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 Post subject: Angle Cube zeroing - question for MadRookie
PostPosted: Mon Oct 21, 2013 12:16 am 

Joined: Wed Feb 20, 2013 2:22 am
Posts: 695
OK, this has been keeping me up at night for months now. And yes, it is super far in the weeds, has no practical effect on knife performance, blah blah blah.

In answer to the eternal question whether to zero the Angle Cube on the blade table or on the knife, MadRookie has espoused zeroing on the knife to take the grind of the knife into account (assuming a flat ground knife). Makes total sense.

To quote the esteemed MR,

"Zero the angle cube on the knife blade while it is in the sharpening position on the blade table.

I call this method the "True Angle Cube" edge and log it in my database as TAC.

Found the angle to vary from as much as two to three degrees lower, than using the conventional method without taking the slope of the knife blade into consideration."

and,

"Once you have measured the difference in angles for a particular knife/blade, add the difference in degrees to the normal method of zeroing on the blade table and adjust accordingly.

eg: my Suisin 240 is sharpened to a true 13* edge, measured by zeroing on the blade itself.

This equates to a 2.7* difference (less, zeroed on the blade table), so I just add the 2.7 to 13 & set the angle arm to 15.7* by zeroing on the blade table.

So now you have a true 13* angle without the mission of zeroing on the blade."

What's eating me is - Assuming the blade is perfectly symmetrical along the longitudinal axis, wouldn't one want to measure at the half angle, which is the difference between one blade face and the blade centerline?

Let's say you zero the Angle Cube on the blade table and you then place the knife on the blade table and see a 4* reading on the angle cube. The centerline of the knife would thus be moved up 2*. So for a desired 15* bevel angle from centerline you should set the stone arm at 17*.

Is everybody moving their hands about like Kung Fu masters yet?

Opinions welcome. And of course, this is no slam on MadRookie. If we lived on the same continent or even hemisphere I would buy him a liter of my favorite rum in payment for all he has taught us. (Lamb's Navy 151 BTW)

Cheers,

Rick


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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Mon Oct 21, 2013 4:28 am 
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"Is everybody moving their hands about like Kung Fu masters yet?"

...hahaha....yeah, exactly what I was doing......

I think the short answer is we are talking degrees PER SIDE in my method.

You can test this by marking the already sharpened bevel with a sharpie - then zero the AC on the blade (while the blade is resting in the sharpening position on the blade table) - then adjust the angle arm with a fine stone inserted until you make 100% full contact with the marked sharpie edge (check with microscope) - then measure this angle again with the AC placed on the stone holder arm with the stone still inserted and resting on the sharpened bevel.

....but I am still doing the kung fu thingy......

PS: The wife just walked in, took one look and asked if I am feeling ok......

:)



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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Tue Oct 22, 2013 12:34 am 

Joined: Wed Feb 20, 2013 2:22 am
Posts: 695
Well, I'm no Leonardo da Vinci, but I think the attached jpg describes my thought process. (Blade grind is exagerated and I took the EP blade table right out of the equation.)

No comment on the rum? I'm guessing that at 0528 SA time rum was the furthest thing from your mind.

Cheers,

Rick


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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Tue Oct 22, 2013 5:35 am 
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....oh no man....I'm back to doing the kung fu thingy again....

:)

Ok, I have some serious thinking/analysis to do here.....

:)



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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Wed Oct 23, 2013 3:50 pm 

Joined: Wed Sep 12, 2012 5:21 am
Posts: 76
I just re did a couple customer knives, a big scimitar and a nakiri. 15 degree angle measured at the center of the blade on the table was 15.40 with the stone resting on the table. I keep a log book for consistancy as MR has suggested to save some time on repeat knives. Is this what we're talking about or am I missing something?


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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Wed Oct 23, 2013 4:11 pm 
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Mrmnms - no, I believe what they're describing is:

A) Zeroing the angle cube on an empty blade table, where the knife would be, then placing the knife, setting the angle for proper sharpening (whatever that happens to be) and then measuring the angle of the rod with the stone resting on the blade edge.

vs.

B) Placing the knife on the blade table flat or pressed down in the sharpening position, then zeroing the angle cube on the knife itself, setting the angle for proper sharpening, and then measuring the angle of the rod with the stone resting on the blade edge.

The offset will essentially be the angle that the knife itself is introducing into the equation.

Rick and Madrookie's described shortcut method (which is great, by the way) is to:

1) Zero the cube on an empty EP, then place the knife in the sharpening position, placing the angle cube on the knife in the proper position that you tend to use when measuring, and seeing the difference in angle between the two.

2) Use that information to make it easier to set repeat angles on a given knife (with a given stone reference) using the blade table to zero, instead of trying to zero on the knife itself each time.

Now for the fun part :-).

I think Rick's question and analysis is spot on from a purely geometric/mathematical point of view. If you had an imaginary knife with a pure wedge shape of let's say 4 degrees from spine to edge as Rick suggested in his example then...I agree with Rick that true angle of the EP to a particular point on the blade edge when sharpening, based on the blade center line would be an offset of 2 degrees, or half the measured offset.

I think the water gets cloudy, though, when we get in the real world of knife geometry with non-50/50 side grinds and convexing, etc. making exact measurements difficult, depending on how the blade is held to the table, and if it's held consistently throughout the sharpening process, from session to session. An to make it even more interesting, the angle will change with the distance that the blade edge is hanging over the end of the blade table, further away (bigger knife or more overhang) = shallower angle and vice versa.

My conclusion is that if you're trying to match an existing bevel, or you're just sharpening to an approximate angle that works for your intended purpose, Madrookie's method makes perfect sense and is easier to execute. If the bevel works for that knife, it really doesn't matter if, say, a measured angle is 15 degrees, but the true angle to centerline is 13 degrees. Also, if the knife is not perfectly symmetrically ground on each side, in a manner that translates into consistency on the EP, then the true angle of the bevel on each side to the blade center line will vary a bit.

If your intention is to truly set a bevel that is exactly, say, 15 degrees to the knife center line - and if the knife blade is pretty symmetrical, then I'd say, yes half the offset and move the rod/stone angle up by that amount in addition to the 15 degrees. In the above example - 17 degrees.

IMO you can also help repeatability and consistency by keeping your blade edges positioned to a similar amount of overhang off the blade table edge - both from knife to knife and session to session.

Damn - my head hurts now! Two things keep swirling in my head:

That song "Kung Fu Fightin'" and Eddie Murphy saying: "GI Joe with the Kung Fu grip" in Trading Places.


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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Thu Oct 24, 2013 2:57 am 
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T, D And S , I got what you were getting at in your first post , I have been putting some idle brainpower into this over the past couple days and Ithink that your onto something. Heres my input...

I just zero my Angle Cube on the knife sitting on the sharpening position on the EP , then set my angle arm accordingly. It kind of takes the knives grind out of the equation for me. I don't think that it is precise as going from the centerline , But I just measured my one pocket knife. Which is 4 degrees. So I have been calling this knife in at 10 DPS (20 inclusive) where in reality it is 12 DPS (24 Inclusive).

Im going to have to compare this to the MR method and see how much it is out by. But honestly I think that in either case it is negligible....



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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Thu Oct 24, 2013 6:10 pm 
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Exactly Sadden! If you're happy with the edge at a given angle, then it's really just a reference number to use for consistency. I think Madrookie's take is that he likes zeroing on the blade table, so he uses an offset for each blade.


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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Thu Oct 24, 2013 7:40 pm 
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SteveG wrote:Exactly Sadden! If you're happy with the edge at a given angle, then it's really just a reference number to use for consistency. I think Madrookie's take is that he likes zeroing on the blade table, so he uses an offset for each blade.


Correct - exactly that - makes life a breeze.....

My analality has its limits...


:)



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 Post subject: Re: Angle Cube zeroing - question for MadRookie
PostPosted: Mon Oct 28, 2013 2:01 am 

Joined: Fri May 11, 2012 12:17 am
Posts: 132
Zero the cube on the knife on the table. Remember, if you can't keep the knife on the spine, zero in the position that is most comfortable to keep the blade solid.


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