Losing temper when sharpening? Has anyone tested this?

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leozinho

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Jan 7, 2005
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I'm posting this in Shop Talk rather than Tinkering because this subforum gets a different set of eyes.

I've posted a few questions about sharpening. It's something that interests me.

I saw this comment, and rather than muddying the thread with a reply I decided to start a separate thread. I hope that's OK.

bladsmth said:
DO NOT POWER STROP a blade if you want your sharp edge to last. The heat build up can severely lower the hardness of the actual edge. It won't feel hot, and will look OK, but the edge life may be greatly reduced. Just a few strops on a charged leather strip mounted on a board is all you need.
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I'm wondering if anyone here has tested this? I'm very curious about how different sharpening methods affect the life of the edge.

As best I can tell, there are a few different schools of thought.

One is blade doesn't get hot enough to matter as long as you are careful. After all, you do the final grinding post-heat treat, so sharpening with a powered belt or powered strop doesn't necessarily mean the blade gets too hot.

The second theory is that while that, yes, you grind post heat treat without affecting the performance of the blade as long as you are careful to dip blade in water, etc, but if you sharpen with a powered belt, the thin portion of metal at the very edge will get too hot and lose temper no matter what precautions you take.

(Bladsmth may be somewhere in between - powered belt is OK but not a powered strop isn't.)

The second theory seems almost probable and intuitive, yet, outside of maybe Cliff Stamp, I don't know that I've seen anyone test this. There are an awful lot of big production knife companies that sharpen with belts. As best I can tell, a lot of ABS mastersmiths and journeymen sharpen their test knives on powered belts. Probably bladesport competitors, too.

I just have to think that if there was a performance disadvantage to sharpening with a belt or power strop, the guys above would have discovered it and found a work around.

Has anyone taken two identical knives and sharpening then in different manners to see which one held an edge the longest? I know so many people experiment with and test the edge holding properties of the various steels, and that gets debated ad nauseam. I don't see people testing the edge holding properties of the different sharpening methods. Or have I just missed it?

Thanks.
 
I forget his handle but a scientist over in Germany did some work on this. You won't ruin the temper of the bulk of the knife, but the edge has so little metal that the temperature increase is huge. So, yes you can ruin the temper of the edge thereby decreasing the hardness and durability of said edge. I forget the best solution to the problem but the thread is within the last few years. Also, someone might chime in with more info.
 
Another problem I have with powered strops is that I simply don't see why they'd be necessary. If you need that much speed to strop an edge, it wasn't sharp to begin with. It's no different than finishing a whole blade... if it's not clean and even at 220 grit, there's not much point skipping right to 2000 grit or the buffer...

As for the heat issue, the cat you're thinking of is named Roman Landes. A quick search will turn up many threads and discussions about his research.

I err on the side of caution, and just barely set my edges with a crisp fine belt and a light touch; that might take as many as three passes per side for a medium to thick edge, but usually one smooth pass on each side will form an apex and just about start to form a burr.

Then I sharpen manually on stones with a light lube and strop lightly with a bit of compound on smooth leather. The stropping takes only seconds if I've done my job right up to that point.

On blades that don't need a super fine/polished apex and have thick edges (machetes, tomahawks, big choppers etc.) I have no problem just putting a quick'n'dirty convex edge on with a 400 grit belt and knocking away the burr.
 
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As a note, Moras come sharp but their factory edge will often bend or chip. I have found that after sharpening a couple of times the Moras develop a finer, sharper, more durable edge. And I read this time and time again from other posters about other factory knives.
 
Exactly! I've long felt that the number one cause of people having troubles with production (and some handmade) knives that won't keep an edge isn't "bad HT", but over-grinding of heat-treated steel.

This is borne out by the many, many people who report much improved success after reprofiling their edges or simply sharpening them normally a few times. They're wearing away the over-tempered steel and getting back down to the good stuff.
 
Bo T said:
As a note, Moras come sharp but their factory edge will often bend or chip. I have found that after sharpening a couple of times the Moras develop a finer, sharper, more durable edge. And I read this time and time again from other posters about other factory knives.
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james terrio said:
Exactly! I've long felt that the number one cause of people having troubles with production (and some handmade) knives that won't keep an edge isn't "bad HT", but over-grinding of heat-treated steel.

This is borne out by the many, many people who report much improved success after reprofiling their edges or simply sharpening them normally a few times. They're wearing away the over-tempered steel and getting back down to the good stuff.
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That's still anecdotal, but something that I might actually be able to test. I've got a Mora that that I've never touched. It is very sharp (from the factory.)

I could test the factory edge, resharpening on stones, and then retest.

What's a simple, repeatable way to test edge holding properties? Some sort of rope cutting test?
 
leozinho said:
That's still anecdotal, but something that I might actually be able to test. I've got a Mora that that I've never touched. It is very sharp (from the factory.)

I could test the factory edge, resharpening on stones, and then retest.

What's a simple, repeatable way to test edge holding properties? Some sort of rope cutting test?
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If you want real research, as has been mentioned above look up the research by

Roman Landes


He tested the heating of the micro bevel in dry sharpening


He has a book, but it's in the German so not as useful to most of us

http://www.amazon.ca/Messerklingen-Stahl-Roman-Landes/dp/3938711043
 
While I sharpen using my 2 x 72 belt grinder I only go about 35% in speed, dunk the really thin ones and keep the blade moving fast! Then buff on my buffer loaded with green chrome. I'm not a papered metallurgist by any means, and have always sent my knives to Paul Bos HTing for consistent excellent work but I think you would need a RC tester and perhaps an electronic microscope to document this question of yours?

I agree that there is no need for a power strop and my personal anecdotal findings are that many factory knives are a bit over heated during sharpening etc and if you clean off a micro level of steel there does seem to be an improvement in edge holding abilities of many knives.

A test would need to be done with a few knives of a known hardness of the same grind geometry and sharpen identically.
Rope, paper or just about any consistent cuttable material would need to be used along with the same pressure for each cut to truly do a "Repeatable" scientific test to document any improvement after any consistent amount of steel was removed from each knife edge..
 
Roman did tests that proved that even sharpening a knife on a STONE BY HAND can cause unwanted tempering of the edge at the micron level, if the blade is honed dry, without any lube. Now I'm thinking no way.....there are many sharpeners out there who swear they get the best edges sharpened dry without water/oil....but it is true. Aparantly....you should always use some sort of lube...no matter how you are sharpening. 2000 degrees celcius was reported in his tests, of course just the first few microns and for a split second...but that is serious heat.
 
Setup:
  1. 2 knives with almost 100% match in profile & geometry. scandi grind to avoid thickening behind-the-edge due to subsequent sharpening.
  2. Lot of 1cm radius dry clay bars. use submicron abrasive-mixed to speed up the process.
  3. Sheets of newsprint.
  4. Sharpening methods - best practice/skills of Ps/Pm/Pf(dry power slow/medium/fast) and H(wet/water hand). Another VERY important parameter is abrasive sharpness & durability.
  5. Keep statistics for Ps & H for 6 rounds. Throw away the lowest 2 and highest 2. Use average of 2 middle scored rounds.

e.g. Params:
* 2mm thick [add your optimal ht steel here]
* sharpened edge thickness: 0.5um
* slice/saw cut the clay bar
* failed when: no longer cleanly slice newsprint

Test:
* Find stable angle - start from 10dps, increment 1dps until edge can withstand at least 5 cuts without roll/chip
* start with clean steel
1. rotate knives
2. sharpen with removal about 0.1mm (or whatever ensure clean steel) of edge (to removed burned/damage edge)
3. cut clay and test slice newsprint every 5/10 cuts until failed
4. repeat 1-3 for 6 times.

leozinho said:
...What's a simple, repeatable way to test edge holding properties? Some sort of rope cutting test?
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Landes, and I and others have had this discussion many times .Yes it's not only possible but probable that the grinding has damaged the blade. I've had blades that needed sharpening [by hand a few times] to get a decent edge and I've even had points break off [without any miss use] . Just the facts ,grinding of any type creates heat which tempers the edge and tip. Be gentile !!
 
Yes I've tested it, yes it matters.

For most applications, a thicker edge that is power sharpened dry is okay because the affected metal wears away and you get the mediocre edge most schleps haul around with them. But on a thinner edge and in an application where a sharp edge is important, such as Roman's razor blades, it makes a big difference.

Grinding on a finished hardened knife is okay if you're careful and know what you're doing. However sharpening under power is risky. Yes factories do it, and quite frankly the edge on almost all factory knives is a compromise between quality and expediency. Very few factory edges are even decent quality.

Even if you accept that you can't get a truly sharp edge to hold up very well when ground under power, the biggest problem that I see is that even very experienced sharpeners have a bad day. And a cooked edge doesn't necessarily run colors. So how do you know?

I sharpen under power. I do this at low speeds and sopping wet. I don't dip, I'm running a dripping wet belt on an ice cold platen chiller. And even then, if it really matters I still finish on oil stones. You just can't avoid burning those last few microns of the edge that makes or breaks a seriously sharp knife.

That said I sharpen my lawnmower blades and my mother's shitty kitchen knives hot and dry. Some applications it just doesn't matter.
 
Nathan, that most made me spit out my drink. My mom seems to have a neverending supply of old kitchen knives for me to sharpen. Some of them are quite bad, others are not so bad. I've seen some pretty neat 40's to 50's era "production" knives with hand forged blades. They get impressively sharp as well. I guess my grandma used to always buy a couple at the state fair. That was back when even the cheap ones were pretty darn good.

I'm becoming suspicious that my mother is buying knives at the swap meet and "finding" them for an excuse to come visit the shop.


On topic, yes this happens, but it isn't an end of the world type of thing as long as you are aware of it.

This happens on all sorts of stuff. The edges of a drill bit reach mind blowing temperatures right at the site of the cut, but the thing is, how far is that high temperature actually conducting into the metal, and how low is it actually dropping the temper?

Another concern is that a dull tool, just like a dull belt, will create heat in the edge. When I grind my blades, I'm not grinding, I'm cutting chips and swirls just like back when I was a machinist. Proper speeds and feeds and sharp tooling will make sure that the minimum amount of heat makes it into the part per material removed.

Using another machining example, you can quickly burn a bit when cutting something such as plastic that doesn't conduct heat away. On metals, you have a bit of a buffer in that the total heat load is usually a small enough ratio to the tool and workpiece's mass and thermal conductivity, that you get slight tempering and subsequent erosion of cutting edges, but the bottom line is they last to get the job done, and when they go dull you sharpen them (sometimes).

I think power sharpening on slow speeds with coolant is more than sufficient for a working/utility edge. I also support refinement or the whole enchilada by hand work. Depends on what you are looking for and need.
 
It's been a couple of years, but I took 2 knives and sharpened them on a HF sander with a 320ish grit belt, then a leather stropping belt. This produced tree topping edges very quickly. I then used them both to chop through a 2x4. Afterward, both were rolled. I resharpened the larger knife and used it to do some yard work. This resulted in a damaged blade. So, I resharpened them both on my water stones and Sharpmaker. After resharpening, both blades went through the 2x4 at least twice and would still shave my arm. I tried the reverse as well. I had an old chef's knife that I used for yard work. I sharpened it on a large water cooled wheel and refined the edge with the Sharpmaker. The edge lasted a long time. Later, I resharpened it on the belt sander as above. After use, it had small dents along the edge. I resharpened it again using water stones and some 0.3 um honing film, and had no trouble. For a fine edge, no more power sharpening for me. If I'm cutting roots in the flower beds, that's different.
 
There's quite a significant difference between final grinding and stropping. For one thing, when stropping you are dealing with a microscopic edge which naturally heats up much faster then the steel during final grinding would.
 
Nathan the Machinist said:
That said I sharpen my lawnmower blades and my mother's shitty kitchen knives hot and dry. Some applications it just doesn't matter.
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I don't dispute the findings of Landes and I am sure that that on something as thin as a razor blade it could make a big difference in the amount of steel that gets overheated. On the other hand many Scissors, drill bits, and as you mentioned Lawnmower blade along with lots more in industry are sharpened dry. So one of the things I always avoid while sharpening with power wet or dry is dull belts. Nothing builds up heat faster than a dull 120 grit Hermes super flex belt. I do a few light passes and keep the edge moving when sharpening knives.
 
I lose my temper all the time while trying to sharpen my knives but I am learning to control myself better each time. At least I've stopped throwing them across the room. :o





;):):D
 
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