sharpening un even convex bevel?

D

DavidL41

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Oct 12, 2014
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For a knife that had an uneven bevel that is too thick in some places and too thin in others(bevel) how would you sharpen by hand to flatten the dips, without accidently sharpening a spot too narrow or get the bevel to be straight, yet ground at thicker at one spot and thenner at the opposite end (off y axis)

Taking it a step further for a convex from a v grind would you start from a wide microbevel and then sharpen w/ a rocking motion or just start a rocking motion from a v grind, which do you prefer?
 
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Unfortunately, you are asking about highly advanced sharpening techniques that cannot be well explained. It's something that needs very good sharpening skill and tools to accomplish.
 
Hi,
Some knives have some parts of the edge fatter and some are thinner on purpose,
so to maintain that, to spot sharpen one part of the blade and not another,
all you need is abrasive narrow enough to only work different part, like sandpaper on a stick.

Similarly if you want to flatten parts of the edge and not other,
you just need abrasive narrow enough to only work one part of the edge.

Use a permanent marker and sharpen within the lines :)

Regarding convexity, it depends on how severe it is, on what grind its closest to.
If the blade grind is more like double bevel/multi bevel/compound bevel,
then color all the bevels with permanent marker, and work each seperately
and then "blend" in between the bevels after and then microbevel to set sharpness

If the convexity is more like flat grind, then simply pretend there is no convexity
and sharpen freehand like you would a v-grind,
you'll put some convexity in there without trying at all,
afterwards just microbevel to set sharpness:)
 
bucketstove said:
Hi,
Some knives have some parts of the edge fatter and some are thinner on purpose,
so to maintain that, to spot sharpen one part of the blade and not another,
all you need is abrasive narrow enough to only work different part, like sandpaper on a stick.

Similarly if you want to flatten parts of the edge and not other,
you just need abrasive narrow enough to only work one part of the edge.
Click to expand...

No, they are not there purposely and that is a very bad way to go about grinding a blade. Whenever you are trying to flatten out over and under grinds you must grind to the lowest spot in the blade but you must grind the WHOLE blade at once. Grinding in small sections with small stones will only make things worse.
 
Jason B. said:
No, they are not there purposely and that is a very bad way to go about grinding a blade. Whenever you are trying to flatten out over and under grinds you must grind to the lowest spot in the blade but you must grind the WHOLE blade at once. Grinding in small sections with small stones will only make things worse.
Click to expand...
What? Do you know what blade hes talking about?
 
DavidL41 said:
For a knife that had an uneven bevel that is too thick in some places and too thin in others(bevel) how would you sharpen by hand to flatten the dips, without accidently sharpening a spot too narrow or get the bevel to be straight, yet ground at thicker at one spot and thenner at the opposite end (off y axis)

Taking it a step further for a convex from a v grind would you start from a wide microbevel and then sharpen w/ a rocking motion or just start a rocking motion from a v grind, which do you prefer?
Click to expand...

On the first point:
I've 'fixed' some of my own unevenly sharpened bevels on large, wide spans of wet/dry sandpaper or coarse grinder belts cut & laid flat over a hard surface, like wood or glass/stone. I emphasize a large, wide abrasive surface, because it enables a much fuller heel-to-tip stroke on each sharpening pass. That long, full, sweeping pass tends to even things out along the bevels; and if done over a coarse grit (maybe down to 120-150 if necessary), it also goes very, very fast.

On the second point:
If wanting to make a convex from a V-bevel, I've previously started by using a guided setup to make a very clean & low-angle V-grind. Then, using a wide & large setup much like I described above, convex the acute V-bevel with maybe one or two sheets of plain paper underneath a piece of sandpaper, using TRAILING heel-to-tip strokes across the paper, as if you're just 'stropping' on the sandpaper. The one or two sheets of plain paper under the sandpaper will give enough cushion to easily start rounding out the shoulders of the V-grind. Make sure to keep the angle LOW when doing this, because the cushion under the sandpaper will also tend to make rounding of the apex a greater likelihood, if not careful. After convexing the shoulders of the grind, I'd recommend finishing the edge/apex itself with no cushion under the sandpaper (will leave the edge much crisper). Convexing from a V-bevel will result in an apex angle that's a little wider than the original V-edge, so that's why you want to start with a relatively acute V-grind (I'd start at 25°-30° inclusive or lower, at least).

If really wanting to do the convexing on stones, I've found a back & forth scrubbing motion on the stone will induce a little more convex than simple edge-leading passes (in my hands, anyway). I'd strongly recommend NOT deliberately rocking or rolling the hands to induce the convex, which may create too much of it, resulting in a very thick and/or rounded edge; a simple freehand motion will introduce enough on it's own. Finishing on a firm strop of denim with an aggressive polishing compound, like black (SiC), grey & white (aluminum oxide) compounds will add a little more convex to the shoulders and will also refine & polish the convex and apex. As an example, the pic below is of my Buck 301's spey blade (tip reground to a spear), after I'd started the convex with the 'scrubbing stroke' on a DMT diamond hone, then finished on a denim strop with some grey aluminum oxide 'cleaning' compound from Sears (their #2 stick compound).

David
 
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bucketstove said:
What? Do you know what blade hes talking about?
Click to expand...

Your question is irrelevant, you don't know the blade either.

I do know that I correct this exact problem a few thousand times a year on Japanese Kitchen knives so you could say I know exactly what it takes to correct a blade with such issues.
 
DavidL41 said:
For a knife that had an uneven bevel that is too thick in some places and too thin in others(bevel) how would you sharpen by hand to flatten the dips, without accidently sharpening a spot too narrow or get the bevel to be straight, yet ground at thicker at one spot and thenner at the opposite end (off y axis)

Taking it a step further for a convex from a v grind would you start from a wide microbevel and then sharpen w/ a rocking motion or just start a rocking motion from a v grind, which do you prefer?
Click to expand...

Without seeing it is tough to give solid advice.

From a background of creating convex from sabre grinds by hand, I'd say make radial sharpie lines from spine to apex, and only grind where you want steel removed. As you thin it out you'll get into issues around the plunge line if it has one, and getting the tip geometry thin also a challenge. It can take a lot of patience to do it to your satisfaction.

Since it is a reduction process you will be working to match to the thinnest region if you take it to a clean conclusion. You have some leeway in how thin you grind the tip region but the bulk will be ground to match the thinnest area. If the spine is thick, you might approach a FFG type configuration, so be advised it can lead to a lot of stock removal.

Upside is it will perform a lot better.

Use hard stones to shape the convex. Once set you could transition to softer media to blend the scratch patterns or you can mechanically blend them on a hard abrasive with subtle wrist rolling.

Edit to add:
when sharpening convex on hard stones I spend extra time on the area right behind the apex to keep it thinned out. If the cutting bevel becomes too obtuse you lose most of the benefit of a convex geometry.

Also, on second read you should also be aware that warps in the blade can make thicker/thinner edge width on V bevel even if ground to the same angle. Take a real close look before jumping in.
 
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Jason B. said:
Your question is irrelevant, you don't know the blade either.

I do know that I correct this exact problem a few thousand times a year on Japanese Kitchen knives so you could say I know exactly what it takes to correct a blade with such issues.
Click to expand...

Do you have an example picture of such an issue?

Maybe hes talking about a granton type of edge and wanting to maintain a pattern like that, like serrations,
thats why I mentioned the narrow abrasive idea ....

I've sharpened a few choil areas, I did it on stones and sandpaper on a stick, it was easier spot sharpening with sandpaper on a stick because it was narrower.... it was trickier with a wider abrasive, too much effort to avoid a recurve

maybe if hes trying to remove the cranton type edge dips and lower it its easier with flat/waterstones,
but yeah , I've not seen picture
 
If you knew anything about granton edges you would know how silly all that sounds.
 
My camera isnt working. Its pretty simple to describe. One side bevel has bevel too short near the point, the other bevel on the opposite side is too short near the bottom/rear. Both are not parallel. The grind is smooth but has high spots, or low spots.

Where it gets difficult to fix is to grind the bevel, where the bevel is not parallel + there is many angles to grind + hard to find the right angle. It also is difficult to grind because I over sharpen and the bevel has a low spot on the left of the blade and right, the middle is the highest the I grind the middle to try to blend. Is that how you would go about it? I guess I could get digital caliper for an accurate look.
 
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Would you recommend using a low grit stone for this or only a water wheel/ sander? I think part of the problem is that its hard to maintain an angle consistenly by hand, even more because you cant push against a flat section of the blade.
 
Hi,
So are you talking edge bevel, blade bevel (behind edge), or both?
How long is the blade, what kind of blade is it?
Any idea about the angles? 10 degrees per side? 15dps? 20 dps?


I've had a ~4 inch paring knife kinda like that,
what I did was grind left side for a while until raised a burr (scrubbing passes),
then cut the burr off with a direct pass into the stone (90 degrees),
then grind the right(opposite) side until I raise a burr
I repeated two more times lowering the angle each time
went from about ~20-25 unevenness to about ~15 dps,
the longer I worked it the easier it was to maintain the
angle because the edge start to balance on the stone
it left a very covexish edge, maybe like 14-17 dps,
getting flatter and closer to 15 the more I sharpened

I've had fatter/unsharpened choil sections on pocket knives, this is where sandpaper on a stick helps to spot sharpen, easier than balancing stone or turning stone sideways

For a v-grind I once used clothespins to make this fancy primitive clamp
jig2.jpg


A simpler jig ( right angle triangle ) that leaves a convex edge can be seen at Simple sharpening jig - stefanwolf88 / How to Sharpen a Knife with Paul Sellers
 
DavidL41: What make/model knife is it? Sounds like a jig or guide of some sort would benefit you in re-establishing the edge. You would then also be able to pick the angle appropriate for its use, rather than trying to use the bevel itself as a guide to setting the angle.

You don't need a water wheel or sander necessarily... you can do it on a coarse stone.

(I think pics might help more than digital caliper measurements if you can swing it).
 
Picks would be a big help. If it just uneven factory grind on a V bevel, you can fix that over time by hand on a coarse stone.

Again, you need to verify the blade is not warped.
 
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