(Why) Are Coarse hones faster than Fine hones?

[ToddS]

If we imagine two ideal hones, one having 5 micron abrasive grit and the other having 10 micron grit, with all else equal, we could expect the dimensions of the metal swarf from the 10 micron hone will be twice as thick and twice as wide as those from the 5 micron hone. For the same distance traveled, the length of those swarf particles would be the same for both hones. Therefore, the volume of the individual swarf particles should be 4 times larger for the 10 micron hone than for the 5 micron hone.

It may also be reasonable to assume that the number of swarf particles produced will be proportional to the density of grit particles in each hone. If we consider the following drawing, where the red circles represent the 5 micron particles and the blue circles represent the 10 micron particles, we see that there can be four times as many 5 micron particles as 10 micron particles in the same area.

Combining these two assumptions, the 10 micron hone removes one quarter as many metal particles as the 5 micron hone, but the particles will be four times the volume. In other words, shouldn't we expect the material removal rate to be the same for both???

 

[cbwx34]

No, because you're thinking in 2D. Think 3D.... how far they "stick out".

 

[ToddS]

No, because you're thinking in 2D. Think 3D.... how far they "stick out".

I'm saying that the 10 micron cuts twice as deep as the 5 micron, do you think it cuts deeper than that?

 

[Jason B.]

It cuts as deep as the applied pressure will allow. You can make stones cut coarser or finer by a simple variance of pressure. I would guess that the variables are too high to get a good caculation though.

 

[Insipid Moniker]

No, because you're thinking in 2D. Think 3D.... how far they "stick out".

This. You're getting volume and area mixed up. Area = 10x10 vs 5x5. Volume = 10x10x10 vs 5x5x5, so you're looking at closer to ten times the volume than 4 times.

 

[Steel_Drake]

This. You're getting volume and area mixed up. Area = 10x10 vs 5x5. Volume = 10x10x10 vs 5x5x5, so you're looking at closer to ten times the volume than 4 times.

Shouldn't that be 10x10x10 = 1000 vs 5x5x5x4 = 500, since there are four of the 5 micron particles for every one of the ten.

Also, doesn't it follow from the assumptions given that the total width and depth covered by abrasive will always be equal since abrasive density increases exactly to offset decrease in abrasive particle size, and will not cutting speed thus vary exactly according to abrasive particle height alone?

 

[Insipid Moniker]

Shouldn't that be 10x10x10 = 1000 vs 5x5x5x4 = 500, since there are four of the 5 micron particles for every one of the ten.

Also, doesn't it follow from the assumptions given that the total width and depth covered by abrasive will always be equal since abrasive density increases exactly to offset decrease in abrasive particle size, and will not cutting speed thus vary exactly according to abrasive particle height alone?

Exactly, though I honestly have no clue if the assumptions made in the original question actually bear out under scrutiny.

 

[ToddS]

It cuts as deep as the applied pressure will allow. You can make stones cut coarser or finer by a simple variance of pressure. I would guess that the variables are too high to get a good caculation though.

You are correct about pressure, but what if we use the same pressure?

I'm just suggesting a simple model to point out that all else equal (including pressure) the removal rate should be the same for the two hones.

 

[ToddS]

This. You're getting volume and area mixed up. Area = 10x10 vs 5x5. Volume = 10x10x10 vs 5x5x5, so you're looking at closer to ten times the volume than 4 times.

No, because the length of the swarf curl will be the same for both, determined by the length of the pass. Each particle of grit interacts with the steel for the same distance. Only the cross-section AREA of the swarf curl changes.

 

[awestib]

If the "volume" of the sticking out abrasive is exactly double between the 5 and 10 micron that hits the steel and if we assume that all of that abrades the steel, than the volume of abrasion should be the same. However, if the pressure is the same, the 5 micron dig deeper than the 10 so in reality, the 5 micron should abrade a larger volume (pressure x surface area) - No?

 

[ToddS]

If the "volume" of the sticking out abrasive is exactly double between the 5 and 10 micron that hits the steel and if we assume that all of that abrades the steel, than the volume of abrasion should be the same. However, if the pressure is the same, the 5 micron dig deeper than the 10 so in reality, the 5 micron should abrade a larger volume (pressure x surface area) - No?

It's true that the 5 micron particle would dig deeper than the 10 micron for the same force because the pressure would be 4 times higher (force/area) - the same reason a sharp knife cuts better than a dull knife.

In this model, the contact area for the 10 micron particle is 4 times the contact area for the 5 micron particle (assume they are the same shape) but there are 4 times as many 5 micron particles per unit area, so the total contact area is the same. The pressure (force/area) at each contact point (grit particle) should therefore be the same (for the same downward force applied to the blade onto the hone).

 

[cbwx34]

It is a good question. Honestly, I'm not sure if there's a flaw in your reasoning... or just in reality abrasives aren't "perfect" like your example.

But here's how I think about it. I think the answer in part, lies in how much metal (or whatever) one particle of abrasive removes. So, a 10 micron abrasive (assuming 10x10 is sticking out) removes 4 times as much as a 5 micron. (I'd also ignore pressure... just trying to simplify that the abrasive removes the maximum amount it is capable of).

The next part might lie in the fact you can't quadruple the abrasive in the same area... and assume they're all effective. If you put 2 abrasives in a line... one right behind the other, wouldn't the "2nd" abrasive basically do nothing... it would just follow in the groove created by the "lead" abrasive? So, in reality, it might not be as easy as just "filling up" the space of a larger abrasive with smaller ones.

Anyway... I guess my thinking is... the real comparison lies more in the amount of work each individual abrasive does? In typing this, I'm thinking that your theory might be correct if you doubled the width... so a 1" wide 10m and a 2" wide 5m would remove the same amount? One 10m and four 5m side by side would remove the same... but twice the width.

Anyway... maybe not really an answer... just spitballing some thoughts on the subject.

 

[Jeff Clark]

I think we are dealing in too many dimensions and using spheres where we don't need to. Think of saw blades. A coarse tooth saw cuts a lot faster than a fine tooth saw. It is real conspicuous when you use a fine tooth band saw blade and a coarse one. The pressure at the contact points is much higher with the coarse tooth blade. There is a lot better clearance for swarf removal. If you use fine toothed blades you not only make contact on more teeth you are also riding on some residual swarf. Spheres don't really cut at all and are a model for a horrible abrasive. I exchanged emails with Sal Glessar at Spyderco and he remarked at how much sharper diamond grit is than even nicely cubic boron nitride.

It is also wrong to presume a nice flat packing of the coarse grit. In reality there is a large fraction of high points doing the work with a lot of low-lying filler between. For most solid hone materials you rely on breakdown that knocks loose grit, it doesn't just wear the grit down flat. That is the nature of waterstones for example. There is actually a softer (yet slightly crumbly) matrix supporting the harder abrasive grit. Water helps that matrix to loosen up and release worn grit to keep replacing it with fresh sharp grits. Oil stones tend to be tougher, but also break down to reveal fresh sharp grit--but still with gaps between the serious projecting cutting grit. In the case of diamond hones you can really feel the roughness of a fresh hone as sharp uneven large grit slash through the surface you are trying to hone.

Anyway a coarse hone is a lot like a coarse saw or coarse file. It is much easier to engage the material you are trying to cut with coarse teeth. If you don't get high enough pressure per grit you skip across the surface. In the case of a sanding belt or grinder you can also detect how much more heat you generate per micro-gram of metal you remove. There is just more dragging friction relative to the amount of constructive work (metal removal) with a fine grit (or fine tooth blade).

 

[ToddS]

I'm not sure I see a saw blade as a fair analogy. A coarse blade (lower TPI) can cut faster because you can feed the stock faster, which in turn is possible because it clears material from the cut more efficiently. I think we would compare speed in that case as how fast can you feed stock without binding the blade.

I don't mean to suggest that a real hone is as simple as uniformly packed spheres - I just use this to simply show how the number of abrasive particles per unit area will increase as the size of the particles decreases.

Shapton Pro 320, for example:

It's true that fine diamond hones with a few anomalously large diamonds will cut much faster than a plate with a uniform distribution of diamonds - but that's more complicated than coarse vs fine.

 

[bucketstove]

Hi,
my image, these are profile views of two stones, two rakes,
5 micron on top, 10 micron below,
the 10 micron taller rake digs deeper removes more metal,
bigger shovels are bigger

After putting up this image I remembered I read a bit about this in
Characteristics of Abrasive Particles and Their Implications on Wear | InTechOpen

Paper has pictures/drawings of abrasives, a formula, and a rate of material removal versus grit/grain/micron chart I include below

Things to note, they used a single abrasive grain a tooth, bigger grain digs deeper, after 70 micron/P220 grit removal rate slows, no longer linear
abrasive-micron-removal-rate.jpg

 

[eKretz]

I wonder if the swarf doesn't limit the downward travel of the item being abraded after a certain amount of movement - since the abrasive particles stick up less, there's less room for swarf before it might bind up and limit the depth of cut. I need to do some thinking on this one.

 

[ToddS]

Hi,
my image, these are profile views of two stones, two rakes,
5 micron on top, 10 micron below,
the 10 micron taller rake digs deeper removes more metal,
bigger shovels are bigger

After putting up this image I remembered I read a bit about this in
Characteristics of Abrasive Particles and Their Implications on Wear | InTechOpen

Paper has pictures/drawings of abrasives, a formula, and a rate of material removal versus grit/grain/micron chart I include below

Things to note, they used a single abrasive grain a tooth, bigger grain digs deeper, after 70 micron/P220 grit removal rate slows, no longer linear

Thanks for that, it's quite interesting, but those data are from a scratch test, which is a single particle (as you point out). A hone is a bit different in that the smaller the particles, the more there are.


PS
I found what I was looking for in one of the references in that paper, (Gahlin, R. & Jacobson, S. (1999). The particle size effect in abrasion studied by controlled
abrasive surfaces. Wear, Vol. 224, No. 1, pp. 118-125).

The wear rate is the same for fine/medium/coarse densities of sharp tips.

 

[ToddS]

I wonder if the swarf doesn't limit the downward travel of the item being abraded after a certain amount of movement - since the abrasive particles stick up less, there's less room for swarf before it might bind up and limit the depth of cut. I need to do some thinking on this one.

When I look at the swarf, it doesn't seem like it's being significantly damaged by repeated passes. Which makes me skeptical that the metal swarf plays a major role. The broken and dislodged grit particles are likely more important.

 

[cbwx34]

Thanks for that, it's quite interesting, but those data are from a scratch test, which is a single particle (as you point out). A hone is a bit different in that the smaller the particles, the more there are.

I think bucketstove's picture/post is what I alluded to earlier... and what makes the difference here. It's not that the finer hone contains more particles, and therefore should work just as fast, as you stated in your original post... it's how much work they can do at a given point in time. So, yes, a 5m stone would work as fast as a 10m, (all other variables being equal) if, at a given point in time, there were 4 of the 5m available for 1 of the 10m... so twice the width.

 

[ToddS]

The obvious question that follows is why use a progression of grits if the coarser stones aren't faster?

If I can remove metal at the same rate with a 4k as I can with a 1k, why bother with the 1k and 2k?