Thread: Honing "Feel"

Nobody really knows why two very very flat surfaces stick together. Nobody has ever bothered to investigate it in depth I think. There have been several theories that have settled out as most likely. Some say Van der Waals forces, others the atmospheric/pressure thing, others think it's microscopic contaminants such as oil or humidity in the air. Maybe a combination? All I know for a fact is that once two surfaces are flat enough, they can stick together with very significant force. One good example I can give from the machining world is what are known as "gage blocks." These are dimensionally accurate steel, ceramic, or tungsten carbide blocks that have been lapped to super fine and extremely flat surfaces. They are used for inspection and measurement down to .0001" and here's the kicker: you can take two of the blocks when in good condition with no scratches or other blemishes and clean them with acetone and a lint-free cloth so that there is no oil or any type of contaminants on the surfaces and stick them together with a slight twisting motion and they will stick like they're glued. If you try to pull them apart in a straight opposing direction (i.e. no angular motion or twisting) they will hold quite fastly. Some of the old advertising materials for certain brands of the blocks would hang quite a bit of weight from one of the blocks like in the old crazy glue commercials to demonstrate their holding power and hence flatness. When assembling them for measuring they are put together like this - it's called "wringing" and there is a defined amount to add to the length of the block stack for each wrung joint (though it is mostly included in the block's advertised length, so it is compensated automatically), so the surfaces are not considered to make perfect and full contact even though they are as flat and smooth as the dickens. Once there is any surface defect or they aren't nearly perfectly flat anymore they will not wring together properly anymore. I have a set I use for dimensional inspection.

Here's a link to one manufacturer's explanation:

Wringing Gage Blocks

I can give from the machining world is what are known as "gage blocks."

Ah, many thanks, could not find the exact term to what I was refering in the previous post as "calibrating tiles in metrology"

They indeed were what I had in mind when I developped my first explanation. I was told at the time it was due to Van Der Waals forces (ingeneering scholl), it's interesting to know the explanation is not that simple.

Still, I stand by the fact that after much pondering, such may not be what happens in the current case. The main reason behind that being that a dry stone does not suck a razor's edge, AFAIK (or at least, not at the grits I am accustomed to).

Therefore my conclusion that the phenomenon known as "wetting" my be a stronger candidate for the correct explanation.

Fascinating discussion, heartfelt thanks gentlemen

I have to admit I do like the idea of a low pressure being created by perfectly flat surfaces. The same effect would apply (to a lesser degree) to a surface that has some sort of wetting agent between it to compensate for imperfections. The low pressure would then create a suction effect because, in order to seperate the two flat surfaces you would have to overcome the force of the displacement of air between the two surfaces. The whole subject is a bit of a mind warp though.