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Jack Devereux

Sharpening Bridge Knives

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I've been working on improving my bridges lately, which has led me to thinking and experimenting with various knife shapes and geometry. I haven't found anything I really like, or a repeatable, relatively simple way of sharpening. I feel pretty comfortable sharpening other tools, and know what something that's working the way it should feels like. However, even when they're cutting, I'm fighting these knives. Does anybody have any thoughts or a method you're fond of? I really like the look of some of the knives pictured in the Triangle Strings bridge cutting article, but can't seem to replicate any of those grinds.  Thanks! 

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One thing that I think is very important is to have knives with removable handles (or no handles).  I don't see how it's possible to precisely grind a knife blade with the handle attached.

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1 hour ago, JohnCockburn said:

Bevel shape is so important. A knife can be as sharp as sharp can be, but it the bevel shape is wrong for the task at hand it will be useless.

Do you like a rounded (convex) bevel or flat?

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I hollow grind everything then hone on a flat stone. However on my smallest bridge knife I will keep honing even after the hollow has disappeared so I guess it is slightly convex at that point and can follow the smallest curves without "prying".

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i like my knives ever so slightly convex. The other thing I do is to ease or slightly round the non cutting edges on the top of the blade. I believe this helps cut a smoother inside curve. pictured are my bridge knife (for fine inside curved parts) and the one I use for almost everything else.

IMG_0513.JPG

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My favorite bridge knife (3mm wide folded steel Japanese blade blank).  The length of the blade (cutting edge) is purposefully ground very slightly convex (I find the control is better and when a long straight blade is repeatedly honed it tends to go concave).  The knife works just fine hollow ground, but I tend to hone it repeatedly between grindings, so the hollow disappears for a while :) IMG_0673.JPG

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7 hours ago, Jim Bress said:

Do you like a rounded (convex) bevel or flat?

pretty much the same as Nathan says above. Personally, for bridge feet I like to have the "bottom" face (the one that touches the feet) to be hollow ground initially becoming flat on honing, then will regrind it when repeated honings have made it too convex.

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2 hours ago, JohnCockburn said:

pretty much the same as Nathan says above. Personally, for bridge feet I like to have the "bottom" face (the one that touches the feet) to be hollow ground initially becoming flat on honing, then will regrind it when repeated honings have made it too convex.

Thanks John and everyone else.  It's unusual and nice when everyone does something pretty much the same way.

-Jim

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8 hours ago, Jeffrey Holmes said:

My favorite bridge knife (3mm wide folded steel Japanese blade blank).  The length of the blade (cutting edge) is purposefully ground very slightly convex (I find the control is better and when a long straight blade is repeatedly honed it tends to go concave).  The knife works just fine hollow ground, but I tend to hone it repeatedly between grindings, so the hollow disappears for a while :) IMG_0673.JPG

Jeffrey, how thick is that blade, please?

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8 hours ago, Jeffrey Holmes said:

My favorite bridge knife (3mm wide folded steel Japanese blade blank).  The length of the blade (cutting edge) is purposefully ground very slightly convex (I find the control is better and when a long straight blade is repeatedly honed it tends to go concave).  The knife works just fine hollow ground, but I tend to hone it repeatedly between grindings, so the hollow disappears for a while :) IMG_0673.JPG

That's mighty impressive grinding for a 3mm blade. I'd probably lose the ends of my fingers if I attempted that!

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47 minutes ago, violins88 said:

Jeffrey, how thick is that blade, please?

2.4 mm

35 minutes ago, JohnCockburn said:

That's mighty impressive grinding for a 3mm blade. I'd probably lose the ends of my fingers if I attempted that!

Who needs fingerprints anyway?  Wait...  maybe that explains why I'm having trouble unlocking my iPhone! :) 

I have a plexiglass table (platform) I made for the Tormek that allows me to grind long knives without too much hassle.

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25 minutes ago, Luis Martins said:

No one uses scalpels?

IMG_20170803_151602.jpg

Not I.  I much prefer a bridge knife.

Vascular knives are great for certain situations when cleaning cracks though (used under a microscope).

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Thanks for all the input! Yeah, I think it's a matter of geometry. I'll sharpen them to the point where they shave easily, but then the seem to dig into the wood and chatter. 

 

Does anybody use anything like this? The picture is from Ryan Hayes's article on the Triangle Strings website- https://trianglestrings.com/carving-a-violin-bridge/ (if reposting this image isn't kosher I apologize and will take it down). I've gotten a version of this to work pretty well but have a hard time consistently getting that shape right and sharpening it effectively. Plus it just looks cool...

IMG_0041.JPG.316aa248a517a27568b5c5e66549e6b8.JPG

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6 hours ago, Luis Martins said:

No one uses scalpels?

IMG_20170803_151602.jpg

Surgical scapels are a little crude and dull, for our purposes, I'd say.

Perhaps things have gotten better since the last time I tried them?

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1 hour ago, David Burgess said:

Surgical scapels are a little crude and dull,  -snip-

Hi David - nothing that a little honing doesn't cure.

The only knife that was satisfactorily sharp out of the box was a Zwilling ceramic kitchen knife that I bought in Germany. Over time the edge lost a little bit of it's ziing and I've been unable to get it back. There's some diamond lapping paper that I'm going to try.

cheers edi

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I have knives pretty similar to Mike Spencer's photos and in the ankles I use something like Jeff Holmes' photo. Recently I've been playing around with a slight curve on knives rather than dead straight, and I think I might like it once I get accustomed to it. 

Bevel angle is important. With really high quality steel you can get a nice edge on a blade with a higher bevel angle. This makes it easier to make a tight inside turn without chatter. You can also grind a rounded portion on the back of your blade, so there aren't corners on the back side of the blade. You only need to do this in the portion that actually rides on the bridge, which is just on the tip end.

On my 3mm knife I have a bevel angle of more than 35 degrees (maybe even above 40, I forget, but it's far steeper than any other blade) and on my 6-12mm blades they're around 35 degrees. If you are using a blade to cut long flat stretches (as you do on a bridge foot) then a longer bevel can work well for you. A lower bevel angle like 25 degrees (wouldn't generally go any lower) will take a sharper edge, but the edge will not last as long. Some hard, but brittle steels are well suited to a slightly higher blade angle depending on your use as it will make it less likely to fracture at the edge under use. I've looked at blades under a microscope, and at the lower blade angles you can get little micro-fractures that are broken off sharply as opposed to a rounding off over use. The gradual rounding seems to be the effect when a blade is wearing normally rather than breaking under too heavy of pressure for the type of steel and bevel angle. 

Also worth noting, issues with short edge life can also be from burning the blade during grinding. Always start at the tip of the blade and work back. Heat builds during each pass, so if you work towards the tip the heat builds and goes to the tip where there is nowhere to dissipate. The tip is most prone to burning. Once you burn it that portion of the steel has lost it's temper and you must grind past the burned area slowly without burning again. Grinder speed, type of wheel, amount of pressure, and use of coolant (just swish it in water if it's getting warm) can all make it easier to work without burning. 

Size of handle is also important. If you're turning tight curves it is easier to do with a smaller handle. My 3mm knife has no handle, my 6 or 8 mm knife has a handle the diameter of a dime or so. Works well for me.

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2 hours ago, TimDasler said:

I have knives pretty similar to Mike Spencer's photos and in the ankles I use something like Jeff Holmes' photo. Recently I've been playing around with a slight curve on knives rather than dead straight, and I think I might like it once I get accustomed to it. 

Bevel angle is important. With really high quality steel you can get a nice edge on a blade with a higher bevel angle. This makes it easier to make a tight inside turn without chatter. You can also grind a rounded portion on the back of your blade, so there aren't corners on the back side of the blade. You only need to do this in the portion that actually rides on the bridge, which is just on the tip end.

On my 3mm knife I have a bevel angle of more than 35 degrees (maybe even above 40, I forget, but it's far steeper than any other blade) and on my 6-12mm blades they're around 35 degrees.

Yes...  the cutting edge on my 3 mm knife is slightly curved, and I much prefer it that way.  The Japanese folded steel blanks are slightly rounded (dull) on the back side too, which allows them track pretty well without much modification. I use the same knife I illustrated for almost everything but fitting the feet for violin & viola bridges.  Can't beat it for tight curves, and I just move to the thicker part of the blade for gentle curves.  I have it ground at around 35 degrees.  I like the control a handle gives me, though... and I think I like familiarity (all my knives have handles).

For 'cello bridges I use a similar knife, but it's 5.5 mm wide.

For fitting feet, I switched from a straight ground knife (like Mike Spencer's second knife) to one with a slight curve to the cutting edge several years ago. Much faster for me... but I know some restorers who prefer a chisel. Too each their own. :)

What I notice at Oberlin, or when visiting other teaching venues, is that a number of technicians cutting bridges may not fully have a grasp on when to use a slicing, rather than shearing, motion during bridge cutting.  Maintaining the integrity of the line is difficult if this is not understood...

 

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10 hours ago, Jack Devereux said:

Thanks for all the input! Yeah, I think it's a matter of geometry. I'll sharpen them to the point where they shave easily, but then the seem to dig into the wood and chatter. 

 

Does anybody use anything like this? The picture is from Ryan Hayes's article on the Triangle Strings website- https://trianglestrings.com/carving-a-violin-bridge/ (if reposting this image isn't kosher I apologize and will take it down). I've gotten a version of this to work pretty well but have a hard time consistently getting that shape right and sharpening it effectively. Plus it just looks cool...

IMG_0041.JPG.316aa248a517a27568b5c5e66549e6b8.JPG

Yes, mine are quite similar.  The trick in retaining the curve is how you position your hand when sharpening.  Once you get the flow of your hand movement, the curve evolves;  the curve follows the hand not the other way around....the cool follows shortly thereafter.

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1 hour ago, Jerry Pasewicz said:

the curve follows the hand not the other way around....the cool follows shortly thereafter.

Excellent advice, no matter how much curve is involved.

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22 minutes ago, JohnCockburn said:

any chance of a pic?:)

Not grinding knives at the moment, but I'll illustrate the parts. It's pretty low tech, but easy to set up.The adjustable aluminum rest attaches to the Tormek's slide arm. The plexiglass rest is placed on top of the aluminum rest with the cut-out portion at the wheel (it's beveled there, so it can be set very close to the grinding surface).  This allows support for the tool at, and on both sides, of the wheel. I use some small clamps to secure the platform once the angle is set and the rest is where I want it. If I wasn't so cheap, I'd buy a second aluminum platform and attach the plexiglass to it permanently.IMG_0675.JPG  

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