Fast thinning behind the edge with a rotary tool/dremel

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palindrome

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I want to share a technique for thinning behind the edge (knocking down the shoulders or making the edge bevel more acute) that I discovered when making a knife from M35, which is so difficult to work that no reasonable knife maker would use it.

I'd say this technique needs some alpha/beta testing, since the final finish still had some scratches, and there's no way to prove the heat stays under control.

The problem
Your blade is too thick behind the edge. This can manifest in an edge bevel that's longer than a mm (which hurts cutting performance and makes sharpening difficult), or an excessively obtuse edge bevel that doesn't cut well--and when you try to set a steeper bevel you end up just sharpening the shoulders, unless you are willing to spend several hours with a stone.

TL;DR
Iteratively grind down the shoulders, and make a more and more acute edge bevel. Do not grind the cutting edge at all, and wet the knife between passes.

The technique
Start by sharpening it at whatever angle feels natural for the blade. If you are making a new blade, it can be as obtuse as you like to save time. The goal is to have a primary bevel around 1 mm.

Use a colored permanent marker or layout fluid to mark the primary bevel. You don't need to mark the whole thing, but it's helpful if you mark the shoulder and it's critical to mark all the way to the edge.

Secure the blade with a clamp or vice so it is horizontal with the edge toward you (tip pointing to the side). Put a fresh (not dished) grinding tip on the dremel. Use a high speed.

Hold the dremel at a near horizontal angle and take metal off of the shoulder. Make passes and always keep the tool moving. I suggest you skip the tip because you are likely to round it off. With each pass, the colored line on the bevel will get narrower. However the coloring at the very edge should never be grinded away! This would mean you are holding the tool wrong and ruining the cutting edge.

After a pass, pour or spray water on the blade. At the very least, wait a few seconds for some of the heat to flow from the grinding region into the rest of the blade. The thinner it gets, the more you need to worry about heat. The lower alloy the steel is, the more you need to worry about heat. (On the other end of the spectrum, you don't need to worry at all if it is a HSS.)

Do as many passes as necessary to get the edge bevel coloring very thin, then flip the knife and do the other side. You are almost at the point of "no bevel", however the knife is still probably not thin behind the edge as you would like.

This image I borrowed shows where the marker (shaded area) will be removed. The lower marker band will get progressively thinner with each pass.
1685182344618.png


Now start again, making a more acute edge bevel this time. It will become wide again, and you can knock it down with the dremel again. If you don't want a more acute edge bevel, you are done--the blade is as thin behind the edge as you can safely make it. You can refine the blade on whetstones unless it is too hard and requires tools.

Tool selection
The cylinder shape works well; a disc shape would be too difficult. Aluminum oxide would be good, but I found they can't cut hardened steel. A wider head is better than a narrow one because its wider surface contact is less likely to cut a groove.
1685171271771.png
1685182525167.png

I used a coarse sanding drum (it was 60 or 80 grit). If it cuts, you will see sparks. If not, you are wasting your time and may overheat the metal. When using a coarse grit, you must keep a perfect angle--if you point downward too much, the scratches in the upper part of the blade will never come out. This happens because the middle of the sanding drum gets dulled from the work, while the top is still fresh and sharp. The top of the drum is capable of cutting much deeper furrows than the middle.

Warnings
First off, this technique can't give perfect evenness. (But if you find a high spot, you can always knock it down later, using a stone or a power tool as you prefer.) Next, the scratch pattern will be in the "wrong" direction. I wonder if there would be high spots that would interfere with using a whetstone on the shoulder. If so, sandpaper (maybe a sanding stick with leather backing) would probably be the way to take care of it.

I would not use a dremel on a knife I considered a work of art. Fortunately, the only way I'm going to own a work of art knife is if I make it. And I'll probably make it with power tools.
 
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palindrome

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I would been worried about over heating the edge and ruining the temper.

How did you get around this problem?
M35 is a high speed steel and practically can't overheat, but I believe this would work for a normal steel as well. The key is that you don't grind the edge, just the shoulders. So the head can spread out inside the steel (and dissipate) before it reaches the edge. Also, you can touch it after a pass to confirm that it's not terribly hot. But as your grind gets flatter and the sharpie line gets thinner, you can make faster passes, because this is the time when it gets risky (you are closer to the cutting edge).

Do you have any suggestion about how I might test this? Tomorrow I will thin a 9Cr18MoV blade's shoulders/edge bevel and see if I can improve its performance--if I don't notice an improvement in time between sharpenings, I'll conclude I messed up the tempering. However that test is neither fast nor objective.
 
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ian

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Tomorrow I will thin a 9Cr18MoV blade's shoulders/edge bevel and see if I can improve its performance--if I don't notice an improvement in time between sharpenings, I'll conclude I messed up the tempering.

You may see an improvement in time between sharpenings just because the knife is thinner, too.

I totally get that we use what tools we have but I can’t help having a worried reaction to using a Dremel for this kind of thing. It’s going to be super hard to get any sort of uniformity in the geometry of the knife where you’ve Dremel’ed it. Some parts are going to be thicker, some thinner. I suppose you could use a Dremel and then try to even it out later on stones, or you could just be ok with it being irregular, since it might not affect performance much. I’d also just be pretty paranoid about working close to the edge with a tool that likes to travel like a Dremel. A huge mistake can happen in under a second.

That said, more power to you for playing around with making knives thinner! It’s good work. :)
 
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I'm with Ian on this.
I onced used my Dremel to remove carbon buildup from the inside of a chainsaw cylinder using a brass wire wheel thingy.
I slipped, accidentally touched the switch and it went to max RPM, the attachment caught the wall and 'rode it'. Caused a major scratch in the cylinder wall...

It's a great tool but nerve wracking when used on stuff that has no room for error.
 

KasumiJLA

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Always find cool to try new things and technique, but when I saw the words "dremel and thinning" I was a little surprised!

Personally I could not live with a kireha which is not uniform and completely homogeneous. I may give too much importance to the aesthetic finish and that the performances can be interesting on a beater knife though.

Thinning on stone is tedious for sure, but it doesn't need to take several hours if you use the right stone at first. I do use a dremel mainly for correcting and polishing choil and this thing can go wrong quickly, even at the lowest rpm. I would be curious to see an example that you can do with this technique 💁‍♂️

88EBD2C9-0374-4543-A91E-712BE98736CB.jpeg
A small example of the dremel that slipped and caused scratches.. It's not the end of the world but it took me more work afterwards to clean everything.
 

palindrome

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This is unpolished M35 after neglect. (Don't worry, I know about pitting so I took care of the rust spots.)

The difference among carbon steels is amazing! The CCK steel slowly turns orange in humidity and/or stinks of sulfur when cutting, even after having a patina forced.
 

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This is unpolished M35 after neglect. (Don't worry, I know about pitting so I took care of the rust spots.)

The difference among carbon steels is amazing! The CCK steel slowly turns orange in humidity and/or stinks of sulfur when cutting, even after having a patina forced.
Is this picture from after using the Dremel?
 

palindrome

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Is this picture from after using the Dremel?
The scratches by the heel are from the Dremel, due to using a coarse sanding drum. This weekend I'll update the top post with more info about that. I think a coarse drum will inevitably scratch since it's not very flat.

The general blade texture is because I shaped it with an angle grinder and didn't want to use dozens of sanding pads to get the scratches out. It's almost ungrindable, though carbide and very fresh sandpaper can do it.
 
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The scratches by the heel are from the Dremel, due to using a coarse sanding drum. This weekend I'll update the top post with more info about that. I think a coarse drum will inevitably scratch since it's not very flat.

The general blade texture is because I shaped it with an angle grinder and didn't want to use dozens of sanding pads to get the scratches out. It's almost ungrindable, though carbide and very fresh sandpaper can do it.
I think @M1k3 is asking about the color.
Yes. The coloring looks like the steel has been overheated.
 

palindrome

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Ahh, the color/heat? No, but some darker notes are from the angle grinder.

I tried the Dremel on ordinary steel, doing it with one or two passes at a time. The steel didn't even get warm. I'd say it's no concern unless you hold it still or work on a single area.

To be very clear, I noticed the patina after using it at barbecues and not washing it afterwards. Abusive grinding (and I choose to abuse it because it's HSS and can take it) caused darkening only.
 

tostadas

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You are far braver than me if you're willing to thin with a dremel. If it slips and the edge of the grind wheel scuffs the blade, the work to fix that would exceed any time savings I think I could gain from the dremel.
 

palindrome

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If you hold it horizontally with two hands and he blade is marked and clamped, it's hard to slip or grind at the wrong angle. The tool doesn't wander or chatter. That's why this technique is worth sharing, in my opinion. Though it would need a lot more guidance to be foolproof.
 

tostadas

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If you hold it horizontally with two hands and he blade is marked and clamped, it's hard to slip or grind at the wrong angle. The tool doesn't wander or chatter. That's why this technique is worth sharing, in my opinion. Though it would need a lot more guidance to be foolproof.
Haha, you have a lot more trust in my hands than I do. I can see some potential benefit in doing the super coarse grinding maybe near the shoulders when thinning bevels. But I personally wouldn't dare get anywhere near the cutting edge with my dremel at 15k+ rpm. I've done plenty of choil work with my dremel, but even with the greater steel mass in that area, it can generate heat pretty quick.
 

palindrome

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Yeah, that's a judgment call. The wider the surface area and the thinner the steel, the more risk. And the Dremel can get the steel hot and blackened or keep it cool to the touch, depending on how you work.
 
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Blank Blades.

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Do you have any tips? What tools do you use? I'm hesitant to use it again because of the difficulty but it takes a beautifully keen edge. Like a razor
Any steel like that diamond is your friend.

But also it helps if you can get it as thin as possible of the grinder. So variable speed and watercooling is usually necessary for getting them the thinness we want, as well as fresh ceramic belts. Going up to about 120 grit. Which is where they stop cutting these steels efficiently.

Then i personally will usually go to my venev stones to do the last tiny bit of thinning, and getting the convex geometry exactly where i want it. Ill also usually go from venev f80-f400, because it makes the work of polishing A LOT easier which i also do with diamond.

Diamond microfinishing films. They make polishing any high wear resistance steel a breeze. And after the venev really its just a matter of evening out the polish.

And sharpening. Same deal. Use diamond. If you dont have resin bonded diamond, just diamond plates work really well actually. One of the dmt coarse interrupted surface plates, followed by 1 micron diamond spray on leather can get you very sharp with any steel.
 
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You’re right, a Dremel can round things pretty well. And used even at 15000 rpm you won’t see any bad trace of heat when you move fast enough.

The only thing to worry about is that they always leave low spots.
I am sorry if this sound inappropriate and you may see this as a very different use of a dremel. When you try to get a mirror surface on a single bevel, once you‘re touching the shinogi line, even a bit, it has became wavy.
If you‘d want to get rounded (not crisp) but straight shinogi, what would keep you from using a benchstone?
 
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palindrome

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Sorry I haven't followed through with photos. However I discovered something--I worked a bit more on my blade using a diamond whetstone, as well as a toothbrush-shaped resin bonded diamond tool I recently bought. Both messed up the look of the blade. The whetstone left polished spots going too far up the blade, and the toothbrush left scratches. You guys had had me convinced the dremel isn't appropriate for use on beautiful blades, but I am rethinking that--for thinning (on the shoulder specifically) I think it has more precision than stones, not less.

@nutmeg Yes, it leaves high and low spots, and I'd expect them to show up during subsequent thinning operations. My thinking on this is still evolving--I thought the blade looked perfect after I burnished it, but that finish is vulnerable to both scratches and unintentional polishing.
 

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Sorry I haven't followed through with photos. However I discovered something--I worked a bit more on my blade using a diamond whetstone, as well as a toothbrush-shaped resin bonded diamond tool I recently bought. Both messed up the look of the blade. The whetstone left polished spots going too far up the blade, and the toothbrush left scratches. You guys had had me convinced the dremel isn't appropriate for use on beautiful blades, but I am rethinking that--for thinning (on the shoulder specifically) I think it has more precision than stones, not less.
I don't know what grit the resin bonded thing your talking about is, but it will certainly leave scratches if its coarse. Also who knows what quality the abrasive is on that.

The diamond stone. Idk what kind you used also. But if its polishing too high, it could be as simple as user error. Depending what you did, and what you mean by "polished spots". It could be because of the not flat bevel mentioned above or whatever else.

Also. I personally didn't say not to use the dremel. But i did say what works well for me. Quality of the abrasive being used will likely make a big difference there.
 
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