cutting theory

[Marsot]

When we cut a piece of wood (for example) with a blade, it needs less efforts if the blade is inclined.
I'am looking for information of the thercal explanations of that (articles, formula, etc.)

thanks in advance

 

[CarlRexHubbard]

When you say inclined are you referring to the angle of the edge as it relates to the matter you are cutting or the angle of the blade leaning left or right. Basically are you talking of left and right tilt or up and down tilt? I think that the first scenario deals with the human shoulder joint. If you were to think of your arm's full rotation as a circle, the area in front and just below your hip usually allows intertia and gravity to take over while still being fueled by muscular contraction. This specific flexion of the shoulder would put the blade you were holding at a specific angle, which is inclined. If you were to chop something at this area, your effeciency would certainly increase as opposed to something at slightly below chin level, where the blade would be parallel to the ground. This placement would not be as beneficial because you joint angles are not at their strongest position and intertia would not be as strong. The intertia would not be as strong because your blade velocity would not be as high at that point as opposed to chopping something at hip level. As for left and right tilt I would say that after a few "practice swings" you naturally adjust your tilt so that the edge will contact the desired point. Tell me if I answered your question correctly, I was not totally sure what you meant. Hope this helps.

 

[Cliff Stamp]

There are a number of factors at work in that case. First off wood has a grain structure and the amount of force required to make a cut is vastly different depending on how you are cutting as compared to the grain. In the most extreme case take a piece of strapping and drive a blade into the middle of the board while it is on a scale and look at the amout of force required to make the cut. Now take a similar piece of board and split it through the edge, once you start the cut the grain actually splits around the blade and there is little cutting. The amount of force required is tremendously different.

A similar process is at work when you cut straight down into a piece of wood as compared to when you slice into it an at angle. When you cut straight into the wood, the sides of the cut must be compressed so as to allow the blade to pass through. Wood requires a tremendous force in order to be compressed. Again just put a piece of wood on a scale and put say 200 lbs on a point contact and see how much compression resu
lts. And to make matters worse you are cutting often right through the grain which is very difficult, it requires maximum force.

On the other hand when you slice into the wood at an angle, the wood is not compressed at all. What happens is that the free side of the wood splits off and thus opens up allowing the blade to pass through. As well you are slicing through the grains with a large percentage of the cuts being along them intead of perpendicular which reduces the force required.

For example :

http://www.physics.mun.ca/~sstamp/images/meuk_wood_cut.jpg

This is an angled cut through a small piece of wood. The lines marked are 2" apart. Note that the blade was able to shear right through the piece of wood, past the halfway point with both cuts. If I just try to cut down I only penetrate a small fraction of an inch.

The blade was a MEUK modified by Ed Caffery to those that are curious.

Another consideration is the angle of the blade, as the blade angle is changed relative to the cut, the amount of force required to make the cut drops. This is because you are slicing rather than doing a complete push cut, and slicing requries far less force as you are tearing through fibers intead of compressing them until they burst.

For more information you can check out "The complete guide to sharpening" by Leonard Lee which contains a wealth of information along the lines of the above and much more.

-Cliff

 

[Esav Benyamin]

If I understand your situation correctly, you wouldn't have a problem if you cut a piece of wood end-on, going in the same direction as the grain. But when you cut against the grain, the knife edge is encountering those fibers backing each other up very closely. The more you angle the knife edge, the fewer fibers support each other: you are cutting them one at a time, before encountering the next one.

You get the same effect with a piece of meat, which is why when you carve a roast you're supposed to cut against the grain of the meat, so the diner will only have short fibers to separate on his plate.