Sharpening Concave Edge Without Radiused Wheels

[RickKrung]

I recently sharpened a set of Henckels (not high end, the whole set cost around $150US), including one paring knife.  The paring knife is one of the Bird's Peak blades which are concave.  I think the knife I worked on is this one. 

I thought I was careful about keeping the blade canted such that the only part touching/grinding on the stone was at the very edge, but apparently, I didn't do it quite right, as the customer commented some days later that the knife no longer had the "hook" that she liked so much when paring fruit for canning.  I do not yet know exactly what it is that she isn't happy with and thinks that is different from before I worked on it, but I have that question out to her.  She and her husband were quite thrilled with the sharpened knives in general and he, being manager of the local high end steak house, is bringing to me, tomorrow, 10 or so of the restaurant's knives for sharpening over the two days per week they are closed.  I do not know what sorts of knives these are but the one he showed me appeared to be more of the mainstream stainless sort than high end.  I've asked him to bring back his wife's paring knife so that I may fix what is wrong/not liked about it. 

Okay, so, given that, how do I fix it?  Assuming the problem is that there is not as much "hook" as the original, it seems I need to grind away some of the center area of the trough.  I've studied Knife Grinders page on sharpening concave edges.  Took a little bit of study, but I think I understand, and it is clear I did not do things as he describes.  I can correct that.  What I cannot do, however, is use either 1) radiused grindstones or 2) honing wheel.  All I have, in service, are: 1) Tormek SG, SB stones, 2) Tormek diamond wheels (DC, DF and DE) and 3) Tormek leather honing wheel.  I do have some paper wheels and one hard felt wheel, all 10 inch, but I've not set any of them up. 

I could radius a paper wheel or the felt wheel for honing, but that does not address the need for radiused grinding stones/wheels.  I could radius the SG and SB stones, but I'm not at all excited about doing that.  I do have the 8" Norton 80 grit 3X grindstone that I use for shaping bevels on really dull knives and did use on all of these Henckels. 

Is it going to work, say, if I follow Knife Grinders' procedure, in particular about using a collar on both sides of the jig shaft to limit travel but that allows for following the LOC across the curvature, while using relatively sharp cornered stones?  I think I would opt to use the SB and SB stones as they can handle some raduising, whereas the diamond wheels cannot. 

I'm interested to hear your thoughts. 

Thanks,

Rick

[Ken S]

Rick,

Your post brought to mind a you tube by Steve Bottorff:

https://youtu.be/KrhZULmd18g

Must dash; I will add more later.

Ken

[RickKrung]

Rick,

Your post brought to mind a you tube by Steve Bottorff:

https://youtu.be/KrhZULmd18g

Must dash; I will add more later.

Ken

Yes, thanks, Ken.  I'm sure that is what I had in the back of my mind going into this initially.  I had cruised past Vadim's page on concave edges in the past but had never looked at it closely.  My bad. 

I like the looks of that Bird's Beak paring knife and we really only have one paring knife, so I may buy one, one so I have one, but second, so I can practice sharpening it.

Rick

[wootz]

I've experimented with rounded edge 10" CBN wheels made to our order, to find out the minimal wheel edge radius that works well for sharpening curved blades. I tried radius of 2mm, 4mm and 6mm. It was a costly way, but as CBN wheels had to be machined to exact specs, now I know exactly the minimal edge radius that works.
The rounded edge wheels we make now are designed with that min radius on the corners, so that the vast majority of regular knives can be sharpened on them as well.

The technique is showed in my video https://youtu.be/Ug_GvJco0vM

Of your stone wheels, I would use an SG wheel, and grade its corners FINE, because the high pressure per point on the corners makes coarser # unsuitable for sharpening concavities in curved knives.

All that said, the Bird's Beak knife you have does NOT require a round corner to sharpen - the little concavity it has we sharpen on a flat fine wheel the regular way, as we would a straight edge knife.

[RickKrung]

Thanks, Vadim.  That is very helpful. 

Rick

[wootz]

No worries, Rick. It was cool to find out that curved knife sharpening requires smaller radius than they put on wheels for sharpening woodworkers' tools.

[John_B]

I think the curve on most bird's beak knives is more of an optical illusion due to the curve along the spine. I have done a number of these and they turned out fine using the regular wheel.

[Ken S]

I have not sharpened bird's beak knives. If I did, I think I would start with the square edge grinding wheel. If I wanted to experiment with radius wheels, that seems a good use for a well worn, but dtill usable wheel. I also have an unused non Tormek 600 grit, which I might convert to radiused corners, but only if necessary.

Ken

[Herman Trivilino]

Okay, so, given that, how do I fix it?  Assuming the problem is that there is not as much "hook" as the original, it seems I need to grind away some of the center area of the trough.

It's almost impossible to believe you could have changed the radius of that "hook" significantly. So what is it that causes the knife to perform differently? I would ask her to demonstrate. Seems more likely that you've sharpened the edge to a slightly different angle, or polished the bevel to different extent. Either of these would affect the way the knife performs the task.

[RickKrung]

Okay, so, given that, how do I fix it?  Assuming the problem is that there is not as much "hook" as the original, it seems I need to grind away some of the center area of the trough.

It's almost impossible to believe you could have changed the radius of that "hook" significantly. So what is it that causes the knife to perform differently? I would ask her to demonstrate. Seems more likely that you've sharpened the edge to a slightly different angle, or polished the bevel to different extent. Either of these would affect the way the knife performs the task.

I have a hard time believing it too.  She did not say it performed differently.  I think she means it "looks" different.  I have ordered one for myself and will try to get my hands on hers to compare them.  She says she is happy with the knives and declined to my offer to fix the birds beak one. 

Dang, I was just at their house today, delivering the restaurant knives.  I should have asked to see the paring knife. 

Oh well.  I leave in two days for almost two weeks.  By then, I'll probably have forgotten the whole thing and I'll go merrily on my way to other things. 

Rick

[RickKrung]

Hair brained thought here.  What about angling a concave blade across the surface of the stone/wheel to approximate the curvature of the blade?  I've doing some mental gymnastics to visualize this and what issues there may be with maintaining the angle, bevel width, etc., as the blade moves across the stone at an angle.  I will go mock this up as a trial, but I am wondering if anyone has tried this.  I am thinking it could reduce the point pressure of trying to use the corner of a stone or diamond wheel. 

Rick

[Sunrey]

Angling the blade across the top of the stone sounds like a good idea.  I have used that technique to speed-sharpen low value knives.  It works like hand honing on a flat stone, but--liberated from the task of pushing the blade forward and back incessantly-- your senses can focus on keeping the angle and rate of diagonal draw across the stone steady.  It's messy near the handle though, so its not something to always. 
.
I recently aquired a large billhook on a 36 inch handle, with four inches of recurve; it's pitted with rust.  I was thinking about how to sharpen it, and this angled straddle will probably work wonders.  A 250mm stone just barely fits in there.  A tighter curve would call for smaller radii.  Just using the (substantial) weight of the tool I will brace my hands on the top of the housing and pivot the tool in an arc following the recurve edge at a 20 degree angle from level.