How do you know if your hones are flat?
There is no absolute rule that a whetstone must be flat; at least - none that I’ve read anyway. Thing is – I do get better results when my hones are flat, so I lap them and I keep them flat.
I don’t believe people used dished stones for any reason other than those people didn’t know how to flatten their stones. Every manual and guide I’ve read has referenced using flat stones to sharpen everything. So there’s that.
Uneven stones – the kind with dips and waves across the top, are bound to cause issues, problems, and trouble. If nothing else, they will definitely cause uneven wear in places and that is, in my opinion, to be avoided at all costs.
Anyway – people usually ask me – How do I know if the stone is flat?
Well – here’s one way to tell for sure.
Get a good straight edge.
Put the edge of that straight edge on top of the stone, and shine a bright light onto where the edge meets the stone and view it from the other side.
In the photo – that is an un-lapped Les Latneuses Hybrid Coticule, my un-ruled Starrett straight edge is sitting on top of it and there is a flashlight behind them.
As you can see – there is considerable light-spill on the right side sneaking out from between the stone and the straight edge. Had the stone been lapped – that gap would not be there and you would not see any light coming through.
For this to work – a quality straight edge should be used. Cheap Chinese rulers from the local big-box store might not cut it. An alternative to a machinists ruler would be a decent stainless steel ruler from an art supply shop, the kind with cork on the back; they usually work ok.
