3

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

Scott Hilbert
  • 435
  • 4
  • 9
  • 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 01 '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 01 '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 01 '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 02 '17 at 06:34
  • Re. contamination, have you tested all portions of the plate by the way? If you can sharpen a chisel in each of the four corners and either side of the centre and the pattern is consistently the same I'd say it's extremely unlikely the contamination didn't happen at source! I've accidentally gotten grit on a stone and on plates a few times and it tends to be very localised, just a handful of grains spread over a smallish area. – Graphus Jul 02 '17 at 06:38
  • Another thing occurred to me after I Commented last night, how does this side compare to the "fine" side? It's a key issue if this side is not significantly finer. – Graphus Jul 02 '17 at 06:39
  • So I tried various parts of the EF side using some spare washers, and I tried comparing both sides by scratching up either side of a putty knife. It seems pretty consistent all over the stone. I feel like there is a small improvement from the 600 Fine to 1200 Extra Fine but it's small -- maybe just better consistency in the scratch pattern. They both produce a "brushed metal" surface. I'm posting a comparison picture. – Scott Hilbert Jul 03 '17 at 22:30
  • Now that you've posed more pics I'll have to compare with my diamond plates and hold up my results to the screen, but even before I do that, shouldn't the 1200 be essentially twice as fine as the 600?? Going by grit-comparison tables it should be nearly exactly twice as fine. 9 micron is what I would know as P2000 which is extremely fine, easily good enough to produce a nearly scratch-free surface and a good step along the way towards a reflective finish. In short, way way finer than in the new photo. – Graphus Jul 04 '17 at 11:29
  • 1
    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 05 '17 at 21:15
  • "diamond sharpening stone coarser than it should be, or am I crazy?" Both could be true. – Alaska Man Mar 28 '20 at 18:13
  • Try smooth e z lap hones on colored plastic handles, I quit using the ones with plastic dots in them, they are not as good and wear rather quickly, If you can't sharpen good pay someone to do it. I freehand and got my kabar d2 extreme scary sharp but I've been sharpening for umpteen years. You can feel burr on one side go to the other side, it's not rocket science folks – Darryl Finley Mar 28 '20 at 08:45

2 Answers2

2

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

Montaray Jack
  • 21
  • 2
1

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.

jbord39
  • 970
  • 2
  • 8
  • 17
  • 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 03 '17 at 06:41
  • 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 03 '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 04 '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 05 '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 01 '17 at 22:12
  • I ended up having to return+replace a DMT Extra Coarse continuous stone for the same reasons given by Hodgibabs's 3/5 customer review, specifically This raised edge also caused the chisel to lift slightly out of plane .... – bgoodr Jun 08 '19 at 21:54