Fixed Angle Knife Sharpening "MYTHS"

T

TrumanHW

Joined
Jan 2, 2024
Messages
52
So, apparently, it's a "total myth" that the slope doesn't change between the belly and tip of a long knife using a fixed angle sharpener. Or at least, that's what Outdoors55 very confidently says ... and literally 425 comments accepted as "fact."

watch the video yourself; see if you're convinced (link removed by staff...as video was not embedded)


Yet, here's a simple refutation:

The height of the blade's edge is fixed.
The height at the rod's eye is also fixed.

Yet, the amount of rod between those points ... changes.

That is, the "rise" is static .. but the "run" is changing.
And SLOPE = RISE / RUN

(in fact, at 5:42, you can literally watch his angle change over just 2 inches)

I trust he's going to get a laugh out of that, bc he's a very very smart guy. And I truly love his videos.
Thing that amused me was people claiming to be ENGINEERS & Mathematicians praising his analysis. 🤣

Test for yourself:
Measure the rod (marks or actual measurement) when it's touching the belly of the knife
Then, extend it out to the tip and see if there isn't more "rod" between those to points.

Or, via Reductio ad Absurdum: Get a SWORD, 3 feet long and see if the angle doesn't change. 🤣
It's literally axiomatic. QED
 
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Yeah the angle DEFINITELY changes. It's generally a small enough change that it doesn't make a big PRACTICAL difference in the case of the small pocket knives and fixed blades that they're typically used on, but it's a fact that it does produce a shifting edge angle.
 
FortyTwoBlades said:
Yeah the angle DEFINITELY changes. It's generally a small enough change that it doesn't make a big PRACTICAL difference in the case of the small pocket knives and fixed blades that they're typically used on, but it's a fact that it does produce a shifting edge angle.
Click to expand...

Right. Of course, his point wasn't that it was 'insignificant. lol.
The only way it wouldn't change..? Is if it were a round blade.

What's hysterical is literally watching his angle change (on the Klein) as he moves it. lol

If you extend the distance between the 'eye' (post) & the edge it'd reduce the significance.
And it's not that he didn't get it, but literally, 425 other people all agreed. I mean, unreal.
Not a single person disagree with him on that.
 
It's actually one of those engineering questions that occasionally keeps me up at night trying to think of the best and most cost-effective way to provide a guided system that would ACTUALLY hold a consistent angle. There's a million ways to do it, it's just the matter of how to make it EASY and ECONOMICAL and the devil's in the details there.
 
FortyTwoBlades said:
It's actually one of those engineering questions that occasionally keeps me up at night trying to think of the best and most cost-effective way to provide a guided system that would ACTUALLY hold a consistent angle. There's a million ways to do it, it's just the matter of how to make it EASY and ECONOMICAL and the devil's in the details there.
Click to expand...


That's funny, I was just thinking about that. Maybe a horizontal rod between two posts in which a an eye that allows some horizontal range yet, keeps the amount of cosine change to a minimum.

Though honestly ... the Tormek is an exceptional system. And when you compare its costs to say, Resin Bonded Diamond and a good fixed-angle sharpener (and all the mounts and attachments) ... it's a pretty danged good value. And if you factor consistency and speed..? Well, if I ever started sharpening as a hobby-business I'd have to at least try one out, as I'm pretty sure it'd be more efficient and consistent. And consistency is what businesses require.

Though, "Gunny" did a video in which he used nothing but a strop (and some GJ) to massively refine a brand new edge down to a finish that looked about like 5,000 level mirror.

But when you compare the FAS mirror finishes ... they don't seem to have the consistency of something like a Tormek.

Your thoughts..?
 
TrumanHW said:
That's funny, I was just thinking about that. Maybe a horizontal rod between two posts in which a an eye that allows some horizontal range yet, keeps the amount of cosine change to a minimum.

Though honestly ... the Tormek is an exceptional system. And when you compare its costs to say, Resin Bonded Diamond and a good fixed-angle sharpener (and all the mounts and attachments) ... it's a pretty danged good value. And if you factor consistency and speed..? Well, if I ever started sharpening as a hobby-business I'd have to at least try one out, as I'm pretty sure it'd be more efficient and consistent. And consistency is what businesses require.

Though, "Gunny" did a video in which he used nothing but a strop (and some GJ) to massively refine a brand new edge down to a finish that looked about like 5,000 level mirror.

But when you compare the FAS mirror finishes ... they don't seem to have the consistency of something like a Tormek.

Your thoughts..?
Click to expand...
The problem with the Tormek is that it still uses a clamp and the contact point on wheel changes the contact angle still. Really what you need is a device that locks the blade to a single plane of movement and then allows the edge to contact the abrasive regardless of specific distance. There are lots of ways to accomplish that, but many of them are mechanically complex, and mechanically complex = more expensive, involved to set up, and so on. There are ways to simplify the build but it's one of the persistent conundrums that I continue to chip away at late at night.
 
As long as the edge of the knife is straight and at a right angle to the guide rod pivot the angle does not change. I can visualize it best if I think of the roof of a house. Put the pivot at the center of the ridge and the eave is the knife edge. As long as your imaginary stone is flat against the eave/ blade edge the angle does not change.
 
Diemaker said:
As long as the edge of the knife is straight and at a right angle to the guide rod pivot the angle does not change. I can visualize it best if I think of the roof of a house. Put the pivot at the center of the ridge and the eave is the knife edge. As long as your imaginary stone is flat against the eave/ blade edge the angle does not change.
Click to expand...
I've cut in my share of roofs and can vouch for this being true.

The only way the angle changes is with a belly or recurve in the blade not the length of the blade.
 
I see lots of folder pictures with a nice mirror edge but obviously a much more larger angle 1-2” from the tip (compared to belly).

Doesn’t happen with a benchstone. Obviously the angle is then way less accurate anyways.
 
I've set my clamp sharpener at the center of the radius of the tip to minimize the change in angle this works much better on smaller knives like folders and it does on a large fixed blade. I'm surprised people don't readily notice the edge looks wider near the tip.
 
A lot of knives are ground thicker near the tip.

I'm not a fixed angle sharpener but I'm not changing my angle to make the bevels look even.
Grinds can be all over the place even on some expensive knives.
 
Diemaker said:
As long as the edge of the knife is straight and at a right angle to the guide rod pivot the angle does not change. I can visualize it best if I think of the roof of a house. Put the pivot at the center of the ridge and the eave is the knife edge. As long as your imaginary stone is flat against the eave/ blade edge the angle does not change.
Click to expand...
The issue is, of course, that most knives are not straight their entire length. While there are some that are, they represent the minority, to the point where may guided sharpeners either suggest themselves or their user base recommends that the knife be clamped consistently at a given site so that the angle remains consistent during subsequent sessions.
 
thebrain said:
I've set my clamp sharpener at the center of the radius of the tip to minimize the change in angle this works much better on smaller knives like folders and it does on a large fixed blade. I'm surprised people don't readily notice the edge looks wider near the tip.
Click to expand...

Yeah, it shocks me that people don't understand the absolute most remedial geometry / trig.

If the distance between two points of at different heights changes, then, the angle between them MUST ALSO. lol

Where else would that added material go!?? lol
 
garry3 said:
A lot of knives are ground thicker near the tip.

I'm not a fixed angle sharpener but I'm not changing my angle to make the bevels look even.
Grinds can be all over the place even on some expensive knives.
Click to expand...


Exactly. And that partially explains why the bevel's width increases at the tip; the angle becomes more shallow.

What do I say to illustrate it to the people who reject it..?


Perhaps, imagine if the belly of the knife were 18" from the pivot point,
and the tip were 3 FEET away. Would the angle still be the same!?

But something tells me the cognitive dissonance would "find a way."
 
It's essentially a simple problem of drawing a circle. All guided sharpening devices (including systems like Tormek when sharpening curved blades—imagine using the KJ-45 and needing to swing up and down along the horizontal bar) will inevitably produce slight angle variations whenever there's a change in the distance between the guide rod and the edge of the blade. This angular deviation decreases as the distance from the pivot point to the tip of the jig increases—basically, the larger the arc you can draw, the smaller the variation.

In theory, this issue can be addressed by guided systems like the Toohr #3, which features a lateral track. However, in practical use, anyone who has tried it will understand that if you aim to finely refine a blade edge with zero error, it's nearly impossible to pause precisely at every "corner" of the edge, adjust the guide, and then resume sharpening without introducing inconsistencies. Human hands are not perfectly accurate; any such adjustment and re-alignment inevitably introduces a secondary angle at the overlap.

So when using guided systems, it's important to understand the fundamental nature of sharpening. I believe most people choose these tools for consistency between the two sides of the edge, not with the expectation that they will produce a completely uniform edge.

Blade edges come in all shapes and forms. I don’t believe there is a single device in the world that, while remaining as user-friendly as a guided sharpener or Tormek system, can perfectly handle all the variations in edge angles with complete precision.
Sorry if it sounds weird—English isn’t my first language. Blame GPT! 😅
 
TrumanHW said:
Exactly. And that partially explains why the bevel's width increases at the tip; the angle becomes more shallow.

What do I say to illustrate it to the people who reject it..?


Perhaps, imagine if the belly of the knife were 18" from the pivot point,
and the tip were 3 FEET away. Would the angle still be the same!?

But something tells me the cognitive dissonance would "find a way."
Click to expand...
Yes.
Provided it was a straight edged knife.
 
It's not quite as simple as logically imagined with your example of rod length. It is a common misunderstanding that comes up regularly.

The fact remains (just as D Diemaker explained) that if you clamp a straight knife at a right angle to the guide rod pivot, then the length does not affect the sharpening angle. (Sharpening angle, not relative angle)

The pitch of a roof as D Diemaker used above is a very good example that often clears up this misunderstanding. Imagine you attach a string to the top of a roof in the center of the ridge. Now pull that string straight down to the eave / gutter. Now pull this string all the way to the left side, or right side of your roof. The string gets longer & shorter as you go left or right and the relative angle between straight down and your current position changes, but the pitch angle of the roof does not change at all. Your sharpening angle is the pitch angle of the roof.

As you move left & right, your relative angle changes and your sharpening stone rolls left & right too but the actual sharpening angle does not change. Unfortunately, a regular digital angle cube has to be rotated at a right angle to the ridge of the roof to measure the pitch of the roof, you can't swivel that angle cube in-line with your string as it's not designed to measure at an angle on the third axis. Some advanced angle cubes are 3 axis cubes, but most are not. This is also where a lot of confusion and erroneous "proof" comes into play with people who do not understand how a basic angle cube should be used.

Now, if your knife is NOT straight, then most definitely the sharpening angle changes. Imagine a tip that sweeps up excessively, closer to your pivot point, or a belly that curves away excessively. Those angles do change. An excessive belly will have a lower sharpening angle and an excessively swept up tip will have a higher sharpening angle.

I hope this makes sense?
 
Ilis said:
It's essentially a simple problem of drawing a circle. All guided sharpening devices (including systems like Tormek when sharpening curved blades—imagine using the KJ-45 and needing to swing up and down along the horizontal bar) will inevitably produce slight angle variations whenever there's a change in the distance between the guide rod and the edge of the blade. This angular deviation decreases as the distance from the pivot point to the tip of the jig increases—basically, the larger the arc you can draw, the smaller the variation.

In theory, this issue can be addressed by guided systems like the Toohr #3, which features a lateral track. However, in practical use, anyone who has tried it will understand that if you aim to finely refine a blade edge with zero error, it's nearly impossible to pause precisely at every "corner" of the edge, adjust the guide, and then resume sharpening without introducing inconsistencies. Human hands are not perfectly accurate; any such adjustment and re-alignment inevitably introduces a secondary angle at the overlap.

So when using guided systems, it's important to understand the fundamental nature of sharpening. I believe most people choose these tools for consistency between the two sides of the edge, not with the expectation that they will produce a completely uniform edge.

Blade edges come in all shapes and forms. I don’t believe there is a single device in the world that, while remaining as user-friendly as a guided sharpener or Tormek system, can perfectly handle all the variations in edge angles with complete precision.
Sorry if it sounds weird—English isn’t my first language. Blame GPT! 😅
Click to expand...

Ironically, I also own the Toohr #3. And I NEVER use that feature.
I'm not mentioning this bc I think it's a requirement or matters (always).
My only point..? Was that it's Trigonometry, math identities, and he was wrong.
And (redundantly) what "impressed" me most was that literally EVER COMMENTER was wrong! lol

Wow, your English is excellent. As is your logic / visualization.
 
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