Sharpening Angle

[bobnikon]

I have a few different systems. I like the sharpmaker a lot for touchups, but find the adjustable guided systems a little easier for fixing bevels and such.

I am not strong enough to math this out right now (or ever?), but I am curious if anybody else has. If you take the 15 deg "setting" on a system like the worksharp precision adust, then the angle when the rod is perpendicular to the blade is presumably 15deg (+/-). How much does this vary as you move end to end on say a 4" blade, so a good couple inches on each side of that point? Obviously the angle would be consistent if the blade were a segment of a circle with the radius equivalent to the arc the rod scribes. But assuming a straight blade, as at least one end likely is and possibly both if you have a wharnie or some such thing, what is the variance of the angle bevel being produced from end to middle?

I have tried to draw it out a couple times, but I can't get my head into three dimensional geometry right now.

Cheers
Erik

[TkoK83Spy]

Around 6 months ago I sharpened my Guardian 3 Wharnie using the Precision Adjust. I don't get, or know how to get all nerdy with all that stuff, but from these photos I took after the reprofile the bevels look pretty even.

[bobnikon]

Thanks for your response.

Yep definitely looks good in the picture, and nice knife. I have noticed, mostly on my 4+" folders that when I move back to the sharpmaker after the WS Precision Adjust that I tend to hit the apex of the bevel mid blade and move up towards shoulders of the bevel as I reach the ends. It may well just be me, but I do notice it, so was curious if it is just me... Only explanation that didn't put it on user error, is that there must be some variance in the angle.

Cheer

[TkoK83Spy]

With a longer blade, pressure could also factor in as well. I had a heck of a time using the Precision Adjust on a friend of mines filet knife. Once I got away from the centered, the tip area was definitely troublesome. Granted it was a filet knife so it was extremely thin.

[bobnikon]

With a longer blade, pressure could also factor in as well. I had a heck of a time using the Precision Adjust on a friend of mines filet knife. Once I got away from the centered, the tip area was definitely troublesome. Granted it was a filet knife so it was extremely thin.

In that case, I am guessing it is a confluence of factors, the angle thing I am still trying to wrap my head around, pressure and the flex in the blade.

I got a math friend of mine thinking of throat punching me now, because I asked him the same question. He thinks he is close, but we are both supposed to be working on something else right now. As a plus, he is now interested in picking up a WS Precision Adjust... Lol...

[bobnikon]

Oh man. I think I just broke my brain. I tried a different search term than my original search. And this popped up:

viewtopic.php?t=61891

Assuming he is right, and no reason not to, it sounds like it is my error/issue and not necessarily a measurable difference.

[RustyIron]

I have tried to draw it out a couple times, but I can't get my head into three dimensional geometry right now.

The hardest part of any problem is bouncing it around in your head until it becomes simple. For this problem, break it down into two, two-dimensional right triangles. Your guide rod is the hypotenuse. Now it's easy, huh?

When the guide rod is in the middle of the table, the hypotenuse is at its shortest length. Now rotate the guide rod way out to the tip of the blade. The hypotenuse of the new triangle is longer. The angle of your edge is now more acute. How much the angle has changed is dependent on your sharpener and how it's set up. You can measure it if you like, and the math is just a simple trig problem. Just remember that you're working with two right triangles.

It's pretty easy to spot the variation in the width of the edge with the naked eye. As long as it's not a dramatic difference, I don't consider it a big deal. On a longer knife, or one where I'm trying to do really pretty work, I sharpen one section of the blade, then move it over and work on the next. You still get variation, but it's less, and it's over a shorter distance. If you sharpen a long knife and don't shift the blade, you'll end up with a dramatic difference in the width of the bevel, and it will look ugly.

[bobnikon]

I have tried to draw it out a couple times, but I can't get my head into three dimensional geometry right now.

The hardest part of any problem is bouncing it around in your head until it becomes simple. For this problem, break it down into two, two-dimensional right triangles. Your guide rod is the hypotenuse. Now it's easy, huh?

When the guide rod is in the middle of the table, the hypotenuse is at its shortest length. Now rotate the guide rod way out to the tip of the blade. The hypotenuse of the new triangle is longer. The angle of your edge is now more acute. How much the angle has changed is dependent on your sharpener and how it's set up. You can measure it if you like, and the math is just a simple trig problem. Just remember that you're working with two right triangles.

It's pretty easy to spot the variation in the width of the edge. As long as it's not a dramatic difference, I don't consider it a big deal. On a longer knife, or one where I'm trying to do really pretty work, I sharpen one section of the blade, then move it over and work on the next. You still get variation, but it's less, and it's over a shorter distance. If you sharpen a long knife and don't shift the blade, you'll end up with a dramatic difference in the width of the bevel, and it will look ugly.

That is exactly how I was trying to work through it, but I couldn't help but think there must me an "arc" contribution in there. As you say though, it should just be the two 90deg triangles and being able to sohcahtoa the sharpening angle on the second triangle. But for some reason that arc thing keeps creeping back into my head. Just for closure I will math it up when I get home, but like you say should mostly be negligible for short blades, and is manageable by moving the blade for longer ones.

Or, I guess I could just get better at freehand...

[Coastal]

Either Mules are ground really obtuse at the tip, or it's this phenomenon that allows 15 minutes to be sufficient for most of the edge, but forces me to spend an hour on the last 1/8 inch at the tip. Maybe both.

[Eli Chaps]

Bench stones.



Another Erik

[Coastal]

Bench stones.

I used them for years, then discovered the Sharpmaker, and then guided systems. Some day I'll pull out the bench stones and give them another shot.

[Eli Chaps]

Bench stones.

I used them for years, then discovered the Sharpmaker, and then guided systems. Some day I'll pull out the bench stones and give them another shot.

I can for sure relate. I used bench stones mostly poorly for years. Way before the birth of the internet so "learning" was shaky at best.

Guided systems really helped me mostly because I gained confidence. As we all know, they aren't foolproof and still have a learning curve but they surely can help. I was able to reset and really start focusing on understanding THE FUNDAMENTALS of sharpening. Then I could out those old stones and start transferring those lessons. Sure, I still had issues and to this day, many, many years later I am still learning. But the freedom of using bench stones can't be rivaled.

And, having a guided system in reserve also inspires confidence as you know you have a system that can help you fx things p if needed.

But one of the beauties of bench stones is your concern about exact angles disappears. It just becomes the edge that works for your knife and your needs.

[Coastal]

^^^^^ This sure sounds like wisdom.

[TkoK83Spy]

I'm like you Eli, I enjoy both. Many of my friends beater knives and mine that I've already flawed the edge or bevel one way or another, those knives I use on the bench stones. I also like to use my diamond DMT stones to start a reprofile where I'm removing a lot of steel, and then switching to my PA to refine it a bit.

I really enjoy using both methods, it's a great skill to have and from my personal experiences, others are very appreciative of it as well!