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how exactly do you grind a convex edge?
- Thread starter obsessis
- Start date
- Joined
- Oct 30, 2002
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- 3,974
The best way that I've found to get a nice convex blade is to start with a partial flat grind on the platen. That will establish your geometry, and you finish it off on the slack belt by placing your flat against the belt and tipping the edge side of the blade more firmly into the belt to remove more material and work down to a cutting edge.
What you may want to avoid is starting with a full 1/4" thick blade and trying to go straight onto the slack belt. I've found that what often happens is that you will get a near full thickness of the blade through the majority of the width of the blade and a very obtuse convexing to the edge. That works well for axes and heavy choppers, but isn't good for much else. By starting with a flat grind first, you can avoid that. How far you take your flat grind before convexing is up to you and the intended purpose of the knife.
--nathan
What you may want to avoid is starting with a full 1/4" thick blade and trying to go straight onto the slack belt. I've found that what often happens is that you will get a near full thickness of the blade through the majority of the width of the blade and a very obtuse convexing to the edge. That works well for axes and heavy choppers, but isn't good for much else. By starting with a flat grind first, you can avoid that. How far you take your flat grind before convexing is up to you and the intended purpose of the knife.
--nathan
- Joined
- Jan 19, 2007
- Messages
- 123
Thanks to Nathan, I now have a good mental picture of how to do this. I myself was also wondering how to do a good convex edge. I received Charlie's knife and it has a nice convex edge on it. Just couldnt quite figure out how it was done.
Thanks Nathan,
Bryan
Thanks Nathan,
Bryan
- Joined
- Oct 30, 2002
- Messages
- 3,974
Glad I could help.
Of course, you could just buy a KMG and a rotary platen and make some really sick convex blades.
That reminds me...I need to pick up a rotary platen...and a MAP tool arm...and a forge...and build a PID setup for the forge...and buy a real heat treating oven...and someday get a mill/drill...and learn to forge. Oh well. Something to keep me busy and poor 
.
--nathan
Of course, you could just buy a KMG and a rotary platen and make some really sick convex blades.
That reminds me...I need to pick up a rotary platen...and a MAP tool arm...and a forge...and build a PID setup for the forge...and buy a real heat treating oven...and someday get a mill/drill...and learn to forge. Oh well. Something to keep me busy and poor .--nathan
- Joined
- Aug 6, 2007
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- 5,060
Nathan, I do the same thing but I always end up with a small strip of material in the center of my fat ground bevel that refuses to touch a slack belt. How do you solve this?
What I mean is, I will go from 80 grit to 120, and at the ridgeline there will be 120 grit scratches and along the edge there will be 120 grit scratches but in the center there is still 80 grit scratches.
What I mean is, I will go from 80 grit to 120, and at the ridgeline there will be 120 grit scratches and along the edge there will be 120 grit scratches but in the center there is still 80 grit scratches.
- Joined
- Oct 30, 2002
- Messages
- 3,974
I've found that happen as well, Sam. What I've done in the past (don't do many convex blades anymore) is make sure my belt is tensioned tightly, and then very gently apply the flat of the blade to the slack belt. What happens is that no matter how hard you push into the belt, the top and bottom edges of the blade will always contact the belt more as the belt itself is pushed into a concavity. By very gently touching the blade to the belt, you decrease the amount of concavity in the belt and increase the contact with the blade center. I've also taken a smooth piece of scrap leather and gently pushed the belt into the knife from the non-abrasive side of the belt just to remove those stubborn low grit scratches. This will make heat, and make sure you are very careful if you are near any of the moving parts (i.e. sheaves). No loose clothing at all.
Mostly what I've done with my convex grinds were more of a convex finish rather than a true convex grind. I've seen others create a faceted flat grind, if that makes sense, with multiple flat planes that they smooth into a convex grind on the slack belt. This reduces that tendancy for the "untouched center". I've even take a blade with such troubles and taken it back to the platen and rocked the blade in horizontal to get the deep scratches out before finishing on a high grit gentle-push slack belt.
--nathan
Mostly what I've done with my convex grinds were more of a convex finish rather than a true convex grind. I've seen others create a faceted flat grind, if that makes sense, with multiple flat planes that they smooth into a convex grind on the slack belt. This reduces that tendancy for the "untouched center". I've even take a blade with such troubles and taken it back to the platen and rocked the blade in horizontal to get the deep scratches out before finishing on a high grit gentle-push slack belt.
--nathan
- Joined
- Jan 13, 2006
- Messages
- 1,655
I use my platten. At the bottom of the platten I just adjust it so the bottom wheel sets out a little. then I can hold my flat on the platten and let the flex in the belt create the convex grind.