Rigid vs. flexible blades

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DangerZone98

Joined
Dec 7, 2019
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My question here is when do you want one and when do you want the other? To clarify, rigidity here means that you can’t easily bend the knife blade sideways while flexibility means the opposite.

My basic understanding (feel free to correct me on this) is that flexibility is something that we’re looking for in machetes and other light bushclearing tools. When you’re hacking away all day, the flexibility allows the blade to better absorb shock, to roll with the impact rather than to break. Conversely, rigidity is something we commonly associate with kukris and large camp knives, though I’m not entirely sure why. Would it be to decrease vibrations and increase momentum when chopping wood?

What about for folders and smaller fixed blades? When do we want a rigid or flexible blade steel?
 
If someone made a folding fish filet knife, I'd want it flexible. That said, cleaning out blood and guts from the pivot area is not fun.
Of course one could take it apart for cleaning and oiling after each use, which also doesn't sound like much fun.
I really can't imagine any other purpose for a flexible folder. Maybe there's some other purpose I;m not aware of.
My 25-30 year old Rapala filet knife with a 4 inch blade is the most flexible fixed blade I have.

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KenHash said:
If someone made a folding fish filet knife, I'd want it flexible. That said, cleaning out blood and guts from the pivot area is not fun.
Of course one could take it apart for cleaning and oiling after each use, which also doesn't sound like much fun.
I really can't imagine any other purpose for a flexible folder. Maybe there's some other purpose I;m not aware of.
My 25-30 year old Rapala filet knife with a 4 inch blade is the most flexible fixed blade I have.

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I see. Is that an Adamas at the bottom? Awesome knives there. :)

I’m also not a fan of using folders for messy stuff. Fixed blades are my go-to for anything involving food.
 
Flex seems to be good in long blades like swords. Watch the slo-mo of the "strength test" they regularly do on Forged in Fire. Wobbledy-wobble-wobble. Or they shatter.
 
Some bushcrafter want their knife to be harder so it can keep an extremely sharp edge for the entirety of the trip, and they won't have to worry about carrying a sharpening stone.
Softer steel wil flex and bend instead of snap, but they will get dull quicker, meaning they'll have to carry a sharpening stone. Smaller stones will do the job, but some people don't want the hassle.
 
When it comes to long chopping blades that will see impact, I think you want as little flexibility as possible. While it may seem intuitive for flex to absorb shock, it can actually cause failure, especially in things like swords and machetes. I've seen swords take warps and sets and fail when biting into a tree or cutting stand. Because the part that is lodged in the wood will stay clamped into the wedge it bit into then the rest of the blade at the foible or tip will take on all of that shock and vibrate and twist, if you watch and record it in slow motion, you will see the blade distort and if it flexes too much it can snap, i've seen blade ends fly off before after hitting into thick targets with bad edge alignment.
Thats the thing that is worst for overly flexible blades, if you do land a hard hit with incorrect edge alignment then it is much more prone to failing than a thicker stiffer blade.
Usually you want only the end of the blade to flex slightly, you don't wan't any flex near the hilt, it should be very stiff about 70% up the blade if you are going for some flex. You don't want to ever make the entire thing an equal spring, you want the center of percussion to have managed levels of vibration and flex, a stiff rigid edge will chop better and transfer energy more efficiently than a blade that shakes and flexes on impact, where the energy is dispersed and wasted.
 
For small blades, A lot of it is user preference. I have only seen less rigid knives used in meat processing. Impact or prying stresses are a frequent cause of chipping, which is less an issue in small knives which is why they generally run harder Rockwells than larger blades. Its especially common for fish, in the form of fillet knives. I have seen a lot of people use fillet style knives for deer, cows, or anything having to do with cutting meat off though. Its not my preference currently but I have never owned a super high quality fillet knife either.

For larger blades, Flex is partly to help the steel overcome shock. Its also due to the impact of super high hardness can lead to chipping. So we try and strike the perfect balance of an edge that won't be to hard and chip, but also isn't so soft it rolls or dulls quickly either. Also its not uncommon to get a large knife stuck even briefly while chopping and you may pull it out slightly to the side or put bending stress on it that smaller knives are not frequently subjected to.

In the case of machetes a lot of the rigidity is due to the choice of steel generally being 1/8th or under. This is mostly do to trying to make them lighter and easy to swing for extended time as well as to keep it thin for good slicing on branches etc. Machetes are also generally not something most people care about high edge retention either, so a lot of company's will cheap out on heat treats because most people don't use them enough to care. The best machetes would probably be the Rockwell of a camp knife on the cutting edge with a differential heat treat and would probably not have as much noticeable flex as much as most of the cheap machetes.

Most kukris and thicker camp knives in my experience will flex to, Its just not as noticeable. This is due to shorter blade lengths, and thicker stock thickness of steel. Blade length and thickness of the steel are probably the 2 biggest factors in if a knife will be rigid or not. Also just because a machete has less rigidity, doesn't mean it actually flexes as far as a thicker knife. Many top Bowie knives will have to be bent 45degrees in both directions and be able to be bent back. So true flexibility is somewhat relative in my opinion.

As for when you want each, that depends what you intend to use it for.:)
 
Flex on machetes is a by-product of creating a durable light tool with a efficient thin blade for soft vegetation. The traditional machete is something that is designed to be used for long periods of time. Flex to the extent that it is there is not a product feature, but represents a waste of the energy transfer. This is particularly true with swords where too much flex will impair the weapons thrusting ability.

n2s
 
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