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+--- Thread: Same point, different score (/showthread.php?tid=260)
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Same point, different score - SHARPCO - 01-23-2018
I marked the point to check on the knife. However, even if I check the same point, the score will come out differently.
295 >>> 265 >>> 290
Of course, I pushed down the knife smoothly. Is there a problem with the accuracy of the PT50B?
RE: Same point, different score - Ken S - 01-23-2018
Sharpco,
My first Edge on Up tester is the original KN-100. It is not as fast to use as the PT-50 series, however, I like it. It sits on a user provided postal scale. The knife is held in a clamp. Pressure is applied with BBs or lead shot. For very quick testing, I filled a plastic 35mm film can with enough BBs to weigh exactly 100 grams. I used this as a "go-no go" standard. If the knife edge cut the test media, I knew that the edge was equal or better than 100.
This 100 gram film can is a good way to test your scale. Whenever you get readings which don't seem right, weigh the film can. If it reads 100 grams, your scale is working properly.
With readings in the 260 to 295 range, my first thought would be that you may have some residual burr. Perhaps the test media is moving the thin burr slightly. I would suggest working some more on deburring and retesting. If the tests improve, your tester is not only measuring sharpness, it is a diagnostic tool.
Keep us posted.
Ken
RE: Same point, different score - SHARPCO - 01-23-2018
(01-23-2018, 05:46 AM)Ken S Wrote: Sharpco,
My first Edge on Up tester is the original KN-100. It is not as fast to use as the PT-50 series, however, I like it. It sits on a user provided postal scale. The knife is held in a clamp. Pressure is applied with BBs or lead shot. For very quick testing, I filled a plastic 35mm film can with enough BBs to weigh exactly 100 grams. I used this as a "go-no go" standard. If the knife edge cut the test media, I knew that the edge was equal or better than 100.
This 100 gram film can is a good way to test your scale. Whenever you get readings which don't seem right, weigh the film can. If it reads 100 grams, your scale is working properly.
With readings in the 260 to 295 range, my first thought would be that you may have some residual burr. Perhaps the test media is moving the thin burr slightly. I would suggest working some more on deburring and retesting. If the tests improve, your tester is not only measuring sharpness, it is a diagnostic tool.
Keep us posted.
Ken
Ken,
This knife is my utility folding knife. I don't sharpen it before testing but a month ago, I honed it with a leather wheel. So it seems burr is not the cause.
After that, I tried testing with a freshly sharpened knife and OLFA cutter. But these also have the same result.
RE: Same point, different score - grepper - 01-23-2018
The test media has a .009 inch in diameter. Chances are the blade is not being tested in EXACTLY the same place, or there is burr, or the blade has a "toothy" edge, or the edge is not sharpened perfectly evenly.
Please read:
http://www.edgeonup.com/Support_Document.pdf
RE: Same point, different score - EOU - 01-23-2018
Hello SHARPCO and Ken. SHARPCO this is an excellent question and very happy that you have brought it to the Exchange. Learning to interpret and apply measurement results correctly is as important to new users as the measurement data itself. Let us first put you at ease and then we will address the problem. Your PT50B will not yield 35 point differentials under identical measurement circumstances and, even more reassuring, the PT50B will not tell you that an imperfect edge is perfect.
We want to speak more generally here to all new users but will try to address some your specifics SHARPCO at the same time. As much as we would like to visualize edges as very homogenous cutting surfaces they typically are not. To the eye, perhaps so but on a microscopic scale, no. The microscopic scale is the measurement basis that all of our instruments operate on. Case in point; your delta (difference) reported here was 35 points. 35 points on the BESS represents a difference in apex radius of 35 nanometers. An almost infinitesimal measurement. While you may think that you are measuring in exactly the same spot it is very difficult to do so and location changes of only a few thousandths of an inch (a distance, perhaps, 50,000 times greater than the apex radius) can easily induce the sort of variability that you describe. Additionally, the PT50B is a manually operated device and some differences between measurements may be induced through variability in technique. These are typically reduced through experience. Experienced instrument users and sharpeners frequently report +/- 10 point differentials (and better) along the length of the blade. They often credit their PT50B's for having pointed out their previous sharpening inconsistencies.
Even DE razor blades do not hold perfectly homogenous apex radii. The DE blades work nearly perfectly as a sort of standard for the BESS scale though because their apex fluctuations are on a smaller scale than the diameter of BESS test media (.009"). Thus, the greater test media diameter averages most of the fluctuations out. If you had experienced 35 point differentials when measuring new DE razor blades (nearly impossible unless the error is induced intentionally) we simply could have told you to look to your measuring technique. In this case, we assume that you are measuring an edge that has seen some use. Under these circumstances we would not have been surprised if the differential you measured was much greater. Remember, even one partial cut through the paper wrapper of a DE razor blade can roll (dull) the DE edge 35 to 100 points.
The most important takeaway here for everyone is, please, don't shoot the messenger. A PT50B will tell you how it is, not how you think it should be. If a PT50B tells you that a given point on an edge is duller or sharper than another point or a previous measurement, it most very likely is. It now becomes your very pleasant and interesting, sometimes disconcerting, job of figuring out why.
EDIT REMARK: We see that Grepper stepped in and addressed some of our points before we could get this posted. Thank you Grepper for your help.
RE: Same point, different score - SHARPCO - 01-25-2018
I understand how hard to check the same point again. But sometimes we need to it.
For example, --> http://www.bessex.com/forum/showthread.php?tid=229
If the increment of 5 scores measured by KG is a valid value, it is necessary to assume that he has checked the exact same point.
RE: Same point, different score - Tman - 01-25-2018
I just have to comment here so sorry. The same point? The same point +/- what? No two 1/4" drill bits drill the same size hole. No two drops of paint are exactly the same color. I've been using the KN100 at work for two years now and I would guess that it will put me on the same spot +/- .010" pretty consistently. I doubt that you can match that with an electronic tester without a test stand of some sort. But who cares? I'm not developing an edge tester. Somebody else already did that for me. I'm measuring edges that are used in high precision steel rule dies and when the guys in the shop sharpen them correctly it doesn't make any difference where I measure because the measurements are going to be within 6-8 pieces of #8 lead shot of each other . That's 3 or 4 grams so what more do I want?
RE: Same point, different score - grepper - 01-25-2018
A applaud your attention to detail Mr. Sharpco, and I think Mr. Tman states it well in his reply.
It’s important to understand that the sharpness testers are not specifically designed to measure sharpness in exactly, precisely the same spot on the blade every time you take a measurement. It’s not the goal, purpose or mission of the device. Even with the knife in a vice and jig for lowering the blade there could be a variance in repeatability simply from reloading the test media between measurements.
In most cases, super accuracy in same spot measurement repeatability doesn’t really matter for measuring knife edge sharpness. Remember that that test media is only .009 inch in diameter, so unless for some reason you are trying cut something that small and need it to be in exactly the same area of the blade with each cut, being able to measure that exact same spot again is unnecessary and would not improve the usefulness of the data.
I usually take two or there measurements along the blade. If they are all pretty close I just call it good. Sometimes I’ll get lucky and they all will be within 5 – 10 grams of each other. Other times more, maybe 20. That’s OK for my kitchen knives. Depending on how lazy I feel at the moment, I might even put up with a 30 gf variation and just call it good enough. I'm not all that concerned about the sharpness in one tiny spot on the blade, but rather an indication of how sharp and evenly sharpened the blade is overall. It's also useful to indicate that there is still remaining burr, especially if there is large variation in one area.
Zero out your sharpness tester and using your finger run it up to 30. That’s the amount of pressure difference we are talking about when cutting something. That’s very little pressure. I know I would not notice it or even be able to detect that variation of sharpness when slicing a zucchini, a potato or even tomato.
But that’s just me. I guess it depends on what data you are looking for and how you are going to utilize that data when taking sharpness readings on a blade.
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RE: Same point, different score - Bud - 01-25-2018
Sharpco asks good questions and pretty much the same ones I had when I first got my tester. I can't add much to Mr. Grepper's very good post other than to say that the better my sharpening got the better my tester worked.
RE: Same point, different score - grepper - 01-25-2018
“…other than to say that the better my sharpening got the better my tester worked.”
Chuckle. ROFL!
I don’t want to spill any beans, but you have stumbled upon a secret, hidden feature built into every sharpness tester. It’s generally not discussed because only experienced sharpeners like you are able to comprehend that feature.
