Strange Burr

C

Cait_Sith

Joined
Oct 5, 2010
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381
So I tried sharpening a really dull knife of CPM154 today with a guided sharpener and DMT course stone. I marked the edge with marker and started to reprofile quite a bit of metal, but there was no burr forming on the other side. I was afraid that I was just grinding too much steel, but I still continued.

After a while, I decided to just give up and try the other side, a couple passes and there was a huge burr on the other side. Is there a reason why I couldn't get a burr on the first side with so much grinding, and yet have a substantial burr on the other with minimal time?
 
A burr doesn't form until you reach the apex and "spill" the steel over it. You were grinding down lots of steel, but there was no edge. You run the risk of having an off center edge with too much heavy removal on one side and then building a burr really quickly.

-X
 
Is there a chance that the grind is not symmetric?

EDIT: Actually picture might be better.
 
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I'm betting there's an extra bevel (or microbevel) between the primary shoulder and the edge on one side. If so, and if you're grinding above that extra bevel, you haven't reached the edge yet, and therefore you're not producing a burr when grinding from that side. The grinding on the other side, as was indicated by the quick burr formation, has reached the edge.

This can be verified by inspecting the edge with a magnifier under very bright light.

Please excuse my lack of artistic ability ( :eek: ), but here's an illustration. Black arrows indicate primary bevels, red indicates the extra bevel on one side, and green highlights the apex and the burr formed on one side. Assuming one is grinding on the primary bevels, no burr will be formed at the apex while grinding on the side with the extra bevel, until that extra 'shoulder' is completely ground away and the primary bevel intersects the cutting edge (apex):



David
 
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It's strange. Let's say I was grinding a lot on side A (which now has a much larger relief edge showing sigh) and it built no burr. Yet on side B, the burr formed so fast (so it has a shorter relief edge sigh again). But when I switched back to side A, the burr formed super easy. I was able to get a sharp edge, I'm just confused why it took so much effort with side A initially.

Everytime I look at it, it's disappointing to see the relief edge so high up on one side.
 
This is exactly why I do pyramid steps in sharpening. If I sharpened a new knife with no edge, and just went unil I got a burr on one side, it would have a chisel edge. Counting strokes is important when reprofiling or bevel setting so you don't end up with an off center edge like you did.

Working until a burr forms is fine with an established edge. A very dull knife does not have an established edge.


Consider a flat bar of steel, you want to sharpen. The end is 90*, a normal cut. You want a centered V edge on it. If you start grinding until a burr formed, you would have taken the bevel all th way down to the other side of the bar. Then when you flip it over to work the other side, the burr forms very quickly because the apex is on that side, a chisel edge.

Counting strokes allows you to remove equal amount on each side pogressively, until they meet in the middle. Do not continue after your specified amount of strokes to reach a burr, and do not stop early because a burr was formed. Stick to your determined amount of laps/strokes to keep it centered. If after all your laps are done and you still don't have a burr, start again.

Hth.

-Xander
 
Ah thanks a lot for the information. I think in the future I'll take it slow and work both sides at a time until the apex is reached. In the meantime, I'll start using the knife and just focus on shifting the sharpening to the other less ground side.
 
Another possibility is assymetrical primary grind. With this, any symmetrical edge will result in one side of bevel wider than the other. Imagine one side is 10 degree, the other 15. A 20 degree edge will be wider at the 15 degree primary.

Sorry, on phone, can't provide a diagram
 
The shallower angle will have more 'abrupt' change when the 20º edge applied, so the edge bevel will be narrower. A pics might make it easier, but again, sorry, am using the mobile devices (now on iPad, the Kingston keyboard can type 'º' using alt-0 ;))

Let me try: a | being sharpened with / compared to a / sharpened with / (almost paralel, thus the bevel will run almost full lenght).

Learnt this from my experience having some asymmetrical blade.
 
Cait_Sith said:
Wouldn't a shallower angle have a wider bevel?
Click to expand...

A wider bevel on one side of the blade will result after taking the thicker side of an asymmetrical grind to a more acute angle than it was before, even if that finished angle is the same as on the other side. More metal has to be removed from the thicker side to achieve that result, so the bevel produced would be wider than seen on the more thinly-ground side of the blade, at the same resulting edge angle. In other words, even if the edge angle is identical on both sides of the finished edge (relative to the centerline of the blade, drawn from spine-to-edge) , the side of the blade with the thicker primary grind will still have a wider finished bevel than the other side's bevel, as measured from the edge apex to the shoulder of the bevel.


David
 
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