Needing Serious Convex Help

Outdoor Gear Dad

Outdoor Gear Dad

Joined
Jun 1, 2019
Messages
211
Hello BF, hoping I can get some advice here as I am wasting a lot of steel (thankfully using the cheaper stuff). I have been on a mission to nail a convex blade, but I just can't seem to do things right. I recently got a Beaumont Rotary Platen. Let me explain my process and what is happening.

Ideally, what I am going for is a convex blade where the bevel goes about half to 3/4 of the way up the knife....I would like to keep some of the flat in tact.

I start by doing 2 different flat grinds and get my bevel a little lower then where I want it to end up. I usually do these flat grinds with a 36 or 50 grit ceramic belt (I post heat treat grind). Then I take it over to the rotary platen. I usually start here with a 120grit ceramic belt. But the problem is there is no "BLENDING HAPPENING." Every time I make a pass on the slack portion it adds another bevel.

So questions:

1)What are the different segements of the rotary platen and what are their purposes. Meaning, I can rotate the platen to have different slack portions...What are the advantages / differences between the shorter slack section and the longer.

2) Why is it not blending and rather creating a new bevel? How much pressure do I need to be putting on this?

3) What belt progression should I be using.

Sorry for the long cry for help, Im just frustrated. Thanks in advance.
 
I find convex to be about the easiest grind, but I do it differently. My reading of what you are doing seems difficult (keeping a bevel-to-flat crisp transition).
I flat grind about 70-80% of the way, basically reaching a full flat grind right to the spine, but leaving the edge thick.
To convex, I find I need a fairly coarse belt to actually remove much material, and not just round over the edge.
I use the "slack" section of the belt, but with as much tension as I can get.
I place the blade very close to flat across the belt, just passing under the spine (spine up), and then lean it in with only a very slight angle towards the edge.
I couldn't really do it that way if I was trying to preserve a flat above the bevel...
I go 36, 60 to get the shape, 120, 180, 320 to get it smooth.

I made a nice chopper with some guidance from Scott Gossman.
6Ixfw6v.jpg
 
Richard338 said:
I find convex to be about the easiest grind, but I do it differently. My reading of what you are doing seems difficult (keeping a bevel-to-flat crisp transition).
I flat grind about 70-80% of the way, basically reaching a full flat grind right to the spine, but leaving the edge thick.
To convex, I find I need a fairly coarse belt to actually remove much material, and not just round over the edge.
I use the "slack" section of the belt, but with as much tension as I can get.
I place the blade very close to flat across the belt, just passing under the spine (spine up), and then lean it in with only a very slight angle towards the edge.
I couldn't really do it that way if I was trying to preserve a flat above the bevel...
I go 36, 60 to get the shape, 120, 180, 320 to get it smooth.

I made a nice chopper with some guidance from Scott Gossman.
6Ixfw6v.jpg
Click to expand...
Thanks for the response...Im not opposed to having the convex go all the way up, but I was trying to go for a convex similar to how AA Forge does it.
 
I convexed my tantos by hand using EDM stones and sand paper. If you take the edge fairly thin to start, then that won't be too much work and you can keep precise control.
 
Richard338 said:
I convexed my tantos by hand using EDM stones and sand paper. If you take the edge fairly thin to start, then that won't be too much work and you can keep precise control.
Click to expand...
I think a big part of my problem is figuring out how much pressure and what sections of the rotary platen to use.
 
Easy peasy, just make a hollow platen. The hollow doesn’t need to be very tall, make it shallow also. A thick, hardened platen will also reduce vibration. I never liked the rotary platen.

Hoss
 
DevinT said:
Easy peasy, just make a hollow platen. The hollow doesn’t need to be very tall, make it shallow also. A thick, hardened platen will also reduce vibration. I never liked the rotary platen.

Hoss
Click to expand...
I’ve never heard of a hollow platen.
 
Are you grinding edge up or edge down?
I always got more control edge up and fresh belts
36 to 50 leaves some deep crap to get out later Try using 80 grit gator to start (knock off the edge with a worn belt first) Use the two closest wheels from start to finish if you want easier control and fresh belts
As you gain more control you can progress to the wider spaced wheels
120-220-400 then whatever finish grit you want
 
do you have a flat platen? take the platen off and you will have a slack belt attachment. the rotary platen seems very short, the shorter the slack, the smaller the radius. this will also depend on if you have a spring belt tensioner or a ratchet. i would think it is easier to do on a spring tensioner.
 
john april said:
do you have a flat platen? take the platen off and you will have a slack belt attachment. the rotary platen seems very short, the shorter the slack, the smaller the radius. this will also depend on if you have a spring belt tensioner or a ratchet. i would think it is easier to do on a spring tensioner.
Click to expand...
I do have a platen, and prior to getting the rotary platen I was doing exactly that. Pushing the platen back and creating a slack potion. But my results sucked, so hoped the rotary platen would fix that.

I have a spring tenstioner.

sounds like the trick to the rotary platen is learning how Much and where to put pressure.
 
Has anyone ever set up a tool test with a rotary platen, and used a slack portion of the rotary platen with a grinding jig?
 
Outdoor Gear Dad said:
Hello BF, hoping I can get some advice here as I am wasting a lot of steel (thankfully using the cheaper stuff). I have been on a mission to nail a convex blade, but I just can't seem to do things right. I recently got a Beaumont Rotary Platen. Let me explain my process and what is happening.

Ideally, what I am going for is a convex blade where the bevel goes about half to 3/4 of the way up the knife....I would like to keep some of the flat in tact.

I start by doing 2 different flat grinds and get my bevel a little lower then where I want it to end up. I usually do these flat grinds with a 36 or 50 grit ceramic belt (I post heat treat grind). Then I take it over to the rotary platen. I usually start here with a 120grit ceramic belt. But the problem is there is no "BLENDING HAPPENING." Every time I make a pass on the slack portion it adds another bevel.

So questions:

1)What are the different segements of the rotary platen and what are their purposes. Meaning, I can rotate the platen to have different slack portions...What are the advantages / differences between the shorter slack section and the longer.

2) Why is it not blending and rather creating a new bevel? How much pressure do I need to be putting on this?

3) What belt progression should I be using.

Sorry for the long cry for help, Im just frustrated. Thanks in advance.
Click to expand...
Did you try with different tension on belt on rotary platen ? That make huge difference .........I MEAN on tension on rubber belt not grinding belt
 
I might be reading you wrong, but I would think that you would get the result you want by flat grinding with a thick edge almost to where you want your final bevel to be. The flat bevel should be easier to give a crisp line. Then use the rotary platen to take the edge down, not trying to fully blend with the flats. Final operation is blending. For blending I would look at something like a Norax, or maybe better, a Gator belt A45. I would think you could influence the blend by shifting thumb pressure on the ricasso.

I have only a Coote grinder, and like full flat with a convex edge. I use the slack between the platen and the wheel, but find that there is a little too much give even there to easily get the low convex that I really like. I take my bevels to almost finished before I start on the convex, maybe not the best way, but I have to alter the lean of my grinder between flat and convex work, so don't like trying to do both with each grit at the same time. I do that only at the end, for the last blends and finishing, and expect to roll the blade a little.
 
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