Stabilizing concave blades for sharpening

CKTG has a large amount of Edge Pro products so we've dedicated a forum to questions on Edge Pro sharpening systems, accessories and techniques.
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Celt16
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Stabilizing concave blades for sharpening

Post by Celt16 »

Blades that are not flat, but concave from the spine to the cutting edge can rock several degrees from what I will call the “true flat plane” to the plane that the concave blade shape presents when pressed against the Apex bed. I’ve thought of a few hacks to bring the edge back to the “true flat plane”, but wonder whether I’m over-thinking this one. It was of mild concern before I measured several degrees difference, now it’s a larger (if not yet validated) concern.

Thoughts?
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Re: Stabilizing concave blades for sharpening

Post by d_rap »

Concave or hollow ground blades, blades with faceted faces, certain convex blades, what have you--any of these and more may rock and be difficult to stabilize on the EP blade table.

First, it doesn't matter what the most stable position is, whether it's the "true flat plane" (if there is one) or not. If you can stabilize the blade, then just find your angle using that knife position. The key question is can you find a way to stabilize the blade that is secure and close to perfectly repeatable as you turn the blade side to side.

For me, EP's magnet was a deal changer, just fantastic. It's not for every blade, and it makes it hard to move a blade as you sharpen, which is desirable, especially for longer blades, but time and again my magnet has allowed me to achieve a rock solid position on the blade table with even the most odd-shaped blade faces.
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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

Hi Celt. You are probably overthinking it a bit and David has some good stuff above. If you have a thicker blade with a decent blade road it can rock around a bit. I don't use magnets on my EP's and occasionally the spine rides up over the top of the guide plate, which can cause problems especially on a polished knife. (Don't ask me how I know!!).

So for what it's worth, I try to minimise the "overhang" of the cutting edge past the end of the blade table and nowdays I almost always use the plastic clip on the end of the guide plate. From there I try and lay and keep the blade road onto the blade table. As David says the magnet(s) will help keep it there I just use a little continuous pressure. These things will help maintain the angle at the edge-of-the-edge more consistently. The only thing that you might need to adjust for, is that the centre-line of the blade (spine to edge) will be at a slightly different angle to the blade table, meaning that the edge cutting angle will be somewhat more acute.

I don't specifically adjust for the concavity of the blade road, as the above technique tends to minimise any angle changes.

Hope this helps
Cheers Grant

Just because you're not paranoid doesn't mean they're not going to get you!!
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Re: Stabilizing concave blades for sharpening

Post by Celt16 »

Thanks David & Grant. I’ve had magnets on my unit for quite some time, and only recently noticed an almost 7degree difference between the “true flat plane” and where the magnet holds the blade, hence the question. I was thinking of adding a small adjustable riser block at the front of the EdgePro to support the blade edge, but that could create more problems than it solves. I could also incorporate that idea into the bed of the Apex, but not sure I want to go there either. I was an analyst by profession for 20+ years - I’m prone to overthinking! ;-/
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Re: Stabilizing concave blades for sharpening

Post by d_rap »

This thread involves the kind of analysis that results in pleasure and sharp blades. It's what we all come here for!

Unless I'm missing something, the difference, even if it's 7 degrees, between your "true flat plane" and where the blade sits with the most stability is immaterial. As long as you can lay the blade on the table securely, in a very similar fashion on each side, then just find the point on the EP pivot arm such that the stone removes metal perfectly from the entire bevel and sharpen.

For me, modifications like a riser block would be desirable if it made sharpening x blade easier. Adding a riser block to match a hypothetically correct (true flat) position would create more problems (and certainly more work and fiddling) than it solves, really because there is no actual problem. A stable blade on the table that is easy to reach with the stone on the pivot arm is what you need; ignore the numbers corresponding to the colored dots on the rod and let blade stability be the deciding factor in placement.
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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

I did play around with this a very long time ago, and should you want to explore it, this is where I landed.

Assuming a 50 / 50 grind on the blade face, I would get the nearest I could to one half of that, using flat, wooden toothpicks and then taping them using masking tape along the leading edge of the blade table. You can get even more pernickety and either sand one of the toothpicks or simply move less toothpicks further back down the blade table.

I can get pretty OCD about some of this stuff, but I gave this one away ages back.
Cheers Grant

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Re: Stabilizing concave blades for sharpening

Post by SeattleB »

Celt16 wrote: Fri Dec 11, 2020 9:47 am Blades that are not flat, but concave from the spine to the cutting edge can rock several degrees from what I will call the “true flat plane” to the plane that the concave blade shape presents when pressed against the Apex bed. I’ve thought of a few hacks to bring the edge back to the “true flat plane”, but wonder whether I’m over-thinking this one. It was of mild concern before I measured several degrees difference, now it’s a larger (if not yet validated) concern.

Thoughts?
I'm new to all of this (this is my second post) so pardon me if I'm asking a stupid question or one previous covered. One thing that strikes me about the engineering design of the Edge Pro is that the vertical rod is not very far from the blade edge. If it were farther away (like 20in / 500mm) these small differences in blade position would make only an insignificant difference in sharpening angle. It seems that other guided sharpeners that I've seen have a similar design. Has anyone made a guided sharpener design with the pivot farther away?
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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

Yes, I designed one back in the late 1990's and had a friend build a proof of concept model. By the time that was done I had discovered the EdgePro was in production, so that was the end of that. My friend still uses this and it can get knives very sharp.

You can check it out here <You can check it out here <https://www.dropbox.com/s/vl7ycm92cre0n ... 3.jpg?dl=0> and here <https://www.dropbox.com/s/dkw4k3ps5i919 ... 2.jpg?dl=0>
Cheers Grant

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Re: Stabilizing concave blades for sharpening

Post by Celt16 »

Radar53 wrote: Tue Dec 15, 2020 10:03 pm Yes, I designed one back in the late 1990's and had a friend build a proof of concept model. By the time that was done I had discovered the EdgePro was in production, so that was the end of that. My friend still uses this and it can get knives very sharp.

You can check it out here <You can check it out here <https://www.dropbox.com/s/vl7ycm92cre0n ... 3.jpg?dl=0> and here <https://www.dropbox.com/s/dkw4k3ps5i919 ... 2.jpg?dl=0>
Interesting ... fixed sharpening angle?
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Re: Stabilizing concave blades for sharpening

Post by Celt16 »

Radar53 wrote: Sun Dec 13, 2020 2:11 pm I did play around with this a very long time ago, and should you want to explore it, this is where I landed.

Assuming a 50 / 50 grind on the blade face, I would get the nearest I could to one half of that, using flat, wooden toothpicks and then taping them using masking tape along the leading edge of the blade table. You can get even more pernickety and either sand one of the toothpicks or simply move less toothpicks further back down the blade table.

I can get pretty OCD about some of this stuff, but I gave this one away ages back.
I started messing around with washers, as the magnet holds them, but didn’t want to mar the knife.
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Re: Stabilizing concave blades for sharpening

Post by Celt16 »

d_rap wrote: Sun Dec 13, 2020 12:18 pm This thread involves the kind of analysis that results in pleasure and sharp blades. It's what we all come here for!

Unless I'm missing something, the difference, even if it's 7 degrees, between your "true flat plane" and where the blade sits with the most stability is immaterial. As long as you can lay the blade on the table securely, in a very similar fashion on each side, then just find the point on the EP pivot arm such that the stone removes metal perfectly from the entire bevel and sharpen.

For me, modifications like a riser block would be desirable if it made sharpening x blade easier. Adding a riser block to match a hypothetically correct (true flat) position would create more problems (and certainly more work and fiddling) than it solves, really because there is no actual problem. A stable blade on the table that is easy to reach with the stone on the pivot arm is what you need; ignore the numbers corresponding to the colored dots on the rod and let blade stability be the deciding factor in placement.
I think I agree with the above opinion only if I can match the 7 degree difference with sharpening angle AND hold it stable (keep it from rocking spine to edge). The latter wasn’t a huge problem, but at least conceptually, if I’m intending to sharpen at say 17 degrees, but actually sharpening at 10 or 24 degrees ... that is significantly different, but I’d need someone with better geometry skills than I to validate that hypothetical difference actually amounts to 7 degrees in sharpening angle (multiple angles going on there). As to a riser block being fiddly, I’m not sure I agree ... my angle cube would dial that in to the same angle as the blade-bed accurately and in no time flat.
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Re: Stabilizing concave blades for sharpening

Post by d_rap »

Celt16 wrote: Wed Dec 16, 2020 8:50 am I think I agree with the above opinion only if I can match the 7 degree difference with sharpening angle AND hold it stable (keep it from rocking spine to edge). The latter wasn’t a huge problem, but at least conceptually, if I’m intending to sharpen at say 17 degrees, but actually sharpening at 10 or 24 degrees ... that is significantly different, but I’d need someone with better geometry skills than I to validate that hypothetical difference actually amounts to 7 degrees in sharpening angle (multiple angles going on there). As to a riser block being fiddly, I’m not sure I agree ... my angle cube would dial that in to the same angle as the blade-bed accurately and in no time flat.
First, I agree, a riser block is no big deal. I have EP's small knife attachment, a riser block any way you slice it, and it works well. Changes your angle by a single degree.

Very interesting discussion too.

Here's what I'm saying, put perhaps another way. Yes, being able to match the angle side to side and keep the blade stable are the keys. But no matter what angle the pivot says you're sharpening at, the actual angle is always going to be the angle described by the stone on the knife bevel relative to an imaginary plane, not the EP blade table exactly, since even the thickness of the blade stock, no matter how we position the knife, is going to change that angle slightly. And of course, as you are saying, the way we position the knife will change that angle as well, even fairly dramatically.

So the bottom line is having the stone move on the knife so you sharpen the entire bevel. No matter what angle you have the EP set at, as long as you are removing metal along the width of the bevel, the actual angle is the same. When I use the EP small knife attachment, I have to raise the pivot arm to compensate, but after I have done that the actual angle I am sharpening at is the same. So set the knife up for it's best stability and repeatability, draw sharpie along the bevel, and set the EP so that you removing all the sharpie. When the stone does that, that's your actual angle.
David
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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

^^^ David sums it up well above. I've been using the EP for 21 years now and as a final check, the last thing I do after all the setup is done is "Sharpie" the edge bevel and with almost no pressure run the stone along the edge.
Cheers Grant

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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

Celt16 wrote: Wed Dec 16, 2020 8:39 am
Radar53 wrote: Tue Dec 15, 2020 10:03 pm Yes, I designed one back in the late 1990's and had a friend build a proof of concept model. By the time that was done I had discovered the EdgePro was in production, so that was the end of that. My friend still uses this and it can get knives very sharp.

You can check it out here <You can check it out here <https://www.dropbox.com/s/vl7ycm92cre0n ... 3.jpg?dl=0> and here <https://www.dropbox.com/s/dkw4k3ps5i919 ... 2.jpg?dl=0>
Interesting ... fixed sharpening angle?
Yes and no :D :D

The three holes in the left hand upstand give pretty closely the three edge angles I was most using at the time ~ so fixed for convenience if you like. However if you look closely at the relatively small blade table, on the right hand side, you will see that it is adjustable for angle. The intention at the time was to adjust for either fine corrections and also to be able to set up any angle between each the big holes. So by choice of hole and blade table adjustment combined, the jig was to be able to provide any angle between the bottom hole and the top hole.

Just for completeness it needs to be said that ken123 of this forum has also designed his own jig called the Gizmo?? if memory serves. Didn't mention it before because I didn't want to steal his thunder. I have only seen a picture, but it appeared to be an all singing & dancing, bells & whistles piece of kit ~ probably the best designed jig I've seen. I was hoping he might contribute to this. Sad to say, but it's my understanding that the Gizmo is no longer available.
Cheers Grant

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Re: Stabilizing concave blades for sharpening

Post by SeattleB »

Thanks for the info. This gives me an idea or two. I think I'm going to take a run at engineering a modification to the Edge Pro or similar sharpener. I want to shoot for:

- Long sharpening arm (is there another name? Control arm? Stone arm?)

- Easy to adjust on the vertical rod, with markings for the angle. I calculate that with a 20" sharpening arm a one degree change in sharpening angle means an adjustment of roughly 10mm on the vertical rod, so it will be possible to easily adjust for desired angle.

- A means to hold the knife securely. EP has one method, and there are others with clamps. I like the clamps that rotate, that looks handy.

I see a new-ish guided sharpener called the Ruixin Pro. I like the blade clamp because it rotates, and I like how the device can clamp to a cutting board (my board is thick, heavy and would make a very stable base). The trick will be to move the vertical rod farther away and hold it in place, and extend the sharpening arm with a coupler and another length of stainless rod. Hmmmm...

Stand by. This might take a while due to the holiday distractions.
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Re: Stabilizing concave blades for sharpening

Post by Radar53 »

I'll be keen to hear about what you do and how you get on. ("Long sharpening arm" is called the Stone rod I think.)

Just a couple of things here.
(i) I always check the angles independently of the angle markings on the vertical rod. Different thickness stones and other things can change the angle from any "preset".

(ii) If you're heading towards getting really acute angles say <= 10dps then be careful about clamps, rotating clamps and other things. In my experience so far, pretty much all the clamping systems have bits and pieces on them which stick up & interfere with how acute an angle you can actually grind, before you start grinding some part of the clamp.

HTH
Cheers Grant

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Re: Stabilizing concave blades for sharpening

Post by SeattleB »

Good point about the stone thickness. Curious, I did a quick calculation. The answer is that if the stone rod is long enough the change in angle becomes very small. For example, I am thinking about a rig that would clamp to an 18 x 24" cutting board, so the vertical rod would be roughly 18" from the edge of the knife. Let's call that 500mm for a round number. Let's imaging setting the height of the stone rod where it meets the vertical rod so that our target angle is 14 degrees. If one EP-compatible stone is 3mm thicker than another that would change the cutting angle by 0.3 degrees.

You're right then, we would have to be aware of the adjustment. But it seems pretty simple. I could put markings on the vertical rod for the target angles, based upon my typical stones (e.g., Shapton Glass). If another stone were a different thickness then the trick would be to raise or lower the stone rod by the same amount.
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