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Adrian Lopez

Can't get my knife sharp enough

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51 minutes ago, Roger Hill said:

Adrian, you have now shown what an almost perfectly sharp edge looks like.  Note that for a considerable length on the knife, the edge cannot be seen.  A little more time on the stones with this one and it will be perfectly sharp.  Good illustration that with no surface on the edge that is transverse to the blade, there is no reflected light.

So... what might I be doing wrong that I'm not getting this on the knife I've been working on? Like I said, I have managed to develop a burr on each side after working the opposite side, so how come I'm getting a burr but the apex is still wrong? Is it that that I have to work it longer, or is there perhaps some common mistake I've committed?

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Adrian, you have to get a 30 degree (or maybe a little less) angle at the edge. There's just no getting around that. Back to work.

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Normal working direction is with the cutting edge pointing into the direction of motion on the stone.  At the end of (say) a dozen forward strokes on one side, work the edge back and forth for three or four strokes in a short scrubbing motion.  Rotate the knife blade 180 degrees to work the other side in the same manner.  A burr is not what you are working toward.  You have not yet spent enough time getting the bevel planes to intersect, rather than just enclose, what will become the cutting edge.  Also, do this with NO pressure on the stone, otherwise you will be rounding the edge, not sharpening all the way to the edge.   Google "razor sharpening bevel setting" and you will probably find better explanations than In am providing.

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3 hours ago, Adrian Lopez said:

So... what might I be doing wrong that I'm not getting this on the knife I've been working on? Like I said, I have managed to develop a burr on each side after working the opposite side, so how come I'm getting a burr but the apex is still wrong? Is it that that I have to work it longer, or is there perhaps some common mistake I've committed?

Adrian,

i will mail your PM-X knife on Saturday. Let’s talk by phone when you receive it. I ground it on an 80 grit worn wet slow speed disc grinder. The effective grit might be 180, since the wheel is worn. Then went right to the 4000/6000 grit wet disc wheel. But the angle was increased just a little so I don’t waste time and abrasive polishing the whole bevel. Only the edge. I can accomplish the same thing with a 220 grit cbn stationary plate, a 1000 grit cbn plate, then increase the angle and use 2000 grit Shapton and 4000 grit Shapton. Finally, strop on 3 micron cbn paste on leather. I use a jig to hold the angle.

 

If you can hollow grind using a cool process, hand grinder or Tormek, that’s great. Then you won’t need a jig to hold the angle.

BTW, instead of a microscope, you can use the thread test. 40 wt. Sulky rayon embroidery thread attached to a 65 gram weight (26 pennies). You try to lift the weight using the knife edge on the thread. If it cuts through, it’s sharp. Every knife that leaves my shop passes this test.

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I think you would have an easier time with a slower speed grinder. Establishing a bevel and shaping the edge is much harder to do on stones unless you have a good system for maintaining the pressure and angle of attack. It’s a pain to have to buy more equipment, especially if you recently made a purchase, but getting a good grinder set up will save you a lot of frustration and time in the end. 

When I grind a blade, I aim the middle of the bevel so it contacts the wheel and grind as evenly as possible, working until the whole surface is even and there’s a burr. Once both sides are finished, I move to a stone and lightly hone. My goal in honing is to remove the burr with the minimum removal of material. The luthier who taught me to set up tools told me that an edge is at its finest and sharpest at the point that the burr falls off. I test my edges by shaving a few arm hairs off or by taking a tiny shaving  off a thumbnail to see how easily the edge will bite.

Finding the best way to hold the knife and making smooth passes that don’t roll the edge is where the mastery of sharpening lies.  

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5 hours ago, Roger Hill said:

Normal working direction is with the cutting edge pointing into the direction of motion on the stone.  At the end of (say) a dozen forward strokes on one side, work the edge back and forth for three or four strokes in a short scrubbing motion.  Rotate the knife blade 180 degrees to work the other side in the same manner.

I've spent about an hour doing this on the 200 grit stone and I think it's looking better, though it's not nearly there yet. I don't know how long it'll take me at this rate to develop a proper apex, but at least the knife can cut paper now (with some difficulty), even without proper sharpening.

I wonder to what degree the edge in the first picture is exaggerated by the burr that was present, which this process has reduced quite a bit.

20190727_021705.jpg

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4 minutes ago, Adrian Lopez said:

I've spent about an hour doing this on the 200 grit stone and I think it's looking better, though it's not nearly there yet. I don't know how long it'll take me at this rate to develop a proper apex, but at least the knife can cut paper now (with some difficulty), even without proper sharpening.

I wonder to what degree the edge in the first picture is exaggerated by the burr that was present, which this process has reduced quite a bit.

20190727_021705.jpg

Hi Adrian - you might well be at the limit of sharpness that you can achieve with a 200 grit - just a thought.

Time to move on to finer grits and micro-bevels.

http://www3.telus.net/BrentBeach/Sharpen/bevels.html

cheers edi

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4 hours ago, violins88 said:

i will mail your PM-X knife on Saturday.

Thank you. I look forward to receiving the knife and trying it out.

 

4 hours ago, violins88 said:

If you can hollow grind using a cool process, hand grinder or Tormek, that’s great.

I think I might just get one. I'm finding out sharpening is not much fun (though this may have a lot to do with inexperience), and it looks like a Tormek might turn what feels like a chore into a relatively easy process.

 

3 hours ago, The Violin Beautiful said:

I think you would have an easier time with a slower speed grinder. Establishing a bevel and shaping the edge is much harder to do on stones unless you have a good system for maintaining the pressure and angle of attack. It’s a pain to have to buy more equipment, especially if you recently made a purchase, but getting a good grinder set up will save you a lot of frustration and time in the end. 

A Tormek with a diamond wheel might be good for shaping and beveling. Not sure I want to spend $300 on a diamond wheel though, especially since I'll be doing more sharpening than shaping.

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A new development is the cbn grinding wheel. Apparently you can grind at speeds like 1100 to 1800 rpm without overheating the tool. 

 

 

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8 hours ago, The Violin Beautiful said:

I think you would have an easier time with a slower speed grinder. Establishing a bevel and shaping the edge is much harder to do on stones unless you have a good system for maintaining the pressure and angle of attack. It’s a pain to have to buy more equipment, especially if you recently made a purchase, but getting a good grinder set up will save you a lot of frustration and time in the end. 

When I grind a blade, I aim the middle of the bevel so it contacts the wheel and grind as evenly as possible, working until the whole surface is even and there’s a burr. Once both sides are finished, I move to a stone and lightly hone. My goal in honing is to remove the burr with the minimum removal of material. The luthier who taught me to set up tools told me that an edge is at its finest and sharpest at the point that the burr falls off. I test my edges by shaving a few arm hairs off or by taking a tiny shaving  off a thumbnail to see how easily the edge will bite.

Finding the best way to hold the knife and making smooth passes that don’t roll the edge is where the mastery of sharpening lies.  

This exactly right.

While it is possible to make an edge that will shave hair, cut thread or slice paper  by honing on a stone or with a belt or strop the job of a violinmakers knife is to SHAVE WOOD. If the knife has a rounded or variable angle at its apex then the only thing keeping it at a consistent angle of attack on a microscopic level is the operators moving, pulsing ,shaking hands. This is also true while honing which makes the geometry worse with each sharpening. As explained above if the blade is ACCURATELY hollow ground then you have only two points of contact  on the stone which if maintained as you sweep the blade along the stone will take only a tiny amount of metal at the edge and at the heel removing the marks from the grinder to leave a polished but FLAT surface just at the edge. Do the same on the other side working gently back and forth one sroke on each side and  the blade will be as sharp as it can be and can be used to shave wood like a plane (which is by the way only a knife blade held at a consistent angle to the work.) As the hollow disappears with subsequent honing it requires more pressure and control by the operator to remove the metal and the quality of the edge deteriorates to the point where the tool must again be hollowed on the grinder.

Other types of edge shape such as a rounded bevel on axes so the blade won't stick in in the tree or tools which are designed to cut a concave chip such as some carving tools or violin bridge knives can be other shapes but for cutting a flat chip you need a planar shape at point of contact

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7 hours ago, violins88 said:

A new development is the cbn grinding wheel. Apparently you can grind at speeds like 1100 to 1800 rpm without overheating the tool.

 

I just looked these up on Google and it seems they're only for grinding high-speed steel. Other steels will damage the wheel.

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8 hours ago, edi malinaric said:

Hi Adrian - you might well be at the limit of sharpness that you can achieve with a 200 grit - just a thought.

I'll give that a try. 1500 stone, using the technique described by Roger Hill.

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1 hour ago, Adrian Lopez said:

I just looked these up on Google and it seems they're only for grinding high-speed steel. Other steels will damage the wheel.

Where did you read this? That has not been my experience. The new CBN wheels are a revolution, in my opinion. Please excuse the pun.

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42 minutes ago, JacksonMaberry said:

Where did you read this? That has not been my experience. The new CBN wheels are a revolution, in my opinion. Please excuse the pun.

On the other hand, these forum posts offer a bit more nuance than "HSS only":

https://www.tormek.com/forum/index.php?topic=3252.msg20404#msg20404

https://www.tormek.com/forum/index.php?topic=3461.msg25671#msg25671

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I just bought two CBN wheels 200mm x 35mm 80 grit and 180 grit  to run on a Axminster 'Trade' rated grinder at 3000rpm after reading this thread. I like the sound of them and will report back with my experience. Anything that speeds up the lost downtime of sharpening is a bonus. I recently changed to DMT diamond stones for honing with no regrets although I must admit I do the final finish on a china white Arkansas oilstone still.

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4 hours ago, Adrian Lopez said:

I've seen this on websites that sell them, and on this piece from the Arizona Woodturneers Association: http://azwoodturners.org/pages/tips/WhatIsCBN.pdf

CBN wheels.png

The Arizona Woodturner's Association needs a knowledgeable proof reader. CBN wheels are for hardened steel, whether it's HSS or not.

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6 hours ago, Adrian Lopez said:

I just looked these up on Google and it seems they're only for grinding high-speed steel. Other steels will damage the wheel.

 

No steel that you ever use to cut wood will be a mild steel.  That's what they're referring to.

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4 hours ago, Adrian Lopez said:

And the problem these Tormek users are having with CBN wheels seems to be related to the fact that they're running the CBN wheels on Tormeks, in a water bath. There's no reason to run a CBN wheel in a water bath.

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28 minutes ago, MarkBouquet said:

The Arizona Woodturner's Association needs a knowledgeable proof reader. CBN wheels are for hardened steel, whether it's HSS or not.

 

28 minutes ago, Thomas Coleman said:

No steel that you ever use to cut wood will be a mild steel.  That's what they're referring to.

This manufacturer says to only grind HSS on these wheels, but it looks like they're making the same mistake of lumping together HSS and other hardened steels. Anyway, your mileage may vary depending on the precise composition and heat treatment of the tool.

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Do not believe that you can sharpen using this wheel without much heat.  The boron nitride is a very thin coating on the wheel surface, and it will soon load up with ground particles from your tool.  Now it is tool steel grinding on your tool.  Heat will result.

How do you clean this wheel so that there is a fresh grinding surface?  This is easy to do with a traditional ceramic wheel either by fast wear or by the ability to renew the surface with a diamond shaper.   

HSS is a special tools steel that can take higher temperatures than traditional carbon steel.  It was developed for lathe cutting tools; not wood cutting tools.  These grinder wheels were developed for HSS.

This sounds like another great idea that falls flat on it face.

Mike D

 

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49 minutes ago, Mike_Danielson said:

Do not believe that you can sharpen using this wheel without much heat.  The boron nitride is a very thin coating on the wheel surface, and it will soon load up with ground particles from your tool.  Now it is tool steel grinding on your tool.  Heat will result.

How do you clean this wheel so that there is a fresh grinding surface?  This is easy to do with a traditional ceramic wheel either by fast wear or by the ability to renew the surface with a diamond shaper.   

HSS is a special tools steel that can take higher temperatures than traditional carbon steel.  It was developed for lathe cutting tools; not wood cutting tools.  These grinder wheels were developed for HSS.

This sounds like another great idea that falls flat on it face.

Mike D

 

You'll have tried it then????

 

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1 hour ago, Mike_Danielson said:

Do not believe that you can sharpen using this wheel without much heat.  The boron nitride is a very thin coating on the wheel surface, and it will soon load up with ground particles from your tool.  Now it is tool steel grinding on your tool.  Heat will result.

How do you clean this wheel so that there is a fresh grinding surface?  This is easy to do with a traditional ceramic wheel either by fast wear or by the ability to renew the surface with a diamond shaper.   

HSS is a special tools steel that can take higher temperatures than traditional carbon steel.  It was developed for lathe cutting tools; not wood cutting tools.  These grinder wheels were developed for HSS.

This sounds like another great idea that falls flat on it face.

Mike D

 

You're thinking the wrong stuff. The Boron Nitride that you're thinking of it the microscopic sputtered layer put on tools to make them "slippery", and prevent wear. CBN is CUBIC Boron Nitride. Super hard crystals that are fused to the wheel surface. You can't get "grit sizes" if you're dealing with a very thin layer.

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