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TOPIC: Question with a sebenza

Re: Question with a sebenza 2 years 1 month ago #4329

  • BassLakeDan
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CliffStamp wrote:
.. Bias is the constant bane of any experiment ..

ain't it the truth!

Well here are some 'un-biased numbers'... I have sent this data to Cliff, who will 'crunch it' and make his own comments later.. he knows what numbers really mean, I am just a hacker, but here is my following take on the situation so far:

The ZT steels performed almost identical to the sebbie.. The basis sharpness was a little worse (higher numbers mean less sharp) than the sebbie (I think just because my 'dressing' of the blade before the test was a little hurried..) . Also per cbwx34 I re-ran the ZT through the 10 slices of copy paper. It, again, performed much like (actually exactly like!) the sebbie: i.e. after 10-15 cuts it started to fail on the 90 pushes, then I put it back on the test jig and noted the following numbers from random points along the blade: 352; 362; 344; 342; 346; 370. This is just like the sebbies numbers after the paper cutting, in other words about a 100 gram jump from baseline numbers after sharpening:

you can get the xls raw data here or just look at the following chart:



for those of you that want to see how all this stuff was collected, here is a shot of the 'test-bench'. You can build yourself your own test jig for under $50 bucks and settle all those arguments at the local beer-bar about whose blade is sharper than yours:

Last Edit: 2 years 1 month ago by BassLakeDan.
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Re: Question with a sebenza 2 years 1 month ago #4331

  • cbwx34
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So, just out of curiosity...
BassLakeDan wrote:

Keep in mind: It is important to note that push cut apex testing reveals only very limited information about real world “cutting ability”.. No matter what the results of my testing tomorrow of the ZT0550 I stand by my statement that the ZT holds up 4x better than a sebbie in the real world.

... did you change your mind? :)

I'm curious, and I know your test method is new, so you may not have this info yet, but can you give a little better idea of what a "350g test" blade can still do? You said it started to fail cutting paper... can it, for example, still shave arm hair at all, or whatever description you want to add.

Also, do you have a parts list avail. for your testing setup? I'm getting tempted...

Thanks.
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Re: Question with a sebenza 2 years 1 month ago #4340

  • BassLakeDan
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cbwx34 wrote:
..

... did you change your mind? :)
.

Well, I am going to return them both to pocket carry over the next week then re-test them both. But it is probably clear that there have been some zealous overstatements that have been made prior to the testing :lol: Cliff had an interesting series of posts once about how knife ergonometrics can effect "apparent sharpness feedback" to the user. i think we need to find those comments and re-post them somewhere..
cbwx34 wrote:
..

..can you give a little better idea of what a "350g test" blade can still do? You said it started to fail cutting paper... can it, for example, still shave arm hair at all, or whatever description you want to add.
Also, do you have a parts list avail. for your testing setup? I'm getting tempted...
.

that is still a rather sharp knife, it will easily shave arm hair, it will not direct push cut tomateo skin, but will cleanly slice a tomateo using an easy draw cut under the weight of the blade, it will still 90 push cut copy paper if you handle it carefully and keep the finger hold on the copy paper within 1/4 to 3/8" of the blade contact. -Will work on getting a parts list together this weekend. the thing is so simple you can basically figure it out from the videos and the posted pictures. The part of it that necessary to understand more fully is the proper placement and use of the testing media, how to run a 'test series' properly, the nature of the whole concept of "testing sharpness", and a few other details that we need to put in some sort of documentation sheet. The whole thing is still a work in progress. .. Cliff has been very kind to work out an Excel template that crunches the numbers. From a recent email to Cliff, here is how I feel about it so far:

... I am not yet confidant enough in the 'testing jig' to accept it as a tool to measure anything but gross ranges of data. In fact because of what it is, and how it is built, it probably has limits that must be "respected" (if that is the right turn of phrase -grin!-)..

I feel an acceptable level of comfort with it as a rough gauging tool, within 20 grams push cut increments starting from commercial razor blades thru a range to kitchen knives, in that range I feel it is testing well, and the data it is returning is meaningful for that use and can be relied upon. When we are taking about testing subtle variations on very sharp high end blades with subtle variations in sharpening technique, well then, ... humm not so sure.
Last Edit: 2 years 1 month ago by BassLakeDan.
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Re: Question with a sebenza 2 years 1 month ago #4343

  • CliffStamp
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A simple fit will turn up :

Sebenza has an initial sharpness of 237 (5) the ZT was 283 (6). This difference of about 15% is significant (meaning it isn't random) both from the standard errors being much larger than the difference (it would pass a t-test for significant at 0.05%, and if you look at the data you can clearly see that while they are scattered, the Sebenza scatters around a lower value (and thus higher sharpness).

Now that just means *for this run* it was sharper, it does not mean that it always would be, Dan would need to sharpen a few more times and if the Sebenza was consistently sharper then an inference could be made about the steel. But I would bet that this small difference in sharpness is simply the expected variation, it isn't like you would expect the initial sharpness to be exactly the same each time you sharpened a knife.

The slope for the Sebenza was 1.35 (12) and for the ZT 1.39 (14), both of them showing a strong non-null value. This can be further checked as the F-statistic for both (136 and 92) easily pass the test of significance which show that the data do not remain constant, there is a statistically significant decrease in sharpness during the test. This is also, as Dan noted, easily verifiable in the data itself because you can see that towards the end the data flattens to a value about 100 g higher.

Dan if you want I can generate a template for that type of work which will generate all of these values and the graphs and email it back to you.

If you have the time/inclination you could carry the Sebenza and ZT and then periodically just record the sharpness and see how well the numbers match your perceptions. The ideal way would be for you to make your notes on the physical handling, and ask a friend to measure the forces and only look at the forces when you have decided to stop the experiment. This way you have reduced one of the largest possible sources of bias.
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Re: Question with a sebenza 2 years 1 month ago #4347

  • BassLakeDan
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CliffStamp wrote:
..
(from an earlier post...-->) If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).

..Now that just means *for this run* it was sharper, it does not mean that it always would be, Dan would need to sharpen a few more times and if the Sebenza was consistently sharper then an inference could be made about the steel.

This is interesting, and I will put this on the 'to-do'list. I was looking through my stack of testing that I have done with the jig and came across one early on that was done with the ZT and the baseline numbers started as low as 175g so I know that I can get at least that far with it. It would be interesting to see if that could be brought even lower, and if the sebbie could be brought along with it. It would be interesting to see if I could 'find the bottom' (so to speak..), but with S35vn I don't think I have the equipment or the skill here to really find out the answer to that, but it would make an interesting afternoon of sharpening :) I do know that I have found such "bottoms" with certain knife steels: for example I tried pushing a CRKT M16 every finer and found out that the jig told me I could never (no matter what I did!) get it sharper than 180 or thereabouts. CRKT says the alloy is 8Cr15MoV.
Last Edit: 2 years 1 month ago by BassLakeDan.
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Re: Question with a sebenza 2 years 1 month ago #4349

  • CliffStamp
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It might be interesting to set a few benchmarks which should be extremely high (or low as it were) :

-standard utility razor blades as-new
-double sided razor blades
-"shave ready" straight razor
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