Large blade physics question.

[Laurentius]

This is something that has been taxing my mind of late being as I am not an engineer and have no more than an O level in physics from more than forty years ago.

Given that a long curved blade (unless it is double edged) is usually sharpened on one side only ..... which kind is the most effective from a mechanical point of view (science not folklore).

Bill hooks, kukri's, slashers, scythes and some machetes are sharpened on the inside of the curve.

Parangs, alos some machetes, sabres and samurai swords are sharpened on the outside.

I would guess it comes down to a question of a little more than which side it is sharpened on, and the weight distribution would have an effect too, but ultimately I expect the effectiveness for an equally sharpened artefact. (that is to say with the same grind) would come down to what degree of force can be brought to bear on the object for how long.

I have personally used more blades that are sharpened on the inside, eg bill hook for snedding, scythe for mowing, and machete of long bill hook configuration for clearing relatively soft vegetation. Are these the most effective and if so why?

It does make me wonder, because parangs are often cited as effective cutting tools, but they are sharpened on the outside so far as I am aware (I have never used one)

 

[sunndog]

I'll chuck my clumsy two penneth in.....but side step the physics

The difference between the two blade styles you describe (curved and re-curve?) to me is a reflection of what they are meant to cut

Take an axe for example (curved) it is shaped in this way so that a smaller surface area is first in contact with the wood to enable it to bite deeper
A scythe is shaped (re-curve) so that the grass is forced along the edge to a greater degree than a straight or even worse, curved edge.....this is simply because grass would bend away from the axe blow thats so good at biting into solid wood

So i s'pose what saying is that to me, there is no "best"......its different strokes for different folks

 

[Goatboy]

Without getting into the physics of it it's probably more down to end use. External sharpened curves will deliver more cutting power. Think axe. But it wont collect the material to be cut like an internally sharpened blade, hence why sickles, scythes and billhooks which are often used to cut multiple small things at once are more prevelant. Whereas axes, scimitars and parangs which often cut bigger things one after the other are of more use.
Also how things are weilded makes a difference. A sickle shape would be harder to pary with and would more likely get stuck in what you are forcing it into. So it's more down to what and how you use it.

Sent via smoke-signal from a woodland in Scotland.

 

[Goatboy]

As an aside if you look at what all the folk (I don't agree with) that say the Katana is the finest cutting blade ever. Yes it's a great cutting tool but Japan's traditional fighting technique has very little of what we Europeans think of as swordplay. I think of it more like a quickdraw gunfight. Over in one or two strokes with little or no blade on blade contact. The curve of the blade doesn't lend itself to it. Where aa our generaly straight blades are goog for blended fighting. Cavalry sabres again offer huge cutting power but are used in different ways.

Sent via smoke-signal from a woodland in Scotland.

 

[Laurentius]

I'll chuck my clumsy two penneth in.....but side step the physics

The difference between the two blade styles you describe (curved and re-curve?) to me is a reflection of what they are meant to cut

Take an axe for example (curved) it is shaped in this way so that a smaller surface area is first in contact with the wood to enable it to bite deeper
A scythe is shaped (re-curve) so that the grass is forced along the edge to a greater degree than a straight or even worse, curved edge.....this is simply because grass would bend away from the axe blow thats so good at biting into solid wood

So i s'pose what saying is that to me, there is no "best"......its different strokes for different folks

I guess you have a point there, that there is more to it than Newton's laws. If you struck a blade of grass with the same force as a tree trunk, the grass would move out of the way and whatever you hit the tree trunk with would make some mark. Like for like though, blade of similar weight and lenghth agains the same kind of object, small branches, tree limbs, I know some people favour parangs and others big up the kukri as a tool of all work, but which in a scientific experiment, equal force against an immovable object of the same density would make the deepest cut?

As for me I was clearing the communal areas on the allotment the other day, The most effective way is with a machete in one hand, stick in the other to keep the vegetation upright and in the path of the swing rather than just hack at it like a demented samurai. Unless of course you are in favour of power scythes and hedge trimmers, which I personally do not like.

 

[Laurentius]

As an aside if you look at what all the folk (I don't agree with) that say the Katana is the finest cutting blade ever. Yes it's a great cutting tool but Japan's traditional fighting technique has very little of what we Europeans think of as swordplay. I think of it more like a quickdraw gunfight. Over in one or two strokes with little or no blade on blade contact. The curve of the blade doesn't lend itself to it. Where aa our generaly straight blades are goog for blended fighting. Cavalry sabres again offer huge cutting power but are used in different ways.

Sent via smoke-signal from a woodland in Scotland.

I have to admit that I don't come across that many Ninjas in my travels, but then again if they were there, I expect they would be that stealthy that I would never see them anyway

 

[sunndog]

which in a scientific experiment, equal force against an immovable object of the same density would make the deepest cut?

Axe......Most weight over the smallest cutting surface and among the least resistance

Internal curve will be worse in general simply because you are trying to cut the most material at once

 

[Laurentius]

Axe......Most weight over the smallest cutting surface and among the least resistance

Internal curve will be worse in general simply because you are trying to cut the most material at once

Yes that would satisfy Newtonian physics, and I suppose the longer the handle the better as it will have more velocity even for a small head.

 

[rik_uk3]

Get a bunch of different blades together, wack the crap out of some wood and just ignore the science and see which one(s) works for you.

 

[British Red]

Convex is more efficient at impact cuts, concave at longer slicing cuts, straight at planing cuts.

 

[Laurentius]

Get a bunch of different blades together, wack the crap out of some wood and just ignore the science and see which one(s) works for you.

I already know what works for me insofar as what I have used, I was curious though as to what scientific merits different kinds of blades would have.

 

[Goatboy]

I have to admit that I don't come across that many Ninjas in my travels, but then again if they were there, I expect they would be that stealthy that I would never see them anyway

Sorry just a few years of martial arts, renactment and a couple as an international man of mystery. That and working with cutting tools for a living.


Sent via smoke-signal from a woodland in Scotland.

 

[HillBill]

It just comes down to chopping( inside) or slashing (outside)

There is the odd exception, sickles and scythes for example... tools for grass, they are slashers but the edge is on the inside to act like a hook. Grass is very light and tends to slip off or be pushed away by blades with the edge on the outside.

 

[mrcharly]

Sickles aren't really slashers. they are slicers.

You don't use them by taking a swing at the grass/grain/brambles. You gather a handfull (or hookful) of the grass in left hand, put sickle blade behind the grass and pull towards yourself while sliding the blade away. So the curve of the blade slides the edge along what is cutting.

It sounds inefficient, but is actually very very fast. I've had someone watch me clearing an allotment (3ft deep in brambles and grass) and declare it was quicker than using his petrol strimmer.

 

[OldJimbo]

Sickles aren't really slashers. they are slicers.

You don't use them by taking a swing at the grass/grain/brambles. You gather a handfull (or hookful) of the grass in left hand, put sickle blade behind the grass and pull towards yourself while sliding the blade away. So the curve of the blade slides the edge along what is cutting.

It sounds inefficient, but is actually very very fast. I've had someone watch me clearing an allotment (3ft deep in brambles and grass) and declare it was quicker than using his petrol strimmer.

Yep and there's quite a technique to things when you are cutting lots that way. It's easy for most to forget that everything we have become is based on six inches of topsoil and the sickle. Now of course we have huge combine harvesters but we only got that far due to our ancestors with sickles.. What an incredible tool!

Longer thin bladed thin and light sinuous parangs work in a similar way to sickles and i was curious as to why such a long bladed hand tool rather than a sickle or scythe. So I asked Wandi who founded Valiant Blades. He replied that while I'd probably cut lots with a sickle and scythe, there probably were fewer cobras in the stuff I dealt with.. Realistically, whenever we look at a traditional tool. it is the best for the job where it is found no matter how we misunderstand it. or its use

A basic primer on blades:
http://www.oldjimbo.com/survival/sharps.html
http://www.oldjimbo.com/survival/valiant4.html
Thanks to Jean-Marc, the rest of us mostly became safer with high performance blades and expected them to sail through stuff which nothing could cut cut through, so we have less stitches holding us together.

 

[tallywhacker]

From an applied physics POV....

A straight blade scatters force out upon impact, a curve will direct it better with less scattering. Scattered force is wasted energy in the context of chopping, obviously. The amount of curvature will impact how efficient a tool is at applying force when chopping, however, it does not take much curve/belly to become efficient and greatly reduce scattering of force.

More curve also means more cutting area for a length of blade [as the crow flies] when slicing. It doesn't matter which way the curve is and unless you are talking about the likes of a sickle opposed to a parang the amount gained is negligible.

An outward curve will minimise surface area, ofc. A forward curve may increase it. However you can also have a forward curve with a decent belly at the strike point, like a kukri. So you can have both.

No matter which way you look at it, which is best depends on both yourself and what you want to do with it.

 

[petrochemicals]

I do not know really, but a flame bladed sword was given to increase cutting power, by increasing pressure on the serrations.

https://en.wikipedia.org/wiki/Flame-bladed_sword

A outside curve increases the pressure on the point, and allows for deeper penetration, by sbreading the material. An inside curve gathers the material together and forces it to be cut by not letting it escape. The jamming action thoughcan create a wedgeing effect. A straight edhe is good to carve with!