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Wide bevels
- Thread starter joesrx
- Start date
thats what I would do
and as somebody noted before, more the clamp closer to the tip so that the angle is roughly the same and the heel and tip. it may not be perfect all along the bevel, but it shouldn't be quite as noticable
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The sharpener has an upright column. A double pivot slides up and down the column. An arm attaches to the double pivot. The arm can swing both up and down and also left and right. A rectangular stone is attached to the arm. The stone can rotate about the arm.
A knife is held horizontally in a clamp. The stone rests on the knife edge. As you swing the arm left and right, keeping the stone resting on the edge, the angle between the edge and the arm changes. But that angle is not the bevel angle. The bevel angle is the angle between the plane of the knife blade and the plane of the stone. If the edge is straight, the stone rotates about the arm to maintain a constant bevel angle no matter how far you swing the arm left or right.
If you have an angle cube attached to the stone, it will read the correct angle if and only if the vertical plane of the angle cube is perpendicular to the edge. Therefore, to get the correct angle reading, you must twist the angle cube back and forth on the stone as you swing the arm left and right in order to keep the angle cube perpendicular to the edge so that it gives the correct bevel angle.
If you have an angle cube attached to the arm instead of attached to the stone, you have to keep the vertical plane of the angle cube really vertical, and the arm perpendicular to the edge, in order to get correct angle readings.
What this boils down to is that if the knife edge is straight and you measure the angle correctly, you will get the correct bevel angle along the entire length of the straight edge.
If the belly is a circular arc, and the upright is at the center of the circle, everything works out right, also. But the upright is almost never at the center of the circle, so you are likely to get a varying bevel angle along the belly.
There is an article about such matters somewhere on the Wicked Edge web site, although the article can be hard to find. Wicked Edge also has some sort of gizmo to help you position the knife, but I do not know how it works.
A knife is held horizontally in a clamp. The stone rests on the knife edge. As you swing the arm left and right, keeping the stone resting on the edge, the angle between the edge and the arm changes. But that angle is not the bevel angle. The bevel angle is the angle between the plane of the knife blade and the plane of the stone. If the edge is straight, the stone rotates about the arm to maintain a constant bevel angle no matter how far you swing the arm left or right.
If you have an angle cube attached to the stone, it will read the correct angle if and only if the vertical plane of the angle cube is perpendicular to the edge. Therefore, to get the correct angle reading, you must twist the angle cube back and forth on the stone as you swing the arm left and right in order to keep the angle cube perpendicular to the edge so that it gives the correct bevel angle.
If you have an angle cube attached to the arm instead of attached to the stone, you have to keep the vertical plane of the angle cube really vertical, and the arm perpendicular to the edge, in order to get correct angle readings.
What this boils down to is that if the knife edge is straight and you measure the angle correctly, you will get the correct bevel angle along the entire length of the straight edge.
If the belly is a circular arc, and the upright is at the center of the circle, everything works out right, also. But the upright is almost never at the center of the circle, so you are likely to get a varying bevel angle along the belly.
There is an article about such matters somewhere on the Wicked Edge web site, although the article can be hard to find. Wicked Edge also has some sort of gizmo to help you position the knife, but I do not know how it works.
Guided sharpeners.... it's all about geometry.
Lets say, I put my EDC in the clamps of my sharpener. On my sharpener the distance from the pivot to the edge of the blade (in the center of the clamps) is 220 milimeters. If this distance from the pivot to the edge changes then sharpening angle also changes.
Lets say, I put my EDC in the clamps of my sharpener. On my sharpener the distance from the pivot to the edge of the blade (in the center of the clamps) is 220 milimeters. If this distance from the pivot to the edge changes then sharpening angle also changes.
joesrx
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Yep, pulled out the angle cube to help. Geometry, guess I should have paid attention in class more.
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Yes, he would get the same bevel angle and same bevel width if the metal is uniform in thickness. I tried to explain this above.
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Many people get confused on this issue because they think in terms of plane geometry, but the issue is solid geometry.
Say you have a house. Nothing weird. The footprint is a rectangle. The roof is gabled. The edges of the eaves are straight and horizontal. The eaves are of constant width. The roof ridge is straight and horizontal. Put a very low, double pivot of some sort on the ridge. Attach a long, straight, rigid pipe to the double pivot so that it rotates freely. Lay the pipe down so that it contacts the slope of the roof all the way down to the eaves. Swing the pipe left and right. The pipe can stay in contact with the roof all the way down to the eaves no matter how far you swing it from side to side.
Now attach a flat board to the pipe so that the board can rotate freely about the pipe. Lay down the pipe with the board so the the board is flat against the roof. Swing the pipe left and right. The board can rotate to stay flat against the roof. The angle of the board with the horizontal plane that passes through the edges of the eaves remains constant because the angle of the roof with that plane is constant.
Yes, I understand exactly what you described with the roof.
When we talked about different width of the bevel when sharpening knives on guided systems we established - if the distance from the pivot to the edge changes then the sharpening angle changes and with that also the width of the bevel shanges.
So, if you put for example a ruler in the clamps and measure the distance first from the edge of the ruler in the center of the clamps to the pivot and then from edge of the ruler at far left to the pivot you will measure different distances so the angle should be different.
Right?
We would get the same angle if the blade would be rounded.. same radius as sharpener radius.
When we talked about different width of the bevel when sharpening knives on guided systems we established - if the distance from the pivot to the edge changes then the sharpening angle changes and with that also the width of the bevel shanges.
So, if you put for example a ruler in the clamps and measure the distance first from the edge of the ruler in the center of the clamps to the pivot and then from edge of the ruler at far left to the pivot you will measure different distances so the angle should be different.
Right?
We would get the same angle if the blade would be rounded.. same radius as sharpener radius.
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I found a nice example if anyone stil listening to me...
Lets say we still have straight piece of metal in our clamps and we want to sharpen it.
If we want to have the same edge angle (same bevel width) along the whole edge of our straight metal we would need to have our stone perpendicular to the edge of our metal in each point along the whole length.
With our sharpening systems we can't do this because our pivots are fixed. We would have be able to move our pivot from lets say left to right.
We would need a system like in link below. The pivot is on rails and we can move it left to right so we can maintain the same distance from pivot to edge across whole length of our straight piece of metal and the rod with sharpening stone is perpendicular to the edge in all points along the edge.
Only in this case we can have the same bevel width across whole edge of our working piece.
Just to say, I'm not promoting this sharpener. I just found an image on internet so you could understand what I'm trying to say.
https://drive.google.com/file/d/17f6RoWO5R_xWpZzxwHC_4p1OdHpJr8Oh/view?usp=drivesdk
Lets say we still have straight piece of metal in our clamps and we want to sharpen it.
If we want to have the same edge angle (same bevel width) along the whole edge of our straight metal we would need to have our stone perpendicular to the edge of our metal in each point along the whole length.
With our sharpening systems we can't do this because our pivots are fixed. We would have be able to move our pivot from lets say left to right.
We would need a system like in link below. The pivot is on rails and we can move it left to right so we can maintain the same distance from pivot to edge across whole length of our straight piece of metal and the rod with sharpening stone is perpendicular to the edge in all points along the edge.
Only in this case we can have the same bevel width across whole edge of our working piece.
Just to say, I'm not promoting this sharpener. I just found an image on internet so you could understand what I'm trying to say.
https://drive.google.com/file/d/17f6RoWO5R_xWpZzxwHC_4p1OdHpJr8Oh/view?usp=drivesdk
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No, because the stone can rotate about the arm of the sharpener.
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Here is another explanation with photos and diagrams and videos:
Does the angle change along the length of longer blades on the Wicked Edge?
Does the angle change along the length of longer blades on the Wicked Edge?
The farther the "guide rod" (or whatever you want to call it -- the thing to which the stone is attached) slides out from the holder, the longer the hypotenuse of that triangle will be, and therefore, since the length of one of the adjacent legs of that same right triangle remains constant (where the guide rod slides through), the angle between the stone and the edge will change. Specifically, the angle will get smaller, and the length of the bevel will increase (unless the knife has a linear edge, like the framing square used in the video).
It's akin to measuring "minutes of angle" of precision when shooting. An inch at 100 yards ≈ 1 minute of angle. If you double the distance -- akin to doubling the length of the guide rod -- to 200 yards, that 1 inch ≈ 1/2 minute of angle. As the distance increases, the angle gets smaller. The only way to keep the angle the same when you double the distance is to double the length of that leg of the triangle (in other words, move the pivot point for the guide rod farther away).
The only reason the Tiltbox doesn't show the change in angle is because they keep the Tiltbox plumb. Yes, the angle with respect to a plumb line remains the same, but the angle at which the stone contacts the knife edge -- at least on a knife with a rounded tip -- changes. And the bevel gets longer. The only kind of knife that won't get a longer bevel out near the end is a knife with a linear edge, like a Wharncliffe ... or a chisel. So their use of a framing square in the video -- akin to a knife with a linear edge -- is misleading. (I got one of those Tiltboxes about 15 years ago for woodworking, and it's handy, but like any instrument, you need to know how to use it ... and how to interpret what it's telling you.)
It's akin to measuring "minutes of angle" of precision when shooting. An inch at 100 yards ≈ 1 minute of angle. If you double the distance -- akin to doubling the length of the guide rod -- to 200 yards, that 1 inch ≈ 1/2 minute of angle. As the distance increases, the angle gets smaller. The only way to keep the angle the same when you double the distance is to double the length of that leg of the triangle (in other words, move the pivot point for the guide rod farther away).
The only reason the Tiltbox doesn't show the change in angle is because they keep the Tiltbox plumb. Yes, the angle with respect to a plumb line remains the same, but the angle at which the stone contacts the knife edge -- at least on a knife with a rounded tip -- changes. And the bevel gets longer. The only kind of knife that won't get a longer bevel out near the end is a knife with a linear edge, like a Wharncliffe ... or a chisel. So their use of a framing square in the video -- akin to a knife with a linear edge -- is misleading. (I got one of those Tiltboxes about 15 years ago for woodworking, and it's handy, but like any instrument, you need to know how to use it ... and how to interpret what it's telling you.)
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