Lapping stones dead flat

S

Slim7

Joined
Oct 11, 2015
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May sound overkill, but i try an aim to get my sharpening stone dead flat, with straight edge and feeler gauges. I can never succeed with loose grit. Some hard stones (India, Arkansas, ceramics) need to be lapped with loose grit because a fixed abrasive will glaze them. In my experience loose grit always leaves a 3-4 thou crown/convexity in my stones, and it drives me nuts... I bought one of those diamond labridary wheels thats talked about here, but mine wasn't flat. I placed it on flat granite and the disc was out of true bad. Those diamond disc will turn a white Hard translucent into milk pretty quickly or i a decent time, but it won't be flat, at least mine wasn't because the disc was out of true.



The only way i have been able to get a hard stone dead flat is to use Sic powder to do the grunt work. Then go to a DMT xxc to get dead flat, then go back to sic and recondition the surface with sic, just enough to get the proper surface texture, without staying on it too long (45 seconds ) to re introduce the crown back into the stone . Any suggestions?
 
I use an Atoma 140 to flatten hard natural stones or waterstones (but not ceramics!). It leaves a healthy, usable surface up to the medium or finer grit stones. If I am not happy with the rougher surface after flattening a fine stone with the 140, I use a 400-800 Atoma to get the surface cutting smoothly.

An Atoma 140 will flatten a few square feet of hard, natural stones, or a lot more waterstones. It is probably more expensive than using loose grit but it is fast and Mr. Starret says it leaves the stones dead flat. When the ends of the 140 wear down from lapping dished stones, I just relegate that plate to less-critical jobs around the shop, and unwrap a new one. I think they are still less than $100 US.

I have never had a glazing problem using the diamond plates this way. I glazed stones with broken-down loose grit several times. It might have been operator error, but I found loose grit was more tricky to use than the diamond bench stones, and offered no advantages that I could see.
 
I have 3 questions, mostly out of curiosity, but they may be helpful:
how are you measuring flatness?
What’s the uncertainty of the measurement?
What’s your tolerance for “dead flat”?


My DMT super pricey lapping plate is +/- 0.0005”, half the thickness of a human hair. 0.003-0.004 inches is the thickness of a piece of notebook paper.

You’re a 100% right that this is overkill. We’re well beyond meaningful measurement, particularly if you are sharpening by hand.
 
Slim7 said:
with straight edge and feeler gauges.
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EngrSorenson said:
how are you measuring flatness?
Click to expand...

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.

EngrSorenson said:
What’s the uncertainty of the measurement?
Click to expand...

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.
 
Not that it necessarily applies here, but I think some people get a little overly hung up on these infinitesimal measurements sometimes.

For example, on woodworking forums, I see folks getting all haughty about getting their table saw blades within a mil or two of parallel to their miter slots.
I got sucked into this whole wild goose chase for a while, before I realized that:
a) it's almost impossible to measure such small distances precisely on a table saw;
b) since table saw blades are tensioned and move when the saw is running due to centrifugal force, and since there's always going to be some runout not only in the saw blade itself, but also in the arbor shaft that it's attached to, the only meaningful way to measure this is while the saw is running -- which is hard on dial indicators, not to mention your fingers;;)
c) wood moves a lot more than the distances we're talking about due to humidity changes, the release of stresses within the wood when the wood is being cut, and 100 other reasons.

In my opinion, all that matters is whether the tool does what you want it to (cut wood with acceptable precision, or sharpen knives decently).
Whatever the case, I suspect "operator error" and "measurement error" account for 100X if not 1000X more variation and "noise" than anything we could measure in Planck Lengths.

To go a little deeper into the rabbit hole, I think it's fair to say: There is always going to be some error/imperfection -- in the flatness of the stone being measured, in one's measurement of that flatness, or most likely, both. Which means that even if one achieved a perfectly planar surface, one might not recognize it as such, due to errors in measurement.

I'll flatten a stone if it gets an obvious belly in it, but I don't think I'd ever break out a feeler gage. Therein lies madness, because "perfectly planar" is an imaginary ideal, not something attainable. So then the questions become "How close to perfectly planar is 'good enough'?" and -- more importantly -- "What is the standard deviation of my sharpening technique in milliseconds of angle?" Even using a sharpening jig, I'd bet the standard deviation is >10X anything resulting from any nonplanar condition that one might measure with feeler gages.

YMMV.
 
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Cementite said:
I think some people get a little overly hung up on these infinitesimal measurements sometimes.
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Yes, this is true. Accuracy and precision are two things we aim to achieve in many endeavors, and we put so much effort in attempting to achieve perfection that we forget perfection isn't possible. Instead we should spend time thinking about what's "good enough".

That being said, measure small, miss small. Measure big, miss big.

I support OP in his goals for flatness, but at the point of "being driven nuts" over it, we must help OP overcome his obsession and settle for what is practical.
 
My grandpa would be laughing at you guys. His big bench stone (which I learned on at age 6) was dished out almost 1/2”, yet he always had the sharpest knives of anybody I knew.

Wish I had that old beast now (the stone, I mean).

Parker
 
catspa said:
My grandpa would be laughing at you guys. His big bench stone (which I learned on at age 6) was dished out almost 1/2”, yet he always had the sharpest knives of anybody I knew.

Wish I had that old beast now (the stone, I mean).

Parker
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I imagine it'd be hard to sharpen scandi grinds, chisels and gouges with.

It's not that dished stones are unusable- they just aren't suitable for all situations.
 
Slim7 said:
May sound overkill, but i try an aim to get my sharpening stone dead flat, with straight edge and feeler gauges. I can never succeed with loose grit. Some hard stones (India, Arkansas, ceramics) need to be lapped with loose grit because a fixed abrasive will glaze them. In my experience loose grit always leaves a 3-4 thou crown/convexity in my stones, and it drives me nuts... I bought one of those diamond labridary wheels thats talked about here, but mine wasn't flat. I placed it on flat granite and the disc was out of true bad. Those diamond disc will turn a white Hard translucent into milk pretty quickly or i a decent time, but it won't be flat, at least mine wasn't because the disc was out of true.



The only way i have been able to get a hard stone dead flat is to use Sic powder to do the grunt work. Then go to a DMT xxc to get dead flat, then go back to sic and recondition the surface with sic, just enough to get the proper surface texture, without staying on it too long (45 seconds ) to re introduce the crown back into the stone . Any suggestions?
Click to expand...
Loose grit results in a crowned stone every time and flattening in a steel pan exacerbates the problem if that is what you are doing. I usually go to a wet dry paper after loose grit. I put down a worn wet dry paper first to not wear the surface of my granite plate.

As you probably know hard arks eat diamond plates up so don't use any you care about. Wet dry paper works fine for maintenance or dressing the surface as does old worn diamond plates. I also use the old three stone method a lot because they are at hand when I sharpen.

 
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.
 
garry3 said:
I use my stones for tools, knifes and straight razors. Flat matters to me, a lot.
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Again, the question is- what's acceptable for flatness? +/- 0.001?
Because, and I promise you this- no stone you've ever used or will ever use is truly flat.
And then, as soon as you use it? It's even less flat than it was when you started.
 
Diemaker said:
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.
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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?
 
Slim7 said:
... In my experience loose grit always leaves a 3-4 thou crown/convexity in my stones, and it drives me nuts...
Click to expand...

Just for grins, I ran the trig.

If your stone is 6" long, and it has an even crown (in other words, with the surface of the stone forming an arc or segment of a circle) that is .004" higher in the center than at the edges, then the angular difference from the center of the stone (at the highest point of the crown, where it will be "level") to the edge of the stone would be 76/1000 of one degree, or less than 1/10 of 1°.

Doubling that would give you the angular difference between the left and right edges of the stone (6 inches apart), which would be 152/1000 of one degree (less than 2/10 of 1°).

At its very worst, assuming the stone is 2" wide, the angular difference between the front and back edges of the stone (in the "narrow" direction, 2" apart) would be less than 1/2 of 1°.

(The figures would be the same regardless whether the stone was concave or convex, so long as the difference was .004" and the stone was 2" x 6". If your stone is larger than 2x6, the angular differences will be less.)

I don't know about anyone else, but I don't think I could freehand sharpen and keep my bevels to a tolerance of ± 1/4 of 1 degree, let alone ± 1/10 of 1 degree, if my life depended on it.
 
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