Edge angle for skinner in M4?

[bludog2]

Hi All,

I'm having a knife built patterned after the Loveless Skinner. It's coming in M4 at 63-64 HRC, and primary use will be for skinning and processing game. The maker is currently planning on 21 degrees per side. Would a steeper angle with this hard steel, say 17 degrees, provide better overall performance for this intended use?

Thanks in advance for your feedback,

bludog2

 

[chiral.grolim]

Hi All,

I'm having a knife built patterned after the Loveless Skinner. It's coming in M4 at 63-64 HRC, and primary use will be for skinning and processing game. The maker is currently planning on 21 degrees per side. Would a steeper angle with this hard steel, say 17 degrees, provide better overall performance for this intended use?

Thanks in advance for your feedback,

bludog2

The standard edge-angle for pretty much ANY cutting is 15-dps (30' inclusive), be it a chainsaw, a hand chisel, an axe, or a knife for any purpose. Even a straight razor, after stropping the edge, ends up ~15-dps. Think about that - if your chainsaw is sharpened at 30' inclusive, why would you sharpen tools that experience less stress to something thicker?
For most knives, the edge bevel is <1/16" wide, and for a skinning knife with thickness at the bevel shoulder <0.02, it should be only ~1/32" wide. How thin will your knife be ground in the primary bevel? Will it be <0.020" behind the edge? <0.010"?
The main reason to go thicker than 15-dps is if you encounter chipping or rolling/squashing at the edge. A thinner edge will most certainly provide better overall performance if chipping/bending isn't an issue, and at 63-64 Rc you are unlikely to experience anything but chipping. IF you experience chipping, adding a 20-dps microbevel to each side will likely suffice to prevent further damage in your intended use.

HOWEVER, I do know some folk who prefer a thicker edge-angle for skinning BECAUSE it doesn't slice as well - 20-dps does a better job wedging the hide away from the meat without risking cutting through the pelt or into the flesh, and that can be advantageous. *shrug*

Below is a schematic of edge-angle profiles and relative performance. The cutting efficiency of 15-dps is 1.35x that of 20-dps.

 

[tim37a]

Those are interesting graphs. How did you derive the curves for edge strength and cutting efficiency?

 

[chiral.grolim]

Those are interesting graphs. How did you derive the curves for edge strength and cutting efficiency?

Edge strength is determined by thickness (cubic), and cutting efficiency is a matter of mechanical advantage (wedge). Each are easily derived from basic trigonometry.
The equations do not account for impact toughness as that is a matter of matrix composition.

 

[GB940Rookie]

20/21 degrees per side will work fine.

 

[bludog2]

Thanks for the replies, and unfortunately, I don't know what the thickness of the primary bevel will be. Should maybe try to find out.

 

[tim37a]

chiral.grolim - Sorry, your answer makes absolutely no sense to me. Please use real numbers and show the the trig formulas you use to derive the curves. As far as I can see, you have simply pulled edge angle and "units" (what are those?) out of thin air.

 

[tim37a]

For a skinner made of S35VN, hardened to Rc 61 by Peters, I would use 12 dps with a micro-bevel of of 14 dps. That edge is based on real world experience, not calculations.

 

[chiral.grolim]

chiral.grolim - Sorry, your answer makes absolutely no sense to me. Please use real numbers and show the the trig formulas you use to derive the curves. As far as I can see, you have simply pulled edge angle and "units" (what are those?) out of thin air.

The units for angle are given - degrees per side. This is a measure of the space between intersecting lines/planes.
There are no units for mechanical advantage - it is a ratio.
Edge strength for a blade of given material at given hardness is largely a factor of stiffness which is derived from intrinsic factors and, most especially, thickness. So if you measure the thickness, your units will involve that measurement. These are general principles governing edge geometry. They translate to real-world cutting performance - the gains and losses you can expect from different edge geometries based on the geometry alone.
I am sorry that the principles don't make sense to you. That is a shame. Is there a reason that you then referenced a different steel at a different hardness to respond to the OP?? "I have no experience with what you mention, but here is a random reference point." Most of my post was about experience. Knowing the principles and knowing the geometry of your knife informs your understanding of why it performs the way it does and how altering the geometry will impact that performance.

Why do you want the microbevel? How do you know that the angles are actually 12 and 14? Have you ever tried the angles mentioned by the OP?

As I mentioned in my earlier post, everything from chainsaws to straight-razors uses ~15-dps for the edge (like tim37a's microbevel). This can be adjusted according to what the user experiences in terms of damage or performance. The charts give the relative impact on the edge of changing the angle, the principles tell you how much strength you gain or lose for a given increase/decrease in cutting performance by changing the edge angle. HOWEVER, if the cutting you do proceeds deeper than the edge-bevel, then shoulder thickness (BET) will have a much greater impact on performance. An edge that is 15-dps and 0.010" thick will behave VERY different from one that is 0.030" thick. The math predicts it to be so based on the principles at play, and experience testifies to the reality of the principle.

 

[marthinus]

I would second what chiral.grolim said above regarding primary grind thickness.

The grind thickness would determine what edge angle I would use but I would generally not go above 15 degrees per side for my skinning applications if the primary grind and heat treat is done correctly.

 

[herisson]

On a thin (less than 0.1") flat ground blade, I had very good results with a 20°dps or even 25°dps secondary bevel (good as in "more resisting to hitting a bone"). chiral.grolim : based on your graph there is a sweet spot at 25°. I can second that but only with very thin ground blades. A thicker stock blade could benefit from a steeper angle. Type of steel and temper play a major role, too. I have a Becker 15 I thinned out at 15°dps for better cutting performance with cardboard boxes. Didn't quite work out. Sharpened it back to 20°dps... now it works fine.

 

[chiral.grolim]

On a thin (less than 0.1") flat ground blade, I had very good results with a 20°dps or even 25°dps secondary bevel (good as in "more resisting to hitting a bone"). chiral.grolim : based on your graph there is a sweet spot at 25°. I can second that but only with very thin ground blades. A thicker stock blade could benefit from a steeper angle. Type of steel and temper play a major role, too. I have a Becker 15 I thinned out at 15°dps for better cutting performance with cardboard boxes. Didn't quite work out. Sharpened it back to 20°dps... now it works fine.

I have a similar story - I compared the slicing performance of a Stanley Box cutter blade to a knife made by Tim Johnson ( timos- ): https://www.bladeforums.com/threads...s-and-a-timos-handmade-knife-feature.1169142/
The box cutter has a much finer apex angle, only 8-dps, and is just 0.017", while the utility knife (which I use as a skinner) is 1/8" at the spine but slimmed down (when new) to 0.005" before a final bevel which I adjust 15-20 dps depending on need. In use, not only does the thinner edge of the timos- blade enhance cutting efficiency dramatically, it is also far more durable and I use it for a lot more than just boxes

I would caution, however, reading too much into the point of intersection on the graphs. 25-26 dps is pretty dang robust, I am not sure that any of us would deliberately apply such an angle to our knives except perhaps as a final swipe on the hone for a true 'microbevel', and it may be that a well sharpened apex will quickly stabilize to this geometry (on the very fine, nanometer level) in use despite what angle we sharpen at. As might be noted, we don't experience how strong an edge really is until it becomes damaged. If 12-14 dps is working for someone no reason to increase an angle and add strength, sacrificing cutting efficiency, if the strength they currently have appears to suffice. *shrug*

For unbelievably awesome details about apex geometry and performance, I highly highly recommend Todd's website https://scienceofsharp.com/home/

 

[herisson]

Great post ! Thanks for the information. I bet that Tim Johnson knife is an awesome slicer. And if the edge holds up well to various tasks, that's a testimony to AEB-L, a steel I like more and more.

 

[timos-]

wow, nice to see that little one is still alive and well!

 

[TRfromMT]

We could "science this to death", or ask the maker for his recommendation. Who are we to presume we know more that the maker, right?

Then, I'd go on the thinner side of the recommendation (in terms of shallower DPS, I don't mean edge thickness). Start with as thin of an angle as he'd recommend. You can easily take it thicker of you want based on field use and actual performance (or maintain it at the maker's setting if you like it) but taking hard M4 thinner will be a mother.

 

[chiral.grolim]

wow, nice to see that little one is still alive and well!

It remains my favorite skinner, used on every chicken, rabbit, woodchuck, and deer that has passed from my property to my cooking pot

My other favorite hunting knife is also one of yours. Keep up the good work!

 

[timos-]

i dont have experience with m4 steel. I am not sure about how thin you can take this steel at hrc 63-64. It would be best to ask the maker this question while also how describing how you use a skinning knife. That will allow the maker to better understand your needs.

 

[sodak]

... As I mentioned in my earlier post, everything from chainsaws to straight-razors uses ~15-dps for the edge ...

I measured the edge angle on 3 of my Dovo's (German straight razors), and they all are between 12 - 14 degrees inclusive, or 6 - 7 degrees per side. And that was from the factory.

 

[chiral.grolim]

I measured the edge angle on 3 of my Dovo's (German straight razors), and they all are between 12 - 14 degrees inclusive, or 6 - 7 degrees per side. And that was from the factory.

Here is my quote, I will bold the relevant part:

...Even a straight razor, after stropping the edge, ends up ~15-dps.

This is at the micron level, and you may be correct that your razors come finer but how are you measuring? Are you just using the spine-to-edge angle?
Below is SEM from ToddS' blog showing a straight-razor stropped edge - measuring just a few microns back from the apex, the angle of this stropped edge is ~25' inclusive, but the convex curvature continues to the very apex - measured closer, it would be very close to 30' inclusive or 15-dps (if you consider 12.5 insufficiently close for my statement to be accurate):

If you ignore the final apex but measure the angle BEHIND that, the geometry is 16.7' inclusive, approximating that <10-dps geometry ground prior to stropping the final edge-bevel:

In their razor blade patent, Gillette includes the following text:

... an ultimate edge sharpened to a tip radius of less than 1,000 angstroms, preferably 200 to 300 angstroms, and has a profile with side facets at an included angle of between 15 and 30 degrees, preferably about 19 degrees, measured at 40 microns from the tip...

 

[sodak]

I am measuring the width versus the spine thickness, and using trigonometry to calculate the angle. I grind the edge with the spine and the edge flat on the stone. I do not strop. Period.

Look up the definition of arc tangent and maybe then you will understand what I am talking about.

My angles are 6 - 7 degrees per side. Is that clear enough?