Another question about the burr...

[Starfish]

Thanks to all who helped out in this thread where I asked about different techniques for detecting the existence of a burr. While practicing freehand sharpening, I find that it is difficult for me to feel the burr when I am grinding the primary bevel. However, when I grind the secondary bevel, I get a burr that I couldn't miss if I tried.

Why?

Thanks,

Matthew

 

[djolney]

Maybe because of the higher angle.

You've already thinned the edge, so force applied at the higher angle moves more steel.

 

[HoB]

Are you grinding the primary bevel all the way to the edge? If not, that is your answer right there. If you are trying to grind the primary bevel to the edge you are grinding at, what, 5-7 deg. per side? At such a low angle it takes a lot of time to really get to the edge. You always think you are already there, because the blade is so thin behind the edge, but in reality you still have some more work to do. Some time ago I calculated that it takes about 1.8 times more work to reprofile a secondary grind that is initially 22 deg to 12 deg than to take it to 15 deg. and 8 times more than to take it to 20 deg. At low angles that can add up quickly.

 

[Starfish]

Are you grinding the primary bevel all the way to the edge? If not, that is your answer right there. If you are trying to grind the primary bevel to the edge you are grinding at, what, 5-7 deg. per side? At such a low angle it takes a lot of time to really get to the edge. You always think you are already there, because the blade is so thin behind the edge, but in reality you still have some more work to do. Some time ago I calculated that it takes about 1.8 times more work to reprofile a secondary grind that is initially 22 deg to 12 deg than to take it to 15 deg. and 8 times more than to take it to 20 deg. At low angles that can add up quickly.

HoB,

I think I did not use sufficient detail when I said "grinding the primary bevel." That's what happens when I try to post at the end of a long day :yawn:

At any rate, during this first real foray into sharpening freehand with benchstones, I have limited myself to either cleaning up the primary edge bevel or slightly reprofiling the edge bevel. As I am sure you and a significant portion of the experienced sharpening hands are aware, it is not unusual to come across a production knife with an uneven edge bevel. I am just now discovering this for myself via the magic marker trick...

I have felt the burr...I'm pretty sure...when cleaning up the primary edge bevel. I KNOW that I have felt the burr when I lift the spine up a little and put in the secondary edge bevel.

I guess what I'm really wondering is if the magnitude of burr formation is dependent in some way on the angle at which the edges meet.

Thanks for your help,

Matthew

 

[Quiet Storm]

Hey, if you got a new knife with a tiny burr near the tip, you could always pull off a "Vinny" and send it back!

j/k

 

[HoB]

Ok I understand now, I usually use Joe Talmadge's definitions. I call the main bevel of the blade the primary bevel, the edge bevel the secondary bevel and everything on top of that (by lifting the back of the edge a bit) a microbevel.

The burr is the steel at the very edge that got so thin during the sharpeing process that it bend away from the stone and doesn't get cut off anymore. So in principle, you can get a larger burr at a low angle (because the edge stays thin deeper into the blade) than at high angles, but by the same token, it takes much longer to build the burr and it doesn't bend over as much so its a bit harder to feel. Best think is probably is to take a protractor (or a ruler at least) and a piece of paper and draw an acute wedge with a pencil on the paper. Now you erase the tip of the wedge and round it of a bit. This is now a blow up of the edge of a blunt knife. Now you draw pencile lines at the angles at which you are grinding. You see imediately the difference in material you have to remove at different angles. The pencile line also show you were the burr ends up: along that "grind line" bending a bit away from the stone.

Don't know how to better explain it. Hope it helps a bit. I started of by drawing a lot on a piece of paper to visualize what is going on during sharpening. You can play around with different angles and see whats happening. Good luck.

 

[Starfish]

Ok I understand now, I usually use Joe Talmadge's definitions. I call the main bevel of the blade the primary bevel, the edge bevel the secondary bevel and everything on top of that (by lifting the back of the edge a bit) a microbevel.

Thanks HoB - I thought maybe we were having a definitions problem. As an engineer, I should have been more careful to define my terms up front

An even better idea would be to start using the generally accepted definitions!

Off to go chew on your explanation a bit....

Matthew