Edge Pro cutting angle relation to blade table vs bevel angle

Joined
Dec 10, 2014
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I'm sure it's been brought up, but I couldn't find a discussion about this concept.

When I set up the angle on an Edge Pro, I zero out an angle cube on the blade table, and then set the cube on the bar that holds the stone. Of course there's compensation for stone thickness.

In this situation, the cutting stone's angle is based on its relation to the blade table. When I put a blade on the table, the primary bevel will always be at a more obtuse angle (or acute depending on your perspective) than the blade table. This would apply to a blade that has flats big enough to lay on the table, or a FFG blade. So what I'm seeing is the cutting stone will always be at a more acute angle with the blade than it is with the blade table. To find the cutting stone's angle in relation to the actual blade, I'd need to zero out an angle cube on the plane of the blade's primary bevel, and then compare it with the cutting stone's angle.

This would seem to apply to all guided systems really.

Of course none of this matters if I sharpie the secondary, and try to match the existing angle.

Am I confused, or does someone else see this?
 
You seem to have a good idea of the dynamics of the system.
It does require a level of intuition working with a multitude of blade grinds.
You can't expect a ffg to sharpen at the same angle as a grind with a flat that can support the blade on the deck.
The full grind will need to be calculated differently.
I log the angles when I get it right so I remember the next time.
This thread kind of needs a couple of simple pictures to make things clearer.
Maybe tomorrow I'll sharpen something.
 
You seem to have a good idea of the dynamics of the system.
It does require a level of intuition working with a multitude of blade grinds.
You can't expect a ffg to sharpen at the same angle as a grind with a flat that can support the blade on the deck.
The full grind will need to be calculated differently.
I log the angles when I get it right so I remember the next time.
This thread kind of needs a couple of simple pictures to make things clearer.
Maybe tomorrow I'll sharpen something.

I believe that even a blade with flats that can lay on the deck, the blade will be on the same plane as the deck, but the primary bevel will still be on a different plane. It's not necessarily a problem, but something to be conscious of.
 
This is a funny coincidence. Over the past week or so I've discussed this in another forum and even did a (poor) video about it! I was going to post it to my youtube channel, but I didn't like how it came out and was going to redo it.

Your idea is correct, but you're not 100% there. Let's define terms. The angle you hope to set is A. This will be referenced from the arm to the table and measured with your angle cube.

The blade itself will have a primary angle that includes both sides. You can measure this angle by zeroing the angle cube on the table and then laying the blade flat (as you will hold it while sharpening) and then measure that with the angle cube. This angle should be somewhere in the 3 to 8 degree range for most blades, but can be HUGE on some big knives. My Cold Steel Recon Tanto measures ~14 degrees! Let's call this angle P. P is an included angle. (both sides). But A is a single side angle.

Now, the angle that will be imparted to the blade, as measured from the center line of the blade, to one of it's cutting bevels, is A - (P/2). In other words, one half of the included angle of the blade body (P) is subtracted the angle of the Edge Pro.

So if you set the Edge Pro arm to 15 degrees, and then put down a blade with a 4 degree body, you will get 15 - (4/2) = 13 degrees.

Or you can do the math backwards. Say you want a 15 degree bevel on this blade that has a 4 degree body.

Arm Angle = Desired Angle + (Body Angle / 2)
17 = 15 + (4/2)

So, set the arm to 17 degrees in order to get a 15 degree true bevel angle on your blade.

I'm probably going to re-make my video and post it. Eventually. :)

I hope this helps some.

Brian.
 
The method you are currently using will give you the important elements. Consistency and repeatability.

The actual angle matters little unless it's just to satisfy your curiosity.
 
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