Appreciation of the Double bevel... No disadvantages?

Gator97

Gator97

Joined
Apr 10, 2000
Messages
3,794
Hi All,
Not so long ago, with the purchase of Edge Pro Apex ( Ben, mega thanks for that device
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I was able to produce Double bevel edge on my knives. Well Spyderco Sharpmakes would do that but a) Apex is much more versatile ( anglewise), b) gives a better edge, polished etc...
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Anyways, I've tried to find some info about double bevel edges. Couldn't find much, interesting, what are the disadvantages or drawbacks of the double bevel?
The way I understadn the advantages it makes the edge stronger & more durable?
Knowing the common truth that nothing comes for free, interesting to know what am I sacrificing when putting the double bevel? Just extra efforts & time to get the secondary bevel?


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[This message has been edited by Gator97 (edited 08-22-2000).]
 
It makes the edge stronger and more resistant to rolling, but a big part of that is because it sort of blunts the edge or at least makes it thicker. That means that you won't be able to just touch up the edge with a strop or fine stone as many times in between full blown sharpening sessions where you completely redo the bevel. But thats not necesarrily a bad thing. If the edge is more durable then you won't have to sharpen it as often anyway, so the the fact that you have to spend more time sharpening each time doesn't matter as much. And if the edge doesn't get damaged as easy you won't be spending as much time polishing out knicks in the blade. Overall I'd say a double bevel edge is better for a working knife than a single bevel edge.

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It'll feel better when it stops hurting.
 
I doubt that there is that much sacrifice in cutting ability. The second bevel created is very small (or should be anyways) and therefore I doubt that there is much sacrifice at all. Sure there may be some, but since the second bevel is really tiny, how much of a difference could it make, really! Besides, the Razoe's Edge bunch say that you should grind in relief much thinner than what most people do anyways, so the entire edge is likely thinnert than those found on non-double ground blades, given that you sharpen it the way the Razor's Edge bunch say to. That is, grins in thin relief, sharpen in an edge, and then lift up the knife a few more degrees and put on the secondary bevel. The very edge may be a little thicker, but this part of the edge is tiny and the relief behind the blade is thin.

This of course is all theory. You have tested more knives than me Cliff!! But, in your experience, with a properly sharpened, double ground edge, how much cutting ability is given up?

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"Come What May..."
 
The edge itself is a very thin line regardless to edge angles, so you can make the edge hair popping at any angle if honed properly. Double bevel will give more thickness, and also some drag by the angled part between the first and the second bevel.
If you round the area, the drag will decrease at ignorable cost. This is how convex grinding was made and is still favored by many people including me.

But I'm sorry I can't show any specific data. All are just my personal opinions.

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\(^o^)/ Mizutani Satoshi \(^o^)/
 
I think the logic here is in reverse: assuming one uses the 20 degree angle as the primary bevel, the "double bevel" INCREASES cutting ability. I know Cliff and some others dispute the "industry standard" 20 deg. angle as adequate for utility/outdoor style knives; but since the Spyderco system is the reference pt here, let's take that as a given for this thread. (Sacrilege, I know; we'll sacrifice a young virgin later ...)

Spyderco's 30 deg. angle is designed for two purposes:

1) to hone the thin edge on "kitchen cutlery"; OR,

2) to grind down the "shoulders" on an edge established by the Sharpmaker's 20 deg. angle. By reducing the acuteness of the primary bevel, it reduces the resistance or "drag" up the face/width of the blade, and therefore *improves* cutting ability.

It only "sacrifices" cutting ability if you establish the primary bevel with the 30 deg. angle, then re-hone it with the 20 deg. angle. But that wouldn't make much sense, would it?

The "disadvantage" is that, by removing more material from immediately behind the cutting edge, it somewhat "weakens" the edge (more or less, depending on how deeply you grind the shoulders). Depending on your steel and its hardness, this may result in increasing its tendency to roll or chip.

In practical terms, outside serious chopping, what Gator97 calls the "double bevel" is probably a better edge to put on knives, if one is using the Spyderco system. By doing so, one is essentially putting a semi-concave grind on the edge. In fact, for those wanting to "convert" single bevel edges to concave can start by using the Spydie Sharpmaker to establish a 20 deg grind, pull back the shoulders with the 30 deg angle, then use sandpaper to round the angles. (E.g., I do this, then use sandppr wrapped around newsppr, wrapped around a wooden dowel ...)

This won't satisfy those who swear by benchstones and/or those who believe the initial 20 deg angle is too "blunt," by it may be a good starting pt to the "holy path of sharpening enlightenment" for not-so-skilled users like me.

$0.02
Glen
 
Crayola :

with a properly sharpened, double ground edge, how much cutting ability is given up?
Click to expand...

On a scale of 1 to 100, with the acute single bevel at 100, and the obtuse single bevel at 1; the cutting ability of a double bevel will rank somewhere between 1 and 100. The extent of the blunting depends on the depth of the additional bevel you put on the blade as well as the type of material you are cutting.

The softer and looser the material you are slicing the more the force will be concentrated at the very edge and thus the more the blunting effect you will notice from putting a more obtuse double bevel on top of the acute edge. On some very loose materials all the material actually "sees" is the very edge (the obtuse second bevel) and thus the knife behaves if the entire bevel was ground to the obtuse one, ie, the cutting ability is 1.

WrongFriend makes an excellent point by noting that by rounding out the shoulders you can have a significant effect on the drag and thus raise the cutting performance.

-Cliff
 
Ahhh, thanks Cliff. I'll have to read you rpost a couple more times to be sure though!! Sometimes you sound like a physicist and not like a knife nut! The virtues of the Moran edge definitely require some investigation on my part. So, can we say that a double ground edge is a quasi-moran edge for those with stones and not sand paper/slack belt grinder on hand?



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"Come What May..."
 
Cliff, don't you sharpen your knives at about 15-degrees? I find myself doing that more and more. I spent an hour on my BM910 and my Edge Pro Apex reprofiling the edge to be very thin.
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Cuts so much better now.

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Chang the Asian Janitorial Apparatus
 
I typically use a double-bevel not to make the edge stronger, but to finish off whatever is left of the burr. I often do two or three very light swipes at 20 degrees, after I've done my primary sharpening at 15 degrees. As long as you do your swipes very lightly, whatever was left of the burr will disappear, and you won't blunt the edge noticeably.

Especially for people who are having problems grinding off whatever is left of the burr -- and if your razor sharp knives seem to lose their razor edge quickly, that's probably your problem -- double-grinding will improve cutting ability, not decrease it.

I also note that storyville's point about how you look at things is valid. If you normally just sharpen at 20-degrees, and now are going to double-bevel with a 15/20 edge, the performance will improve, period. If you normally sharpen at 15 degrees, and will now double-bevel at a 15/20 degree edge, the performance could go down some, unless you've been having the burr problems I described above, in which case performance could still improve.

Joe
 
Crayola :

can we say that a double ground edge is a quasi-moran edge for those with stones and not sand paper/slack belt grinder on hand?
Click to expand...

A crude one but yes, it has the same basic idea. If you round out the shoulders as described above it basically is a Moran edge with a very low curvature.

Chang :

don't you sharpen your knives at about 15-degrees?
Click to expand...

For some large knives yes, on the smaller ones they are usually under 15 degrees, about 5-10 or so. These are primarily cutting blades, I don't do any scraping or poking with them. I carry another blade for that kind of thing.

Note if you double bevel in the way that Joe describes then the depth of the additional more obtuse bevel is very low, less than .05 of a mm. In this case it is difficult to notice any lose in cutting ability. Of course there is not a significant gain in durability either. The primary purpose as Joe comments is to insure that the weakened burr is removed.

-Cliff
 
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