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Power sharpening: how fast is too fast?

Posted by WobblyHands 
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Re: Power sharpening: how fast is too fast?
November 06, 2016 07:55PM
Here are a couple quotes from the thread razoredge cites, that better articulate what I had been trying to get at. Hyperlinks didn't format, so look on the other site. Plenty to discuss, but read first. I still have to read over everything myself. So far just excited to see I am not the only one in the world of the same viewpoint or concerns... grinning smiley

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So I found a paper from UMass published 1995 in the JOURNAL OF ENGINEERING FOR INDUSTRY that may be applicable:
[manufacturingscience.asmedigi...icleid=1447963]

The authors embedded thermocouples into steel (including AISI 1020 at 40 Rc and also 52100 and O1 steels hardened to 62 Rc, none "high speed" or stainless alloys) and ground the surfaces repeatedly using abrasive wheels running 30 m/s (for reference, the HF 1x30 runs ~15 m/s) cutting to a depth of 25 microns on each pass, with the steel plate feeding at ~13 cm/s - I am not skilled enough to translate this to applied pressure of the grinder against the work-surface, it may be that the pressure is lower than hand-sharpening but I doubt it...

They found that the total energy generated by the grinding-action could be converted to a maximum heat <850'C for 4 milliseconds.
They found that 60-75% of this energy is transferred to the workpiece as heat when using Aluminum-oxide wheels under these conditions, resulting in heating the first 10-20 microns of the surface to ~500'C for 4 ms.
They found that only ~20% of the energy was transferred to the workpiece as heat when using CBN wheels under these conditions, resulting in heating the first 10-20 microns of the surface to ~120'C for 4 ms.
They found that you could not use the CBN wheels under these parameters even with lubrication to achieve good cutting of the 1020 plate because it is too soft and glazes readily, the harder steel plates cut better.
The CBN abrasive has higher thermal conductivity and retains its sharpness longer (so they suppose) such that significantly more heat is conducted away from the work-surface...


And another article from the journal Materials Research 2002: [www.scielo.br]
Among the conclusions is this:
The cooling properties of the cutting fluids can be neglected and don't cause any improvement in the reduction of the grinding zone temperature and in prevailing compressive residual stresses.
Now both of these papers are not specifically related to knife edges, and both DO recommend the use of lubricant while power-grinding, but they are also discussing grinding at much higher speeds and cutting depths, and the lubricant is recommended primarily to preserve the life of the abrasive which tends to be more heat sensitive than the work surface. *shrug*

Here is another paper, 2008: [wumrc.engin.umich.edu]
This one is trying to develop a more accurate method of measuring total grinding temperatures (not just the amount absorbed by the work-surface) via thermocouples.
They were grinding (dry compared to wet) with an alumina-oxide wheel into The workpiece material was Dura-Bar 100-70-02 (carbon content of 3.5–3.9%, Rockwell hardness HRC 50, thermal conductivity of 63 W/m K, and thermal diffusivity of 1.63).
They measured a maximum temperature rise at the very last microns of contact to be 500'C with ~85% transfer to the work-surface (425'C) correlating closely with the first paper posted above. Once again, this was noted:
Grinding experiments and heat transfer analyses showed that grinding fluids provide negligible cooling within the grinding zone.

Are these data points relevant to the discussion?
52100 is tempered at ~200'C for two hours, what is the impact of heating even the last 10 microns to 200'C for 4 ms?

It is mentioned in these and other papers I've found that water or majority-water lubricants largely retain the thermal properties of water and so vaporize before they reach the point of contact, such that their impact is not felt by the contact-area itself which responds as if it were being cut dry. The primary reason for using these liquids at all is their lubricity (reducing friction) and to wash away swarf that can clog the abrasive and cause irregular grinding.

Again, none of this means that you cannot over-heat an edge through power-grinding or maybe even hand-grinding, thereby ruining the temper at the very apex. I think we know that it CAN happen, bluntcut has put forward (on various occasions) the factors involved in getting it done, but I challenge the assertion that it MUST happen without use of a coolant/lubricant or that the use of these matters at all for that last few microns of apex.

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This is why I appreciate what Cyrano is doing here - clearly presenting ALL of his methods/materials along with the results for criticism, etc. which he takes back for further testing. Regarding common usage that heats an edge, it would be interesting to see what his method evinces on a blade used for cutting cardboard or chopping wood - I have personally felt how uncomfortably hot the bevel of my knife or axe can get after doing a lot of cutting. I guess I should be using coolant whenever I'm cutting dry material with my knife? When we talk about knives having more/less abrasion-resistance via carbides, do we think the CATRA tests do not generate much heat at the apex of the knives? Or do we think the heat generated is insufficient to significantly impact performance?

The fact of the matter is that, just as Cyrano indicated in his first post, there is STILL NO DEFINITIVE ANSWER regarding damage to apices through over-heating during grinding - we know that it CAN happen, we certainly know HOW to cause it if desired (Cyrano shows us how in this very thread), but it remains to be shown that it MUST happen if specific coolant is not used - steel type, grit, pressure, time, other circumstances.
Landes offered limited evidence of a specific tool made from steel that very likely has low heat-resistance (since we don't know the steel, no conclusion can really be drawn), ground in an unspecified manner (he gives the grit, but no time or pressure which are key components without which no conclusions can be drawn), and the hardness tester used finds a change in hardness within the first 100 microns (NOT the first 300) but we do not have the control measurements of the hardness at those points prior to sharpening. *shrug* So what he offers is a warning that power-grinding dry might damage the first 100 microns of your edge. And stay away from crok-stiks?
Re: Power sharpening: how fast is too fast? (3 fps dry)
November 27, 2016 04:01PM
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Any Cal.
... 1 hrc difference would not be measurable in cut testing

A few points :

-a drop in hardness from increasing tempering isn't the same as what happens from a difference in austenization

-most people would not be able to tell a difference from m390 and 440C in cut testing either

Here is the only real relevant point :

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oldsailorsknives
I grind at low speed wet to prevent the chance of overheating. If have spent the last 5 hours heat treating a blade and getting the hardness I want, why risk overheating and creating soft spots on the edge.

If a maker :

-spends a lot of money on a very expensive steel

-uses very expensive and intricate processing

and then at the end subjects a steel to random time/temperatures then it just makes no sense at all. If during the above any of the manufacturers did the same then they would rightly complain as they are paying for exact scales to get a particular result. But then in the final stage where the actual cutting performance is actually created they ignore a significant risk for what purpose exactly?

It is trivial to use a coolant, the cost is literally a few dollars worth of parts for a basic setup (drip cooler) and not much more for a mister. Beyond the cooling it also makes it easier to grind (more control, less pauses) and saves belt life as it reduces friction losses and wear and it keeps a lot of the dust out of the air. It makes no sense to run abrasives dry.
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 02, 2016 10:14AM
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CliffStamp
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Any Cal.
... 1 hrc difference would not be measurable in cut testing

A few points :

-a drop in hardness from increasing tempering isn't the same as what happens from a difference in austenization And?

-most people would not be able to tell a difference from m390 and 440C in cut testing either Exactly

Here is the only real relevant point :

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oldsailorsknives
I grind at low speed wet to prevent the chance of overheating. If have spent the last 5 hours heat treating a blade and getting the hardness I want, why risk overheating and creating soft spots on the edge.
And we still have no way of knowing if he is overheating or not, just as we have no way of knowing if I am. Coolant isn't guaranteeing anything, in fact could be lulling the operator into a false sense of security.

If a maker :

-spends a lot of money on a very expensive steel

-uses very expensive and intricate processing

and then at the end subjects a steel to random time/temperatures Still no proof of this being the case. then it just makes no sense at all. If during the above any of the manufacturersNot sure what that means? did the same then they Who? would rightly complain as they are paying for exact scales to get a particular result. But then in the final stage where the actual cutting performance is actually created they Who?ignore a significant riskStill no proof of significance, occurence, or extent of such. for what purpose exactly?

It is trivial to use a coolant, Incorrect, in my experience. the cost is literally a few dollars worth of parts for a basic setup (drip cooler) and not much more for a mister. Beyond the cooling it also makes it easier to grind (more control, less pauses) Not necessarily. and saves belt life as it reduces friction lossesIncorrect and wear and it keeps a lot of the dust out of the air. It makes no sense to run abrasives dry.

I understand where you are coming from Cliff, and agree to a certain extent. I subscribed to the same theory until I had more experience making and selling knives. Learning some of the machinists trade helped teach me about coolants and how they work- or don't.

What it comes down to is that we are talking about a variable that is MAYBE 1% of the performance of the knife, likely far less depending on how and what we are measuring. If it demands more than 1% of the attention/labor/cost/time, then you have to weigh its importance.

After all the back and forth, there is NO proof, or even existence of a proper test to prove, that careful grinding 1) overheats edges, or 2) that any additional tempering is reducing edge life. We know that an edge can be overheated, but not what is required to keep from doing so, or how much the edge can take before there is a problem.

This is very much like a group of people claiming one supercar better than another because it has a top speed 2mph (kph for you, ha!) faster. Few will have the opportunity to own the car, and of those, even fewer will have the opportunity to see what the top speed is. Of those that can buy the cars, some will choose to buy based on looks, others on comfort, others on handling. The top speed is almost immaterial to actual buyers.
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 02, 2016 12:18PM
ok, so you learned about machining. explain why ALL the cutting and grinding on drive axle pieces is done wet, and here the cost is significant. explain why all the cutting and grinding to make the punches and dies that forge the drive axle pieces are done wet.
please explain "Careful Grinding". do you mean low speed, fresh belts or discs, minimum pressure, cooling the blade?
what is more trivial than coating the belt with water before grinding and at regular intervals after you start? you can do it with a 50 cent paint brush and water.
"saves belt life as it reduces friction losses Incorrect " then why is my belt life increased when i grind wet compared to the exact same belt and grinder speed dry?
all this makes my head hurt. off to the doctor to see if it is hollow like most folks say or full of straw(really just need a refill for my dextroamphetamine)

scott
[www.etsy.com]
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 04, 2016 12:15AM
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Any Cal.

After all the back and forth, there is NO proof, or even existence of a proper test to prove, that careful grinding 1) overheats edges,

Here is the concern, how do you tell when not using coolant, if the edge is over heated? Here are the common answers that people give :

-I can hold onto the knife and I don't get burned
-the knife is quenched often
-when it is quenched it doesn't seem to heat the water much
-I don't see the steel discolor

None of these are sufficient to ensure the steel isn't being over heated, hence the steel is being subject to a random increase in temperature for an uncontrolled period of time and then quenched (in a fluid for which most steels isn't suitable) so the edge is under going rapid expansion and contraction. In combination with this known issue is the fairly steady stream of issue of knives from manufacturers who grind without coolant which have the exact problems you would expect from dry grinding, and the fewer (but still existing) knives which are actually visually over heated (oxidization visible).

Does using coolant mean you can't over heat a knife, well of course it doesn't, it simply reduces the heat transfer by friction. As this is nothing more than waste energy it also increases belt life and general grinding efficiency. The question is why do something that :

-increases risk of heat damage
-reduces belt life
-reduces grinding efficiency

all the while paying massive amounts of money to get extreme performance steels where money is spend massively on tight controls (PM steels, high performance furnaces, cross rolling, etc.) to just subject the steel to random time/temperature at the end?

Now if you are making knives from railroad spikes, sure. With people hardening by eyeball with a torch, random forging without normalization, quenching in random oils, tempering in ovens which are +/- 25 at best and are far from even in distribution - well complaining about a little over heating on the apex is not terribly sensible. That is likely the least of the issues.

But buying expensive PM steel, cross rolled, high performance furnaces/ovens and then at the end - grind with no coolant with no way to ensure that the apex isn't being over heated, all the while reducing grinding and belt efficiency - well it begs the question as to what / why? This however is a moot point as more manufacturers and makers are openly wet grinding and shortly it isn't going to be an issue.

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or 2) that any additional tempering is reducing edge life.

This isn't controversial, and there are lots of data on the effects of over tempering on edge retention.

[www.cliffstamp.com]

In general, strength and wear resistance data can be found for steels based on tempering, patent data is full of it for example.
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 04, 2016 11:41PM
We are still at the original problem- you say there is no way to ensure the steel isn't being overheated, I say there is no proof that it is, and we are both right.

The next step is to say that coolant could take care of it if there is a problem, and with no downside- but both of those statements are false.

The best way to say it is that as currently known, a wet grinding set-up could reduce any issues caused by overheating the edge, though grinding dry is not neccessarily causing a problem in the first place.

If there was data on abrasive types, pressures, speeds, steel types, coolant type or usage to bound that statement, I would be interested in seeing it.
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 06, 2016 03:54PM
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Any Cal.
We are still at the original problem- you say there is no way to ensure the steel isn't being overheated, I say there is no proof that it is, and we are both right.

Roman, has never made the claim that anyone sharpening a knife without coolant is producing over heated edges. In the same way, while Kevin Cashen has been very critical of many common knife making methods, he has never claimed that you can't properly harden a complex steel by eyeball/torch and a non-calibrated toasted oven for tempering, or that you can't make a decent knife out of some random unknown composition scrap steel. The only point that either have made in these regards (and other similar ones) is that these processes are at risk due to significant random influences which are beyond the control of the smith.

It thus makes no sense therefore to do them on knives which have a significant investment and performance is an issue when the means to reduce the risks are easily available. Again, it is a point of risk not certainty. In the same way that modern ovens will use multiple thermocouples which are checked on a regular basis for response. This isn't so as to claim there is no risk of over heating, all thermocouples at some point will malfunction. Just that you are taking steps to minimize said chance of damage.

Here is the same claim with just a different risk factor :

If you drink while under the influence of alcohol then your risk of accident is increased.

Clearly, this does not mean you will always have an accident in such conditions, nor does it mean you can't have an accident when you are not under the influence. It simply means the risk is lessened in regards to that one aspect if you are sober and hence if this risk is of value to you then you would not drive under the influence. It would also not make much sense to take great care in many ways to not have an accident and then drive while under the influence.

The abrasive will produce significant heat without lubrication and coolant by basic thermodynamics (frictional losses). Roman has shown that this can even cause over heating with dry grinding by hand, without power. There is considerable literature on over heating without coolant when power grinding hence the constant research into lubricating coolants for industrial grinding. They are used simply because they increase performance. Just simply ask yourself why do you want power going into heating the knife when it could be cutting instead? Why would any maker argue that it is better to waste energy heating up the blade vs actually cutting it with the abrasive? But yet that is exactly what dry grinders are doing. Further this potential for damage is born out out with raw data because there is a constant stream of overheated knives from manufacturers with obviously overheated blades (which show oxidization damage) all who dry grind.

Now can you use coolant and still damage a knife? Of course you can, just like you can drive sober and still have an accident. However would this mean you would drive under the influence and defend it by saying "Well you can't say that you KNOW I will have an accident just because I have been drinking? You can't even say you KNOW I won't have an accident if I wait until tomorrow when I am sober." Would anyone make that argument? It isn't likely because the response to it is obvious. It is the same as to grinding without coolant. You can't know that a knife which was dry ground was over heated, just like you can't know that a knife which was wet ground was not. What you can know is that the risk is less, and from a maker point of view it is beneficial in many other ways.

There is also the wealth of data from makers who have switched to wet grinding and who notice obvious increases in performance in both grinding efficiency and belt life (costs which would be passed onto the consumer as reductions in price from a more efficient manufacturing) and a reduction in issues with edge life after sharpening. Charges against anecdotal evidence could be made here of course with significant warrant. But again as a consumer, just realize all the maker is doing is nothing more than this :

-using an inefficient process which has a increased risk of damage
-no control at all to know if the damage has happened
-passing it on to you with increased cost and higher risk of defect

And somehow this makes sense to both the maker and consumer? This is one of the things that is only even discussed in the knife industry, in industrial grinding it is never even questioned because it is simple metallurgy.



Edited 1 time(s). Last edit at 12/06/2016 04:40PM by CliffStamp.
Re: Power sharpening: how fast is too fast? (3 fps dry)
December 17, 2016 07:30AM
We are still going in circles here.

There is not an increase in belt life when using water. In fact, ceramics will tend to go dull sooner with lubrication, as it reduces the fracturing they rely on for life- and that fracturing is what keeps them cutting instead of rubbing. They also twist as they dry, which means they may not run as well if removed and put back on. Basically, there is the potential for wet belts to have a significantly shorter life, and wet grinding to lead to higher costs.

Manufacturing has gotten past trying to use coolant, and hasn't used water as coolant since 60s? Many high speed processes do NOT use coolant, as it does nothing in the cut. The coolant is actually not used as coolant, but for part shape stability and chip evacuation among other things. Industry knows that you aren't cooling the cut. Flood cooling was out of style by 1990? or so, it wasn't actually cooling the cut, and did nothing for chip evacuation either. Why is it seen as a cure-all for knife making?

My biggest issue is that wet grinding will tend to cause in increase in grinding speed or pressure, the operator assuming that the water is taking care of things. From that perspective, using water can cause an INCREASE in microscopic edge damage. Remember, THE WATER IS NOT IN THE CUT. And water makes an awful coolant even if it was. Well, that and it is like trying to make a fine steel sculpture in a mud puddle.

If wet grinding was important, then it would be even more important that a proper coolant was used. A proper cooling additive could allow the solution to get closer to the 'heat affected area', and that coolant would have more cooling effect than simply water. Why would someone go to the trouble of wet grinding only to use water, which is just a placebo in this scenario?

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Cliff
But again as a consumer, just realize all the maker is doing is nothing more than this :

-using an inefficient process which has a increased risk of damage
-no control at all to know if the damage has happened
-passing it on to you with increased cost and higher risk of defect

And somehow this makes sense to both the maker and consumer?

So without these 3 arguments, which are incorrect or not applicable, do you have some proof of 'water' preventing microscopic edge degradation while grinding? There was never any solid proof shown of edge degradation while grinding with reasonable technique, either...

This argument is like the Ralph Nader line for knives. Invent a problem, then lobby for a solution that creates additional real world problems.
Re: Power sharpening: how fast is too fast? (3 fps dry)
January 16, 2017 04:25PM
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Any Cal.

There is not an increase in belt life when using water. In fact, ceramics will tend to go dull sooner with lubrication, as it reduces the fracturing they rely on for life- and that fracturing is what keeps them cutting instead of rubbing.

The fracturing is due to applied load, not frictional losses. The compressive force on the abrasive isn't eliminated when wet grinding, just the frictional losses. This isn't any more the case than if you put skates on so that you glide over the ice well compared to sneakers it doesn't stop the ice from breaking if you are too heavy. Now if you run the belts at too low a load they won't fracture and hence you have to maintain the proper contact pressure which is going to depend on the steel, belt speed, applied load and bond strength of the abrasive.


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Manufacturing has gotten past trying to use coolant, and hasn't used water as coolant since 60s?

Water based coolants, or less than full neat oil coolants are in high demand in industry, but not because of raw performance (water isn't a great high pressure coolant or lubricant) but because of other issues (green related mainly). There are lots of reasons not to use pure water, but because pure water isn't ideal isn't a reason to grind dry.

Trying to form the apex of a knife on a low strength stone is very difficult due to the effect of the slurry on the apex, however that isn't a reason to advocate using blunt knives. It however would be a reason to advocate to keep sharpening but just change techniques. There are high pressure additives which when blended with water increase its lubricating and cooling capacity.

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My biggest issue is that wet grinding will tend to cause in increase in grinding speed or pressure, the operator assuming that the water is taking care of things.

That is like saying that driving while not drinking causes problems because people will think they can drive safely, thus the thing to do is drunk drive.
Re: Power sharpening: how fast is too fast?
February 09, 2017 01:16PM
I'll take all the data we have going here... interpreting the data is the tricky part and not caring who's right, that's tricky too.

Any Cal I think you have some good points there. I have also noticed that my Blaze belts seem to last longer running dry, though I have not done controlled testing (not having made the same blade twice that's always an issue)

I thought it might be washing away the factory applied anti-friction coating, actually.

I have burned many blades under water spray and stream, and in a strange way grinding wet can, as you say, result in burned edges...

...but it isn't the fault of the coolant. It's my fault... proper technique required as you say, but proper technique is hard when water spray obscures vision in the air and on eye protection, and hands are soaked, and the workshop is rotting from constant soaking. I tried air blast but that pushed the blade all over the place.

I think I might believe you about "water not being in the cut" though I didn't see your source. It's still a great heat sink for the edge though if you can get it close. Scott's set-up with ultra-slow grinder probably helps as does the finer grits as I think the water sticking to the belt is the only way to get the water to the apex... but not everyone can get this equipment together or grind that slow.

Wobbly hands posted some wisdom up there...