If I want 15 degree cutting bevels, how much should my relief bevel be?

[Rey HRH]

I'm been pondering this. I don't think any of the four books i've read addressed the relationship of the relief bevel to the primary bevel other than the relief bevel obviously has to be less than the primary bevel.

Looking at Carter Murray's DVDs, it looks like he just about lays the whole blade flat and raises the spine just a couple of spine widths.

On another thread here, I read someone worked a 7 to 8 degree relief to put on a 15 degree cutting edge but that's on a filet knife. I think I'm correct in thinking that if you use too small of an angle for the relief, that will thin the blade too much and weaken it.

Is there like a thumb rule for relief bevel angles depending on the desired primary bevel? I'm also thinking the thickness of the blade and how wide the blade is from the spine to the edge would have an influence of how much the relief bevel has to be.

 

[cheapmovies25]

in a way what you are talking about is a micro bevel on the primary bevel. Don't ever go down to 7 to 8 degrees on any knife, that will probably scratch the knife, and also will not give you a very sharp edge, unless you take it to 18,000 grit lol. In general, just sharpening with 1 bevel is the way to go. just go with 15-17 and keep at it, and you will have a great stronger edge. if you do want to create a micro bevel, sharpen it at say 15 to whatever you want, say 800 grit, then lift to 17 and finish up to whatever grit you want, like 1500 for example. But in general, i would rather prefer a one stage sharpen at any degree, rather than 2. A nice convex or V edge at 17-20, would be better than a relief edge or bevel, and then another one on top of that, imo. however, in the field adding a microbevel is ok if you need a quick sharp edge. doing the above will allow you to cut easier, and with a more acute edge, when say shaving a stick for example, that a micro bevel, which most ppl do not like. But some do, some don't. they are good if you are limited on the amounts of grits you have, and i think the main reason for them, like out in the field as stated.

 

[bgentry]

I'm the guy that wrote about sharpening the filet knife on my WSKO. In my case I had flattened the entire edge and shortened the width of the blade by at least 1/4", so I was thinning the blade out some to restore more of it's factory shape. Really working on the grind leading down to where the "real" cutting bevel would be. This is very similar to primary and micro bevel, but in my case the final cutting bevel was a normal width, as opposed to just a tiny sliver wide like most microbevels. But it really is the same idea; it's just a matter of how far you take the grinding.

Murray Carter is obviously an expert and his advice about primary bevel and cutting bevel mirrors what John Juranich says in his book also. Juranich advises you to take your hunting knife with a pretty picture on the side and scrub that picture right off on the stone. They both know what experienced sharpeners know: Thin Is Sharp.

None of this addresses your question of what's a good ratio of regular bevel to microbevel. I don't think there's a number that answers the question. ...and honestly I haven't done enough of my own experiments to figure out exactly what works yet. But I'll repeat what I've read here many times:

Thin your blade out so that it cuts well and keep going thinner until either you are satisfied, or the blade chips or otherwise fails for your cutting tasks. At that point, add a microbevel to strengthen the edge, and you'll have a thin *and* durable edge.

Carter says something similar and adds that almost no one EVER gets to the failure point in his experience. Currently I'm doing some experiments on my Delica in ZDP-189. I've got the primary bevel at something like 12 degrees per side and hope to end up between 8 and 10 degrees per side. It only has a microbevel from being touched up on the SharpMaker. The next sharpening will remove the micro and drop the edge angle another few degrees. So far, the cutting performance is MUCH better *and* it's holding it's edge longer!

Brian.

 

[chiral.grolim]

...Thin Is Sharp.

...Thin your blade out so that it cuts well and keep going thinner until either you are satisfied, or the blade chips or otherwise fails for your cutting tasks. At that point, add a microbevel to strengthen the edge, and you'll have a thin *and* durable edge.

Carter says something similar and adds that almost no one EVER gets to the failure point in his experience. Currently I'm doing some experiments on my Delica in ZDP-189. I've got the primary bevel at something like 12 degrees per side and hope to end up between 8 and 10 degrees per side. It only has a microbevel from being touched up on the SharpMaker. The next sharpening will remove the micro and drop the edge angle another few degrees. So far, the cutting performance is MUCH better *and* it's holding it's edge longer!

Focusing on angles behind the edge-shoulder is, imho, the wrong way to look at things, as stated in the quote above it isn't angle that cuts or provides strength, it is thickness.

For most knives, the primary bevel angle is ~5-dps or lower to allow a thin knife to slide through material with a minimum of wedging. My Spyderco CalyIII in ZDP-189 is ~3-dps in the primary. The hollow-grind of many knife is even lower, the very point of using a hollow-grind. But that is all in the primary and on knives built of relatively thin-stock to begin with. The CalyIII is ~1/8" thick at the spine, FFG at 3-dps across the ~1" wide primary bevel brings the edge to ~0.015" thick at the shoulder, whereupon I sharpen at 15-20 dps to the apex.

The important measurements to consider here for most cutting applications are that sharpening angle and that edge-thickness.
Sharpening at 15-20 dps produces an edge that is 2:1 height vs thickness, an excellent ration for strength as well as cutting efficiency. Everything from hard-use wood-chipper and chainsaw blades to light-use fillet knives can be sharpened at this angle for high performance in a myriad of applications. There is a good reason so many angle-guide systems are set this way.
But BEHIND that edge is where edge-thickness really comes into play. That CalyIII's 0.015" is sufficiently robust for a wide array of tasks, but it is inferior for specific tasks. For example, I have custom-knives and straight-razors ground to <0.005" at the edge-shoulder - 3X thinner and 3X more efficient at penetrating material. An edge that thin is perfect for cutting deep into soft material, even abrasive material. Keep in mind that a standard utility-blade box-cutter is ~0.017" thick. With an edge that thin behind 15-20dps, the edge-bevel is quite thin as well and so VERY easy to re-establish when sharpening, so little material is removed. The CalyIII bevel is more difficult to maintain because it is so much wider, much more metal must be removed to re-apex a dull blade. However, that 0.005" edges are easily bent/twisted if too much lateral force is applied, such as cutting into hardwood or bone - it is not built for "hard-use" but high-performance precision cutting. Remeber, passed that edge-shoulder the knife increases in thickness at only 3-4 dps. Thickness translates to rigidity (i.e. resistance to lateral stress) in cubic proportions, i.e. when you double the edge-thickness you strengthen it 8-fold. Many outdoor knives are ground to >0.020" thick at the edge-shoulder. Such an edge penetrates relatively poorly but is built to withstand significant lateral stress, preventing the edge from flexing and potentially suffering a catastrophic chip. Axes and dedicated chopping tools are obviously designed to be even more robust to endure the stresses they must encounter.

What i am trying to get it is that your knife blade geometry should be built for the task it's intended for, and that won't have much to do with a ratio of primary:secondary bevel widths/angles. You start with as thick of stock as you need to provide lateral strength to the blade, and as wide as you need to minimize wedging in the primary bevel or the edge-shoulders without compromising the strength of the blade. If you're experiencing wedging at the edge-shoulders and don't want to re-grind the primary, then you cut a 'relief bevel' be it V or convex (better). My recommendation is to cut it at ~5dps lower than your apex angle. so if you have a primary grind ~5dps and edge-bevel ~15dps, cut the relief at 10-dps.

 

[Rey HRH]

Carter says something similar and adds that almost no one EVER gets to the failure point in his experience. Currently I'm doing some experiments on my Delica in ZDP-189. I've got the primary bevel at something like 12 degrees per side and hope to end up between 8 and 10 degrees per side. It only has a microbevel from being touched up on the SharpMaker. The next sharpening will remove the micro and drop the edge angle another few degrees. So far, the cutting performance is MUCH better *and* it's holding it's edge longer!

Brian.

Thanks for your response, Brian. I can mentally (theoretically) agree with the idea; part of my reluctance has been the weakening and aesthetics part. What got me started seriously learning about free hand sharpening was I didn't like how my Lanksy thinned out the tip of my ZT 560 and only on one side. And that thin edge didn't look good to me. Fixing that knife is my objective after I practice with some cheap steak knives I have. (In between, I have to sharpen the turkey carving knife too.)

But thank you for helping to drill into my head: "Thin is sharp" and "no one's ever gotten to the failure point yet" doing it that way.

 

[Rey HRH]

What i am trying to get it is that your knife blade geometry should be built for the task it's intended for, and that won't have much to do with a ratio of primary:secondary bevel widths/angles. You start with as thick of stock as you need to provide lateral strength to the blade, and as wide as you need to minimize wedging in the primary bevel or the edge-shoulders without compromising the strength of the blade. If you're experiencing wedging at the edge-shoulders and don't want to re-grind the primary, then you cut a 'relief bevel' be it V or convex (better). My recommendation is to cut it at ~5dps lower than your apex angle. so if you have a primary grind ~5dps and edge-bevel ~15dps, cut the relief at 10-dps.

Thanks for your detailed answer. You highlighted that blade geometry is dependent on use, i.e. razor blade for shaving and axe head for chopping. I guess in between is a folder and I just have to consider what form it has to be to follow the function for which I want to use it.

On the other hand, I know you said thinking about the angle behind the edge shoulder is the wrong way, but at least you gave me a head start with your suggestion of 5 degrees less. I have considered differences of 1 degree to whatever results from lifting the spine just 1 width's worth but I obviously don't have the experience to know the minimum that will make a difference and the maximum where I would mess things up. You've given me the confidence to start with 5 degrees less so I can see the impact. I know I can always take more away afterwards but I found that it's hard to add metal back in.

On the particular steak knife I've been practicing on, I measured and calculated the smallest angle I can aim without hitting the first thick part of the blade as measured from the edge is 5 degrees. I'll go back and regrind to something way less than 15.

Thank you all for sharing your experiences and knowledge.

 

[Cynic2701]

I subtract 10 degrees inclusive from the inclusive microbevel angle. For example, most of my knives have something around a 10 degree per side bevel with a 15 degree per side microbevel.

If you have the patience to completely apex the edge, I think that it is worth your time. Throw on a microbevel if you start getting rolling/chipping issues.

Here's a left handed Sebenza at ~25 inclusive.

 

[sodak]

Carter says something similar and adds that almost no one EVER gets to the failure point in his experience. Currently I'm doing some experiments on my Delica in ZDP-189. I've got the primary bevel at something like 12 degrees per side and hope to end up between 8 and 10 degrees per side. It only has a microbevel from being touched up on the SharpMaker. The next sharpening will remove the micro and drop the edge angle another few degrees. So far, the cutting performance is MUCH better *and* it's holding it's edge longer!

Brian.

And it sharpens in 1/10th the time!

Thanks for your detailed answer. You highlighted that blade geometry is dependent on use, i.e. razor blade for shaving and axe head for chopping. I guess in between is a folder and I just have to consider what form it has to be to follow the function for which I want to use it.

Exactly. Balance for the intended use is very important.

On the particular steak knife I've been practicing on, I measured and calculated the smallest angle I can aim without hitting the first thick part of the blade as measured from the edge is 5 degrees. I'll go back and regrind to something way less than 15.

Kudos for practicing on a cheap blade first. I actually sharpened/ground my way through several steak knives before becoming satisfied with my own preferences.

I subtract 10 degrees inclusive from the inclusive microbevel angle. For example, most of my knives have something around a 10 degree per side bevel with a 15 degree per side microbevel.

Nice! You can floss you teeth by them! It's great having something that cuts as good as it looks, isn't it?