Knife position Trigonometry

[uilleann]

Knife Vise position seems to be determined mostly in a few different ways. On the KME there is positioning the knife so the CL of the jaws is on the center point of the knife spine parallel to the lines on the jaws when the spine is inserted to one of the lines.  This does not account for a curved blade so it will give you a changing sharpening angle.

Most KME users use the method of drawing a line from the tip to the heel of the edge. Then another line at the midpoint of that line or th midpoint of spine to a perpendicular angle on the first like.  That get you a center position and an angle to mount in the jaws.  Then you do the sharpy thing and adjust if necessary.

On the WE, where repeatibilty is a goal for sharpening to sharpenng on the same knife is the same, the knife is always positioned with the spine touching both pins.  Then using the sharpy the knife is moved along the pins till you get the most constant grind mark to edge distance.  In my limited experience so far I have found that I do this method but sometimes tilt the blade to get the most desireable  bevel consistency…… losing some of that repeatability.

But it occurred to me….  Couldn’t this situation be calculated using Trigonometry.  Better yet reduced to a mathcad or other cad drawing where the angle between the stone and the knife edge could be calculated over the length of a sweep for any knife position.  Best would be an App where you put in some key knife blade dimensions and it spits out optimum knife/vise position.

Anybody ever get close to doing something like that?  If not I bet that would be a great school project?

 

[Marc H]

Bob, this Advanced Alignment Guide (click here) is used when the knife spine does not rest on both pins or any other random placements you may chose to use to allow for repeatability by clamping the knife with the same X/Y coordinates for the tip, then you can flip the guide, around to the rear, on the pin to take an X/Y coordinate for the heel.  As long as you clamp the knife to give these two points aligned again it should be positioned the same without any mathematical computations.

Marc
(MarcH's Rack-Its)

[Organic]

Yes, the advanced alignment guide is the answer to this problem.

1 user thanked author for this post.

Marc H

[Readheads]

A great analysis was done by Anthony Yan quite a while back. He compiled a pdf of it which is 100 pages long with embedded videos (just look for the push pins in the document). It is 150 MB is size so I embedded a link to it below. I also included Verhoeven’s “Metallurgy for Bladesmiths” which is also an outstanding work.  Someone should also add these to the Resources section of the WEPS site.

This also ties into my latest “issue” in that while the angle will change somewhat (around 1 deg per analysis and actual) along the curved sections of a knife (unless the curve is precisely on the perfect circle of rotation), this assumes that the knife is perfectly stiff. While this is true enough for both kitchen and non-kitchen thicker knives, many kitchen knives have flex to them. The flex kitchen knives (and fishing filet knives for that matter) are the ones you want the keenest edge on. When a blade flexes you lose repeatability especially at the tip. I verify this using a 250x USB scope and it is a major PIA. Stopping the flex with your finger, strings, extra klugey fixtures, knife repositioning, etc. goes against the goal of simple elegant solutions especially when you are trying to put perfect edges on a $300+ kitchen knife. So that being said I will throw out 2 points for discussion:

1. I have suggested another set of jaws (similar to the thick knife jaw accessory) which would be for flexible knives which would extend the clamping length from the standard ~1.5 inches to ~6 inches. Does anyone in the forum have the ability to machine such a prototype ?
2. While #1 is being considered, most people suggest a dual position scenario where you move the knife after completing a sharpening scenario into a position which clamps the more flexible part and then do a another sharpening scenario on the flexible part while blending the final edges together. To best “blend” the edges should the second position be along the same horizontal line of the advanced alignment gauge ?

https://drive.google.com/drive/folders/0B5t7FJ9Zmj1hZ2hiZUx1Y3JKMTA?usp=sharing

Attachments:

You must be logged in to access attached files.

[Marc H]

Another suggestion, which I believe I have seen someone already employ, is to mount two WE Gen 3 Clamps in-line, one behind the other on the same base.  Of course they would need be precisely drilled so the are square and straight in-line. If they were mounted separately each onto it’s own flat metal plate base with precisely drilled holes to allow for the plate mounts to be adjusted closer or further apart, say with knurl knobs, or wing nuts, while still maintaining their straight in-line and squared orientation, that would be ideal. The advantage of this setup is the long knife would be very securely held by two clamps and you would have two sets of rods allowing for overlap along the bevel.  This is just an idea I’m tossing around.  Of course there are limitations I haven’t considered which my fellow forum participants will think of and point out.  That’s the beauty of a forum discussion.

This double clamp method may prove to be easier to engineer and less expensive when you consider designing, engineering, and cutting prototypes for a wider clamp adapter and engineering a method to attach it stably to the vice clamp and not reduce the space between the jaws too severely to render them unusable for all but the thinnest blades.  I do think the clamp adapter is a great idea and something I would buy. But the double clamp is still an intriguing idea to me.

Marc
(MarcH's Rack-Its)

2 users thanked author for this post.

Organic, uilleann

[uilleann]

Those resources are great.  Thanks.  Downloaded and will read soon.

[Organic]

Marc H wrote:

Another suggestion, which I believe I have seen someone already employ, is to mount two WE Gen 3 Clamps in-line, one behind the other on the same base. Of course they would need be precisely drilled so the are square and straight in-line. If they were mounted separately each onto it’s own flat metal plate base with precisely drilled holes to allow for the plate mounts to be adjusted closer or further apart, say with knurl knobs, or wing nuts, while still maintaining their straight in-line and squared orientation, that would be ideal. The advantage of this setup is the long knife would be very securely held by two clamps and you would have two sets of rods allowing for overlap along the bevel. This is just an idea I’m tossing around. Of course there are limitations I haven’t considered which my fellow forum participants will think of and point out. That’s the beauty of a forum discussion. This double clamp method may prove to be easier to engineer and less expensive when you consider designing, engineering, and cutting prototypes for a wider clamp adapter and engineering a method to attach it stably to the vice clamp and not reduce the space between the jaws too severely to render them unusable for all but the thinnest blades. I do think the clamp adapter is a great idea and something I would buy. But the double clamp is still an intriguing idea to me.

There as a user on the forum who set up two WE clamps (older generation) and attached the arms to a rail that allow them to slide up and down the length of the blade. I remember they posted a picture with a sword in the clamp, but I can’t seem to find it. The obvious downside to this type of setup is the cost, but it would be awesome.

[Author]

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