9 all that hard, but the melon offered very little resistance when the blade hit and just cut really easily.

But definitely a 'draw cut' will slice much easier than a push cut, moving the blade makes a very noticeable difference no matter how polished the edge.
 
You must log in or register to reply here.
Back
Top