ZDP-189 and EdgePro

R

RipD

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It's sharp but not as sharp as it should be. Doesn't cut news print easily and is just ok on the Murray Carter 3-finger test.

Spyderco Stretch with ZDP-189. It had a ding in the blade edge (small gap) and the edge overall didn't look great. I used a 150 diamond stone on it (minimally, to get rid of the gap) 500 diamond lightly followed by 500, 1000, and 4000 Shapton Glass. Edge looks good under a loupe. Even shape and angle on both side, no burr.

Similar treatment on other knives gets them very sharp. Do I need a different approach for ZDP-189? Is 1K t0 4K stones too big a jump on this steel? Seems like I'm missing something fundamental.
 
ZDP-189 likely won't hold much of an obvious burr, if it's hardened like most blades in this steel, to low-mid 60s HRC. If much of a burr does form, it breaks away quickly, as the steel is very brittle at this hardness. I think that same tendency toward brittleness makes it harder to refine, as a superfine hair-popping edge is pretty fragile in this steel.

I'd suggest going back to the 500 diamond only, and apexing with that. Finish with an extremely light touch, and carefully watch for the first traces of a burr; use well-lit magnification there. Avoid the 150 diamond, as chipping issues at coarser grit could be a problem with ZDP-189 (a chip might've been the 'ding' you saw in the edge). Check sharpness after the 500 diamond, and see how it compares to the edge as it was finished on the waterstones.

Might also be an edge angle issue, if the edge is somewhat obtuse. My knives in ZDP-189 (Spyderco, Kershaw) didn't impress me much for sharpness at their factory angles, which were somewhere close to ~ 40° inclusive or so. I thinned the Kershaw (Leek) down to about ~30° inclusive, and it cuts much better there. Have to be careful with it, in making it thinner, due to the brittleness of the steel.
 
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Obsessed with Edges said:
ZDP-189 likely won't hold much of an obvious burr, if it's hardened like most blades in this steel, to low-mid 60s HRC. If much of a burr does form, it breaks away quickly, as the steel is very brittle at this hardness. I think that same tendency toward brittleness makes it harder to refine, as a superfine hair-popping edge is pretty fragile in this steel.

I'd suggest going back to the 500 diamond only, and apexing with that. Finish with an extremely light touch, and carefully watch for the first traces of a burr; use well-lit magnification there. Avoid the 150 diamond, as chipping issues at coarser grit could be a problem with ZDP-189 (a chip might've been the 'ding' you saw in the edge). Check sharpness after the 500 diamond, and see how it compares to the edge as it was finished on the waterstones.

Might also be an edge angle issue, if the edge is somewhat obtuse. My knives in ZDP-189 (Spyderco, Kershaw) didn't impress me much for sharpness at their factory angles, which were somewhere close to ~ 40° inclusive or so. I thinned the Kershaw (Leek) down to about ~30° inclusive, and it cuts much better there. Have to be careful with it, in making it thinner, due to the brittleness of the steel.
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I dont think edge angle would explain why he cant cut news print. I keep many of my knives at 40 inclusive and they easily pop hair and do curvy cuts in phone book paper. I think i would rather keep zdp more obtuse for the sake of edge stability.
 
microtech85 said:
I dont think edge angle would explain why he cant cut news print. I keep many of my knives at 40 inclusive and they easily pop hair and do curvy cuts in phone book paper. I think i would rather keep zdp more obtuse for the sake of edge stability.
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Any other, more typical, steel, I'd agree with you. Newsprint-slicing is pretty straightforward, with a crisp apex at even relatively wide edge angles. But ZDP-189 is a completely different animal, in keeping the apex crisp while refining it. The finer it gets at the apex, the more vulnerable it is to chipping and crumbling away with a slightly too-heavy touch on the hones. So, any issues with a heavy touch on the hones, especially ceramics, can dull the apex in a hurry. Making the apex crisp, in the first place, is what's more challenging with ZDP-189. If the apex isn't crisp, paper-slicing at 40° or higher basically stops in it's tracks. At thinner geometries, paper-slicing will tolerate some more dulling of the apex before the cutting stops altogether.

The fact that the OP has already mentioned seeing how different it behaves, as compared to other steels given the same treatment, is testament to the above points, and matches exactly what I've seen, and others here as well, in sharpening ZDP-189. A common complaint has been that it's difficult to detect burrs on ZDP, because they immediately break off instead of just rolling over (rolling makes them easy to detect/see/feel). So, with no burr yet seen, many just keep grinding (and grinding and grinding) looking for the burr, which seemingly never comes. That's why it's harder to refine, unless one is aware of the steel's tendencies.

I'm not a big fan of ZDP-189, for essentially these reasons. It's typically heat-treated to very high hardness, and therefore is very resistant to abrasive wear or plastic deformation (denting, rolling, etc). But it's extreme hardness comes with a trade-off of greatly reduced toughness and a vulnerability to chipping or cracking as the cutting geometry is improved (thinned). One basically has to choose between better cutting geometry, or better edge retention at a wider geometry that never cuts as well in the first place. So, if choosing better cutting geometry, the price paid is in reserving the blade for lighter uses (no prying/twisting/poking or heavy chopping), in hopes of not chipping, cracking or breaking it.
 
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Thanks to all for the replies. Helpful, as always. I did get a burr at 500, but no detectable burr with 1K or 4K stones. I assumed there was one that I couldn't feel and did a gentle deburring with the stone toward the blade on both sides (very light pass one direction only). I was going to run it through a cork, but the loupe showed no burr at all.

I'm at 40 since that's what it appeared to be already. I'll start over with the 500 diamond and move closer to 30. Also won't expect a burr and will check more frequently with a loupe as I go. Could be I was one of those who kept cutting too much. I did get the diamonds off it quickly.

Off to read those other referenced threads as well. Appreciate you posting those, Mo2.

If I have time I'll bust out the good camera and get some pics.
 
Pay attention to David (OWE) advice, and over Spyderco forum, Evil D.
Light pressure to form the apex is key. I use diamond all the way (up to DMT EEF).
I like the steel though, as my use is light.
 
Sometimes you leave a lower grit stone with a fine apex but then make a a few mis-strokes on the finer stones that are too steep that destroy the edge to the point you are no longer at a proper apex.
 
Took another run at it this morning but got interrupted by the kids and will have to finish up later. (I hate stopping mid-way on a sharpening.) I put a 30 degree bevel on it which looks great. Will contemplate staying with it or doing another bevel at 35 or so. I always use a light touch with the EP. I'm going extra light on this steel now.

Btw, I'm using an Angle Cube for my angles measuring blade on the table and EP stone. Also using a 20x lighted loupe. No bragging since I generally suck at sharpening, but the EP can deliver a perfectly even bevel all the way across the blade. Always amazed at how even and precise the bevel looks off the EP.

And... I didn't tape the blade b/c I'm was being very careful and cleaning the blade and the table as I go. Still got some scratches on the blade... GRRRR... lesson learned to always use tape.
 
Ok... Reground it at 34 degrees. Use a 500 diamond very lightly to get it there, then just a few passes with 500 Shapton very lightly, then a few passes with the 1K very lightly. It was sharp, but not as sharp as I wanted. I went back with the 1K stone toward the blade only - two single passes per side, one stroke per pass (so one stroke toward the blade all the way across 2x, both sides very lightly.)

Didn't check arm hair, but it easily cuts newspaper quite well. I opted not to go to 5K because somebody may be buying it and they wanted a toothy edge.

A few lessons learned: after reading the posts on the Spydie forum I went extra light. I probably intuitively went harder the first time b/c this is very hard steel. My brain likely translated that to more pressure. I also watched a Youtube video of somebody doing ZDP-189 and they were going to town on it. I figured that's what it took. No. The lightest touch seems to work really well with this steel. Counter intuitive for me. And even the 1K stone was cutting well with no pressure at all. For the last passes I took weight off the EP arm.

Lastly, I realized that didn't reset the arm angle when I changed stones last time. Doh! It looked fine under the loupe so it wasn't way off, but could have been enough.
 
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