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Forum Index > Equipment > More on knife sharpening.

More on knife sharpening.

31 Aug 2018
@ 11:09 am (GMT)

Warrick Edmonds

There's been ongoing and rigorous research into sharpness conducted by a mob in Sydney over the last couple of years. I won't bother you with all the graphs, details and palarva but here's an interesting finding that's arisen. It doesn't really matter what method you use for sharpening, after 24hrs of the knife sitting idle it is half as sharp, that is, it loses a large portion of its sharpness by just sitting around for a short time. For a while they put this down to oxidisation of the cutting edge however recent experiments have eliminated that. Their best determination is that sharpening of any type creates stresses and distortions in the fine cutting edge that afterward relax which removes some of the gains. This happens fairly quickly, in a matter of hours. Sounds like a real bummer however there's a simple solution, that is before you use your knife simply give it a quick and light strop on leather. They've even tested how much stropping is best and concluded two strokes at 12 degrees each side are better than many strokes. Using a steel also works as long as it's not one with large and aggressive serrations. Apparently this light action resets the cutting edge to straight. (They have many micro-photos to prove it).

So, the take home is, sharp or not, strop or steel your knife briefly and lightly before every use if you want the best from it.

Replies

01 Sep 2018
@ 10:19 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Here's a couple of relevant articles for those interested in sharpness. The summary take-home is, if you're using a 350 grit stone well or even a 600 grit, there's no point in going finer because you won't make the knife sharper. Also, diamond stones are better than waterstones.

https://scienceofsharp.wordpress.com/2015/03/01/the-diamond-plate-progression/

and

https://scienceofsharp.wordpress.com/2015/07/09/its-too-big-of-a-jump/
01 Sep 2018
@ 10:40 am (GMT)

Andrew Murray

Re: More on knife sharpening.
These posts are great Warrick.

On a different note... which type of edge do you find best for wear, avoiding chips and knicks if you will... or does it come to the steel more than anything?

I was using my knife like a cleaver to hack off a roo tail, unsuccessfully I might add but it left some gnarly knicks in the edge which came out relatively easy with a steel and some light stone work. Makes me think the steel may not actually be 440C as it claimed. Maybe just hardened outside or along the edge...
01 Sep 2018
@ 11:11 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Thanks Andrew, good to know someone's interested.

"I was using my knife as a cleaver", Might have answered your own question there mate.

The type of edge depends to an extent on intended use. For example, razors use a hollow grind for a thin blade, as do many hunting/skinning knives. Chefs knives often go for flat grinds, mainly so they can stone the entire blade down to an 'infinite' edge that doesn't have much of a secondary or tertiary bevel. Heavy duty choppers benefit from a grape-seed profile/cross-section that brings a lot of steel in support of the cutting edge. And don't forget the chisel grind, preferred by sushi masters and their ilk. I like the chisel grind in the kitchen, it stays sharp and is easy to sharpen. However it does take a bit of getting used to the slight sideways torque you get when pushing a cut.

440C is a good, utilitarian steel that was the backbone of quality knife making for ages. It remains a great standard. Possibly look to the quality of the knife or the profile of the blade. Most hunting/skinning knives are not built for chopping, they're more for fine slicing. And also, any sideways twist during a cut could lead to chip-out.

As for the roo tails, better to take your time and look for a joint to cut through rather than cleaver it.
01 Sep 2018
@ 11:36 am (GMT)

Andrew Murray

Re: More on knife sharpening.
Cheers mate.

In the end I ended up twisting it then a sharp breaking action. It did the job but yes, finding a joint would be much better. I was pressed for time, probably took longer in the end.

Most of my kitchen knives are flat grinds. Though I have a couple that are like a double grind type deal.

My current razor is a hollow grind. I do like a nice big wedge (flat grind) for thick beards. The extra weight is nice. The hollow grinds are great for their manoeuvrability and light handling.
04 Sep 2018
@ 05:12 pm (GMT)

Wayne Woodard

Re: More on knife sharpening.
Very interesting Warrick. It is something I guess I have noticed over the years, but couldn't explain it. I am no knife maker or expert at sharpening but have sharpen knives since I was a kid. You always kept the castrating blade on your pocketknife as sharp as possible but always had to give a quick strop on your boot to get it back to what it was yesterday when you had it mickey mouse. Hunting and butcher knives same sought of deal. I just thought I must not quite be doing it right.
04 Sep 2018
@ 09:03 pm (GMT)

bryan long

Re: More on knife sharpening.
great info, so is a blade going back to half it's sharpness in 24 the same irrespective of the steel used?
05 Sep 2018
@ 10:46 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Bryan

The effect appears to be slightly less in high end steels. It's also slightly less if you use certain types of sharpening devices, especially those that rely on swinging diamond plates, such as TSProf or Edgepro, where you can control the application of force.

However remember that half as sharp is a relative term. Sharpness in these cases was measured in BESS units, which is a machine that measures the force taken to cut a known nylon wire, so essentially given in grams. So, what do I mean by relative? Well, if for example a knife cuts at 50 BESS it is sharper than a commercial razor blade. After 24hrs it is cutting at 100 BESS, which is still sharper than most chefs knives. So, it's not that your knife becomes a hammer overnight. The point is as Wayne says, you may have noticed this effect and now you know it's real. So a quick strop or steel before each use and all's good.

At last years Adelaide Knife Show there was a prize for the sharpest knife. It was won with a kitchen blade then went 35 BESS, which is so sharp it's unbelievable. The same knife was tested at the recent Sydney Knife Show, a year later and though unused in any way it measured 50 BESS, which is still twice as sharp as the best chefs knives, but half as sharp, if you see what I mean. A quick strop on untreated leather and it was back in the 30's. It's good that there is such a simple solution to this.
17 Apr 2019
@ 11:01 am (GMT)

Andrew Murray

Re: More on knife sharpening.
Just on this... I bought a cheap axe/tomohawk from aldi to take camping. I noticed it has been "sharpened" to preserve the edge. It has a 90° edge. 45 each side. I had a go with my 400 grit diamond plate but there is so much material to remove I gave up and am trying to find a grinder. Once done. I plan to heat the edge with a blow torch and quench it to harden it right up.

Any advice that could help would be appreciated.
17 Apr 2019
@ 03:31 pm (GMT)

Len Mattsen

Re: More on knife sharpening.
Warrick,
I spent 20+ years as a kill floor butcher. Eventually I learned to sharpen really well.

You're right, the devices employed are less important than your technique.
Both diamond and ceramic rods/stones are very efficient and cost effective. Care must be taken with rods so as not to over stress the steel. Light pressure works best.

I use flat diamond stones to grind and re-profile an edge, rods to hone and a slack leather belt with 10,000 grit aluminum oxide applied to polish.

This method does not put micro serrations on the edge which quickly dull. Rather it puts a mirror polish on the edge. Additionally, the edge is neither flat ground nor hollow ground, it is convex ground like a Moran edge. As such, it keeps it's edge for a long time.

One last thought, I buy the best possible quality steels I can get, often thought as "super steels" such as ZPD-189. More expensive? Yes but it is more cost effective in the long run. I hope this has helped a bit. Best wishes, Len
20 Apr 2019
@ 10:27 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Andrew

Sorry about my lack of timely response, I've been away on school hols and only got back last night.

Re the axe. Mate there's not a lot I can help you with there. I don't know the steel type and the word 'cheap' is buzzing around in the air between us like a mossie at a BBQ.

As much as I'd like to say something useful, you're on your own with this one I'm afraid.
20 Apr 2019
@ 10:37 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Len

ZPD-189 is a well respected steel. It gets quite hard, up in the RC=65 sort of range which puts it out of contention for anyone trying to sharpen it on normal stones or thinking they're going to do a quick top-up mid butchering job. The benefit is a longer lasting working sharp edge. Just keep an eye on it for rust though. This steel has a lot of chrome but also a lot of carbon so it may turn on you if you let it sit around bloody or wet.

It's been shown through a number of controlled experiments that with all steels, including the new gen 'super steels', that the first, sticky sharp edge degrades at almost the same rate regardless. Where the 'super steels' come into their own is that the remaining sharp edge, what you would call the working sharp edge, lasts much longer, sometimes much much longer than lesser steels. You will need good sharpening kit to bring them back to sticky sharp.
20 Apr 2019
@ 03:07 pm (GMT)

Andrew Murray

Re: More on knife sharpening.
Hi Warrick,

Cheap is right. I figured it is fairly useless now with the edge on it, and if I bugger it up entirely, it won't be any less useful!

21 Apr 2019
@ 12:30 am (GMT)

Warrick Edmonds

Re: More on knife sharpening.
Andrew

I like your way of thinking. You're probably in with a better than even chance of doing something useful to it, so why not give it a go.

After quenching it might be brittle is the only thing. You may consider tempering it, which reduces the stresses from heat treating. Stick it in the kitchen oven (just the axe head) at a couple hundred degrees C for an hour or two, about the same temp you'd cook a chook, then let it slowly air cool. I advise you wait until the missus is off visiting before attempting this.

Good luck !
08 Aug 2019
@ 11:50 am (GMT)

Scott Struif

Re: More on knife sharpening.
I bought an unused Wear-Ever Professional chef's knife with a 12" stainless blade 6 years ago at a thrift store for $5. It gets used daily. Every 3 months or so it needs a dozen swipes on a 4" diamond impregnated homing stick. If only I could find a hunting knife of that quality!
 

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