I don't think blade bending imparts any stiffness

[Seraphim]

I've seen it posted many times that a razor that gives a strong curvature to the blade somehow imparts more stiffness to it. That just does not make any sense. The blade itself is a thin piece of metal, bending will impart stiffness longitudinally, but not in the axis we would be concerned with. The part of the blade exposed from the razor, unsupported gains no benefit whatsoever from any bending that occurs up to that point. It is simply a flat plane unsupported piece of thin metal at that point.



Discuss.

 

[Agravic]

Interesting question.
I see your point about the edge of the unsupported blade presented. My view would be that blade bending does make a difference overall in terms of tensile 'strength' for the summation of the whole length of the blade when it's facing hundreds of hairs during a pass.

 

[dcobranchi]

I agree that bending doesn't really affect the blade per se. I've thought the curvature was a way of allowing the angle between the handle and your face to be decreased (for comfort) while still maintaining a good angle between the bade and face. Without the bend the handle would be sticking out at close to 90°.

 

[kingfisher]

I think you're wrong, but I lack the statics training to prove it.

 

[woodfluter]

Mmmm...you may be both right and wrong.

I see the thin blade as a spring. In fact, I believe K.C. Gillette actually used the term spring steel in descriptions of his idea. Bending the blade transversely (over the short dimension) creates pressure against the cap, quite close to the edge, which imparts longitudinal stiffness. The stiffness comes from the rigidity of the cap and the blade pressing against it.

Yes, you could get that same stiffness without any arching by clamping it very firmly between two plates and equally close to the edge, but then you wouldn't have the needed "blade gap" between the edge and the guard or comb portion of the baseplate, which is needed for good blade contact and flow of lather + hairs away from the edge.

Single edge razors of course don't utilize bending, but they compensate with much thicker blades.

 

[Seraphim]

Mmmm...you may be both right and wrong.

I see the thin blade as a spring. In fact, I believe K.C. Gillette actually used the term spring steel in descriptions of his idea. Bending the blade transversely (over the short dimension) creates pressure against the cap, quite close to the edge, which imparts longitudinal stiffness. The stiffness comes from the rigidity of the cap and the blade pressing against it.

Yes, you could get that same stiffness without any arching by clamping it very firmly between two plates and equally close to the edge, but then you wouldn't have the needed "blade gap" between the edge and the guard or comb portion of the baseplate, which is needed for good blade contact and flow of lather + hairs away from the edge.

Single edge razors of course don't utilize bending, but they compensate with much thicker blades.

I think that's a pretty decent explanation of the dynamic.I see what you are saying about a cantilever effect helping secure the blade in tension.

But, when push comes to shove blade "stiffness" is more a function of how much blade is left unsupported, no matter what is going on behind that point.

Take a piece of paper (model of a DE blade), and bend it in one direction. You can easily see that the bend imparts a great deal of stiffness along the direction of the bend, but adds zero stiffness to the other direction. In fact you can just go right on rolling that sheet up as much as you like, or even start a bend going in the opposite direction altogether with no problem or resistance.

I'm just trying to get a discussion going so that we can better understand what the real dynamics are. I've read a number of posts to the effect such as "The Futur feels rough due to the fact that there is not much blade bending going on, allowing for flex which leads to hazards..." or some such (which I believe to be an erroneous cause)

And there are many other posts saying that razors such as the Progress are so damn good because it gives the blade alot of curve, which therefore gives it alot of stiffness, and thus is the reason why that razor is so great to use.

I think dcobranchi may be on to a stronger contributor to what differentiates a strongly arched blade razor from a flatter one.

 

[moshulu]

Thanks for bringing up this interesting point. I myself have been guilty of thoughtlessly referring to the virtues of razors whose design guarantees good tension through blade bending. On reflection, I can see that there is no effect here, as you say. I also agree that dcobranchi’s argument is a good one.

But here is another wrinkle: bending may be useful by providing some feedback to the shaver when the blade is being installed. That last bit of resistance to tightening the knob at the bottom of the handle allows more deliberate control than would occur when the threads simply bottom out suddenly at the end of the travel. This effect is most easily felt and observed when using TTO models. If you squint down the side of the head, you will see the blade rise and bend as the very last phase of the tightening (and you can feel a bit more resistance as you turn the knob). When the blade is up tight against the doors, all is well. If it stops before it gets there – beware! – the resulting blade exposure can be very nasty (I’ve had this happen, and it’s one of the reasons I don’t care for TTO razors).

 

[woodfluter]

I see what you are saying about a cantilever effect helping secure the blade in tension. But, when push comes to shove blade "stiffness" is more a function of how much blade is left unsupported, no matter what is going on behind that point.

Yep, that was sort of my point, but a picture might have helped the explanation!
Bending the springy blade presses it firmly against the cap edge, along a line parallel to and close to the blade edge.
That support is what imparts the stiffness.

If the edge of the cap is farther away from edge, the blade can flex slightly more.
(Keeping in mind that it is *very* thin, and yet more so where ground).
If cap edge is closer to the blade edge, blade can flex less. Design factors. Some flex may be good.
I could be wrong, but believe the "singing" sound you hear as hairs are cut is blade edge vibration.
Can't imagine what else it could be. And that differs from one razor design to another.

I think dcobranchi may be on to a stronger contributor to what differentiates a strongly arched blade razor from a flatter one.

That part is right too...it's the geometry that allows a convenient angle between the blade and the handle on both sides. But I don't think it is the whole story. Without curving a very thin blade, it is hard to see how you would give it enough support near the edge along with needed blade gap below the edge.

 

[Seraphim]

I used a totally flat DE blade superglued to a pair of chopsticks to shave as a goof.

Zero blade curvature, TONS of blade exposure, but it also allowed for a very shallow blade to face angle, and shaved just fine:

 

[Chester]

The curvature DOES lend stiffness. I'll give you three examples -- three that you can see in the bright world around you without having to do anything.

1) find a window that has metal blinds (the kind that run horizontally). Look at the shape of the slats in these blinds. Notice that they are curved. What would happen if they were not curved? They would sag because the forces (the load of the weight of the slat) exerted on the areas that are held by the little "ladder" strings are exerted over a shorter distance away from them. By curving the slat, the load at those support points is spread across the length, and is actually supported by the arch of the curvature. This is the stiffness -- the resistance to load. R. Buckminster Fuller had students construct an immense dome out of the slats from the blinds in his classroom. It didn't last long, and crumpled almost as soon as it was standing, but the tension of the arch in the blind slats was enough to make them rigid ("stiff") enough to play a role in the structure.

2) If you don't already have one (they're handy!), go buy a metal retractable measuring tape... the kind you pull out and can lock in position with a little brake and when you take off the brake it zips back into its case. Look at the measuring tape. It's curved, isn't it? Why is it curved? It's curved so that you can extend the tape for maybe ten feet before it'll flop over. This is so that you can extend the tape and hook it on a surface and make a longer straight-line measurement without tugging or losing your measuring point. How does the curve do that? It spreads the weight of the tape you pull out over a longer distance (over the arch, which is not a straight line) than it would if the tape was flat. If you extend the tape four feet, it's harder to get the tape to bend than if you have the tape extended for eight feet, because the length (and hence weight of the tape) grows greater while the arch remains the same and at some point the arch can't impart enough tension to hold up the tape.

3) Last one. Think about 1 inch angle iron -- 90 degree bend, maybe 1/8 inch thick steel -- 2 sides that are perpendicular to each other. The bend IS a curve -- a very tight one, but a curve nonetheless. if you put the single edge (where the two sides come together) pointing up, it can support more weight on top than if you have the single edge pointing down. As long as you load it in the right direction, it's stiffer than a flat piece of 1/8 inch thick steel that's 2 inches wide.

The forcing of a curve into the steel of a razor blade does impart stiffness along the edges. How the blade is supported also has an effect.
Does this help?
-- Chet

 

[talibeard]

I am with Chester on this one; forcing steel at low temperature into a different shape stresses the metal by forcing the crystals a (tiny) bit out of their natural position which stiffens the steel in every direction, though maybe not equal in all directions.

@ Seraphim; by how you glued your shark on the chopsticks you took out most of the blades flexibility. Only twice the width bevel is allowed to flex here. When you would have turned your chopsticks by 90 degrees and glued your blade on the thinner part would make a lot of difference and be much closer to how it is held in a razor I think. Interesting topic btw, I love these kind of debates!!

 

[Seraphim]

The curvature DOES lend stiffness. I'll give you three examples -- three that you can see in the bright world around you without having to do anything.

1) find a window that has metal blinds (the kind that run horizontally). Look at the shape of the slats in these blinds. Notice that they are curved. What would happen if they were not curved? They would sag because the forces (the load of the weight of the slat) exerted on the areas that are held by the little "ladder" strings are exerted over a shorter distance away from them. By curving the slat, the load at those support points is spread across the length, and is actually supported by the arch of the curvature. This is the stiffness -- the resistance to load. R. Buckminster Fuller had students construct an immense dome out of the slats from the blinds in his classroom. It didn't last long, and crumpled almost as soon as it was standing, but the tension of the arch in the blind slats was enough to make them rigid ("stiff") enough to play a role in the structure.

2) If you don't already have one (they're handy!), go buy a metal retractable measuring tape... the kind you pull out and can lock in position with a little brake and when you take off the brake it zips back into its case. Look at the measuring tape. It's curved, isn't it? Why is it curved? It's curved so that you can extend the tape for maybe ten feet before it'll flop over. This is so that you can extend the tape and hook it on a surface and make a longer straight-line measurement without tugging or losing your measuring point. How does the curve do that? It spreads the weight of the tape you pull out over a longer distance (over the arch, which is not a straight line) than it would if the tape was flat. If you extend the tape four feet, it's harder to get the tape to bend than if you have the tape extended for eight feet, because the length (and hence weight of the tape) grows greater while the arch remains the same and at some point the arch can't impart enough tension to hold up the tape.

3) Last one. Think about 1 inch angle iron -- 90 degree bend, maybe 1/8 inch thick steel -- 2 sides that are perpendicular to each other. The bend IS a curve -- a very tight one, but a curve nonetheless. if you put the single edge (where the two sides come together) pointing up, it can support more weight on top than if you have the single edge pointing down. As long as you load it in the right direction, it's stiffer than a flat piece of 1/8 inch thick steel that's 2 inches wide.

The forcing of a curve into the steel of a razor blade does impart stiffness along the edges. How the blade is supported also has an effect.
Does this help?
-- Chet

Thank you for contributing to the discussion, but the stiffness in your examples lies along the axis of the bend, not perpendicular to it, as I stated in my OP. And in post #6:

Take a piece of paper (model of a DE blade), and bend it in one direction. You can easily see that the bend imparts a great deal of stiffness along the direction of the bend, but adds zero stiffness to the other direction. In fact you can just go right on rolling that sheet up as much as you like, or even start a bend going in the opposite direction altogether with no problem or resistance.

It would seem that stiffness in the short axis would be more of an issue, as that is the direction of force applied to the blade by your skin/whiskers as you are shaving.

Here's a pic of an example: a 1000Yen bill wrapped around a MG5 aftershave bottle. I can easily deflect the paper back against itself, no added resistance than if it were simply laid out flat.

Yes, there is stiffness added along the other axis, but not perpendicular to it.

 

[Seraphim]

Some further examples.

In the first pic, I have the bill rolled up nice and tight for half its length, and then I could easily completely reverse the bend back on itself, no added resistance.

The second pic shows the greatly improved stiffness along the long axis---sufficient to support a 150ml glass bottle of aftershave, no problem.


So, yes, bending imparts stiffness, but only in one axis.

 

[tinashubby]

I can't see that it really matters if it is supported or marginally stiffer. Some one made a comment on another post about unsupported blades skipping or chattering along your face. Any worries about this are most likely negated by using the wrong technique anyway. The way I see it is that it is a super sharp metal edge vs. a hair on my face. The hair will lose.

 

[Seraphim]

I am with Chester on this one; forcing steel at low temperature into a different shape stresses the metal by forcing the crystals a (tiny) bit out of their natural position which stiffens the steel in every direction, though maybe not equal in all directions.

@ Seraphim; by how you glued your shark on the chopsticks you took out most of the blades flexibility. Only twice the width bevel is allowed to flex here. When you would have turned your chopsticks by 90 degrees and glued your blade on the thinner part would make a lot of difference and be much closer to how it is held in a razor I think. Interesting topic btw, I love these kind of debates!!

I agree it is not a perfect example. However, there is alot more blade exposure on my chopsticks than on a regular razor. Take a look, it is mostly unsupported all the way back to the blade notches, not simply twice the bevel width. That is quite a bit more than a regular razor, as pictured below of my Weber ARC.

 

[TallyShave]

No dog in this fight, just wanted to say that this may be the best shaving video I have seen. Not sure why it craked me up so much, but I really enjoyed it.

 

[talibeard]

I agree it is not a perfect example. However, there is alot more blade exposure on my chopsticks than on a regular razor. Take a look, it is mostly unsupported all the way back to the blade notches, not simply twice the bevel width. That is quite a bit more than a regular razor, as pictured below of my Weber ARC.

You would have to measure from the notches to the edges to know the (semi-) free movement of the blade. It is hard to get it exactly the same with the chopsticks I agree, but it is nice to see you taking that test for the team!!

One other thing; you are comparing paper with steel and that doesn't work at all. Paper is build out of fibres, not crystals like steel. And it are the crystals and the way they behave under pressure or because of tempering that give steel its specific behaviour. It is why cars are never build with flat panels but always curved and bend. If your roof would have been made from a flat panel it would have a dip in the middle because of its own weight and the curves on the side panels are part of the construction of your car; because of the added stiffness (due to the bending) the construction underneath can be made much lighter.

 

[Seraphim]

You would have to measure from the notches to the edges to know the (semi-) free movement of the blade. It is hard to get it exactly the same with the chopsticks I agree, but it is nice to see you taking that test for the team!!

One other thing; you are comparing paper with steel and that doesn't work at all. Paper is build out of fibres, not crystals like steel. And it are the crystals and the way they behave under pressure or because of tempering that give steel its specific behaviour. It is why cars are never build with flat panels but always curved and bend. If your roof would have been made from a flat panel it would have a dip in the middle because of its own weight and the curves on the side panels are part of the construction of your car; because of the added stiffness (due to the bending) the construction underneath can be made much lighter.

paper and steel are not the same, but it is sheet bending all the same.

Good analogy with the car panels. Although they are curved in two directions, not simply one, like our DE blades.


That chopstick shave was from back in March, not a current test. I just dug it up as a possible example.

 

[JDTTO]

Even though I have a bachelor's degree in engineering I tend to go with gut feeling on things like these.

If we accept that bending only provides stiffnes over the axis perpendicular to the bend (which I think is valid) doesn't the preloading of the blade by the bend provide stiffness? A linear spring which is preloaded will require more force to be bent further, right?

 

[woodfluter]

Seraphim, I agree with TallyShave...the chopstick shave video was a hoot!
Also, thanks for using my wife's favorite Beethoven symphony, and one of mine also.

However...
None of the examples or comments address the mechanism I discussed.

I'm very impressed that you can shave WTG using a blade glued to chopsticks, but that isn't a safety razor.
If it works equally well, that's terrific but not sure most would agree!
You do use a lot more care and take more time here.

About the Venetian blinds example, Seraphim is entirely correct. Longitudinal stiffness is a minor factor.
None of that relates to edge stiffness imparted by elastic force pressing against a solid chunk of metal - the cap.

If the blade on chopsticks works equally well, edge stiffness just doesn't matter - regardless of mechanism.
If it does matter, then flexing a blade against a solid support does provide something useful.

Straight razors, and the early SE safety razors, used a thick wedge blade that gave a lot of support to the edge.
Mr. Gillette's innovation was finding a novel way to get similar support with a thin flat piece of "spring" steel.
Much of the initial challenge was finding a way to get sufficient hardness without the blade becoming too brittle to bend.
If that stiffness and edge support doesn't matter, sheesh...they went to a lot of trouble for nothing!

P.S. This is a fun discussion for me, and hope it is equally so for all of you!