(Why) Are Coarse hones faster than Fine hones?

[me2]

Can material in fact be removed as fast with finer grits? That has not been how it works for me. Does a 120 grit belt remove material as fast as a 60 grit belt? Has any one tried?

 

[cbwx34]

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?

Because coarser stones are faster... and you can't remove metal at the same rate with the 4K as with the 1K, everything else being equal. That's the simple answer.

But again, it depends on the variables. If I'm using a sharpener that has a 1" wide stone, and want to do some reprofiling on a 3" long knife... I'd use a very coarse stone, around 100g or whatever is available. But take that same knife and put it on a 3" wide waterstone... I can easily get the same job done at 1000g as fast or faster.

I think the point being missed is how much work can be done at a given point in time. It's rather obvious from a practical standpoint that a coarser stone is faster... everything else being equal. Spend 1 minute on 1 side of a knife with a 1K stone, flip the knife over and spend 1 minute with a 4K stone will tell you that. But add width and a finer stone could become faster, since at any one moment more of the stone is available to do work. The (probably obvious) point also is that, if the knife is only 1" wide, then putting it on a finer 3" stone vs. the 1" coarse stone... now the coarse stone would again win out.

 

[HeavyHanded]

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.

The unit pressure is going to be tremendously increased for the coarse stone. The absolute tip of each contact point shouldn't be a whole lot larger - determined by the abrasive type and condition.

In your 5 micron and 10 micron abrasive, lets imagine that's the amount the abrasive stands proud. that would make them a 100 micron and 50 micron abrasive chunk. These hones are not all that far apart, the difference between 120 and 360 grit or so. If you compare a 100 micron and 20 micron now you have a much larger difference. Now 10 and 2 micron abrasive depth, a difference of 5x the unit pressure on each available abrasive point for the same amount of applied force.

You have to factor in not only increased unit pressure but dramatically increased chip clearance. The rougher stone can accommodate greater volume of swarf.

Another factor depending on abrasive is the coarse hone due to much greater unit pressure will experience break down at a much faster rate, the finer abrasive will experience greater glazing effect. Increased pressure will only accelerate the plugging of the stone with nowhere for the swarf and broken abrasive to go.

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.

Bingo!

 

[eKretz]

Todd, do you have a link for the paper you were able to check from the references? Looks interesting:

(Gahlin, R. & Jacobson, S. (1999). The particle size effect in abrasion studied by controlled
abrasive surfaces. Wear, Vol. 224, No. 1, pp. 118-125).

 

[HeavyHanded]

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?

For the same reason you cannot get a highly refined edge with a coarse hone no matter how lightly you work it. The unit pressure is still too high, and the lack of relative uniformity from one peak to the next as a percentage of overall abrasive size, make it impossible - though there is certainly room for variation depending on amounts of applied force.

One can easily do an experiment with a handful of people on both guided and unguided systems. Then work backward from those results which I can 100% guarantee will show the coarse hone removing material at a much faster rate...all else being equal.

On a powered system, another give-away will be the increased amount of heat being generated from the finer abrasive, all else being equal.

As another consideration, even on imperfectly formed points, such as ones at a poor rake angle or ones that have been blunted, the increase in unit pressure on the coarse hone may allow for some continued scratch removal where on the finer abrasive the imperfect points are incapable of penetrating due to drop off of pressure.

 

[Steel_Drake]

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?

A couple of points on why I think the predictions of this model don't translate into the real world:

1) In most types of sharpening stones the abrasive particles are held in some type of binder. There is no reason to assume that larger abrasive particles do not have a relatively larger proportion of their surface area exposed above the binder than smaller particles would. I would think that this would exaggerate the difference in exposed abrasive size to the point where area density could no longer compensate for the difference in particle size.

2) In reality you cannot abstract away the influence of pressure. Fewer, larger abrasive particles which have sharp edges (and are not flat topped like your model) translate into the same grinding force being applied across fewer, smaller contact points, and thus much higher pressures at each abrasive particle.

 

[David Martin]

Both good factors at work in this equation ^. Pointing us toward faster metal removal with the more coarse grit. Plus, cleaning your stone during use helps create space for swarf. More surface space for waste material. DM

 

[HeavyHanded]

More good reading:

http://slideplayer.com/slide/6137209/

Edit to add: good topic!

 

[cbwx34]

The unit pressure is going to be tremendously increased for the coarse stone. The absolute tip of each contact point shouldn't be a whole lot larger - determined by the abrasive type and condition.

In your 5 micron and 10 micron abrasive, lets imagine that's the amount the abrasive stands proud. that would make them a 100 micron and 50 micron abrasive chunk. These hones are not all that far apart, the difference between 120 and 360 grit or so. If you compare a 100 micron and 20 micron now you have a much larger difference. Now 10 and 2 micron abrasive depth, a difference of 5x the unit pressure on each available abrasive point for the same amount of applied force.

You have to factor in not only increased unit pressure but dramatically increased chip clearance. The rougher stone can accommodate greater volume of swarf.

Another factor depending on abrasive is the coarse hone due to much greater unit pressure will experience break down at a much faster rate, the finer abrasive will experience greater glazing effect. Increased pressure will only accelerate the plugging of the stone with nowhere for the swarf and broken abrasive to go.

Given what ToddS is asking... I don't think pressure (or even swarf) needs to be factored in. (Don't misunderstand, I know in actual sharpening it does). I think he's just asking why a 5m stone won't work as fast as a 10m... everything else being equal. I know realistically, pressure / swarf / etc. all contribute, but to the original question not so much... the way I read it anyway.

 

[ToddS]

You might guess I have a reason for asking this question. I measured the removal rate for a piece of hardened carbon steel (approximately 1cm^2) on a variety of hones with the same downward force (approximately 500g), and found that the removal rate was essentially the same for the Shapton Pro 320, Chosera 1k, Shapton Glass 2k and Shapton Glass 4k. (The rate did drop off with the 8k and 16k Shapton Glass stones). I'm not claiming this was a systematic or highly controlled study, but it does lead me to the question of why do we generally assume coarse hones are faster than fine hones.

 

[HeavyHanded]

Given what ToddS is asking... I don't think pressure (or even swarf) needs to be factored in. (Don't misunderstand, I know in actual sharpening it does). I think he's just asking why a 5m stone won't work as fast as a 10m... everything else being equal. I know realistically, pressure / swarf / etc. all contribute, but to the original question not so much... the way I read it anyway.

Pressure is everything.

Even if you switched the conversation to two 800 grit surfaces, one with an open coat and one with a closed coat, the open coat (for as long as the abrasives stay intact) will cut faster - remove more steel per unit of infeed time. And this with the same approx abrasive depth potential and a smaller number of points per.

The better question to my mind, is at what stage the reduction in points per no longer increases stock removal. From that standpoint, Todd's hypothesis is correct...at some stage outside the ordinary I suspect. Perhaps a 10 grit stone will remove stock as rapidly as a 600 grit stone simply because the possible surface area is so small, and the grind waste is no longer making use of the additional clearance - its wasted.

For woodworking tools there might not be a lower limit (subject to reason), as rasps have no analog to steel removal that I'm aware of.

We tend to think of things as being more or less linear when it comes to grinding but in my experience that is not true. Maybe if we were working with a material that did not have the same structure as steel.

 

[ToddS]

Pressure is everything.

Even if you switched the conversation to two 800 grit surfaces, one with an open coat and one with a closed coat, the open coat (for as long as the abrasives stay intact) will cut faster - remove more steel per unit of infeed time. And this with the same approx abrasive depth potential and a smaller number of points per.
....

look at post 17 again
http://www.bladeforums.com/forums/showthread.php/1449226-(Why)-Are-Coarse-hones-faster-than-Fine-hones?p=16681211#post16681211

They are showing that if the abrasives are sharp, there will be no difference in the removal rate for open and closed coat.

 

[HeavyHanded]

You might guess I have a reason for asking this question. I measured the removal rate for a piece of hardened carbon steel (approximately 1cm^2) on a variety of hones with the same downward force (approximately 500g), and found that the removal rate was essentially the same for the Shapton Pro 320, Chosera 1k, Shapton Glass 2k and Shapton Glass 4k. (The rate did drop off with the 8k and 16k Shapton Glass stones). I'm not claiming this was a systematic or highly controlled study, but it does lead me to the question of why do we generally assume coarse hones are faster than fine hones.

Did you begin with a standard surface finish for every sample? How did you measure the amount of removed material?

Interesting enough. So really you are thinking the added depth of the coarse abrasive is offset by the increased surface abrasion of the finer abrasive. This would be a factor largely overlooked in a general sense, as we shoot for depth when making repairs or corrections, then grind the resulting asperities with finer and finer abrasives. It would appear they are faster, but in reality are working the Y axis more than the X.

Still and all, the larger chip from the coarse abrasive has a much wider surface area at its gouge trough. So much deeper and much wider than a collection of smaller abrasives. Back to the 3D model with its surface area to volume relationship. Applied to chip removal that explains plenty.

 

[cbwx34]

Pressure is everything....

I agree... I'm just thinking that's not what the question is. You could never get a fine stone to cut as fast as a coarse stone... unless you modified something else (like pressure), (which if you then applied to the coarse stone, would again cut faster). It's the question of one working just as well as the other, independent of other factors I thought he was trying to figure out.

Still and all, the larger chip from the coarse abrasive has a much wider surface area at its gouge trough. So much deeper and much wider than a collection of smaller abrasives. Back to the 3D model with its surface area to volume relationship. Applied to chip removal that explains plenty.

Yup... this, (which you added while I was answering) is what I was getting at.

You might guess I have a reason for asking this question. I measured the removal rate for a piece of hardened carbon steel (approximately 1cm^2) on a variety of hones with the same downward force (approximately 500g), and found that the removal rate was essentially the same for the Shapton Pro 320, Chosera 1k, Shapton Glass 2k and Shapton Glass 4k. (The rate did drop off with the 8k and 16k Shapton Glass stones). I'm not claiming this was a systematic or highly controlled study, but it does lead me to the question of why do we generally assume coarse hones are faster than fine hones.

I think you'd have to give more of the specifics of your experiment, because now you have introduced other factors into the equation besides simply the abrasive size.

 

[HeavyHanded]

look at post 17 again
http://www.bladeforums.com/forums/showthread.php/1449226-(Why)-Are-Coarse-hones-faster-than-Fine-hones?p=16681211#post16681211

They are showing that if the abrasives are sharp, there will be no difference in the removal rate for open and closed coat.

I'm not familiar with that experiment, but I do know if I try to sand some oak with 320 grit wet/dry it plugs up in an instant, and 320 open coat sandpaper keeps right on going. A case of the experiment not matching direct observation of similar phenomena.

Again, this isn't theoretical. We're working against a material with a lattice structure and carbides (even cementite will have their effect) that are defined by their size.

 

[ToddS]

...
I think you'd have to give more of the specifics of your experiment, because now you have introduced other factors into the equation besides simply the abrasive size.

I simply took a workpiece and rubbed it back and forth on each hone, and determined the removal rate from the change in weight.

 

[BluntCut MetalWorks]

There is an optimal range of volume displacement (VD / feed rate) by bit configuration per material. In uniform distribution in an area, bit density is square order. Whereas VD is cubic order. In a meaningful comparison, pressure/feed-rate should be within optimal range per bit size. So e.g. 320 to 5K shapton rate of removal are about the same for 500gr, which could mean 500gr still within 5K shapton upper optimal limit vd rate and at the same time it's at the lower limit of 320grit shapton. Now, with 10kg pressure - 5K shapton will not cut well at all (due to grit detachment + loading). A needle is not a jack hammer bit and vice versa.

 

[cbwx34]

I think, just looking at your picture, that other factors now come into play. For example swarf could affect it... might try repeating it under running water where the stone is continually flushed?

I also thought a bit more about pressure... while one route (that you took) was to keep the pressure the same... the other route (that I think Heavyhanded meant, in part anyway), is finding the optimal pressure that the stone works at? So, for example, you could find a set pressure where both stones cut the same... for example very light pressure wouldn't take advantage of the "cutting ability" of the coarse stone, so both would appear to be approximately the same. But add a little pressure and the cutting of the coarse stone could increase dramatically, with little affect on the finer stone.

 

[ToddS]

There is an optimal range of volume displacement (VD / feed rate) by bit configuration per material. In uniform distribution in an area, bit density is square order. Whereas VD is cubic order. In a meaningful comparison, pressure/feed-rate should be within optimal range per bit size. So e.g. 320 to 5K shapton rate of removal are about the same for 500gr, which could mean 500gr still within 5K shapton upper optimal limit vd rate and at the same time it's at the lower limit of 320grit shapton. Now, with 10kg pressure - 5K shapton will not cut well at all (due to grit detachment + loading). A needle is not a jack hammer bit and vice versa.

I did a couple of tests with approximately 5 times more force, basically as much as I could apply with two fingers and still move the workpiece back and forth on the 320, 1k and 4k. These three show a trend where the coarser hone is faster than the finer, but the 320 is still only about twice as fast as the 4k.

 

[ToddS]

I think, just looking at your picture, that other factors now come into play. For example swarf could affect it... might try repeating it under running water where the stone is continually flushed?

I also thought a bit more about pressure... while one route (that you took) was to keep the pressure the same... the other route (that I think Heavyhanded meant, in part anyway), is finding the optimal pressure that the stone works at? So, for example, you could find a set pressure where both stones cut the same... for example very light pressure wouldn't take advantage of the "cutting ability" of the coarse stone, so both would appear to be approximately the same. But add a little pressure and the cutting of the coarse stone could increase dramatically, with little affect on the finer stone.

I'm not sure I see why swarf loading (assuming it is a factor) would affect the coarser hone more than the finer hone.

I suspect you are correct that there is a "lower pressure threshold" and this is likely a bigger issue with the diamond plates where the coarse particles aren't very sharp.