# Blades and edges - why do they cut? **URL:** \ **Category:** Factual Questions\ **Created:** [March 13, 2007, 2:35pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749 "2007-03-13T14:35:59Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Merijeek](https://avatars.discourse-cdn.com/v4/letter/m/f05b48/32.png) [@Merijeek](https://boards.straightdope.com/u/Merijeek)\ **Post date:** [March 13, 2007, 2:35pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/1 "2007-03-13T14:35:59Z")
If I take a knife and put it against, say, some softer wood, and I lean a lot of weight against it, it will probably dig into the wood a little bit. However, if I pull the knife back and forth (while applying much less pressure), the blade will cut much more deeply into the wood. Why? What’s special about the back-and-forth motion, and why is it necessary to get the most out of a blade? Is it just because each pull/push is sliding some of the just-cut material out of the way or something more? -Joe ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [March 13, 2007, 2:45pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/2 "2007-03-13T14:45:36Z")
When you push down into the wood, the blade crushes the wood to a certain depth. Sawing back and forth moves the sawdust away, letting you crush to a further depth. Serrated blades have pits and channels to shovel away more of the cuttings than a non-serrated blade. ---
**Author:** ![Turek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/turek/32/18782_2.png) [@Turek](https://boards.straightdope.com/u/Turek)\ **Post date:** [March 13, 2007, 2:51pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/3 "2007-03-13T14:51:12Z")
And “non-serrated” blades have serrations. They’re just too small to see unaided. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [March 13, 2007, 3:28pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/4 "2007-03-13T15:28:25Z")
It’s the tearing motion of the blade ridges. ---
**Author:** ![Merijeek](https://avatars.discourse-cdn.com/v4/letter/m/f05b48/32.png) [@Merijeek](https://boards.straightdope.com/u/Merijeek)\ **Post date:** [March 13, 2007, 3:37pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/5 "2007-03-13T15:37:29Z")
[QUOTE=Turek] And “non-serrated” blades have serrations. They’re just too small to see unaided. [/QUOTE] Really? That’s interesting - I had no idea. If I sharpen something with a whetstone, is it actually creating tiny serrations that I can’t see? -Joe ---
**Author:** ![Turek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/turek/32/18782_2.png) [@Turek](https://boards.straightdope.com/u/Turek)\ **Post date:** [March 13, 2007, 3:45pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/6 "2007-03-13T15:45:59Z")
[QUOTE=Merijeek] Really? That’s interesting - I had no idea. If I sharpen something with a whetstone, is it actually creating tiny serrations that I can’t see? -Joe [/QUOTE] That was my understanding, although after some googling of “micro-serration” I see that there is some debate as to whether or not they are a good thing. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [March 13, 2007, 3:46pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/7 "2007-03-13T15:46:26Z")
[QUOTE=Merijeek] If I sharpen something with a whetstone, is it actually creating tiny serrations that I can’t see? [/QUOTE] As I understand it, yes. That’s why you move it across the whetstone perpendicular to the blade edge. The rough surface of the whetstone gouges microscopic serrations in the edge. You’re supposed to use a steel the same way, as seen in [this YouTube clip](http://www.youtube.com/watch?v=9Gmbzs5UgdI). ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [March 13, 2007, 3:49pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/8 "2007-03-13T15:49:16Z")
[QUOTE=Merijeek] Why? What’s special about the back-and-forth motion, and why is it necessary to get the most out of a blade? Is it just because each pull/push is sliding some of the just-cut material out of the way or something more? -Joe [/QUOTE] In part the edge of a blade is a force multiplier of sorts. If you apply 50 pounds of force with the edge of your hand, the forces is distributed over several square inches of contact point translating to 10-20psi. The edge of a blade is only going to put a couple hundredths of a square inch in contact surface with the material, delivering the same 50 psi to lets say .1 square inches. Since the force is not distributed to a point that the material can handle/support, failure occurrs at the point of contact. Imagine the amount of force and effect delivered by poking someone with a baseball bat or poking someone with the same amount of force with a rapier. Same force, less distribution. ---
**Author:** ![Merijeek](https://avatars.discourse-cdn.com/v4/letter/m/f05b48/32.png) [@Merijeek](https://boards.straightdope.com/u/Merijeek)\ **Post date:** [March 13, 2007, 4:20pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/9 "2007-03-13T16:20:27Z")
[QUOTE=drachillix] Imagine the amount of force and effect delivered by poking someone with a baseball bat or poking someone with the same amount of force with a rapier. Same force, less distribution. [/QUOTE] Sure, but that doesn’t explain the back-and-forth motion. By your logic I should actually achieve as much with pressing the blade in as I should with slicing. However, the micro-serration and debris-clearing seems to have covered everything pretty well. -Joe ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [March 13, 2007, 5:07pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/10 "2007-03-13T17:07:48Z")
[QUOTE=Merijeek] Sure, but that doesn’t explain the back-and-forth motion. [/QUOTE] My very amateurish guess has always been that the back-and-forth makes the blade effectively sharper than it appears to be. Let’s say that the two sides of the blade, which meet at its edge, form an angle of one degree. Well, wouldn’t you think that an angle of one-half degree would be sharper? Of course it would, but it would be thinner and therefore weaker and more likely to bend or crimp. However, you can acheive the effective sharpness by using the back-and-forth motion. Sure, I know that none of the above makes any sense, but consider this: Suppose you are on the side of a hill which is at a 40-degree angle to horizontal. That will be a very difficult hill to climb if you try to go straight up. However, if you go off to the side, you’ll cover the same altitude over a much longer distance, effectively lowering the slope of the hill, and making the climb much easier. Similarly, if you are cutting through an object of a given thickness, moving the blade back and forth allows you to cut through the same thickness over a greater distance, making it easier to descend through that object. ---
**Author:** ![daffyduck](https://avatars.discourse-cdn.com/v4/letter/d/ec9cab/32.png) [@daffyduck](https://boards.straightdope.com/u/daffyduck)\ **Post date:** [March 13, 2007, 5:29pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/11 "2007-03-13T17:29:59Z")
When you press down on something, you’re exerting a compressive force. By sawing back and forth, you add a tension force as the micro serrations pull the surface apart. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [March 13, 2007, 5:32pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/12 "2007-03-13T17:32:03Z")
I think **drachilix** was meaning to explain why any knife manages to cut in the first place. In all instances of cutting described, the knife is able to multiply the force put on it because it is so thin. Although this : > [@](#): > > …delivering the same 50 psi to lets say .1 square inches. should read 50 lbf (pounds force), not psi. A typical sharp blade might be .001" thick at the edge, giving in the range of 10,000 psi with 50 lb behind it. Though I doubt most people put 50 lbf on their knives. You could get nearly the same results by pressing the knife in, removing it, cleaning the material, and repeating (as with an axe). This depends on the blade - as **Harmonious Discord** pointed out, the back and forth motion is important as well because the uneven parts of the blade can tear the material (again, due to the force being applied on such a small area). ---
**Author:** ![Sleel](https://avatars.discourse-cdn.com/v4/letter/s/73ab20/32.png) [@Sleel](https://boards.straightdope.com/u/Sleel)\ **Post date:** [March 14, 2007, 3:50am UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/13 "2007-03-14T03:50:55Z")
If microserrations were a good thing then obsidian blades wouldn’t be good for [detailed surgery](http://en.wikipedia.org/wiki/Obsidian), and sharpening processes like [scary sharp](http://www.shavings.net/SCARY.HTM) wouldn’t produce good results. Generally, the finer the polish—i.e. the smaller and fewer the microserrations—the cleaner, better, and easier the blade cuts. I’m not a physicist or an engineer, but I think there are some differences between compression and shear forces that are probably at work here. The other thing is that moving the blade rather than just putting more force behind it keeps the wider material of the blade from jamming in the cut. After all, it’s not just your edge there, there’s this bigger wedge-shaped piece of material supporting the edge that has to force its way into the relatively small area of the cut. As you force the blade deeper into the cut, the sides of the blade are having to push away whatever is on either side of it. Sooner or later, the force you exert on the blade isn’t sufficient to overcome those side forces and the sharpness of your actual edge becomes moot. I suspect that if you were able to make a “blade” that was all edge, like a microfiliment cutting tool, you’d see all the debates about microserrations pretty much disappear. ---
**Author:** ![gromehown](https://avatars.discourse-cdn.com/v4/letter/g/a5b964/32.png) [@gromehown](https://boards.straightdope.com/u/gromehown)\ **Post date:** [March 15, 2007, 11:30am UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/14 "2007-03-15T11:30:50Z")
I think the key to your understanding this is to adapt your view-point from the macro middle-world human-view, to something microscopic. I had no luck finding anything with Google now, but if you can find electron microscope images of both blades and materials they cut, you’ll have no problem understanding the forces involved. (I could never understand how velcro worked until one day I happened upon a highly magnified image of the hooks and loops, and the mystery resolved with a quickness) ---
**Author:** ![Mr.Goob](https://avatars.discourse-cdn.com/v4/letter/m/c77e96/32.png) [@Mr.Goob](https://boards.straightdope.com/u/Mr.Goob)\ **Post date:** [March 15, 2007, 2:28pm UTC](https://boards.straightdope.com/t/blades-and-edges-why-do-they-cut/395749/15 "2007-03-15T14:28:24Z")
As you cut, the material removed, sawdust or chips are called swarf. (I just wanted to say swarf in a sentance.)