Help with Flat Grind

Outdoor Gear Dad

Outdoor Gear Dad

Joined
Jun 1, 2019
Messages
211
Hello all, I am trying like hell to get a full flat grind down and I am riding the struggle bus hard. Here is what I am doing.

First things first I am using a jig...Relatively new and just feel more comfortable. I am scribing 2 center lines, 20 thousands apart. Grinding 45 degrees into the scribe line Freehand. Then I throw it on a jig and calculate my angle using Dan Comeau Angle calculator....What I am left with though is a saber grind. In theory the angle should take the grind down to the spine, but I hit the center line before I am anywhere close. Is my angle wrong? Do I need to just keep walking the angle down more?

Any advice would be great.
 
Can you confirm that it is the angle you think it to be relative to the platen (digital angle gauge)? Also with your jig is it possible you are applying much more pressure towards the edge than the spine? Even in a jig if it’s not totally rigid that’s possible. Otherwise maybe a more experienced maker will have an answer. Oh and if you tell me thickness of blade and grind height desired I could confirm angle, just a little geometry math.
 
imill3567 said:
Can you confirm that it is the angle you think it to be relative to the platen (digital angle gauge)? Also with your jig is it possible you are applying much more pressure towards the edge than the spine? Even in a jig if it’s not totally rigid that’s possible. Otherwise maybe a more experienced maker will have an answer. Oh and if you tell me thickness of blade and grind height desired I could confirm angle, just a little geometry math.
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I use a digital angle guide to calculate the angle. I used a 2.9 degree angle on a 1/8 blade for a height grind of 1.25”
 
Drop the angle guide for now. How I learned was to start with your off hand. I’m a righty so I start with my left hand. Grind to the top. Slowly check you final edge grind. I also work the blade in by the tip giving a distal grind. Once you resort of close , go to your other hand. Try to match it to your non dominate hand grind .. I use a 60-120 grit progression . A reall thick or long blade. It’s 36-60-120G . Once you get the hang of free hand grinding you’ll be fine with FFG grinding. Good luck!
 
Did you subtract the 20 thou? I get a height of just over 1" for 2.9 degrees when subtracting the the 20 thou, and 2.4 degrees should get you to 1.25".
 
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Rhinoknives1 said:
Drop the angle guide for now. How I learned was to start with your off hand. I’m a righty so I start with my left hand. Grind to the top. Slowly check you final edge grind. I also work the blade in by the tip giving a distal grind. Once you resort of close , go to your other hand. Try to match it to your non dominate hand grind .. I use a 60-120 grit progression . A reall thick or long blade. It’s 36-60-120G . Once you get the hang of free hand grinding you’ll be fine with FFG grinding. Good luck!
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Hubert S. said:
Did you subtract the 20 thou?
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No but would that make that much of a difference? Y’all think I should drop the jig?
 
Outdoor Gear Dad said:
No but would that make that much of a difference? Y’all think I should drop the jig?
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Not sure I did the math right, I just punched it in a calculator and got an 0.5 degree difference. It appears to be quite sensitive to small changes. I would lower the angle of the jig and rely on what you observe vs. trusting a spreadsheet or some random guy on the internet like me doing math...
 
I agree the angle should be 2.4 and that will make a difference but freehand is always an option. I have started to do it on larger or odd shaped knives. It is a learning process and my off hand is still only alright.
 
Obviously geometry calcs do work but I suggest putting that aside. You already did the 45 degree pass to the scribe line, now just walk it up to the spine. The only tough thing is making a nice clean transition and straight bevel if you leave some flat area, but for full flat, it is hard to go wrong.
I find it easier to get the grind more or less complete at 36 grit, then just run quickly through the higher grits to smooth it.
 
While there is nothing wrong with using a Jig, I taught myself to grind freehand and found it not only made my grinding technique better, but I also find it to be almost zen-like and relaxing, but then again I do it as a hobby :)
 
navman said:
While there is nothing wrong with using a Jig, I taught myself to grind freehand and found it not only made my grinding technique better, but I also find it to be almost zen-like and relaxing, but then again I do it as a hobby :)
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It’s a hobby for me too, so I really have nothing to lose. Thanks for the tips
 
Even with a jig, trigging out the angle is just a starting point because you don't have complete control over all of the variables going into the grind profile. It seems like it's just angle and depth of grind, but it's not. You're manually manipulating the jig and could be putting uneven pressure towards the edge. Your belt may be binding slightly at the edge and billowing which causes the edge to grind faster and actually produces a slightly convex bevel instead of flat. You could be resting too much weight on your workrest and putting flex into it which changes the angle relative to the platen. You may have a worn platen that's dished out in a way relative to the height of your workrest and jig that causes a convex profile.

Just walk it up until it's where you want. The math doesn't matter.
 
"Just walk it up until it's where you want. The math doesn't matter."

Bingo. Add .1 or .05 to whatever setting you're on till the grind line reaches as high up the knife as you want.
 
kuraki said:
Even with a jig, trigging out the angle is just a starting point because you don't have complete control over all of the variables going into the grind profile. It seems like it's just angle and depth of grind, but it's not. You're manually manipulating the jig and could be putting uneven pressure towards the edge. Your belt may be binding slightly at the edge and billowing which causes the edge to grind faster and actually produces a slightly convex bevel instead of flat. You could be resting too much weight on your workrest and putting flex into it which changes the angle relative to the platen. You may have a worn platen that's dished out in a way relative to the height of your workrest and jig that causes a convex profile.

Just walk it up until it's where you want. The math doesn't matter.
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Just walking it up seemed to work. It’s not perfect but a lot better! Thanks a ton.
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