Blade Geometry Questions

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Jun 1, 2019
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I recently got my hands on some precision lapping paper. I have been sharpening all of my blades to a polished mirror edge. In the process I have noticed different blades sharpen in different ways. In the picture that I provided, I have a folding Nencchmade pocket knife, a Morakniv chisel knife, and a cheapo amazon straight razor. Of these 3 blades, the chisel knife is the best at shaving. The chisel blade blade is set to a 25 degree angle while my straight razor is set to roughly a 10 degree angle. Why is the chisel such a good shaver? Is it the angle of attack? The angle of the cutting edge? Could it even be the hardness of the steel? I know for a fact that the razor is soft. While shaping the razor's cutting edge, I managed a decent foil bur from it.

Image: https://imgur.com/a/nChi6vm
 
It's probably a combination of factors. The biggest one, in my mind, is how well you formed a burr and how well you completely removed that burr.

Chisel ground blades are generally easy to sharpen, especially one with such a wide flat bevel like the Mora you pictured. Deburring is usually very easy also. Just lay it almost flat and do a couple of strokes. Boom, no more burr.

The shape of the Mora is helpful also. A completely straight blade is the easiest to sharpen. You probably formed a very even complete burr on the whole length of that blade. Blades with curves, like the Benchmade are harder to form a complete burr. The straight razor even appears to have a small amount of curvature from base to tip.

The straight razor should have the sharpest edge, as it has the smallest included angle. 10 degrees per side is a total of 20 degrees inclusive. Which is smaller than the chisel ground blade at 25 degrees inclusive. The benchmade was probably sharpened at 15 to 17 degrees per side, or 30 to 34 degrees inclusive.

So, we wonder why the straight isn't as sharp or sharper than the Mora. I can think of two reasons: Uneven burr formation. Did you sharpen this blade by placing the spine flat on a stone as a guide? If not, you might not have formed a nice even bevel. Your bevel might not be fully apexed in some parts of the blade. Angle control is important. The chisel ground blade has kind of a built in angle guide since the bevel is so wide. Straights have their own angle guide in the form of the spine.

Another possible problem is what you said: The straight has soft steel. If this steel is very ductile (bendable) the burr it forms would be hard to remove. You might have to flip the burr back and forth many times in order to get it to come off. If you have burr left on your straight, it's a little bit like a foil blanket sitting on top of your edge. There's a sharp edge there. But it doesn't feel sharp because there's a blanket on top of it.

It's hard to diagnose these things over the Internet. Hopefully I'm closer to the truth than fantasy.

Good luck.

Brian.
 
It could be as simple as what you've noted about the soft steel & foil burr on the razor. Very narrow geometry can cut well. But if the supporting steel for the edge is weak or soft, that very thin edge is much more likely to deflect or roll, even in light use. That could make the difference you're seeing in shaving.

I think Mora knives in their carbon steel are well-known for good hardness. Mora claims HRC 58-60 in current models. At 25° geometry, that could be a very good shaving edge. Most anything at/below 30° inclusive can shave pretty nicely, if it's finished cleanly and the steel is hard enough to support it. With a few of my own knives in various steels, I've noticed they get pretty delicate at the edge if I thin them much below 20-25° inclusive. These are knives in simple carbon or low-alloy stainless in mid-50s HRC hardness range. They'd do better at just a few points harder, such as 57 or above.

Edited to add:
Really good shaving edges are made even better if the steel is also very fine-grained, meaning it'll take a very fine edge and will also be stronger at a given hardness. A lot of that also comes from good heat treat & tempering, and purity of the steel itself at manufacturing stages. This is an aspect not often seen in cheap blades, such as how you've described the razor.
 
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It's probably a combination of factors. The biggest one, in my mind, is how well you formed a burr and how well you completely removed that burr.

Chisel ground blades are generally easy to sharpen, especially one with such a wide flat bevel like the Mora you pictured. Deburring is usually very easy also. Just lay it almost flat and do a couple of strokes. Boom, no more burr.

The shape of the Mora is helpful also. A completely straight blade is the easiest to sharpen. You probably formed a very even complete burr on the whole length of that blade. Blades with curves, like the Benchmade are harder to form a complete burr. The straight razor even appears to have a small amount of curvature from base to tip.

The straight razor should have the sharpest edge, as it has the smallest included angle. 10 degrees per side is a total of 20 degrees inclusive. Which is smaller than the chisel ground blade at 25 degrees inclusive. The benchmade was probably sharpened at 15 to 17 degrees per side, or 30 to 34 degrees inclusive.

So, we wonder why the straight isn't as sharp or sharper than the Mora. I can think of two reasons: Uneven burr formation. Did you sharpen this blade by placing the spine flat on a stone as a guide? If not, you might not have formed a nice even bevel. Your bevel might not be fully apexed in some parts of the blade. Angle control is important. The chisel ground blade has kind of a built in angle guide since the bevel is so wide. Straights have their own angle guide in the form of the spine.

Another possible problem is what you said: The straight has soft steel. If this steel is very ductile (bendable) the burr it forms would be hard to remove. You might have to flip the burr back and forth many times in order to get it to come off. If you have burr left on your straight, it's a little bit like a foil blanket sitting on top of your edge. There's a sharp edge there. But it doesn't feel sharp because there's a blanket on top of it.

It's hard to diagnose these things over the Internet. Hopefully I'm closer to the truth than fantasy.

Good luck.

Brian.

You are totally right about the burr. I did shape the edge using the spine as a guide. In this process, I was chasing the apex of the tip for a short time. I then realized in this process what I thought was the apex was actualy a thick burr. Roughly 1/128th of an inch wide. So in the process of trying to remove this, I kept going back and forth on the stone but would never leave. Just kept flipping back the other way. So to remove the monster burr currently on the blade, I lightly ran the blade in some end grain pine to remove the foil. Then back on the stone brought a smaller burr back to 1 side and finished the sharpening.
And once again you where right, I looked at the edge using my phone camera+ zoom, the edge looks so much worse this morning then it did last night before shaving. There is a considerable burr (you can actualy feel it) and there is an actual small chip in the blade that didn't exist after I sharpened it... Looks like my cheapness bit me in the end.
 
And once again you where right, I looked at the edge using my phone camera+ zoom, the edge looks so much worse this morning then it did last night before shaving. There is a considerable burr (you can actualy feel it) and there is an actual small chip in the blade that didn't exist after I sharpened it... Looks like my cheapness bit me in the end.

Buck up cheapo :) now you get to work on your technique ... what exactly did you do?

https://scienceofsharp.com/2016/04/14/simple-straight-razor-honing/
 
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