Convexing thoughts

[Ken S]

Whenever I encounter a new piece of equipment or technique, I tend to categorize it as immediately useful; hopefully useful at some point; maybe useful for me at best; or not for me. Sometimes a jig or technique may fit more than one category for this user.

Different aspects of the KJ-45 self centering knife jig fall into different categories for me. The main event, self centering falls into immediately useful, although not in a way one might expect. My thickest knife is my Mora Garberg, with a thickness of 3.2mm. I have several Mora knives, none of which I need. I have reached the age where bushcrafting is only a mental interest. I just like Mora knives. Looking ahead, if I ever need to sharpen my Garberg, it will be far less frequently than I sharpen my several thin paring knives. It is my thin paring knives which seem candidates for self centering. If I am being honest with myself, workarounds for my thin knives do not seem difficult.

While the inner stop of the KJ-45 might seem a possible substitute for the US-430, the ideal US-430 is the US-430.If I did not already have one, for knife blades longer than eight inches and cleavers, the genuine US-430 would be high on my wish list.

I am glad to own a jig which can handle convexing, although I doubt I will ever convex an edge. None of my Moras seem heavy enough, nor are my future plans demanding enough to warrant convexing.

These are all just personal thoughts. If I was a serious knife collector or an active bushcrafter, I would feel differently. I welcome other points of view.

Ken

[tgbto]

As I wrote in other threads, I remain very skeptical of the convexing ability of the KJ-45...

First of all convexing on something hollow seems unnatural. I can see how it theoretically works, but unless it is done by a machine - or maybe Wolfgang who could probably convex freehand anyway - I think the pattern will be uneven. The fact that one will probably spend more time at the stopped positions will result in a kind of hollow tip, sorta convex edge and hollow shoulder. Maybe that is a bit dramatic a depiction, but it will not be a smooth convex edge, and the result will depend on timing. How much lateral travel for each up-down move for the convexing to be efficient but not so much so that it will indent the edge ?


It is also hard enough to not create high or low spots when grinding a straight hollow edge on a blade. Combining the movement parallel to the shaft with a up-down movement perpendicular to it will not make it any smoother. It introduces a jittery variation to the process when the purpose of a jig is to keep everything as steady as possible. I even think one may enter the dreaded "static friction" realm when moving from the main stop to the secondary one, and mess up the grind in the process.

Nor does it allow for adjusting convexity the same way adjusting a belt slackness would.

I am glad to own a jig which can handle convexing, although I doubt I will ever convex an edge.

Ken

I think this perfectly sums up the idea behind the convexing with the KJ, and if I may allude to something you wrote in another thread, that is indeed a marketer's argument: now we can say there's a way to convex with a Tormek, so the hollow grind argument against Tormek doesn't stand any more. But it will perform convexing at its best when convexing just in our minds. My attempts at convexing knives with the KJ have yielded dubious results, though maybe it was the Cassandra effect.

All in all I would put convexing in the same basket as grinding hawkbills on the Tormek, and playing the violin with boxing gloves. Doable by a very skilled user, but sacrificing result quality and efficiency for everyone. A small belt grinder will convex much more quickly and consistently than a Tormek, and allow for much easier thinning and reshaping. At the cost of a spare Tormek wheel.

As a side note, I thought the general consensus was that hollow grinds were only an issue for thick knives. Good luck precisely convexing a thick edge, removing all this metal along the entire edge with the KJ's induced "schlak-schlak" maneuver.

[Sir Amwell]

Agree wholeheartedly with tgbto.
I make no apologies for asking again: why not a self centering knife jig with an adjustable stop? PLEASE!

[HaioPaio]

I would like to keep the double stop and make it adjustable.

[Ken S]

Interesting thoughts.

I generally follow Tormek's recommended sharpening procedures. They have been factory and user tested for decades. The key word in my statement is "generally".
All too often, I have found that certain tools do not work well with the Tormek because the manufacturers failed to design them within Tormek specs. Some are too large; some are too small; some are not made of a Tormek friendly alloy; and some are not perfectly machined parallel, flat, and square. A related set of problems are Tormek buyers who may not want to purchase all thirty nine jigs to better cover the waterfront and sharpening customers who procrastinate long beyond the "just off sharp" period for which Tormek excels for resharpening.

In my opinion, one of the best examples of forum ingenuity is "Herman's Homemade Small Platform". While the SVD-110 excels with large turning scrapers, it is too large and clumsy for very small knives. While the SVD-00 performs well with round Wooden handled carving tools, it is clumsy with small metal knives.

In the excellent Tormek YouTube channel long video of Glenn Lucas demonstrating sharpening turning tools, Glenn demonstrates sharpening a thin parting tool with the SVD-120 platform. His technique was excellent, quite in contrast to my memory of watching a Tormek demonstrator unsuccessfully butchering a thin parting tool with the traditional multi jig. The tool was just too thin for the jig.

I have stated my opinion that the best US-430 is the US-430. Yes, other methods may work occasionally, but not ideally.

I am keeping the jury out with convexing with the KJ-45. I have not developed the skill to fairly judge it. Also, for my needs, I am not convinced that convexing is necessary or beneficial. I freely admit that my knife sharpening needs are more limited than many users. If that should change, I would rethink the issue.

Ken

[cbwx34]

Have any of you actually tried it?  I took a cheap practice knife and gave it a whirl... overall it comes out fairly decent.  I tried to snap some pics (ignore the scratches above the main bevel, that was from another "experiment".)... pics aren't that great, but gives an idea.

Anyway, if you look close you can see some minor "defects", but to the naked eye, it looks pretty decent and definitely convex.  None of the issues some of you have described.  There are (obviously) better ways if one wanted to convex regularly, but for the occasional use, give it a try, I think you'll find most of the theories here of what might happen don't really pan out.  Probably the biggest issue I saw was how it narrowed in the bevel to tip area.

[Ken S]

Well done, CB.

[tgbto]

Yes, I tried it on three different knives for about one hour total. And maybe we don't agree on what we expect to see after we grind a convex edge on a knife.

But to my naked eye there seem to be two clear zones with something in between (orange) on your pics: what I call the hollow tip (red)and hollow shoulder(yellow). Kinda hard to say without a cross-view of the edge, but at least it looks more three-faceted rather than what I'd call convex. Convex as in the convex grind in Tormek Video #10 advanced sharpening.

Plus it may be that the coarseness of the grind kind of evens out the defects, yet there is still much work to be done to have a nice grind. Maybe not SJ-level polish, but at least the kind you'd get after 5 different grits on a belt sander with some slack, in less time than was necessary to write this post.

So yeah. I'm sure the Tormek can yield the results we see on these pictures. I wouldn't dare do this to any of my friends' knives.

[cbwx34]

I kinda knew that would be confusing... the only part I convexed with the clamp is the part you have marked in red.  That other part is what I said earlier was from a previous experiment.    (I think I tried to thin it on the wheel.)

Shoulda picked a different knife (but these are so cheap in case something goes wrong.) 

[cbwx34]

FWIW, here's another one... 

The pics 703 and 704 are off the stone, 707 and 708 are after a couple of passes on the leather wheel to see if I could tell a difference.  (I can't...) 

I added a comparison to a "regular" edge...

[Ken S]

CB,

You make a very valid observation; I should actually convex a knife. Part of me knows that is the right thing to do. Another part of me doubts that convexing would really benefit my simple knife needs. I need to make that decision based on actual experience.

I like Mora knives. The Companion HD, heavy duty model seems a good balance of a thicker blade at a modest price, a good convexing candidate, and a knife I might actually use. WhTare your thoughts on this? (Other members, please feel free to comment.)

Ken

[cbwx34]

CB,

You make a very valid observation; I should actually convex a knife. Part of me knows that is the right thing to do. Another part of me doubts that convexing would really benefit my simple knife needs. I need to make that decision based on actual experience.

I like Mora knives. The Companion HD, heavy duty model seems a good balance of a thicker blade at a modest price, a good convexing candidate, and a knife I might actually use. WhTare your thoughts on this? (Other members, please feel free to comment.)

Ken

The debate on whether it's beneficial probably started when the first knife was forged. 

But, what's the harm in trying?  If you don't like it, a few "regular" passes should clean things up... you'll just have a bit of a "thinning behind the edge".   (Whether you benefit from convex should be outweighed by the actual experience of trying the method... if for no other reason you get to play on your Tormek!)   

[Ken S]

CB,
I ordered a knife to convex today. It is a Mora Companion HD (heavy duty, as the blade thickness is heavier than the standard Companion. It is within .1mm of the thickness of the Garberg, Mora's only full tang knife. I ordered it in stainless steel and colored burnt orange, so that it will be easily identifiable.
I will post my impressions of convexing it.
Ken

[cbwx34]

I watched the part on convexing in the Tormek video...

https://www.youtube.com/watch?v=JFHqQg5wZEg&t=1720s

... after I tried it.  I don't really see the point of establishing an edge first and then convexing... I just sharpened/convexed at the same time until I raised a burr.  Probably more like this video...

https://www.youtube.com/watch?v=bKXwiSfRqJo

... although he doesn't show very much, I did it more like what he does show.

The other "decision" is whether to sharpen based on angle or bevel width...

You cannot view this attachment.

...  In the diagram the black line is a flat grind, the red line is a convex with the same angle at the edge as the flat grind, the blue is a convex with the same bevel width. If you sharpen based on angle at the edge you get a narrower edge (better cutter?) vs. a flat grind, if you sharpen based on the already established bevel, you end up with a "fatter" edge.  (This seems to be what most refer to when they say a convex edge is "stronger".)  I set mine based primarily on the angle at the edge.  On the Morakniv, you might end up somewhere in between the two.

[HaioPaio]

I think you could move the blue line upwards, so that it lies fully inside of the black lines.
This would illustrate the fact that the grinding wheel cannot add material to the knive.
Changing a flat grind to a convex grind with a fatter edge always reduces the height of the knife.
Grinding the flat grind to a thinner edge maintaines the knife hight, however increases the bevel hight - as you already explained.