Knife dulls overnight after sharpening

[KnifeGrinders]

Our data on effect of subsequent honing at a little higher angle.

The knives are SWIBO, stainless steel with 0.5% carbon and HRC56-58, sharpened at 20 dps.
Measured the initial sharpness, then in 24 hours. After that gave them one pass each side on a paper wheel with 0.5 micron diamonds at a +0.4 degree higher angle, i.e. at 20.4 dps, and let them idle for another 24 hours, then measured the sharpness again.

INITIAL SHARPNESS (BESS) - IN 24 HOURS - IN 24 HOURS AFTER EXTRA HONING WITH 0.5 MCRN DIAMONDS @ +0.4 DEGREE HIGHER
55 - 105 - 60
85 - 110 - 95
90 - 125 - 92
75 - 110 - 85

The final loss of sharpness is <= 10 BESS compared to the initial, which we attribute to oxidation; I believe we've got rid of the stressed steel on the apex completely.

In addition to stropping on clean leather as detailed in Grepper's posts above, we have two more tricks to beat the post-sharpening loss of sharpness:

1) In your sharpening session, deburr at a higher angle with a 5-6 micron honing compound- depending on the steel the angle should be 0.4, 0.8, 1.2 or 1.6 dps higher (some loss of sharpness does occur due to the apex rounding, by 10-20 BESS) - commonly approx. 1 degree higher angle works well.
2) In a day (min in 17 hours) after sharpening, gently hone with a 0.5 micron honing compound at 0.4 dps higher angle (keenness of the edge is preserved).

[grepper]

It seems this edge loss problem is not limited to knives only. I just reshaped a turning skew. With no special attention paid to deburring, it showed a BESS reading of 200. After using it to round one billet, the BESS reading had more than doubled to 420. Apparently, there is something in wood which causes edges to deteriorate.

That is a great real-world test Mr. Ken.  Excellent use of an edge sharpness tester.

It would interesting to repeat the test.  I have no idea how long it takes to turn one billet.  It would be interesting to know how quickly the edge dulls.  Maybe turn for 1 minute and take some readings, then 2 minutes, etc. 

If I was a turner, I would think it wold be cool to know that after one or two or three minutes of turning that I was now working with a 420 edge that started out as 200.  I might ponder the value of sharpening to 200 when after only one billet the sharpness reduces to 420.  If only one billet is a very small amount of use, why not just sharpen to 420 in the first place?

Again, very cool test Mr. Ken!  Real-world, normal use tests are the most instructive of all and we see precious little of it here on the Exchange.

[Ken S]

I recall one of the marketing terms used to promote life insurance is "instant estate". In fairness, this is not a bad idea. With using BESS with turning tools (and any other application), BESS interpretation gives me "instant experience". I make no claim that BESS reading for a beginning turner like me are the same as the expertise I would acquire after years at the lathe. However, using BESS is a "reasonable facsimile" and can give a beginner a good leg up.

My example of rounding a square billet can have many uses. After resharpening, if my initial BESS reading changes from 200, my technique is either improving or needs more work. Does using my leather honing wheel and/or the new 1200 grit diamond wheel improve the edge, or do I need more work?

Rounding a square billet is intermittent cutting, like chopping with an axe or a cleaver. BESS readings can tell me if it is more logical to do the rounding with the skew or the spindle roughing gouge (the tool designed specifically for this purpose    It seems to me that the primary constraint in using BESS as a learning aid is the limits of my imagination.

There is a fascinating interview online (you tube) with the late Allan Batty. He discusses his six year apprenticeship in England (ages 15 through 21) followed by four years journeying before he was considered a fully qualified turner. I don't have that kind of time. This is a hobby, and must be balanced with family and other responsibilities. I am also 68 and not 15.

I think BESS has great potential to advance sharpening in the work that Grepper and Knife Grinders are doing. This kind of research is truly commendable. On a much humbler, less scientific level, I also believe that BESS can be a useful coach in developing proficiency more quickly. I do not pretend that my numbers are as accurate as more scientific work, however, they are useful indicators, and I am grateful for the assistance.

Ken

[EOU]

Thank you very much for the thought here Ken. We are even more grateful to both you and Grepper for demonstrating and talking about just a couple of the things that  can be learned about edges via edge testing. In our view, measuring the final sharpness level of an edge is utilizing only about 20% of the value of the instrument.

[TZ750G]

Hello friends. 

As a new member, I am trying to get up to speed with the rest of you as quickly as I can, so that I don’t bother you with unnecessarily dumb questions.

So, having found this ancient thread that goes almost back to the beginning of BESS time, maybe a comment or two might bring us up to closer to the present…………………
 
The main points of interest in this thread – at least to me – are that sharpness (measured by BESS) worsens over night, but can be brought back by either the passage of time, or by stropping.
 
Some of us are wondering is “metal memory” exists, and what it has to do with our problems of “edging on up.”
 
Having worked years ago with sheet metal, I am familiar with the concept of “springback,” which is when you bend a piece of metal a certain amount, and it “springsback” some.
 
Bend that metal too far, and it will never recover.
 
https://www.thefabricator.com/thefabrica...ingforward
 
One wonders if this is happening with our edges…….

There might also be the consideration of "Coefficient of restitution," where we "impact" our metal blades at the microscopic level, and that induces stress to the edge.

https://www.scienceabc.com/pure-sciences/coefficient-of-restitution-definition-explanation-and-formula.html


 
Another consideration might be that with vigorous stropping, we are “locally heating” our VERY fine edge above the annealing temperature of the metal at the edge – leaving a softer metal that is less good at edge holding.

When I was making knives, I was very careful with grinding after tempering.  Bare hand on the blade close to the grinder, and quick dunks in the water at every opportunity.  Sometimes I could see “color” in the edge without ever feeling the heat in the blade.  Local heating.
 
Afterwards, stropping eliminated the “micro-softened” metal, and then  a return to a hard edge.
 
Combined with eliminating the “metal fatigued” edge left by the “metal memory” rough treatment, we can then strop to an edge better than originally formed.

Remember that springs in general are an easy example of "metal memory." 

But they too eventually wear out or get damaged.

[grepper]

I don’t know why knives dull themselves after sharpening but I’ve seen it with my own testing.  Maybe it’s corrosion or metal memory.  The super sharp edges we are talking about are so thin we may never really know for sure but it’s a fun experiment to try:  Sharpen a blade and measure the sharpness.  Then measure the sharpness the next day.  Let the blade sit around for a week or two and check sharpness every day.  It’s and interesting process to observe.  If you decide to do it, please share your results.

As to heating during sharpening you may find this interesting.  One thing to think about is that even if there is some flash heating of the apex during sharpening it is extremely ephemeral.  Something to ponder when considering if there is any possible tempering or annealing considering these processes generally take time.

This is a long thread but interesting to follow through:

http://bessex.com/forum/showthread.php?t...ht=templaq

[TZ750G]

This is a long thread but interesting to follow through:

http://bessex.com/forum/showthread.php?t...ht=templaq

Mr. G:

That was indeed a great thread!

With all the rabbit holes included,
it took DAYS to finish it up.

GREAT education.

Thanks.