And what do you use for those?
For those it's more of a process.
For knife sharpening "flat" means "I can erase a grid of sharpie lines after lapping the surface lightly about 10 times". And I often don't even bother with lines. But that's the general idea.
For razors, it's a fussier process and flatness counts for a lot more. For this I want "dead flat". This means using a straight edge I KNOW is flat. I lay it on the stone across 8 axes, 3 times on the long axis plus 3 times on the short axis (left, right, center), then the two diagonals. If I see light between the stone and the straight edge, it ain't flat.
When you flatten by hand (as opposed to a milling machine) you inevitably create either a hollow or dome in the middle of the stone. Depends on how much hand wobble you have, and the flatness of your plate. It could be not flat due to a warp the metal plate. It could have a slight concave surface due to uneven diamond wear in the middle. Or both. Fun times!
Long story short, it's pretty challenging (at least for me) to get dead flat by hand.
If a stone is badly dished, I'll take the worst out with a diamond plate or SiC powder on a piece of wet dry paper on top a granite reference block. Depends on how hard the stone is.
Once that's done, I hope to have less than 1mm of light between stone and straight edge. It's easier to do the last step it the stone is slightly hollow - a domed surface needs more concentration. So I usually try to STOP flattening when I have a slight dish. If I over do it, I'll usually make a dome.
Now it's time for dead flat.
Back to the granite reference block with a fresh sheet of 120 grit paper, no SiC powder. With slow, controlled, non wobbling moves, I drag the stone on the paper. I'm not scrubbing, just one direction, deliberate pushes and pulls.
If I was smart and left a dish in the stone, it'll rest on its ends, and those will wear until I hit dead flat. If I was careless earlier, it'll have a dome. Then I need to hold it in the stone in the center, and try not to rock it as I move or I'll just recreate the dome on every pass. Grinding material but not making progress.
This step needs frequent comparison to the straight edge, and stopping just when you're done. It's really easy to push past flat and start making a dome. Stone surface needs to be dry when referencing.
For softer stones like some cotis it's often not worth it to be this flat because honing a razor will get you off dead flat within a few passes. For cotis, you need to be OK with that and work with the stone. For an Ark, it's so bloody hard that having it dead flat is worth the time investment. Especially because if it's domed, your razors apex will never actually touch the stone and you'll get no benefit. It it's hollow, your Apex will become more obtuse the more you hone. And for razors that counts. Obviously it's a matter of degrees, but the closer the better. I'll also condition an Ark to 600-1000 grit after flattening, FWIW.
As a last note, having your bevel setting razor stone dead flat is a huge benefit since the rest of the progression is just polishing an established, sharp apex. Flat is a platonic shape you can return any stone to. Varying shapes of worn stones are not replicable, so starting flat and staying flat through a whole progression means your final finishing stone is going to shine so much more because you did the ground work properly.
For a knife, there's so much hand wobble it's like pfff who cares about dead flat. I'll never be able to hold a knife stable enough to need more than "good enough".
Sorry to hijack. Kinda got going.
" - Significant Other
