Convexing on a belt sander... how?

[REK Knives]

so I got my craftsman 2x42 in the other day and been having a blast with it! i took the platen off of it so that way the belt will flex more to give me more of a convex edge. the belts I have are 2x42: 3m trizact gator i have 80x, 120x, 240x, 400x, 600x, then i have 2 of the aluminum oxide kingspore belts in 220 and 500 grit, along with 2 2x42 leather belts. i'm also waiting on some 1x42 belts that will bring the polish down to 6 microns, at which point i have some ultra fine compound (pink i think) which i will use to finish my edges on the edges i want to have that high of a finish.

while i am grinding the blade on the belt, it does curve in around the edge (i try to push a little bit so it will convex the edge), but ultimately it looks pretty much like a straight edge.... especially on small knives or pocket knives. do you guys have any tips so i can get my edge more convexed? was wondering if i use my higher grit belt (so it won't remove hardly any metal) and push the edge into the belt more , if it would remove the shoulders and make the edge more convexed? thanks guys!

 

[Noctis3880]

I think traditionally you would adjust the belt tension so you can get more slack on the belt without mashing your edge into the belt(which can build up more heat or screw up your grind). If your current belt sander doesn't allow for that, I'm sorry to say that you might need a new one.

 

[230grains]

If there's flex to the belt, its convexing. It may not appear it on thinner pocket knives because it's not needing to remove as much material above the edge. A lot of people when they think of convex they think of wide shiney bevels, which is usually the case. Here's an example of a traditional convex zero grind I did on my kalamazoo 1x42.

Here's a V edge with a bevel so wide it looks convex. This was done freehand on DMT stones.

This edge however IS convexed, with a much smaller bevel than the above knife, because less material needed to be removed above the edge. Done on my belt sander.

 

[REK Knives]

thanks for the pics 230! nice!

so is it normal for you to not really notice the convexing on a thinner blade (like a pocket knife)? i have the craftsman 2x42 belt sander with a spring tensioned top wheel. therefore, if i press into the belt, the belt will flex more and thereby convex the edge more. so what i was wondering is - should i just up the polish (i.e. got to a lower micron belt) and push into the belt more which would give it more of a convex edge?

 

[tabeeb762]

If all you want is a more pronounced convex you could try grinding two or three bevels.
One at a low angle, another at a higher angle, and later blending them together.

 

[230grains]

No need to use a lot of pressure, if there's any slack in the belt and you're not grinding against the platen, you're convexing it. If you use too much pressure you're going to start rolling the edge over with the belt and it's going to look shiney, but the apex is going to be rounded. Take it up to a higher micron depending on your preferences for edges, I happen to like high polish, others like a toothier edge. My spyderco military is convexed, but if you look at it, it looks like a v edge as the bevel, although ground fairly thin, is not very wide.

thanks for the pics 230! nice!

so is it normal for you to not really notice the convexing on a thinner blade (like a pocket knife)? i have the craftsman 2x42 belt sander with a spring tensioned top wheel. therefore, if i press into the belt, the belt will flex more and thereby convex the edge more. so what i was wondering is - should i just up the polish (i.e. got to a lower micron belt) and push into the belt more which would give it more of a convex edge?