Compensating for tilted blade
- This topic has 23 replies, 7 voices, and was last updated 02/01/2016 at 7:16 am by
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This is how I do it ?
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see my comments and photos on setting the sweet spot in the thread ” Advanced alignment guide ” probably better if they were in this thread.
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Thanks for reposting the video Josh. The first time I did it was for a full flat ground blade. To double check that the math made sense I laid the blade on my countertop and measured the angle of the blade from the counter top. Long story short the angle of the blade checked out with the measurements in the vice which was good to know before I stared grinding away.
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One thing just pointed out to me is that I had my math wrong in the video… someone pointed this out to me about a year or two ago and I edited the video with a caption to correct the math. So if you have a browser w/ an add on that disables the captions you will miss it. Just wanted to note this! Thanks and glad it’s helping.
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Actually, Josh has two math errors in the video. He subtracted the left reading (+0.70) from the right (-4.00) when he should have added them. The object is to find the angular displacement between one side of the blade and the other. Imagine that you have the AngleCube on the left side, reading +0.70. Now rotate the ‘cube to the left to -4.00. To do so, you have to rotate the ‘cube 4.70 degrees, not 3.30. This means the median (centerline) is displaced 2.35 degrees from each face. If the blade were perfectly vertical, it would read +2.35 on the left side and -2.35 on the right side. Since the left side reads +0.70 instead of +2.35, the centerline is canted 1.65 degrees to the left. To compensate, you deduct 1.65 from the left setting and add 1.65 to the right.
On another issue, the Gen 2 vise uses face-to-face contact in gripping the blade. If one or both of the jaws is not parallel to that face of the blade, you are in danger of losing grip and the blade might move on you. When you are adjusting the jaws, try to keep them both parallel to the blade faces.
The Gen 3 vise has features to avoid that shortcoming. It makes contact with the blade at two points, roughly positioned at the upper outside corners of the vise. This “point contact” and a designed-in forward-to-back flexibility allows the vise to accommodate both vertical and horizontal variability in the blade’s contours. For those who question the use of such small contact points, rest assured that the principle is technically correct and born out of good mechanical design principles.
The force holding one body against another, preventing lateral movement is dependent on which of two modes of contact exist between them. They are either in “traction” with one another, wherein there is at least some physical engagement between them (think tires on concrete), or in “slip” where one body can slide across the face of the other body (think tires on ice), if enough force is applied.
Metal against metal is generally a “slip” situation. The force required to move one relative to the other is dependent on (A) the pressure applied forcing one face against the other and (B) the coefficient of friction between them. The size of the area is irrelevant.
In the case of the Gen3 vise, making the contact points larger would have added no benefit and simply would have made adaptability to various contours more difficult. Making the contact points smaller might have increased the point-contact pressure enough to have left marks on the blade. All in all, well done.
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Bill: You are right about the included angle being the same even if the blade is tilted to one side or the other. The need to keep it centered lies in (A) the desire to have the bevels appear to be equal and (B) the need to be repeatable – able to find and match the bevels without going thru and extended Sharpie-matching process. I use the “measure and compensate” method every time I touch up my Delica 4 and never fail to hit the angles within 0.1 degrees.
Which brings up your statement about angle cubes not being accurate. You apparently have a bad one, as 15 degree variability is 10 times anything that I would consider bad. My Imaging AngleCube will never be off by more than about 0.1, when used properly. The video above demonstrates the right way. Place the cube against the stone, take a reading after about three seconds, then tip the cube away and replace it, taking another reading. Do this at least three times and choose an angle midway between the highest and lowest readings. If any of my readings falls more than 0.2 degrees from the opposite extreme, I assume the readings are invalid and start over, after trying to eliminate anything that might have contributed to the excess. Lifting and replacing the entire stone, cube affixed thereto, adds the variability of the system mechanics and your technique (how much and where you apply pressure) to the mix. This reduces the effectiveness of your ‘cube and its potential accuracy.
As to the question of how accurate your angles must be, I can say that errors of more than 0.1 degrees are easily seen after the first few strokes. You’ll see that the new scratch pattern seems to not touch the entire bevel. There will be residual scratches from the previous grit showing, either at the bottom of the bevel or at the apex. Such an error will just require additional work to remove those scratches made by a coarser grit.
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Alan: This isn’t a math puzzler by any stretch of the imagination. Any child who’s had 5th grade arithmetic should be able to execute the simple math here.
- Measure the angle of each side of the blade.
- Add the two numbers; result is the included angle of the blade’s primary grind.
- Then divide this included angle by two; result is blade’s centerline.
- Subtract the smaller of the blade side angles from blade’s centerline; result is the lean angle (How the blade leans to that side)
- Subtract the lean angle from your intended sharpening angle; result is rod angle for that side.
- Add the lean angle to the intended sharpening angle for the opposite side; result is the rod angle for that side.
Easy peasy.
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Thanks Tom, I understand everything you say, but even in Josh’s vid.. he is >>??????? UT OH,, I made a mistake.. just realized it.. my cube can be off by 0.15 to 0.30 degrees, not 30 degrees.. You see I’m use to dealing in absolutes, and also learning the touch technique ( the feel ) much like learning the touch, and feel of a micrometer or a caliper. I can repeat those readings blindfolded, and be accurate to a tenth (a ten-thousandth). ( 1/10000 ) or 0.0001 in.
When you have a radial edge with no stable touch point ( No two point contact ) your placement can be off giving you variable readings.. like I said, I’m trying to decide where to apply rocket science, and where to apple stick sharping science. Its just all new to me.
The one thing is my knives are now so sharp its scary. One thing I’ve really never been able to do in the last 55 years.
Thanks for taking the time Tom
One other thought, I use to teach Engineering co-ops in their fourth and fifth year of engineering school, where they did six month classroom and six month in practical application. And in my teaching , I always remembered one of my College Prof saying, “Its easy , if you know how to do it” ,
You have to put your teaching at the level in which your students can absorb it. As a teacher, I could always see those who didn’t get it, so I adjusted , slightly, my delivery and hit it from a different angle. The great thing about being as teacher is when you can see the light bulb go off in your students face. The Point where they suddenly ” Get It “. That’s the biggest reward about being a Teacher.
Also to Alan, there is nothing wrong with saying you don’t understand something… I do it all the time, and I have two degrees. There are no apologies necessary for not knowing something. As another Proff of mine once said, ” It not as important to try to know all the answers as it is to know where to find the all the answers.
Asking for help is the smart thing to do, and continue to do until you get it.. Don’t be too proud to ask.. Seeking knowledge is an intelligent and admirable pursuit.
Bill aka ET
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When you have a radial edge with no stable touch point ( No two point contact ) your placement can be off giving you variable readings..
I guess by definition, what you’re describing is not a full flat grind (FFG). Most of the knives I sharpen are not FFG. If it’s not FFG, there’s no point in measuring the face angles, as they differ from the facets at the vise contact points. But the objective is to set your rod angles so that they are aligned with the centerline of the blade, regardless of the grind. Do what you gotta do.
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