Thread: Zowada Website - double bevel info. updated

Originally Posted by Russel Baldridge

I have to disagree fundamentally with the comment about a double bevel not being as long lasting as a single, the raw physics at work dictate that a steeper bevel produces a more resilient edge.

It's like the difference between a Sushi chef's ultra fine Sashimi knife that would probably shave your face as well as any brand new TI, and his Cleaver, which, while intentionally not sharpened to the same degree, has a much more robust grind to withstand the force of chopping through hundreds of pounds of tuna spine without incurring any damage.

Granted this is a dramatization, but there is no way a steeper bevel should deteriorate faster than it's more acute counterpart, if indeed the steels are of identical quality, and the same abuse imparted to them.

That may not be entirely correct. An acuter bevel might cut with less effort than a steeper one. As a result the material that is being cut could exert more stress on the edge and abrade it sooner. If one would conduct lab test with a given knife and a given material, one could find the ideal cutting bevel that would always be the steepest bevel that could still allow for a fairly strain-less cut, or looked at it from the opposite direction, the acutest bevel that could still offer enough resilience against the cutting forces.

If I'm informed correctly, razor manufacturers have been using bevels of 15 to 17 degrees (someone better double-check this). I think they had enough time (a few ages) to get it right.

I don't know if altering the bevel angle with a degree or so, would be noticeable in the shaving experience. The only one that seems to have done some exhaustive comparison between single and double bevels on the same razors is David (heavydutysg135) and Perhaps Tim Zowada himself.
Actually, the whole hollow grind principle is already a kind of a double bevel thing.
As far as completely removing scratches from lower grit hones, it is my experience that doing enough strokes on my Belgian Blue and there after on the coticule easily takes care of that. I aim for that when I inspect the edge under the microscope between different hones. I understand that Tim Zowada, being a professional knife and razor maker, wants to speed up things a little, aiming for a better production-cost ratio.

Just my random thoughts on this matter,
Bart.

Originally Posted by Bart

As far as completely removing scratches from lower grit hones, it is my experience that doing enough strokes on my Belgian Blue and there after on the coticule easily takes care of that. I aim for that when I inspect the edge under the microscope between different hones. I understand that Tim Zowada, being a professional knife and razor maker, wants to speed up things a little, aiming for a better production-cost ratio.

Just my random thoughts on this matter,
Bart.

I agree with these comments.

Originally Posted by Bart

That may not be entirely correct. An acuter bevel might cut with less effort than a steeper one. As a result the material that is being cut could exert more stress on the edge and abrade it sooner. If one would conduct lab test with a given knife and a given material, one could find the ideal cutting bevel that would always be the steepest bevel that could still allow for a fairly strain-less cut, or looked at it from the opposite direction, the acutest bevel that could still offer enough resilience against the cutting forces.

This comparison deals with cutting in a line parallel to the edge, not perpendicular. I agree when severing a given fiber there is always a "field of stress" that develops as the edge forces it's way through the media. And the the field of stress increases with bevel angle, so the lower the angle the easier it is to cut with that utensil.

But ease of use and edge retention are not the same quality. And the two different cutting motions do not "necessarily" correlate.

This is one of those situations where we are dealing with such a fine edge that a slight increase in angle should not affect the ease of use as much as it ought to increase the edge retention.

Here's a quote about a BIC razor blade taken off a triple bladed cartridge that was inspected under a micrscope:
"The third microbevel is only 0.0009" wide and shows no scratches. It was probably done using an abrasive with grit under 1 micron. In my testing of honing compounds used on leather strops, I have not found any that produces this fine a finish. This leads me to suspect that BIC is using a very fine abrasive paper to finish their edges, or possibly using a sub-micron diamond honing compound. ...Images of a more recent blade, taken from a three blade razor, suggest that the final include(d) angle is actually over 35 degrees."

I think BIC has done their research, and apparently they've found it acceptable to have a dramatic increase in angle if its small enough and polished well. Though I seem to remember seeing a "How it's Made" episode where they attributed the improved edge retention of the micro bevel to laser cutting. but not sure on that one.

EDIT: This is just a frame of reference for the validity of double bevels and edge retention, as that is probably the primary goal of BIC's design, not comfort. It's possible and plausible that 35deg. is too much and would probably suffer in terms of comfort compared to traditional razors, but the point is that the double bevel aids (at least a little) in edge retention.

Originally Posted by Russel Baldridge

I think BIC has done their research, and apparently they've found it acceptable to have a dramatic increase in angle if its small enough and polished well. Though I seem to remember seeing a "How it's Made" episode where they attributed the improved edge retention of the micro bevel to laser cutting. but not sure on that one.

EDIT: This is just a frame of reference for the validity of double bevels and edge retention, as that is probably the primary goal of BIC's design, not comfort. It's possible and plausible that 35deg. is too much and would probably suffer in terms of comfort compared to traditional razors, but the point is that the double bevel aids (at least a little) in edge retention.

I think it would be in BIC's interest to produce blades that cut very comfortably and dull rather quickly. I believe the days that engineers in large multinational firms were actually trying to design something to make it better are long gone. I'm pretty sure the major concern is cost-effectiveness and short term profit. I bet they even conduct tests to make sure that those blades don't stay sharp longer than the marketing division has instructed.

My initial point was that one can draw two graphical curves for each bevel angle when one was to conduct standardized lab tests: one graph that would illustrate decreasing bevel sturdiness with a decreasing (acuter) angle and one graph that would illustrate increasing bevel stress exerted by the material being cut with an increasing (obtuser) angle.
For each given material there is a given point where the best of both worlds meets up.
A slicing motion will lower the bevel stress that the material exerts on the bevel. A guillotining motion will augment that stress. When a bevel hits the material at an angle (that's what happens with a razor 'cause we place in at an angle on our face, in order to cut close enough to our skin), there would be even more stress.
Take all that into account, do the lab tests and you ought to come up with the ideal angle. I don't know what that angle is. I know the angle one holds the razor to his face is another contributing factor.
BIC knows that angle, because they put their blades in a cartridge. That angle could differ from the angle a straight shaver typically uses while shaving. I don't really know that either.

In the end I trust the engineers that designed razors for those lustrous brands like Puma, Dubl'Duck, Thiers-Issard, Dorko and others more then I trust those at BIC.

In the end I don't think the slight angle increase that Tim Zowada suggests would make much difference. I just don't think that you could conclusively say that the edge would last longer, just for the reason that the bevel is a little obtuser. And I'm pretty sure that it would not be sharper than a properly honed edge with a single bevel. Maybe it would not be distinctively less sharp either. All I know, is that my face and the result of the shave tells me a slight decrease in sharpness immediately. Maybe that's what heavydutysg135 meant when he said he liked his razors better with a single bevel.

"I think it would be in BIC's interest to produce blades that cut very comfortably and dull rather quickly. I believe the days that engineers in large multinational firms were actually trying to design something to make it better are long gone. I'm pretty sure the major concern is cost-effectiveness and short term profit. I bet they even conduct tests to make sure that those blades don't stay sharp longer than the marketing division has instructed."

So the double/triple bevels that they spent precious time and money on researching provide a smoother shave? Because then great, we agree on one point. (but not really for the same reason)

And no matter which way you look at it (slicing, guillotining, chopping) on the scale that we are dealing with, the metal composition (and heat treatment/tempering thereof) would play much more of a role than the double bevel in making the blade dull more rapidly. So quicker degradation is not a logical result of the double bevel in this case. It would save them much more to skimp on quality of materials than to spend money grinding double and triple bevels to get the same resultant decreased working time.


"My initial point was that one can draw two graphical curves for each bevel angle when one was to conduct standardized lab tests: one graph that would illustrate decreasing bevel sturdiness with a decreasing (acuter) angle and one graph that would illustrate increasing bevel stress exerted by the material being cut with an increasing (obtuser) angle.
For each given material there is a given point where the best of both worlds meets up. "


But, if things were to work as you claim then both of these graphs would have a similar curvature, like the graph of Ax^2 and Bx^2, both curves are x^2 but are altered by an arbitrary constant, A or B. They would start at 0 and move in a smooth curve up to some asymptotic position or linear slope. More than likely though they'd be different translations of a portion of the sine wave and still separated by some arbitrary constant, which means they wouldn't cross.

And those types of tests are done routinely to determine the best angle to put on a cutting utensil, there just isn't a quantitative point where the graphs cross, the variables have to be weighed in terms of functionality, durability, cost to produce etc.


"In the end I trust the engineers that designed razors for those lustrous brands like Puma, Dubl'Duck, Thiers-Issard, Dorko and others more then I trust those at BIC."


This is a logical inconsistency because they didn't "design the razors", the design was in already place, so it is not a basis for determining whether or not they were correct. And it has no bearing on whether or not BIC's design and the double bevel in general can hold an edge longer. But, if in fact the "traditional design" turns out to be the superior, excellent! I'll re-adopt it.



"In the end I don't think the slight angle increase that Tim Zowada suggests would make much difference. I just don't think that you could conclusively say that the edge would last longer, just for the reason that the bevel is a little obtuser. And I'm pretty sure that it would not be sharper than a properly honed edge with a single bevel."


The argument is not that one or the other is sharper, they would essentially be the same, as the edge can only be as fine as the grit used to polish it. But the double bevel ought to be more resistant to abrasion (chipping really, on the scales we are concerned with) because the cross sectional area would be differentially greater. A=[2x^2(tan(angle~7deg.))].

This argument is like someone saying "my face won't tolerate even the slightest micro chip in the edge." While that statement has a good deal of validity, the truth is that you will never be without some degree of micro chipping, it just depends on the scale you are talking about. The same applies to this. At some point yes, an obtuse angle hinders performance, but at some point it can only help, just like the micro serrations of .5 CrO help to cut through hair, but are too small to be felt by the skin.

BTW I think this is great that we are really getting into the fine details of cutting edges, I wish we could have some more people weigh in with their expertise. It's good to debate, things, life becomes much more interesting with alternate opinions and viewpoints.

"So the double/triple bevels that they spent precious time and money on researching provide a smoother shave? Because then great, we agree on one point. (but not really for the same reason)"
Russel, I don't know why BIC puts a double bevel on their blades. You have a good point that some of my assumptions about their reasons for doing so, are incorrect. I was under the impression that you saw the fact "they do it at BIC" as proof for the superiority of a double bevel. "Because BIC does it" is simply not good enough an explanation for me. Maybe you only wanted to illustrate it is common practice to use a double bevel on razor blades. Which is a good point.

And no matter which way you look at it (slicing, guillotining, chopping) on the scale that we are dealing with, the metal composition (and heat treatment/tempering thereof) would play much more of a role than the double bevel in making the blade dull more rapidly. So quicker degradation is not a logical result of the double bevel in this case. It would save them much more to skimp on quality of materials than to spend money grinding double and triple bevels to get the same resultant decreased working time.

"And those types of tests are done routinely to determine the best angle to put on a cutting utensil, there just isn't a quantitative point where the graphs cross, the variables have to be weighed in terms of functionality, durability, cost to produce etc."

I think that when you perform a edge retention test, with bevel angle as a variable and everything else as a contant (steel alloy, temper, material being cut, cutting angle, etc) the result could be a hyperbolic (or is that parabolic) function (your math is obviously fresher than mine), with the top of the curve being the bevel angle that would have best edge retention in the given situation. I don't know how steep that curve would be. If it would be very steep, there would be not much margin for alteration of the angle without great loss of edge retention. If the curve would be sloping slowly, then it wouldn't matter all that much.

"This is a logical inconsistency because they didn't "design the razors", the design was in already place, so it is not a basis for determining whether or not they were correct. And it has no bearing on whether or not BIC's design and the double bevel in general can hold an edge longer. But, if in fact the "traditional design" turns out to be the superior, excellent! I'll re-adopt it."
Excuse me my poor choice of words. I'm not a native English speaker. What I was trying to say that the engineers at the old razor manufacturers obviously must have been thinking about the ideal bevel to put on their razors. I'm inclined to believe their motivations were to produce the best possible razor. I'm not inclined to believe BIC wants to produce "the best possible razor blade". I think they may have other motivations. But I already made that point.

"The argument is not that one or the other is sharper, they would essentially be the same, as the edge can only be as fine as the grit used to polish it. But the double bevel ought to be more resistant to abrasion (chipping really, on the scales we are concerned with) because the cross sectional area would be differentially greater. A=[2x^2(tan(angle~7deg.))]."
That's the point where we 'll have to agree to disagree. I my book a steeper bevel cuts easier than a more obtuse bevel. In my woodshop I sometimes use obtuser bevels on plane blades, mainly to avoid tear out of wood fibers on difficult to plane woods. It is clear to me that it takes more force to plane with an obtuser bevel. Edge retention is sometimes better, but often worse with such an obtuser bevel. It depends. Not all contributing factors stay constant so there's nothing conclusive to draw out of that observation.

"BTW I think this is great that we are really getting into the fine details of cutting edges, I wish we could have some more people weigh in with their expertise. It's good to debate, things, life becomes much more interesting with alternate opinions and viewpoints."
I couldn't agree more. I'm very interested in the raw physics of razors and how they shave. I'm afraid I'm a bit at the end of what I can contribute with my limited knowledge of these topics. It was nice to exchange some thoughts about this with someone willing to put some thought into all this, although we couldn't always agree. I do think that a guy who puts that much thought in his craft (or art, whatever you prefer), must be a great knife maker. I hope to see your work, some other time.

Kind regards,
Bart

Bic and other disposable blade manufacturers use the triple bevel method because it is the easiest way to cheaply and quickly produce a good edge with their mass production process, not because it produces the best results. They use three progressively higher grit wheels at progressively higher inclusive angles so that the scratch pattern of the previous grit is removed as quickly as possible. I don't remember how long it takes them to put an edge on but it can't be more than a few seconds. Us straight razor guys don't mind spending hours and lots of money to get the perfect edge, so speed and cost is not the primary concern to us; the best edge is.

What I was trying to point out with the BIC under a microscope observation was that one of the few logical (to me) outcomes of the double bevel, is improved edge retention. Because it doesn't make the blade dull any faster to increase sales as that is easily accomplished with lesser means. It shouldn't make their production costs cheaper since it requires extra work, and it doesn't make sense to me that the three bevels make removing grit scratches any easier on a production scale because they are using high powered machines. For a straight razor, sure, it allows more of the weight of the blade to affect a smaller surface area making the honing go faster, but for a mechanized factory production the difference would be negligible as compared to the extra steps needs to produce it. So to me it only makes logical sense that their aim would be to produce longer lasting products and bump out the competition. And seem I remember seeing a "How it's Made" show on the history channel about all things sharp, from razors (straight and cartridge) to debris shredders where they spoke with one razor company and showed their beveling process as an example for why their blades last longer, but there may be other factors, who knows.

And it doesn't matter which variable you put on which axis to make the graphs, what matters would be a comparative graph, of the two plots, which in order to avoid being nonsensical would require the values go a way in which they shouldn't cross. and I know this is true because there are literally volumes and volumes of discussion about lab tests, theoretical applications and daily experience in bladesmithing circles about what the best angle is. But it always boils down to the job at hand, the ease of use desired and how long you want the edge to last. Not a point on a graph. Every factor is relative to every other factor. I just think in our case the difference in ease of use/comfort would be smaller than the increased edge retention.

I think we are talking about two different things though when we say "sharpness". I was referring to how fine the line is where two flat planes meet, and it can only be as fine as the finest grit used to polish it is. The result of the refined edge intersection is what you are speaking of, because the result is a smoother cut with less force applied. And it is true, an acute angle will cut with less force, and often cleaner because of the reduced field of stress around the edge produced. So while you are correct, a lesser angle feels sharper, in actuality if they are refined to the same degree it is just the field of stress that has changed and technically they are equally "sharp" .


David-
I don't know their exact production figures/time constraints either and I don't know anyone who works for them who can verify which one of us has made the correct assumption. But having a decent knowledge of the metallurgical, physical, and thermodynamical process involved, I would tend to believe that for a mechanized mass production, the difference between maintaining one bevel and adding extra steps to create three doesn't result in decreased production costs. So, to me, there must be another reason.