Realistic expectation of flatness for diamond sharpening stone?

C

Colorado Brian

Joined
Mar 3, 2024
Messages
4
When people and sales literature say a diamond plate is "dead flat", what do they mean? Do they mean "flat enough to sharpen a blade?"

I ask b/c my new Dia-Sharp 10x4 double sided stone is dished on both sides by .004" and .0045", which seems like a hell of a lot for $150. I don't know if that's "bad" or not or even if it qualifies for "dead flat" around here. But it seems like I could get that level of flatness from a random $30 Amazon purchase. It can't be hard to get a metal plate to be flatter that that.
 
If you're unhappy with the flatness of the hone, you should contact DMT and/or the dealer you purchased it from to arrange a replacement or refund. DMT has usually been good about making these issues right, one way or another.

DMT does claim, or at least imply, some degree of precision in the flatness of their Dia-Sharp bench stones, although I don't see exactly what their spec is for that. I think they DO spec flatness for their Duo-Sharp double-sided bench stones with the interrupted surface, but I've never seen the continuous-surfaced Dia-Sharp stones publicly spec'd in the same manner.
 
Last edited:
Colorado Brian said:
When people and sales literature say a diamond plate is "dead flat"
Click to expand...
There is no such thing as "dead flat".
Usually people say that when the variation in the surface is negligible, but there's always variation.
I personally wouldn't flinch at 0.005"- at that point, the stone is more accurate than I am.

Keep in mind that 0.0045" of dish on a 10" stone doesn't have the same impact as 0.0045" of dish on a 6 inch stone.
More than sufficient, even for wide primary bevel applications such as chisels.
 
Given that 0.0045" is only a little over 1/10th of a mm (or 114.3 it's very possible, depending on the grit of the stone, that you are actually measuring variation in the grit protrusion of the stone. 106 microns corresponds with about ANSI 120 grit. Remember that this is grit that is electroplated to the surface of the stones, meaning that you have a bunch of diamonds sitting on top of a plate. There is variation in the shape of those diamonds and how they lay on that surface.
 
FortyTwoBlades said:
Given that 0.0045" is only a little over 1/10th of a mm (or 114.3 it's very possible, depending on the grit of the stone, that you are actually measuring variation in the grit protrusion of the stone. 106 microns corresponds with about ANSI 120 grit. Remember that this is grit that is electroplated to the surface of the stones, meaning that you have a bunch of diamonds sitting on top of a plate. There is variation in the shape of those diamonds and how they lay on that surface.
Click to expand...
Exactly.
Not to mention you first need to break-in the plate with machined piece of steel to remove poorly bonded diamonds and to kill spikes of diamonds which sticks out of the rest not to mention those larger diamonds (contamination with larger particles).

And no,
to make a zero dead flat aluminium surface is not that simple.

I suppose you tried to measure the plate with a ruler. Could be your ruler is dished our.
 
Last edited:
<<< What was your measuring method? Do you have 3D measuring machine? >>>
Fair enough, good to eliminate user error before getting excited. I have a 24" machinists flat edge. I forget the specs of flatness, but it was a big deal when I bought it a few years ago. I put it on top of the plate, and you can plainly see light under the flat edge. Same on both sides of the diamond plate. I then got out my feeler gauge and could easily slip the .005 feeler gauge under the flat edge on one side of the plate. That .005 feeler gauge actually measures .0045 on my precision micrometer. On the other side of the plate I can slip the .004 gauge under w/o resistance. That one measures pretty close to .004 on the micrometer. I got the same result if I use either end of the straight edge, or the center. But just eyeballing it, there really is a ton of light under the straight edge, this thing is nowhere near flat.

BUT, my question is does it matter? "Flat" is subjective and context-specific. It's totally possible this qualifies as 'flat' in the diamond sharpening plate world. As imprecise as wood working is, I can certainly hand plane a piece of wood this size within .002 or less. We flatten the soles of our hand planes flatter than that using sand paper on float glass. And I saw a guy use a metal scraper and printing ink on a surface plate to get a jack plane within .0005 of flat across 15 inches. Before I start using a pricey diamond plate, I thought I'd bounce it off the board here to see if .0045 across 10 inches out of flat has a consequence. If it doesn't have a consequence, game on, I may not even call DMT about it. Enter the other reasons I bought the DMT plate. Largely I'm sick of peeling glued-on sandpaper off of glass.
 
Last edited:
Brian,

what are the grit ratings? As 42 mentions above, my guess is the diamond particles are sticking up. Run a hard blade across it and work it some and then measure again. New diamond stones will wear in some, especially in lower grits.
 
Colorado Brian said:
We flatten the soles of our hand planes flatter than that using sand paper on float glass. And I saw a guy use a metal scraper and printing ink on a surface plate to get a jack plane within .0005 of flat across 15 inches.
Click to expand...
Flat less than 0.002 on sandpaper and float glass? Thats extremely impressive. That’s half the size of the average particle in 120 grit sand paper. If you’re actually able to do that, and you intend to flatten your plane with the DMT stone, forget about it. You’re already achieving outstanding results.

If you’re trying to sharpen a knife, the DMT stone will be suitable for your needs.
 
I got to thinking about the comments on grit more or less 'piling up' and causing lumps (fine and extra fine, btw). When I run my hand over the plate I do feel something a lot like little nubbs here and there. Which got me to thinking. So I redid my measurements. I first cleaned the straight edge with soap and water, it wouldn't take much residue to cause error measured in thousandths of an inch. I ran the plate under water (I still want to return it as new if I need to), gave it a good hand rub and towel dry and tried this again. One side of the aluminum plate is undeniably about .0045 out of flat. The flip side measured much flatter, right at .001 after measuring the actual thickness of my feeler gauge. Definitely much better, which gives credibility to the idea that a little buildup of diamond particles here and there could cause a measurment error. I'm at least open to the idea that breaking it in would knock down whatever 'piled up' diamond particles and improve upon that .0045. I'll call DMT about it tomorrow and report back.

BTW, for grins and giggles I got out my favorite jack plane, actually it's a 12" long Millers Falls "Jr. Jack" plane. I flattened it a couple two-three years ago with float glass and sand paper. I can't get any of my feeler gauges between my straight edge and the plane sole. Not even close. That plane is flatter than I can measure. Sub .001 across 12". Definitely sticking to glass & sandpaper for plane soles. The diamond plate is for the plane blades, chisels, and knives.
 
Last edited:

Here’s the best hand tool woodworker in the would sharpening a plane blade. His technique works great and you can see there’s more variation in the stroke angle than 0.0045 every 5 inches. He uses very inexpensive diamond plates as well.
 
Here is an old thread about flatness of diamond plates. I haven't read it so I don't know if there is something in there to help you:
https://www.bladeforums.com/threads/true-flatness-of-diamond-hones.952773/

Otherwise;
As I wrote try first to break in the plate and look for diamond build-ups at the edges or anything sticking out which could mess your flatness measurements. Otherwise I don't know how to help you. You can make a water stone flat if it's dished out but diamond plates...I don't know.
If you are not feeling OK with your plates I would contact manufacturer and explain the problem.
 
Is the body of your plate steel or aluminum? Either way getting them under .001" flatness in production is not difficult. The diamond coating thickness will vary, how much depends on several details. What grits are on your plate? The diamond crystal sizes are mean +/- around 50% so that is also an element of variation.
 
I would think that variation wouldn't make a whole lot of difference. Never measured my DMTs but they still work great after more than a decade.
 
Hopefully this brings some resolution... both to my initial question of what a realistic expectation of flatness is (spoiler alert: flatter that what I got!) and to how significant flatness is.

From carefully reading about the adventures of the waterstone crowd, the magnitude of out-of-flat is far less important than the shape of that out of flat. And what you want to sharpen. Visualize sharpening a plane blade, maybe even with one of those jigs that you roll across the stone. If the out-of-flat is shaped like a skateboard half pipe, then that isn't so bad. The angle of the blade doesn't change much, you just sort of ride the continuous curve. Not a lot of concern. HOWEVER, if the out-of-flat is bowl shaped, often called dishing I believe, then there is a low spot in the center of the stone that becomes less deep toward all four edges of the stone. That is a problem. The wide and straight plane blade will assume the shape of the bowl shape in the stone as you sharpen it - it causes a slight U shape on the blade. That isn't good. Ditto for chisels. Worse, that "dish" in the stone causes the stone to contact the plane blade inconsistently - at one end of the stone you get continuous and fairly flat contact, then toward the middle of the stroke the center part of the stone drops away, leaving contact at the sides of the plane blade. NOW, if you're sharpening a curved knife edge, a bowl shape may not be a big deal at all. There are apparently very competent waterstone people who seem to think of stone flatness in terms of 16ths of an inch for knife sharpening. And you can probably think through the shape of a stone out-of-flatness and find all sorts off scenarios where it does and doesn't matter. < all my non-expert observations >

As for what a realistic expectation of flat is... I contacted DMT via their web page a couple days ago. No answer (yet?). I called the establishment I bought it from (sharpeningsupplies.com) and spoke to a technician. I explained how I measured the flatness and what I'm seeing. It took him about 0.5 seconds to say that was definitely beyond what it should be and he's sending me a replacement w/ a return shipping label for this one. I asked him about breaking it in first and he said I'm welcome to try and that will knock down a lot of the roughness you feel on a new stone, but it's not likely to change that amount of out-of-flat. We did talk about DMT's coyness about stating a flatness tolerance. In the tech's experience these stones should be in that ~1 thousandths territory, but nowhere near 4.5 thousandths. They (DMT) do check for this, but sometimes one slips through. They're happy to replace it and disappointed it happened. < this is why I avoid Ebay! >

Bottom line, out-of-flat may not matter depending on the shape of out-of-flat and what you're trying to sharpen. But if you lay down ~$140 on a diamond plate it'd better be flatter than 4.5 thousandths.
 
Last edited:
Colorado Brian said:
Hopefully this brings some resolution... both to my initial question of what a realistic expectation of flatness is (spoiler alert: flatter that what I got!) and to how significant flatness is.

From carefully reading about the adventures of the waterstone crowd, the magnitude of out-of-flat is far less important than the shape of that out of flat. And what you want to sharpen. Visualize sharpening a plane blade, maybe even with one of those jigs that you roll across the stone. If the out-of-flat is shaped like a skateboard half pipe that isn't so bad. The angle of the blade doesn't change much, not a lot of concern. HOWEVER, if the out-of-flat is bowl shaped, there's a problem. The wide and straight plane blade will assume the shape of the stone - a slight U shape on the blade. That isn't good. Ditto for chisels. Worse, that "U" in the stone is not consistent - on either end of the stone it's barely there and it gets big toward the center. But if you're sharpening a curved knife edge, a bowl shape may not be a big deal at all. There are apparently very competent waterstone people who seem to think of stone flatness in terms of 16ths of an inch for knife sharpening. And you can probably think through the shape of a stone out-of-flatness and find all sorts off scenarios where it does and doesn't matter. < all my non-expert observations >

As for what a realistic expectation of flat is... I contacted DMT via their web page a couple days ago. No answer (yet?). I called the establishment I bought it from (sharpeningsupplies.com) and spoke to a technician. I explained how I measured the flatness and what I'm seeing. It took him about 0.5 seconds to say that was definitely beyond what it should be and he's sending me a replacement w/ a return shipping label for this one. I asked him about breaking it in first and he said I'm welcome to try and that will knock down a lot of the roughness you feel on a new stone, but it's not likely to change that amount of out-of-flat. We did talk about DMT's coyness about stating a flatness tolerance. In the tech's experience these stones should be in that ~1 thousandths territory, but nowhere near 4.5 thousandths. They (DMT) do check for this, but sometimes one slips through. They're happy to replace it and disappointed it happened. < this is why I avoid Ebay! >

Bottom line, out-of-flat may not matter depending on the shape of out-of-flat and what you're trying to sharpen. But if you lay down ~$140 on a diamond plate it'd better be flatter than 4.5 thousandths.
Click to expand...
Tell them to sent it here- I can use a stone like that!
 
You must log in or register to reply here.
Back
Top