I Don't Trust My Aging Eyes to Check Flattness

So I rigged my dial indicator to check my stones. Graduated to .0005" , I observed no movement of the dial at all on my Arkies. When checking my recalcitrant purple slate, I found a gradual .004" dip towards the center. Question is will .004" affect anything while honing? Is there a number, a dimension, that indicates I should go back to the lap? Most folks use a straight edge and look for light, doesn't seem to work well for me. I'm a tool maker and always checking for flat or square at work and shun visual methods do to my eyes.

I would say not to worry about it unless the edges aren't as good as the used to be or the bevel starts to look different under a scope.

Duke your question leads to a question I've had, which I'll throw onto the table for the experienced folk here to answer along with your question.

My question is: if the stone is flat across the width of the stone, does it matter if it is not flat along the length of the stone?


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I understand the obsession, but I doubt it's worth much concern.

I sometimes see heavily used Coticules by old barbers and they're VERY dished in the middle from time to time. It's like honing with a lot of tape on the spine I presume. I don't think that's what you want either, we do want a relatively flat stone.

I flattened all my hones, but I'm sure after years of honing if I recheck they won't be flat, I suspect the outer parts and the centre of my hones especially do be worn a little more than the rest.

I guess it will take an extremely dished stone to interfere with the functioning of the stone.

Stones need to be flat but I don't obsess about flatness, I use my Atoma plate and during lapping I use the edge of the plate to check different areas and aslo do a pencil grid test, I generally lightly lap/clean before every session so they stay pretty flat.

...flatness is a relative thing...yes flat is good but you should not loose sleep over it if the edges are good.

I have a routine when honing...
After a bevel set i give my 1 k hone @8 laps with my 600 grit diamond plate
The higher grit stones get less.

This keeps them flat for the next use and i dont worry about the need to lap them...i do it anyway.

I usualy panick if i put a straight metal ruler on the face of the stone and light sips in between... about a 0,25 mm - 0,5mm will let light sip in easy.

Never neded to mesure themroutinely though...

Best way to check for absolute flatness, if one is really obsessed with the subject, is to pick up some 1/1000" feeler gauge tape from McMaster-Carr. Place a steel rule edgewise along the diagonals of the stone in question and try to slip the tape under the rule. If it slips underneath, the stone surface is not absolutely flat. If it fails to pass underneath the rule, the stone surface is flat. No eyes being really necessary.

I learned this trick from one of Stuart Tierney's videos, on his site toolsfromjapan.com. Check it out there for further info.

I am a rookie honer with only a few years experience. I rarely lap my Naniwa hones as my razors seem to have some sort of warp/twist to them that needs a bit of honing gymnastics to take care of. I just watch how the water moves down the edge while doing X strokes of some variety. No doubt I am in a very small minority here. I'd just say do whatever works for you through experience and you are comfortable with.

Bob