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I have always had very unsatisfactory results from the "extra fine" side of my diamond stone (DMT Duosharp Fine/Extra Fine w/ Hardcoat, W8EF-H-WB.) It is supposedly "1200 mesh / 9 micron" (green in DMT's color code) but it never seemed to provide the results people claim it should online. The edges it produces feel very rough to the fingernail, and I always have to go to sandpaper afterwards to get anything that looks flat instead of ragged under a loupe.

So, I did a test today, flattening the back of a 1/4" chisel on the extra fine side of my stone (stone was cleaned beforehand), and then doing the same on a 1000 grit piece of 3M imperial wet or dry sandpaper spray-adhesived onto plate glass. I believe that the sandpaper is using the CAMI grit system which means 1000 grit is ~9 micron, but even if it was using FEPA-P that just means it would be even coarser at ~20 micron or so.

As far as technique I am holding the chisel flat on the stone/sandpaper and stroking back and forth with light pressure, holding it at a 45* angle to the axis of motion, lubricating with plain water.

The difference is pretty large to me -- the diamond stone produces a very noticeably scratched surface whereas the sandpaper produces a surface that is MUCH smoother. It's not exactly a mirror surface but it is starting to show a cloudy/blurry reflection.

Back of chisel after "extra fine" DMT diamond stone: enter image description here

Now taken to $0.70 piece of 1000 grit wet/dry sandpaper on glass: enter image description here

(This difference in surface quality is similar to the difference in edge quality I experience. It is just much easier to photograph a flattening test than the edge itself. When you run a fingernail along the edge produced by the stone you can feel how rough it is!)

I understand diamond stones would cut more aggressively than silicon carbide, but this just seems like a wild difference. I can think of a lot of reasons why I might be getting this result, but first I want to ask, is this really what a 1200 mesh diamond stone should be doing to my tools?

Comparison of 1200/Extra Fine (left) and 600/Fine (right) on either side of a 2 inch/51mm putty knife for comparison:

enter image description here

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  • The photos of the plate are deceptive because it looks more than fine enough (to me at least) to be classed as "extra fine". My finest diamond plate is I think a 600 (worn-in), visibly coarser than this and yet the surface it produces (used dry!) is better than you show in the first image and there's no roughness detectable to a fingernail. Because your plate appears to be fine enough there's a possibility that it was contaminated with a coarser grit. [contd] – Graphus Jul 1 '17 at 16:38
  • Even loose large particles can be very difficult to completely remove because they can become embedded. But if contaminated at source (something reported too often online sadly) the atypically large particles or clumps are held in place by the plating and may never completely go away until the plate is so worn that you'd want to replace it anyway. – Graphus Jul 1 '17 at 16:40
  • @Graphus Thanks for the sanity check -- I saw similar reports so I think I will get in touch with DMT after the long weekend. – Scott Hilbert Jul 1 '17 at 22:04
  • Welcome. If you haven't seen it there's a clear pic of the scratch pattern produced by my "fine" plate on a chisel bevel in this previous question. That's a 19mm or 3/4" chisel which should give you something you can make a direct comparison with. – Graphus Jul 2 '17 at 6:34
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    I received an uninterrupted DMT extra fine stone in the mail today (I had ordered one a week ago when I was suspecting the polka dots being the culprit) -- and honestly out of the box the uninterrupted EF seems to leave a finer finish than the EF side of the much, much more used duosharp. Going to call DMT and see if I can get them to check it out. – Scott Hilbert Jul 5 '17 at 21:15
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Some (perhaps all) of the DMT Extra Fine and Extra Extra Fine stones are contaminated with coarser grit diamonds. See these Scanning Electron Microscope images here https://scienceofsharp.wordpress.com/2015/09/28/diamond-plate-break-in-part-2/ enter image description here

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Give it a little while, it will ease up. It even mentions this in the DMT guide, that the stones come a little rough and eventually settle at the specified grit.

As a warning, be very careful around the EDGES of those stones until they have some use on them. I was pretty disappointed that my $120 diamond stone (600/1200x) for plane blades had a burr/lip thing near the edges, so after opening the stone and sharpening the plane blade I seriously dinged up the edge. It only needed a little honing. Had to take it back to like 320 grit and grind out the nicks in the bevel.

Keep in mind that these diamond stones never seem to cut as well (quickly or evenly) as sandpaper or waterstones to me. But the benefit is that they don't need to be replaced. Window cleaner works great to help clear out the swarf and lubricate everything.

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  • It's hard to be sure just from one picture but I think given the magnitude of the scratches that they aren't a case of the plate not having worn in yet. Don't the scratches look of a size that you might expect from a coarse? Prior to breaking in I'd expect a 1200 to maybe be about 900 grit but even if it's equivalent to about 800 that's still quite fine, way finer in fact than the grit that used to be used as a finishing surface in many workshops in the Western world. – Graphus Jul 3 '17 at 6:41
  • How much 'break in' would you expect? This thing is not new, I've tried probably at least the equivalent of 25-50 sharpenings on this thing (it's hard to quantify exactly), but tool steel should have gotten over most of this plate several times by now. DMT suggests it only takes a couple sharpenings to "break in" which I feel like I am far beyond at this point. – Scott Hilbert Jul 3 '17 at 21:57
  • @ScottHilbert I'd say "a couple" of sharpenings is much too hopeful based on experience with other plates, I'd put it at far higher than that (50 uses would probably cover it). Discussions on some forums might give a better idea if DMT are being a little disingenuous or if that is about right for their plates specifically. – Graphus Jul 4 '17 at 11:33
  • @ScottHilbert my experience is more in line with Graphus's answer of 50-ish. I sold off a DMT EEF stone because the damn thing never seemed to break in. My DMT EF stone serves well enough as the second-to-last step before the polishing stage, but the edge it delivers isn't that great. – David Peters Jul 5 '17 at 15:43
  • Maybe you should contact DMT about it. If this is a systematic problem or a misunderstanding, I'm sure they'd have heard about it before, and if it is a manufacturing defect, they would probably send you a replacement. – Alexander Gruber Dec 1 '17 at 22:12

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