How do I know if my stone is flat?

[Aaron Liebling]

I use mostly diamond, which (I assume) doesn't need flattening, but my final level for sharpening prior to stropping is a glass sharpening plate...which does need flattening?

If so, how do I know when it needs flattening? And given that all I currently have are diamond plates of the same width, what's my best bet for flattening it?

[lowell holmes]

My diamond hones are flat, you can check with a tri square.

[Don Dorn]

I'm not trying to sound presumptuous though it might come off that way. If the stone is diamond, it's flat enough. Sometimes, we can chase out tails a bit. I feel the same way about my bench surface, it might need "flattening", by a straight edge, but it's flat enough. Just a personality thing I guess - perhaps it's more important to you than me.

[Phil Mueller]

Draw criss cross pencil lines over your stone, then use one of your diamond stones on it.

[Andrew Pitonyak]

I use mostly diamond, which (I assume) doesn't need flattening

If this is a diamond plate, even if it did need flattening, I don't see how you could do that. So, no, you will not flatten a diamond plate.

but my final level for sharpening prior to stropping is a glass sharpening plate...which does need flattening?

The point of the glass is that it is expected to be flat.


A water stone, India stone, Crystolone, Arkansas, etc, they might need to be flattened. Arkansas stones very rarely, India stones seem to stay flat pretty well. Never heard of anyone flattening glass.

[mike stenson]

Float glass is flat, there's no need to worry about that.

[Winston Chang]

I have to disagree with some of the other posts here. I have had diamond plates and float glass plates that weren't flat. If in your sharpening progression you have plates that are out-of-flat in opposite directions (one is concave and another is convex), then it will cause you to spend a lot more time sharpening, compared to if all the plates were flat (or at least have the same profile).

Here's a picture of a diamond plate that was .05 mm (.002 inch) out of flat.

IMG_1209.jpg

I had some pieces of float glass that were also out of flat. Unfortunately, I don't have any pictures of them, and I threw them out, but I seem to remember that they were at least as bowed as this diamond plate.

Note that the out-of-flatness matters most when working the back of the blade; for the bevel side, it doesn't matter as much, because there's less area of the blade. The diamond plate in the picture above is 300 grit on one side and 1000 on the other. The 300 grit side was concave, and the 1000 grit side was convex by the same amount. If I "flattened" the back of a blade on the 300 grit side, the blade would be slightly convex. Then if I moved it to 1000 grit side, the convex blade would be worked on a convex surface, and to get the blade to be concave in a way that matched the convexity of the 1000-grit side would take a lot of extra work and material removal. And then if you move to a finer plate that is bowed in the opposite direction, that step would take even longer.

When I had this diamond plate, I wondered why I was having difficulty getting my blades really sharp; after I replaced it with one that was much flatter, sharpening was faster and easier. Just to be clear, I'm not advocating for flat stones just for the sake of being flat; it's because having flat stones (or at least all the same shape) makes sharpening faster and require less material removal.

To check for flatness, I use an aluminum straight edge I bought on Amazon, which is also shown in the picture above. It was about $25 for a 2 foot long straight edge which I'm confident is very straight (due to various informal tests I've done with it). If you place the straight edge on the plate and can't see any light between them, then that means there's less than a .001" gap. If you want to quantify it, you can also use feeler gauges as I did in the picture above.

If your diamond plates and glass plates aren't flat, there's not much you can do except replace them.

[Tracy Roberts]

Ditto Mr. Chang. I assume that your finish sharpening is on a glass backed water stone like Shapton makes and if so then yes it will require flattening. Most folks flatten their water stones with a diamond stone...plenty of info out there about flattening water stones.

On the issue of flatness, diamond stones are not flat, some are just flatter than others. The issue as mentioned above is are they flat enough for your needs.

Are you having an problem sharpening that you think is due to the flatness of your finish stone?

[Warren Mickley]

Are you having an problem sharpening that you think is due to the flatness of your finish stone?

Bingo! Yes we generally decide when a stone needs flattening based on tool performance. For plane irons we usually like a little camber on the irons, so a small concavity on the stone is beneficial. When we start getting too much camber for our smoothing planes, it is helpful to flatten the stone.

Welcome to the forum, Tracy!

[Tracy Roberts]

You can check the stone for flatness with a straight edge. Hold the straight edge along the stone and hold it up to the light, you should be able to see light where there are low areas on the stone. To flatten, I would use the finest diamond stone, and plenty of water, maybe at the sink. Cross hatch the surface of the stone with a pencil and rub the two stones together and check the lines for progress. When the pencil lines are gone the stone is as flat as the diamond stone.
Hope that make sense.

[Gary Focht]

I have a diamond stone that is slightly humped along the middle. Maybe 15 thousandths. When I sharpened a plane blade with my Mark II, the blade would not cut in the middle when taking a very light cut. I was confused at first as to how I managed to remove more material in the center of the plane iron until I checked my diamond stone. Then I understood why I was removing material with only the outer edges of the plane iron.

Had I sharpened freehand I don’t think I would have noticed because I would not have tracked straight along the hump.