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Adrian Lopez

Can't get my knife sharp enough

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So I got a bench grinder, some water stones (200, 1500, 8000), and an assortment of plastic angle guides to profile and sharpen a 12mm Pfeil knife that I'd like to use for cutting bridges but I haven't managed to get it sharp enough. In fact, when I say it's not sharp enough I actually mean it's dull: it won't cut paper without tearing it and will barely shave wood. I've watched several sharpening videos on YouTube, but try as I might I can't get the knife to cut properly.

I've beveled the knife as close to 30 degrees on each side as I can manage but I don't know if my bevels are precise enough (they don't look all that clean to my eye). I've made sure to develop a burr as I sharpen each side, but even so the knife won't come out right.

I have here a close-up pictures of the edge and one of its sides. Like I said the edge doesn't look very clean to me, but shouldn't it be at least a little bit sharp? It's kind of frustrating.

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Hi Andrian - it looks like you are "rolling" the edge during sharpening. Try keep the edge angle constant all the way through the "lift off" stage of your swipe along the stones.

An edge is the intersection of two flat planes.

Luck edi

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30 degrees on each side adds up to 60 degrees for the whole edge. Is that what you're doing (?), because if it is that's way too blunt. A knife like that should be beveled about 15 degrees per side, for a total of 30 degrees at the edge.

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2 hours ago, MarkBouquet said:

30 degrees on each side adds up to 60 degrees for the whole edge. Is that what you're doing (?), because if it is that's way too blunt. A knife like that should be beveled about 15 degrees per side, for a total of 30 degrees at the edge.

Yes, the bevel angle is too high, and will be even higher than you think because of the rounding.

This is the crux of the problem, I think.

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All of the above.

I think you are making the knife the wrong shape which is making you grind too much down the side of the knife. Have you seen violin making knives before? Perhaps some one could post pictures of their knives. The useful part of the knife should basically be a 20-30 degree angle from the back side which is then given a slight lengthwise curve while being ground to a 25-30 degree included angle along that edge. The sides of the knife are not sharpened at all

Ideally you should learn to grind a single, clean ground, hollowed surface right up to the edge so that when you lay it on a stone only the edge and the heel of the grinding touch. While basic to violin making this is not an easy thing to learn and you need to have patience and be careful not to overheat the knife as you grind or it will soften the steel and you have to start again. The actual cutting is done by the shape right at the edge and it must be two geometrically flat surfaces which meet at the edge in order to cut.

i am on the road and can't send pictures  which are the best way to explain all this.

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Yes, 60 degrees is way too steep. I sharpen my axes to a lower angle than that. Also, your sharpened section is way too long. You probably don't need more than about 30-35mm sharpened for a 12mm blade. Having too long a sharpened section exposes you to a cutting hazard if you choke up on the handle to make use of the curved section..

Note: I sharpen masonry cold chisels to about your 60 deg angle. I sharpen my axes to about 25-30 degrees total angle (I can slice paper or shave with my axes.). My violin knives are probably in the 25 degree range.

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For the shape of the knife I've been going for something resembling the profile on one of Michael Darnton's bridge knives (see first picture). His knife is 18mm rather than 12mm and the angle of the picture might be throwing me off, but I don't think it's that far off.

The flatter section of the edge is so the knife can serve double duty for anything that might benefit from a flatter edge, though it's longer than I wanted as I wasn't watching the length of it too closely at first. The second picture shows what the knife looks like once placed inside its holder/handle.

On "rolling the edge," I suspect you're right, but I'm finding this very difficult to control. I can't catch myself doing it while I'm sharpening, but I must be doing it or the edge would not look rounded.

The consensus appears to be that 30 degrees on each side (60 inclusive) is too steep, so that's one of the first things I'm going to change. I had read here on Maestronet that a 30 degree bevel was a good target to aim for, but I realize now they must have meant 30 degrees total, so 15 degrees on each side. The strange thing is the width of the bevel has come out twice the thickness of the blade (2mm vs 1mm), which what you'd get with a 15 degree bevel each side -- even though I've been comparing against a 30 degree angle guide. I guess I'm not holding it right, in spite of my efforts.

Thank you all for your advice.

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4 hours ago, Michael Darnton said:

That's the Bein and Fushi shape for bridge foot fitting, and it works fantastic if you understand how it works.

I can't imagine how such an open cutting edge angle could be efficient, but obviously I'm not going to doubt it and  I believe you on the word, I simply never tried someting similar.

I could imagine it with a scraping action, not for the thin cuts needed for the feet of the bridge.

What should be the correct (approximate) angle for a similar bridge feet knife?

 

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3 minutes ago, violins88 said:

Are the peril knives M2 steel? If so, that’s a big problem 

I don’t know, John, but I have several and they take a good edge. Not as good as your knives, but perfectly serviceable....

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18 minutes ago, JohnCockburn said:

 Adrian - the Pfeil knives are 2mm thick, not 1mm, so I think your calculations are a factor of 2 out. 

Hmm... 2mm thick blade and 2mm wide bevel would be consistent with a 60 degree interior angle, so that's probably it. If I could find my digital caliper I would confirm this, but it makes sense. Measuring tape is hardly the best way to measure the thickness of a knife.

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aside from the steepness of the bevel, perhaps the steel is simply too soft to hold a proper edge, especially if you have been grinding a lot and perhaps got the blade a little too warm.

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2 hours ago, Michael Darnton said:

I was referring to the shape, not the angle. For angle, I personally make the bevels twice the tool thickness..... around a 25 to 30 degree cutting angle.

Thanks Michael for the clarification, a similar angle sounds more familiar to me. The shape is indeed interesting, many curves available to reach the various points of the foot selectively, sooner or later I will have to try it.

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2 hours ago, Adrian Lopez said:

- snip -

Measuring tape is hardly the best way to measure the thickness of a knife.

Hi Adrian - if you try you should be able to read a measuring tape to 0.2mm. It's just a matter of focus.

cheers edi

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