Rope Slicing Test, Part Deux

S

sodak

Joined
Mar 26, 2004
Messages
6,197
Well, here we are. I'm not as far along as I would like, but I think I have the testing procedures worked out, so I thought I'd show them and see what you all think.

I'm going to follow Gunmike1's lead and stretch some thread/twine across a plastic bucket, holding it on one end with a water bottle tied to the other. I can lift the water bottle a repeatable distance, to the lip of the bucket, and slip the knife under the thread, pick it up about a quarter of an inch (this is the most variable part) and draw/slice the thread towards me. Here are some pictures of how it will work, the thread is from Home Depot, SKU 292 222 twisted mason line. I'll be cutting 5/8 manilla from Lowe's, and will get the SKU in a couple of days when I run out and need to buy some more. I have the knives marked every quarter of an inch, so my precision will be to the nearest eighth of an inch.

The thread:
http://i137.photobucket.com/albums/q203/sodak_photos/p1010578.jpg

The knives:
http://i137.photobucket.com/albums/q203/sodak_photos/p1010579.jpg

The highly calibrated weight:
http://i137.photobucket.com/albums/q203/sodak_photos/p1010583.jpg

The technique:
http://i137.photobucket.com/albums/q203/sodak_photos/p1010582.jpg

I am going to do a run with the knives as is - Gunmike1 knows how to sharpen them, and then I'll standardize the bevel on my Edgepro for both of them, and do 3 runs coarse, 3 runs smooth, then start thinning the bevel.

Comments, criticisms, suggestions are all welcome, thanks!
 
That looks like great technique for the mason line slicing, you are much brighter than me on that one! That should be very consistent, and an improvement over how I measured it. This will be fun to see how things turn out. It looks like you are off to a real good start, these things end up taking way longer than you think they should, don't they? Is this first run as shipped sharpness? They were still pretty darned sharp, with only a couple minor cuts on some food and an envelope or two after getting polished out, so basically almost full sharpness (or as sharp as my meager skills could get them). I should have posted my SKU's like you did, I used the SKU you gave me for the rope at Lowes and need to revisit Home Depot for the twine SKU, though that mason line looks better for the test. I also really like your high tech, highly calibrated weight! I look forward to your results and commentary.

Mike

Edit to add: I am really curious as to how the polished edges vs. the coarse edges will perform in initial slicing sharpness on the mason line, and of course the difference in slicing edge retention. As I have seen from many Spydercos and especially from Tom Krein sharpenings (120 grit finishes that whittle hair!), you can have both tremendous push cutting and slicing agression in the same edge. Me, on the other hand, with my meager skills, have to make do with one or the other, though I'm wondering if the higher push cutting of the polished edge (and it's approximately .1 micron burr, if sharpened right) will make it slice better initially on the mason line, as my better polished edges have some bite on the three finger test that I would think would bite into that mason line. From my experience I would expect it to not cut the rope as well after multiple cuts (or even the first cut) as a coarse edge, but the testing will speak for itself, and we can look at the blunting rates of each type of edge. Who knows, we may see a revalation as to the polished edges performance on the rope, the whole point of the testing is to keep an open mind and see what happens, so I guess I should shut up with my preconceptions. Now you just need 40 hours in a day and 8 days a week (and a family that won't revolt) and you can have all of the testing done!
 
sodak said:
I have the knives marked every quarter of an inch, so my precision will be to the nearest eighth of an inch.
Click to expand...

General experimental precision is 1/10 of marked calibration limits, the last digit is always an estimate. It takes practice to be reliable to that degree but almost everyone can split a division into at least 5 parts accurately by eye.

gunmike1 said:
As I have seen from many Spydercos and especially from Tom Krein sharpenings (120 grit finishes that whittle hair!), you can have both tremendous push cutting and slicing agression in the same edge.
Click to expand...

Yes, there is a bit of a myth about highly polished edges being of very low aggression. Now there is truth of course that coarse edges raise slicing aggression but there is also the common influence of highly polished edges being smoothened. I think the main influence is too much work on very fine abrasives which can not cut the carbides.

I'm wondering if the higher push cutting of the polished edge (and it's approximately .1 micron burr, if sharpened right) will make it slice better initially on the mason line, as my better polished edges have some bite on the three finger test that I would think would bite into that mason line.
Click to expand...

It should, if you get it sharp enough it will simply push right through the cord so will require no draw.

-Cliff
 
sodak said:
slip the knife under the thread, pick it up about a quarter of an inch (this is the most variable part) and draw/slice the thread towards me.
Click to expand...

Sodak,
Do you mean lift the thread up, or lift the bottle up a quarter inch?

From the pic it looks like you are lifting the thread up a quarter inch, so I think the force of the cut will be dependent on the distance you lift the thread, or more accurately stated by the tension on the string and the angle of the thread on each side. I think how close to the edge (either edge) of the tub you place the knife may also affect the force of the cut (not sure, and too late to get the HP out). I would suggest making every cut in the same location of the tub, and you could argue that each cut used the same force better if you had a jig (or something) to help ensure you lift the cord equally high for each test cut. I do not know how cord stretch affects this, but if you can get equal force for each cut I think it wouldn't make a difference. Good luck.
 
Lift the thread with the knife. You're right this is one of the larger variables that I'm running in to. I tried to rig up a ruler of sorts to measure the "lift", but it got too time consuming. If you lift too much, you essentially just cause a lot of tension between the holding hand and the blade, negating the "standardized weight". I'm trying to eyeball it carefully and be consistent, and am finding out it takes a few runs to really make things repeatable.

Kind of like being on the firing line, I need to put a few rounds downrange before I settle down and do some good shooting.
 
Random variations are rarely a concern because of the effect of averaging. The force uncertainty will basically be the height uncertainty, now lets assume that you are really varying the height by 25% with each cut. Further that you take 5 measurements for three runs. When you average this force variation will be reduced to about 5%, not significant. What you need to be concerned of is are yuo doing something consistently different with one knife versus another, the things that randomally change will smooth themselves out.

-Cliff
 
How about putting something in the tub for the spine of the knife to rest on that way you're always at the same level of lift. Heck you could even use a ruler and then measure by how much the knife is moved on it.
 
sodak said:
Lift the thread with the knife. You're right this is one of the larger variables that I'm running in to. I tried to rig up a ruler of sorts to measure the "lift", but it got too time consuming. If you lift too much, you essentially just cause a lot of tension between the holding hand and the blade, negating the "standardized weight". I'm trying to eyeball it carefully and be consistent, and am finding out it takes a few runs to really make things repeatable.

Kind of like being on the firing line, I need to put a few rounds downrange before I settle down and do some good shooting.
Click to expand...


I found I spent as much or more time doing test runs as actual testing. Figuring out the amount of weight to hang from the twine, how to best measure the length of edge cut, and the actual rope cutting set up were very involved. Doing stuff like this for the first time is tough and time consuming, but I know I really appreciate the work you are doing. I'm sure everything will work out fine, it looks like you have a pretty good set up for measuring the slicing length on that line, and as Cliff said with multiple cuts and runs it should equal out as long as you are consistent from knife to knife. I tried my best to do that, but being human we can't quite attain perfection.

Mike
 
Even if you were perfect, the steel itself is not, take a look at some micrographs and the variation is large. You can see this directly in Landes edge stability measurements where the same steel scores near the bottom and near the top in a results table because it is highly carbide aggregated and it depends if the measurements hit a carbide chunk or not.

I meant to note this before as a suggestion in regards to the measurements. At the end of the runs if a smooth steel is available, use it for just 1-2 passes per side and then retest the sharpness. This will allow an estimate of just how much blunting was caused by deformation vs wear as the steel will just align the edge with a mild smearing effect.

-Cliff
 
Thanks for all the good comments! Cliff, I'll add in the smooth steel part, that's quick and easy. Db, I'll try something like a book or block of wood or something like that. It has to be standalone, as both my hands are busy! Mike, you're right, testing takes time to do it carefully!

I'm finding it takes a lot of concentration to correctly read the sharpness results, when the twine snaps and the bottom falls, it's fast enough to make you concentrate hard.... But it's fun, I'll post my first run results next week, along with a "general" review of both knives, ergonomics, etc.

Paul
 
sodak said:
I'll add in the smooth steel part, that's quick and easy.
Click to expand...

It will be interesting to note if there is a difference between the obtuse and acute angles. It would seem obvious at first that there should be more deformation with the more acute angles and more wear based blunting with the more obtuse ones. Mark Henry of Furitech has claimed to have proven this through measurement, but has not released the data.

-Cliff
 
Cliff Stamp said:
Random variations are rarely a concern because of the effect of averaging. The force uncertainty will basically be the height uncertainty, now lets assume that you are really varying the height by 25% with each cut. Further that you take 5 measurements for three runs. When you average this force variation will be reduced to about 5%, not significant. What you need to be concerned of is are yuo doing something consistently different with one knife versus another, the things that randomally change will smooth themselves out.
Click to expand...

I do not think this is the correct way to look at the inaccuracies in this test.

What you have to determine is how much variance there is in the length of cut measurement due to other variances inherent in the test. To say that there is a 25% variance in ht. of twine means nothing until you determine how much this 25% variance in a unrecorded test variable affects the actual measurement you are taking, in this instance the length of cut reading.

So you have to come up with a total inaccuracy in the length of slice measurement inherent in your test from ALL factors. So let's say there is a 25% inaccuracy in force, a 10% factor for variances in the twine, and a 25% factor for variance in each slice (velocity of slice and up and down movement of the hand). Are there any more?

Then you would normally use your hypothesized model to determine how much these factors would affect the length of cut measurement. You'll have to wing it if you're using Cliff's model, because it cannot tell you how much all those factors will influence the length of cut.

Then after that, you will have to add on the inaccuracy from the actual measurement of the length of slice. Also consider that the variance or inaccuracy of the length of slice measurement changes throughout the test (it is greatest, and probably very high, when length of slice is shortest). So early data will have a greater inaccuracy than later data.

Lastly you will use this estimate of total variance to determine how many measurements and how many runs you need to take to get acceptable data.

I'm not saying this is always feasible, but I think it is the correct way to look at this scientifically and mathematically.
 
If you are worried about the height you are lifting being repeatable, you could add a ruller into the setup. Just hold the ruler next the string, and watch as you lift.

This is an interesting point, because with this geometry, the downward force of the string onto the blade varies with the sine of the angle made. For small angles like you have, the force will vary linearly with the height you lift. So if you vary the height unintentionally by 25% (ie, 25% of 1/4 inch = 1/16th) you are varying the applied force by 25%.

Any variation in your standard weight will have the same linear effect. Presumably this will only be a problem when you do the test one day and then the next.
 
Thanks Gator86,

I was trying to come up with a quick stand to set the ruler in, but was thinking about it too hard - duct tape, of course! The force that binds the universe together! :D

http://i137.photobucket.com/albums/q203/sodak_photos/p1010584.jpg

I marked it on the 6, and marked the other side of the tub where I hold the string to it, so now the only (really significant) variable is measuring the length of edge, which is always the challenge.... As you can tell from the pictures, I'm accustomed to working in clean rooms with highly sensitive apparatus....
 
gator68 said:
If you are worried about the height you are lifting being repeatable, you could add a ruller into the setup. Just hold the ruler next the string, and watch as you lift.
Click to expand...

This really is not necessary, it is a small random variance and not even the major one. Errors in measurement do not simply add as in 25% + 10% + 5 % + 5% = 45%. The actual summation of all these errors will only produce 28% which means you can ignore the smaller ones completely. The dominant random errors will be in the materials, what is cut and the actual steel itself, especially for D2 which looks even more inconsistent in composition than this which is ATS-34 :



and the P/M counterpart RWL-34 :



[those images are from Landes text on steel]

D2 vs CPM D2 would be similar, but even more aggregated, with the carbides being twice as large. Now imagine testing an edge which intersected those large carbides vs one that did not. Even more importantly is to not get caught in precision because the real focus should be on accuracy which is determined by the systematic errors. Accuracy is how close your value is the the population value while precision is just how much your value changes from one measurement to the next.

Note that Mike testing already showed that the above slicing is a consistent measurement of sharpness because he did it repeatidly with between 1.5-3% variance in his final results. He also showed that it was accurate as it correlated well with the other tests of sharpness used and with how well the knives were cutting the rope:

http://www.cutleryscience.com/forum/read.php?7,23,23#msg-23

These small deviations in his final sharpness measurements were the actual measured standard deviations in his mean results. You do not determine the uncertainties by theory, you actually measure them just as you measure the value itself. You estimate them coarsely in the beginning to see what needs attention, but these are again just guidelines, the actual values come directly from measurement. Mikes values were very consistent from one run to the next as all the random cutting variances will smooth out by a root n effect. This is basic statistics.

So if you vary the height unintentionally by 25% (ie, 25% of 1/4 inch = 1/16th) you are varying the applied force by 25%.
Click to expand...

This will reduce the cut length by the same amount for small changes in force. This is an insignificant concern as it will simply average out as will any random deviation. Quite frankly, the results from one Military to the next are going to be far larger than the small deviations that Mike saw when measuring the sharpness, 1.5-3%. I already showed this in the past by running the same edge holding checks on multiple S30V and ZDP-189 knives. Ref :

http://www.cutleryscience.com/reviews/blade_testing.html#cardboard



-Cliff
 
gator68 said:
This is an interesting point, because with this geometry, the downward force of the string onto the blade varies with the sine of the angle made. For small angles like you have, the force will vary linearly with the height you lift. So if you vary the height unintentionally by 25% (ie, 25% of 1/4 inch = 1/16th) you are varying the applied force by 25%.
Click to expand...

As long as you make your cut in the center of the tub, right? It changes as you get closer to the edge of the tub (force gets greater for the equal lift heights, the closer you are to the edge of the tub, I think...).

So make cuts in the center of the tub. And account for the inaccuracy in how close to the center of the tub you are, too. ;)

And again, it's all academic unless we can hypothesize (or otherwise estimate) how much a change in force changes the length of slice required to cut the twine. The standard way to determine test uncertainty (or error) is to take the uncertainties in the different variables, and plug them into your hypothetical model equation to get total uncertainty or error (I'm probably butchering the correct statistical terms here).

I know I'm being particular to the n'th degree here, but thought the n'th degree may be warranted given the circumstances. I am still on the fence as to what can be concluded from this test, but think it's a worthwhile endeavour to reduce uncertainty any way that is practical without making the test too difficult. And I wish you good results.
 
If sodak has to worry about lifting the twine an extra sixteenth of an inch to see marked difference in measurements between the D2 and CPM D2, what's it going to mean in a choice of which knife to edc.
 
There is that rather obvious point which is why I said clearly to the evaluation group that no measurements have to actually be taken if the individuals are not so inclined. They can just carry them and report how often they are sharpened over a period of time.

There is a huge problem in this thread with understanding the difference between precision and accuracy, what are the causes of each and how to determine the functional tolerance for both. I will be writing a series of articles for the evaluation group to use on this subject.

-Cliff
 
I thought the point of having your group was for them to be independent from influence and test with their own methods and thoughts on the knives. Seems that writeing articals for them steers them in your direction. Creating bias? I clearly already see Cliffs influence on the tests of Gunmikes, and looks like it could bleed into this one also.
 
You must log in or register to reply here.
Back
Top