the missing knife test

[Herman Trivilino]

Once the edge becomes convex it's very difficult to straighten it back out without a file.  On the other hand, once the concave shape has been established, Stickan is correct in that if you're careful to not spend too much time on any one spot it'll stay convex.

[Ken S]

The moral of the story is sharpen it correctly from the start.

Ken

[grepper]

I've found reprofiling a convexed blade to be flat a frustrating experience and was actually going to post here and ask if anyone is good at it.

On the T7 I've tried pressing harder and/or just spening more time on the on either side of the convexity (call it the "high" parts), but that never worked very well.  I'd end up with an uneven bevel and when I continued sharpening the convexity returned. 

I've used a file to flatten the blade by just removing high parts. That did not work so well either because as the now square edged high parts got sharpened, so did the convex part, and so  the convexity returned.  Very frustrating.

The width of the Tormek wheel, about two inches, makes this more difficult for me on knives where the convexity starts less than that distance away from the bolster.  I find it extremely difficult, if not impossible to sharpen only using half the width of the wheel.

I think what you have to do is to flat file the entire length of the blade to the same dulness or in other words the same cutting edge thickness.  That way when you resharpen, the entire edge sharpens at the same rate.  I've done this, and it works.

I suppose that there might be some way to do it using the edge of the wheel, or maybe the rounded edge of the wheel?

I suspect the correct way to do it is to file the entire edge to the same flatness and then resharpen.

Maybe I just don't know how to do it, and/or it requires practice, but completely resetting the edge worked for me.

[Herman Trivilino]

The width of the Tormek wheel, about two inches, makes this more difficult for me on knives where the convexity starts less than that distance away from the bolster.  I find it extremely difficult, if not impossible to sharpen only using half the width of the wheel.

I don't understand this part.  If the first inch or two of the blade next to the bolster is straight, just keep it straight. If it's concave you have to grind away the edge near the bolster.  Removing some of the bolster itself will likely be necessary.

As to the other problem you mentioned about having to spend more time on the parts of the edge that you ground flat, well, if you watch what you're doing closely with good light I don't see it as a problem.  I suppose you could use a marker to color the knife edge and make sure that you grind into the colored portion only in places where the edge was flattened with the file.

I guess the trick is to make sure the convexity is large enough that you reduce the risk of making it concave.

And yes, Ken, it's best to not ever end up with a concave edge to begin with, but sometimes we inherit them.   

[grepper]

Yup.  I understand what you are saying Herman.  In theory it seems very simple.  Grind on the areas that are not concave.

Where I had problems was when the part that was not concave only went half way across the wheel.  In theory you could just lay the blade across the wheel and it would only remove steel from the part that was not concave.  Should work.

What happened to me several times was that as the part that was not concave was ground down very close to the part that was concave, the wheel started hitting the concave part too, thus maintaining the concavity.

I suppose it's what you suggest, that I just need to pay closer attention, take more time and practice at it.  Nonetheless it gave me problems on several knives.

I've inherited a bunch of knives like that.  Seems to be a pretty common problem with how folks sharpen knives.  I try really hard not to let that happen because it's a pain to deal with it!

[Herman Trivilino]

What happened to me several times was that as the part that was not concave was ground down very close to the part that was concave, the wheel started hitting the concave part too, thus maintaining the concavity.

Can't you just tilt the knife a bit so only part of the edge makes good hard contact with the grindstone?

I've inherited a bunch of knives like that.  Seems to be a pretty common problem with how folks sharpen knives.  I try really hard not to let that happen because it's a pain to deal with it!

Lucky you!  I can see how knives end up that way.  People get frustrated with dull knives and try everything they can think of to get them sharp.  It's really better to pay someone to sharpen them, then invest in a good steel to keep them sharp.  But you have to know where to go to get them sharpened, drop them off, then pick them up.  It's really much easier to just go out to the workshop and turn on the Tormek.   

[grepper]

Yes, Herman.    I can tilt the knife and apply more or less pressure, etc.

Nonetheless, I find it a amazingly challenging to get a perfectly flat edge on an 8" chef knife what has been badly concaved. Especially ones with a heavy bolster that extends to the cutting edge.

To place the thing on a cutting board and and see a good 6" of perfectly flat edge contact with no light at all passing under the blade, (a wet spinach leaf), is difficult... for me at least.  A good strong light behind the cutting board is very telling.

I've sharpened a lot of knives now, and I'm ok at it.  But still, getting a perfectly flat long edge is challenging.  Guess I just need to practice some more.

[Herman Trivilino]

Getting a straight edge is very difficult, I agree.  Getting a convex edge is much easier.

[Jeff Farris]

Grepper,

You need to work on getting a feel for where to cut and where to relax. It sounds like you're trying to keep your pressure even and consistent. That doesn't always work. Like Kenny Rogers sings, you have to know when to hold them and know when to ease off --- or something like that.

[Mike Fairleigh]

Oh great, now that's stuck in my head.

[Ken S]

Good thoughts, guys.  In this age where major crimes are solved on television in less than an hour, becoming really proficient in knife sharpening requires concentrated experience.  (It makes me appreciate my old friend the chisel!)

I spent a long time and hard work flattening my grandfather's oilstone years ago.  Today I would try to do it on a diamond flattening stone, but back then it was rub rub rub on glass with carborundum powder.  Since then, I am very careful to try to use the surface of a stone or grinding wheel evenly.  I will also be sure to know when to hold and when to fold.

Any thoughts on thinning the bolster when needed (other than just do it)?

Thanks.

Ken

[Herman Trivilino]

I've ground off the bottoms of bolsters, but I've never thinned one.  Why would you want to do that?!

[jmforge]

I've ground off the bottoms of bolsters, but I've never thinned one.  Why would you want to do that?!

Because if you have gotten to the point where you need to grind off the bottom of the bolster, you have sharpened away enough steel to where the edge will be too thick.

[grepper]

True on why you would thin a blade, but it's not always the case that a blade needs thinning if you have to grind away the bolster.  I've see a bunch of chef's knives like this one, http://tinyurl.com/mdacxvx, where when the knife is new the bolster is even with the edge.  In this case if any metal is removed from the edge when sharpening, the blade will never lay flat unless the bolster is also ground down. 

[jmforge]

True on why you would thin a blade, but it's not always the case that a blade needs thinning if you have to grind away the bolster.  I've see a bunch of chef's knives like this one, http://tinyurl.com/mdacxvx, where when the knife is new the bolster is even with the edge.  In this case if any metal is removed from the edge when sharpening, the blade will never lay flat unless the bolster is also ground down.

The funny part is that people make knives with those extended bolster primarily because that is the way that Pierre and Klaus always did it.  Some of the German knives are not even drop forged from a single bar of steel anymore.  The 2 bolster pieces and that one small part of the strip blade steel are heated with an induction coil and  are forge welded together and then stamped out. There is a Wusthof video out there showing them doing it.  The selection of steel is driven more by how they have to manufacture the knives than by the performance of the specific alloy. The Japanese bolstered knives often have the bolsters TIG welded onto the blade and they are sometimes hollow. They rarely, if ever have an extended bolster even iof they are actually forge completly.  Bolsters do not mean better quality.  They just mean that it is a western style knife.   Even if I forge an integral bolster knife, it will never have the extended bolster.