Thread: Zowada Website - double bevel info. updated

"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.

Quick question here,

If I've already used 2 layers of tape to hone my razor ( don't aks me why but I couldn't get that damn Dovo Tortoise to work without using it) should I add two more or one more layer if I want to put a double bevel on it?

thanks,

JF