Geometry and Kinematics of Guided-Rod Sharpeners

@eKretz,

I'm not fully understanding how the rails were pivoted. Could you post a picture for us? Even a camera photo of a hand drawing would be very helpful. :)
 
Let me see if I can find it: if not I'll post a drawing. I made it a long time ago before Internet really took off with forums and such, not sure if I have any pics or not.
 
Couldn't find the actual sharpener, but here is a quick sketch:

download.jpg~original


I was able to get a whole bunch of parts from machines we had scrapped, so it didn't cost much in parts, just the time I had invested to machine things and assemble it. I had slots to adjust the linear guide rods at the bottom so i could change the angles, and the back side had a twinned setup, same sort of thing as the wicked edge system. When sharpening I would use two hands, one to slide the bottom guide along the rod, and the other to slide the hone along its own guide rod.
 
eKretz said:
Couldn't find the actual sharpener, but here is a quick sketch:

download.jpg~original


I was able to get a whole bunch of parts from machines we had scrapped, so it didn't cost much in parts, just the time I had invested to machine things and assemble it. I had slots to adjust the linear guide rods at the bottom so i could change the angles, and the back side had a twinned setup, same sort of thing as the wicked edge system. When sharpening I would use two hands, one to slide the bottom guide along the rod, and the other to slide the hone along its own guide rod.
Click to expand...

That is quite a setup you have there! And by the looks of it, it seems it would work very well with limited stone "throw" and still being able to sharpen a long blade.

For smaller blades the side to side motion is probably not needed but the stop collar is genius for the start of the radius.

I think a interesting improvement would be so that at the "pivot point" could be spring loaded to say in the perpendicular direction. Then only when it hit the stop, would it bend and follow the radius of the knife.

This might allow one handed movement.
 
I was asked to do an analysis of sharpening a khukuri knife at 10 degrees per side. The analysis here is for a WEPS-Gen1

I started with photo of a khukuri that I found from www.KnifeCenter.com. I then did a polygonal trace of the knife edge which was then spline interpolated with sample points at roughly every 0.25 inches. The trace was more carefully done for the blade's cutting edge, and then only roughly accurate for the spine and handle. Analysis of the results are graphed as requested.

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Original photo of the khukuri:
1ErU5ML.jpg


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Results:
Please note that the graph and the knife are vertically aligned to help visualize the results.
jkOOB6O.png


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Discussion:

I did not trace the knife edge perfectly, which may have caused some minor artifacts So the slight "hump" at around -4.5 <= X <= -4.0 inches is probably due to inaccurate tracing on my part. Similarly, I may have clicked slightly off on the very tip of the knife, which would cause the change at the tip of the knife at X = +3.5 inches. Likewise, the slight oscillation around 10 degrees for the main edge of the knife is almost certainly due to inaccurate tracing.

The sharp increase in degrees per side for -5 <= X <= -3 is expected from the Dihedral Triangle Theorem in the technical report. The gray line is a tangent of the knife edge near X = -5. As one can see, that tangent line travels very close to the circle center. The distance from the tangent line to the circle center is labeled "d". Because d is much smaller than the radius of the circle, the dihedral angle must be much higher. Hope this makes sense. Feel free to ask questions if it is not clear.

Although this analysis was done with a WEPS-Gen1, the results will be very similar for a WEPS-Gen2. They should only differ by a tiny fraction of a degree, so the graph will be virtually identical. If there is interest, I can run the analysis for a WEPS-Gen2 and then we can compare it to a WEPS-Gen1.

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When I have time, I plan to also analyze a kitchen chef knife.
Let me know if you have other requests or suggestions. :)
 
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brplatz said:
That is quite a setup you have there! And by the looks of it, it seems it would work very well with limited stone "throw" and still being able to sharpen a long blade.

For smaller blades the side to side motion is probably not needed but the stop collar is genius for the start of the radius.

I think a interesting improvement would be so that at the "pivot point" could be spring loaded to say in the perpendicular direction. Then only when it hit the stop, would it bend and follow the radius of the knife.

This might allow one handed movement.
Click to expand...

Yup that would be a good addition. In use it was almost possible to do that without a spring since the slides have such minimal friction but I always felt more comfortable using two hands for whatever reason. Lagrangian's sharpener looks pretty nice too. I like the angle adjustment setup, is that a combination LH/RH threaded rod? Or do you have to adjust them independently?
 
For my set-up, the two angles are just clamped independently with bolts. So you have to do both sides separately.
 
Ahh, I thought there was a threaded adjuster. I thought about doing that with mine using a LH/RH threaded rod so when you turned the rod it would adjust both sides in or both sides out depending which way you turned it. Never got around to it though, just had marks for my normally used angles. And nowadays I just freehand everything...I wish I could find that old sharpener though, might be fun to play with some more.
 
@eKretz,
Oh, the rod ends are threaded, but I'm not using the threading. Instead, those rod ends are glued into place with epoxy. But I like the idea of using LH/RH threads so that turning a single threaded rod could move both sides.

If you do find your old sharpener, please show us a photo of it. :)
 
I'm still digging for it...there are a lot of old projects as well as not-yet-realized project parts up in the garage attic. I'm pretty sure it's up there somewhere. As long as I didn't put it somewhere else! =) I'll post up when I find it.
 
Following a suggestion, I have added three knives to the report: An 8" chef's knife, a 10" khukuri, and the Spyderco LionSpy pocket knife. In the report is a new section,"Case Studies of Curved Blades on the WEPS-Gen1, WEPS-Gen2, and EP-Apex."

You can download the new version of the report here:
https://drive.google.com/file/d/0B8rQYhU8N9ZGSENqc2Q2MlRFbTA/

Alternate Download Link:
http://www.mediafire.com/download/2flrqn7po9um350/Geometry_and_Sharpening_(DRAFT_1.0beta17).zip

If you are just curious about the pictures and graphs, you can see them below but without much description. The report contains the details and discussion. I re-did the analysis for the khukuri with better tracing and improved spline interpolation.
http://imgur.com/a/M0QCx#0

Zt4Q0M1.png

sZkZUo5.png

TlSn8jG.png

mFgzCzO.png

7KKzH7G.png

irOV0ct.png

eGXN0Vk.png
 
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I like the additions! The comparison of deviations between sharpeners is interesting as well!
 
Something that May or may not be interesting, could you analyze a few different knives while at different angles relative to perpendicular?

Like this?

 
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brplatz said:
Something that May or may not be interesting, could you analyze a few different knives while at different angles relative to perpendicular?

Like this?

Click to expand...

Oh, you mean where the long edge of the knife is not parallel to the edge of the EP's main platform? And/or not parallel to the top edge of the WEPS clamp?
 
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