Sebenza blade chipped by cutting Starbucks bag

N

Nenum

Joined
Sep 28, 2014
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A couple weeks ago I purchased a bag of Starbucks coffee beans at Costco. The store has a grinder near the exit of the store so I whipped out my Sebenza 25 and made a clean swipe across the top of the bag to open it. Later I noticed a rough dull section of the blade right where I swiped across the bag. Upon inspection under a 10X loupe it was evident that the edge had sustained multiple small chips. The edge was not rolled, just chipped away. The bag is just a normal Mylar material I think so I'm surprised this could happen to such a high quality steel.

The edge is precisely profiled to 36 degrees inclusive and polished using my EPA. Is it possible this angle is too acute? Or does Mylar, or whatever the Starbucks coffee bag is made of, have some weird attribute that destroys even the finest edges?
 
Not that it makes it any more acceptable or whatever, but am I right in assuming this chipping is invisible without magnification? Anyone? Some of my knives may have this kind of chipping, I don't own a magnification device of this caliber.
 
Don't those bags have a wire imbedded in some paper on the top to help you seal it? Maybe you hit the wire?
 
I would expect a wire edge would be visible under the loupe. How would you know if there is a wire edge? Is there a simple detection method for it?
 
Wanimator said:
Don't those bags have a wire imbedded in some paper on the top to help you seal it? Maybe you hit the wire?
Click to expand...

Well I made the cut about 1/2 inch below the sealed section of the bag. That could explain it but like I said the slice felt very clean. No indication of it catching on something.
 
Nenum said:
I would expect a wire edge would be visible under the loupe. How would you know if there is a wire edge? Is there a simple detection method for it?
Click to expand...

You can feel it the same way you feel for a burr, except its not as pronounced.
Just because your edge is mirror polished, doesn't mean it's a perfect apex.

However, this might not be the issue. Perhaps you did just hit the wire. I've had an edge go dull just from light pressure against metal.
 
Perhaps there were micro fractures already there and whatever u used for just knocked out what was there already ready to dislodge
 
There is no way a coffee bag could 'chip' any blade... maybe a plastic blade, but that would be about it.
 
Does that small 25 say sanrenmu on the blade? Then yeah maybe the 8cr3movish steel would chip on a Starbucks bag.
 
Try it again, see if you get the same results with before and after pics.
 
Any pics available? Are we talking like this?

images


Or more like this?

IMG_0065-2.jpg
 
It sounds to me like the problem is with the sharpening, not the steel. I would recommend starting a thread in (or having this one moved to) MT&E and providing some pictures. There are some very talented sharpeners there, that would be happy to help you resolve this issue.
 
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