Stones That Warp

[Radar53]

This is something that I have been observing for some time. I used to flatten my stones after every reasonable sharpening session so that they were ready to go when I next felt the need. Probably by accident, a couple of times I couldn't remember whether I had flattened a stone or not and when I did then flatten it, a bit of work was needed. Sometimes for any given stone, I had this niggling thought that I was sure I had previously flattened it, but couldn't be absolutely sure.

Stage 2 was that I started pencilling the sharpening date onto the stone so that I would know that it was done & flat. That should have been it done & dusted. But, and I can't remember the why's & wherefore's, I found that some of my "dated" stones weren't always flat prior to their next use and this raised my curiosity as an engineer. Also they could be some way from what my OCD would consider flat.

Stage 3 was that I flattened my stones immediately prior to use, in order to fix whatever the problem was. But, my OCD again, I needed to understand what was going on.

So I purposely did a bit of mix & match, observing the results, and I've noticed that the warping seemed to be greater on stones that I hadn't used for a while.

Here is my thinking. I believe that there is differential thermal expansion between the aluminium backing plate and the stone material so that if I flatten at the height of summer and then check it again in the middle of winter the aluminium has contracted more than the stone material and vice versa creating a slight curvature.

Its no biggie & easily fixed by flattening immediately before use, but I was just curious.

[Rufus Leaking]

I have a Norton stone that I use for plane blades, and every time I flatten it, the same low spots show up in the same two diagonal corners regardless of how the stone and flattening plate line up. I think there are voids of sorts in the stone- a process of inconsistent hardness in the stone itself.

[ken123]

Stones DO flex. I first noticed this with Shapton pro stones ( eg the 5k). I'm sure there are other stones that show this. Indeed the glass of glassstones is a good supporting stricture.

For EP stones the Aluminum plate reduces flex. There can be some variability here - a less than perfectly flat plate, a less than perfectly flat stone and partial separations or the stone warping from a warped plate. Rare but it happens.

I really don't see the major problem being a differential flexing or warping because of differences in thermal expansion characteristics ( a la the copper/constantan wires in thermostats ).

I think the main culprit is one of flattening technique.

The stone should be well supported during flattening so it doesn't bow under pressure. You should also flatten gently - not like some gorilla tossing around luggage . Gentle. Also applying pressure against the stone EVENLY.

There can be other issues but this should be the first things you look at.

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Ken

[Radar53]

Hi Ken, thanks for coming back with your thoughts. I always value your input and look to be open and improve all things in my sharpening, however some things in your response don’t resonate with me. Let’s assume for now that my flattening technique is reasonable & I will provide details further on.

@ken123 “For EP stones the Aluminum plate reduces flex.”
So to my mind, this should assist a stone that has been flattened, to stay flat, which is not what I am experiencing.

@ken123 “ There can be some variability here - a less than perfectly flat plate, a less than perfectly flat stone and partial separations or the stone warping from a warped plate. Rare but it happens.”
Pretty much all my EP stones will exhibit the behaviour described in my OP. Additionally the rare variability noted in your response should largely be addressed in the first flattening. What I am observing is a repeating change.

@ken123 “I think the main culprit is one of flattening technique.
The stone should be well supported during flattening so it doesn't bow under pressure. You should also flatten gently - not like some gorilla tossing around luggage. Gentle. Also applying pressure against the stone EVENLY.”

It’s hard not to get a bit defensive here given that you have no idea of my technique. So I will describe what I do and am certainly open to any improvements that I can make, as I definitely don’t have a mortgage on flattening stones.

First some background. I’m an engineer and have worked most of my life in large industrial plants, including very high-pressure steam boilers & turbines used in power generation. As part of my training I was taught to hand lap high-pressure steam-valve-trims on optically correct lapping tables. The technique I use for my sharpening stones is based on this as described below.

I use 210X75mm, Atoma diamond plates, which I assume are pretty flat. These are a lot bigger than the EP sized stones. I secure the Atoma plate on the bench, with the long edge going left to right in front of me, wet it and put the “cross hatched” EP stone on top.

I place 8 fingers longitudinally along the EP stone, centrally placed edge to edge and equally distributed down the length. Using Pete Nowlan’s P1 to P2 pressure.

1) I will make X light strokes backwards & forwards, while moving the stone from end to end of the Atoma plate (to keep the plate wearing evenly) & then check.
2) I rotate the EP stone 180º & repeat.
3) I rotate the EP stone 180º again and use X large figure “8” strokes & check
4) Rotate the EP stone 180º & repeat
5) Return to 1) as required until the cross-hatching is removed.

Given the above, if anyone should suggest that I am at all OCD, then I would have to resemble that remark!! I would be interested to know if plate-glass-backed EP stones exhibit this behaviour as the coefficient of thermal expansion of Al is about 2.5 times that of plate glass? Or is some other phenomena at play here?

[ChefKnivesToGo]

Grant, being OCD and sharpening seem to go together.

[Kalaeb]

Seems perfectly plausible. I generally lap before, after and sometimes during. I also store/dry my stones on edge rather than flat.

[ken123]

Now I see the problem - you're an engineer! Kidding of course. As Mark mentioned OCD and sharpening go hand in hand. MOST of my customers, including me have some obsessiveness.

My comments were of a general nature to all sharpeners, but from your comments, it looks like the basics I mention you have under control. Do you have an optical flat available?

If it is of help you could back your stone with an intervening layer of glass between it and the plate or go with a thicker aluminum plate to increase rigidity. Just a thought ...

Interesting problem.

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Ken

[Radar53]

Hey Mark, you're dead right there, however, being an engineer I'm further afflicted and I can see an intervention in my future

@Kalaeb, yeah I mix & match a bit too & if I'm having a big sharpening session, I will flatten during as well.

@ken see my comment to Mark above haha - engineers & OCD are common bedfellows!! I do not have an optical flat available to me any more, so like most of my fellow sharpeners I have to rely on the marketing bumph regarding the flatness of diamond plates, which for flattening stones should be just fine I think. The difference noted in my OP is quite defined and I'm just curious from a technical / engineering standpoint as to what's causing it. Differential thermal expansion still seems to me to be the most likely cause. The solution is really easy in that I now flatten my stones immediately before sharpening.

Thanks All for your input / comments

[ChefKnivesToGo]

I had a guy that was convinced that his stone was warping. CONVINCED! I had him send it to me and after a few simple tests figured out that he was flatting the stone improperly and grinding away the ends of the stones by using too much pressure on the stone which was causing the stone to reverse dish. There was no warping going on with his particular stone.

I'm skeptical that stones warp but I'm open to it. I've just never seen one do this on it's own.

[Radar53]

Hi Mark I don't have any deep concerns as its really easily sorted. Its just an interesting technical curiosity to me. My stones go both concave and convex, although I have not checked this against the flattening date and seasonal temperature variations and am unlikely to as I now flatten just prior to use.

I was just curious to see if anyone else had noticed similar behaviour.