How to flatten a badly concave blade

[grepper]

I have a knife that is badly concave towards the spine at the center of the blade.  It needs a lot taken off a good inch at the heel end and some taken from just before the curve towards the tip to get the blade flat.

I have tried increasing pressure at these areas in an effort to remove more metal there, but it does not work well.

I have an old, long, slicing knife that I use for practice and experimentation.  The things that I have done to that poor blade are not things that one should speak of in polite company.  I only do these "things" late at night, down in the basement when I think nobody is looking.

It too was badly concave towards the spine, and needed a good 2mm or 3mm or more removed from the last 1.5" near the heel end to get it anywhere near flat.

So, I just held the knife parallel with the wheel and ground it down unil it was even with the rest of the blade.  This resulted in a "cutting edge" that was .5mm to maybe 1mm wide.  Of course, this was done late at night when nobody was looking...

Then I just put a new edge on it.  It worked!  I can't really tell it from the rest of the blade, and it's sharp.

If you ever run out of knives to sharpen because all of your knives are sharp, this method works very well to remedy that situation.

It worked well, but I have to ask, is this the correct way to do this?

[Herman Trivilino]

It's definitely a faster way to do it, and it may be easier to get the edge straight or convex.  I wouldn't use the Tormek grindstone to do it, though.  I'd use either a hand file or a dry grinder, just to save wear on the grindstone.  The hand file is straight, so it can be used to make sure there are no concave areas on the edge.

[jeffs55]

I would think that any blade that had enough metal to remove a whole inch of stock and still be viable would be grossly misshapen after this treatment. I sure wish you would post a picture to show this butchery. I really cannot imagine people grinding on something so long in one place as to remove an inch of material. Is that a misprint/typo? It must surely look odd in its present condition.

[Rob]

That's an interesting idea Mark. Basically, you just temporarily used the Tormek as a fast grinder knowing you could re-edge it after the fast stock removal. If it wore the grindstone badly I would agree you might consider using a more "blunt" instrument like a hand held angle grinder.  If it didn't affect the tormek stone too badly then why not.  It's very controlled and you'll never draw the temper of the steel....like it.

[Herman Trivilino]

I would think that any blade that had enough metal to remove a whole inch of stock and still be viable would be grossly misshapen after this treatment.

I think he meant to say that the concave spot was located an inch from the heel.  I had a double-take on that one, too.

[grepper]

That's correct Herman.  Maybe I weirded that wordly? "needed a good 2mm or 3mm or more removed from the last 1.5" near the heel end to get it anywhere near flat."

By inch I meant the last inch or so was from the heel towards the tip along the cutting edge, not towards the spine!   That would truly be a knife that need some attention!

It worked well and quickly too.  No heat of course.  I pulled the knife against the wheel rotation over the high area whilst moving it side to side on the wheel so as to be kind to the wheel.  The only issue is that you have to keep checking it for flat.  To finish it off I think Herman's suggestion of a file would be quick and easy.  I'll use a diamond file.

There is not much to show now.  It's more or less flat.

[Rob]

Just a spine with no blade :-)

[Herman Trivilino]

I've got a paring knife like that in my garage, Rob.  It's close to being an ice pick!

[Rob]

 

[Jeff Farris]

So, basically, what grepper is talking about is re-profiling the edge by grinding the edge away, rather than doing it from both sides. It saves time and is easier to see what is going on, since you're seeing all the work instead of half of it. I do this when grinding down bolsters to match blades.