Video: Convex Grinding On A Tormek T8

[iSharpen]

Had a go at convex grinding on the T8. Obviously I'm not an expert at it but a Facebook troll said it was impossible so I had to try - right?

It came out "ish" and with practice and refinement it'd be nicer. I'd love to try it on the Jap wheel. Imagine a perfect mirror convex grind. That'd squash the trolls and would look marty purdy.

I had a search around and couldn't find much in the topic apart from the French version that Tormek officially did. As atrocious as YouTube's translater is, just watching him do it was all I needed to give it a go. My biggest tip is "fast is better than slow". I made a short of me giggling when I floored it.

If I had to give myself a score out of 10, Id score myself a solid "at least he tried" 

Enjoy...

Convex Grinding On A Tormek T8: Is It Really Possible?

[tgbto]

Hey Baz,

Nice video, and congrats for taking the time and effort to detail all this.

I hope you won't count me among facebook trolls ... but ... I have to ask how you happen to be sure it is "definitely convex", and especially how it was so right after the Tormek treatment alone.

The way I see it, there is no avoiding the concavity created by the wheel at the apex. As soon as you start to bring the knife down from the far stop to the near stop, you're working on the shoulder of the concave grind. So all the apex ever sees is a convex wheel grinding a concave grind. And how convex it is beyond the apex is the result of many factors including speed, but also probably at least pressure.

We should just remember how easy it is to overgrind the middle of a fine knife while sharpening on a Tormek, and transpose that to the bevel (y axis) instead of the length of the blade (x axis). Especially given how much less control the KJ allows when "convexing" the bevel.

All this not taking into account the fact that until now we were very careful with controlling pressure and speed to not create the aforementioned concave overgrind, and now we're just adding an energetic, parasitic movement to convex the bevel. I don't think it will help counter the natural overgrinding tendency.

[iSharpen]

Don't worry, this isn't Facebook. I can tell it's convex because of it's shape. It might not come through on the video but it's definitely rounded.

I just wanted to give it a try as Tormek suggested. The results are actually pretty horrible and I'd never do this for a customer but at least it proves it can be done (sort of) on a Tormek.

I found the exercise interesting and there wasn't any detailed videos on how to use the (horrible) new knife jig to grind a convex edge so I thought I'd try it. Mostly to shut up the smart mouthed Facebook trolls. Thought maybe someone in here might get a kick out of it.

The fast moves are essential or it looks like a dog's breakfast with a million obvious facets. It was more of an experiment and exercise than a serious attempt at proper, decent knife sharpening. Barely more than a bit if fun really. A break from my normally serious videos.

[Ken S]

Baz and TGBTO,

Sometimes a good video will activate my "little gray cells". Your going an extra mile refining the convex grind makes me see the possibility of more refined convexing. I am not convinced about the practicality of rapid fire farmers market convex grinding. However, well controlled careful convexing might have a lot of potential, either for the sharpener/owner or a discriminating customer with a fine hunting knife.

I believe the keys to this control are allowing more grinding time with lighter pressure  and a finer stone AND consistent rhythm with the back and forth movement. "Fast or slow" is generally not consistent enough.

The great photographer, Ansel Adams, was also an accomplished pianist. He developed his large negatives in complete darkness. He kept a metronome in his darkroom to keep track of the developing time and insure consistent agitation. After some trial and error, the ticking of a metronome should insure consistency in the motion time. Adjusting the beats per minute would fine tune the consistency of the motion. Both mechanical and digital metronomes are very reasonably priced. Some of the digital metronomes are even USB rechargeable.

EDIT: I just checked the app store. There are numerous free metronome apps available. I spent many hours listening to my children's piano lessons. Today I'm sure people use an app on their phones.


Ken

PS my next little gray cells thought is using the round collar lock of the SE-76/77 as an adjustable second stop with the SVM-45. That is in very early stages.

[iSharpen]

Baz and TGBTO,

Sometimes a good video will activate my "little gray cells". Your going an extra mile refining the convex grind makes me see the possibility of more refined convexing. I am not convinced about the practicality of rapid fire farmers market convex grinding. However, well controlled careful convexing might have a lot of potential, either for the sharpener/owner or a discriminating customer with a fine hunting knife.

I believe the keys to this control are allowing more grinding time with lighter pressure  and a finer stone AND consistent rhythm with the back and forth movement. "Fast or slow" is generally not consistent enough.

The great photographer, Ansel Adams, was also an accomplished pianist. He developed his large negatives in complete darkness. He kept a metronome in his darkroom to keep track of the developing time and insure consistent agitation. After some trial and error, the ticking of a metronome should insure consistency in the motion time. Adjusting the beats per minute would fine tune the consistency of the motion. Both mechanical and digital metronomes are very reasonably priced. Some of the digital metronomes are even USB rechargeable.


Ken

PS my next little gray cells thought is using the round collar lock of the SE-76/77 as an adjustable second stop with the SVM-45. That is in very early stages.

I agree, this is only an initial experiment. There must be a better way to do this although I was impressed that it worked at all. On my channel I've been dared to try to polish that knife with the SJ. It's a good idea for content if not for anything else.

I look forward to seeing what you come up with to refine this technique

[tgbto]

I can tell it's convex because of it's shape. It might not come through on the video but it's definitely rounded.

Hey Baz.

There is no question that the edge is rounded in the area that's away from the apex. But I really don't see how a convex wheel will grind a convex apex. In the same way a flat stone can grind a convex bevel with a flat  apex, a convex wheel may grind a convex bevel but there will be a transition to a concave apex at some point. And the wheel will have the tendency to dig into the material, ie make it more concave than it actually is, whereas the flat benchstone will guarantee that you always remove material in at worst a tangential fashion. Plus the benchstone tends to wear in a fashion that makes it concave, enhancing this phenomenon.

Going back and forth in between the stops kind of blends this into the shape of the blade that is most often naturally convex, which is why you see convexity (for now, and "far away" from the apex). You can think of a more extreme situation with a much bigger blade and/or a much smaller wheel, and you will easily see that you're naturally concaving the bevel, albeit much faster.

That happens pretty much in the same way that the natural tendency of most sharpening devices, being narrower than the blades they sharpen, is to make blades concave. Fighting this natural tendency is something we all know is part of sharpening skills. But we know it's something that needs to be fought constantly, and to that end we use the fact that the Tormek wheel is flat(ish) along the edge axis.

Grinding knives on the shoulder of the wheel is a sure way to eventually grind nice hollow sections. So is grinding the bevel along a round stone.

[tgbto]

I can tell it's convex because of it's shape. It might not come through on the video but it's definitely rounded.

Hey Baz.

There is no question that the edge is rounded in the area that's away from the apex. But I really don't see how a convex wheel will grind a convex apex. In the same way a flat stone can grind a convex bevel with a flat  apex, a convex wheel may grind a convex bevel but there will be a transition to a concave apex at some point. And the wheel will have the tendency to dig into the material, ie make it more concave than it actually is, whereas the flat benchstone will guarantee that you always remove material in at worst a tangential fashion. Plus the benchstone tends to wear in a fashion that makes it concave, enhancing this phenomenon.

Going back and forth in between the stops kind of blends this into the shape of the blade that is most often naturally convex, which is why you see convexity (for now, and "far away" from the apex). You can think of a more extreme situation with a much bigger blade and/or a much smaller wheel, and you will easily see that you're naturally concaving the bevel, albeit much faster.

That happens pretty much in the same way that the natural tendency of most sharpening devices, being narrower than the blades they sharpen, is to make blades concave. Fighting this natural tendency is something we all know is part of sharpening skills. But we know it's something that needs to be fought constantly, and to that end we use the fact that the Tormek wheel is flat(ish) along the edge axis.

Grinding knives on the shoulder of the wheel is a sure way to eventually grind nice hollow sections. So is grinding the bevel on a round stone.

[tgbto]

I'd slap myself for those pesky SAVE/POSt buttons when editing...

[tgbto]

Anyway, here's what I just did as an empirical accelerated test:

I 3D-printed a one-sided 20° flat bevel, as wide as the SG-250 wheel, out of white PLA filament. It's a soft plastic that will be ground easily. You can see it in the first pic.

I sharpied it black as is customary, checked that the angle when against the far stop was correct, then ground it with one-second strokes, up and down, until all sharpie marks were gone (mostly), see second pic. I used a digital metronome, and tried to have even pressure.

I then painted it black again, then rubbed it against a #2000 diamond plate, and guess what, it is concave/hollow at the apex, hollow again at the shoulder, and convexish (or should I say, less hollow ?) in between.

As mentioned somewhere else, physics are stubborn, and they like continuity, such as when an equilibrium results in a concave shape, small displacements around this equilibrium will seldom turn that into a convex shape. It is easy to understand that if you spend more time against the stops, then you grind more, so the result is more hollow at the stops.

Now if we take one step back and think of what we do with our Tormeks :
1) With a typical kitchen knife ground with the KJ, using this "convexishing" technique, you will get a bevel that is hollow at the apex and round out the shoulder, then grind the side of the blade. But honestly, who really cares about concavity on a typical kitchen knife.
2) With a knife with a large primary bevel, the result will depend on where the shoulder of said primary bevel with sit when the KJ rests against the near stop.
 2a) If you move past the shoulder before reaching the near stop, refer to the above.
 2b) If you don't, you will either have this double-hollowed shape, or a completely hollow shape depending on how slowly you move in between stops. This shape will however have less of a concavity in the middle compared to when grinding the traditional way. But the same concavity at the ends.
3) With an axe ground with the latest jig, well no actual difference with 2a above, you'll just take more time to reach this result if the axe is convex to start with.