Lapping stones dead flat

[chalby]

Fixed abrasives will wear the periphery of the stone, or other workpiece, every time I have used them to flatten a surface. Loose abrasives on a FLAT surface are by far the best way to transfer that flatness to your workpiece, which is your stone in this case, but it won't happen without proper care. I agree that dead flat is not needed for any stone used to sharpen knives. With a guided sharpener I find around .01" of variation in the stone thickness, which includes any error from the sharpener, is fine for any finishing/polishing work.

How flat is the flat plate you are using with the loose abrasives? I ask because those abrasives not only wear your stone but your plate as well. I use at least 3 plates so I can true them up as needed.

I used wet and dry sandpaper on a tile for years and never had that problem. Maybe you are talking about beyond what is detectable to the naked eye but all I did was lay the edge of a metal rule across several places and if I couldn't see daylight it was good to go. For a benchstone, I don't see the point in worrying about flatness too much.

 

[Diemaker]

I have heard similar to this before in relation to using the three stone method of keeping stones flat. Not sure if it can be verified though or perhaps it's out of my league of flatness?

With 3 equal sized plates of identical material, the limit on flatness you can achieve is based on how stable the flats are and your patience. I have heard of the same method being used to make extremely accurate ceramic parallels, it's not just for rounds or squares.

 

[Diemaker]

Does that include PSA-backed lapping film on a granite surface plate?

Yes. The action of pushing the part over the fixed abrasive means the contact pressure will be higher at the front of the parts movement, causing uneven wear. I have been sorely disappointed every time I tried to flatten something that needed to be flat using sandpaper on a surface plate. With resin bond stones I can get to within .002" or so of flatness, with steel blocks a bit under .001". Loose grit grinds far more evenly, but is hard on your reference flat. The way to get a surface plate flat is with loose grit and a smaller flat plate grinding the high spots down.

 

[EngrSorenson]

The action of pushing the part over the fixed abrasive means the contact pressure will be higher at the front of the parts movement, causing uneven wear.

This is just a technical comment, but it’s always true regardless of the abrasive. The higher the coefficient of friction the more noticeable the impact. It’s a result of the fact that the force isn’t applied to the stone on the same plane as the source of friction, imparting a turning moment on the stone.

The only way to eliminate that entirely would be to make a stone holder that helps the stone resist the moment imparted from the user.

 

[garry3]

Yes. The action of pushing the part over the fixed abrasive means the contact pressure will be higher at the front of the parts movement, causing uneven wear. I have been sorely disappointed every time I tried to flatten something that needed to be flat using sandpaper on a surface plate. With resin bond stones I can get to within .002" or so of flatness, with steel blocks a bit under .001". Loose grit grinds far more evenly, but is hard on your reference flat. The way to get a surface plate flat is with loose grit and a smaller flat plate grinding the high spots down.

Are you doing this to make precision ground stones? I have always wondered what would happen if a straight razor was put to a stone that flat.

 

[chalby]

Yes. The action of pushing the part over the fixed abrasive means the contact pressure will be higher at the front of the parts movement, causing uneven wear. I have been sorely disappointed every time I tried to flatten something that needed to be flat using sandpaper on a surface plate. With resin bond stones I can get to within .002" or so of flatness, with steel blocks a bit under .001". Loose grit grinds far more evenly, but is hard on your reference flat. The way to get a surface plate flat is with loose grit and a smaller flat plate grinding the high spots down.

Maybe you are using more downward pressure than what I tend to do or the technique is different, or maybe you are talking more from a manufacturing point of view where flatness matters more.

The other thing too is that once the glass wears down a bit I would think that flatness would be harder to achieve with loose grit.

 

[Slim7]

To expound on this more, are you maintaining orientation of the straight edge to some reference surface? Or allowing it to “rock” while you shim it with feeler gauges?

If the straight edge is allowed to rock, your average variation is half of what you measure. You’d be looking at a 0.0015-0.002” problem. That’s of course assuming you’re measuring from both sides and determining the crown is centered.



Expounding further- if your straight edge carries +/- 0.0005 and your feeler gauges have +/- 0.0005, the tolerance stacks. That’s not even considering human error / the feeler gauge deflecting the straight edge. Giving you the benefit of absolute human perfection (you’re welcome), you probably don’t know your number within at least +/- 0.001.

First i use light if light can be seen i know its off, but the only way i know how much is by using feeler gauges to measure

Not allowing the straight edge to 'rock'. I lay the stone on the table and the straight edge on the stone then i check with various feeler gauges .

 

[jiggyfly16]

 

[EngrSorenson]

First i use light if light can be seen i know its off, but the only way i know how much is by using feeler gauges to measure

Not allowing the straight edge to 'rock'. I lay the stone on the table and the straight edge on the stone then i check with various feeler gauges .

You may benefit from devising a better method for measuring a crowned surface. With one point of contact on a straight edge I’m unsure how to avoid moving it while using feeler gauges.

 

[Slim7]

You may benefit from devising a better method for measuring a crowned surface. With one point of contact on a straight edge I’m unsure how to avoid moving it while using feeler gauges.

I hold the straight edge with one hand and check with the feeler gauges with the other.

 

[Mr.Wizard]

This is just a technical comment, but it’s always true regardless of the abrasive. The higher the coefficient of friction the more noticeable the impact. It’s a result of the fact that the force isn’t applied to the stone on the same plane as the source of friction, imparting a turning moment on the stone.

I can feel that drag and instinctively move my point of pressure from the center toward the trailing edge to try to compensate. I don't know how successful I am but shouldn't there be a neutral point where the forces cancel?

 

[EngrSorenson]

I can feel that drag and instinctively move my point of pressure from the center toward the trailing edge to try to compensate. I don't know how successful I am but shouldn't there be a neutral point where the forces cancel?

Theoretically, but it’s not likely to achieve that by feel. Most people overcome that effect by using loose grit and using light pressure. The loose grit lowers the coefficient of friction and less force reduces the net friction forces.

 

[Daedric Panther]

Honest question here, does it matter? I’m not as skilled at sharpening as many of you. I’m capable of putting a hair splitting edge on a knives, but my equipment is basic as is my knowledge on the subject. I’m not familiar with the concept of lapping sharpening stones. Is that something I’m supposed to do? we’re not talking about machining an engine deck here, and I can guarantee none of my oil stones are at that level of flatness.

 

[chalby]

Honest question here, does it matter? I’m not as skilled at sharpening as many of you. I’m capable of putting a hair splitting edge on a knives, but my equipment is basic as is my knowledge on the subject. I’m not familiar with the concept of lapping sharpening stones. Is that something I’m supposed to do? we’re not talking about machining an engine deck here, and I can guarantee none of my oil stones are at that level of flatness.

No, it doesn't matter. I think the OP is probably a bit OCD and most of the posts here are just people having a technical discussion about flatness that has nothing to do with practical sharpening stone considerations most end users. Flatter is better but only to a certain extent IMO.

I think a lot of people over-complicate the subject of flattening stones but nothing wrong with that.

 

[Cementite]

Yes. The action of pushing the part over the fixed abrasive means the contact pressure will be higher at the front of the parts movement, causing uneven wear...

This is just a technical comment, but it’s always true regardless of the abrasive. The higher the coefficient of friction the more noticeable the impact. It’s a result of the fact that the force isn’t applied to the stone on the same plane as the source of friction, imparting a turning moment on the stone...

I've noticed that when flattening stones, but never really put it together why it was happening, so thanks. It makes sense: When you push the stone forward, it wants to "rock" forward due to friction (in the same way that the front of a car dips when you jam on the brakes...if not for the exact same reasons), causing more pressure, and more removal of material, at the front than the rear of the stone, and vice-versa when you pull the stone back. I suspect this effect is even more pronounced with lateral movement (lapping left-right across the stone), since a stone is narrower than it is long.

So I guess when you get "typical" wear on a stone (i.e., a hollow in the middle, with the edges higher than the center), the best way to flatten it is to lap it until the edges are lowered to the same height as the center, but not go a hair more, because if you go "too far" with lapping, you'll inevitably (due to "rocking" of the stone) get a crown in the middle that could even get higher the more you lap. (In other words, if you lap a stone long enough without somehow preventing it from tilting/rocking, it will tend to turn into a sphere.) Would you agree?

 

[TheOne45]

In my experience loose grit always leaves a 3-4 thou crown/convexity in my stones, and it drives me nuts...

Are we talking about hand sharpening stones?

3 to 4 thou…
So; how this un-flatness impacts you knife edges?

 

[EngrSorenson]

So I guess when you get "typical" wear on a stone (i.e., a hollow in the middle, with the edges higher than the center), the best way to flatten it is to lap it until the edges are lowered to the same height as the center, but not go a hair more, because if you go "too far" with lapping, you'll inevitably (due to "rocking" of the stone) get a crown in the middle that could even get higher the more you lap. (In other words, if you lap a stone long enough without somehow preventing it from tilting/rocking, it will tend to turn into a sphere.) Would you agree?

Do what you can to mitigate the impact- flattening the long way, less pressure, etc etc etc.
The technique that you use will only introduce so much out-of-flat-ness, so there's no "over doing it" per say.
You will, however, just start wasting perfectly good stone if you go too long.

 

[EngrSorenson]

3 to 4 thou…
So; how this un-flatness impacts you knife edges?

OP has acknowledged that this is probably over kill.
We have to respect that, perhaps for OP, this hobby will be getting stones as flat as possible.

 

[TheOne45]

perhaps for OP, this hobby will be getting stones as flat as possible

This could be interesting technical challenge. Why not?

We have to respect that

Yes, indeed.
We can only explain Slim he does not need to worry about a few thau of un-flattnes of his stones.
More challenging would be to give him a method to make his stones dead flat. Would be interesting to make some tests how flat can I make the stone just with what I have at home but I don't have stones. I use diamond plates.

 

[Cementite]

We have to respect that, perhaps for OP, this hobby will be getting stones as flat as possible.

Nothing wrong with that, and it's really not too far removed from the pursuit of getting a cutting tool as sharp as possible, when you think about it.
I mean, I recently bought an HFT circular saw sharpener on clearance, and now I'm watching YT videos on how to trick that out with homemade jigs ... and I could see myself going down the "straight razor rabbit hole" just for the sake of learning to reliably sharpen edges to razor sharpness...stop me before I buy more stuff!