Joining the thread cutting club...with surprising results

H

HoB

Joined
May 12, 2004
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Today I rigged myself a quick thread cutting tester. THIS IS NOT MY DESIGN IDEA!. This is a modification based on a design that Nozh I believe showed on the forums about a year ago (found the thread, see here: http://www.bladeforums.com/forums/showthread.php?t=346429). I liked the design because tying nooses with thread drives me nuts.

But now the question becomes: What kind of thread. I would have thought it wouldn't matter much, there would be simple and offset in the numbers... but it seems that is not the case. I started with really thin poly thread and my Yojimbo got 35 g. Not constistently over the entire edge, but that was no surprise since the edge was already used. So only the part that didn't really see any use very close to the choil achieved that performance.

Still the number seemed very low. So I tested my straight razor and it clocked in at 14-18 g!!!! It actually cut the individual fibers while the other knives cut the thread in one snap.

I tested the strength of this poly thread by taking a simple 1 foot plastic ruler and pushed till it would snap. It did so at about 1.5 kg. I figured this would give a way to compare different kind of threads. So, to get more comparable numbers, I dug out some thicker poly thead. This one is labled "button/carpet" and made by singer. The strength of this thread is beyond the measurement capabilities of this setup. It will tear at the screws, not at ruler at well over 3 kg.

My Yo clocked in at about 80 g with this thread. I tested as benchmark the straight razor again and it came in at about 130 g....huh??? Ok, so I tried again...with reproduceable results.

I looked at the straight razor blade and it turned out that the stronger and thicker thread had actually cut a notch into the edge of the razor! Not very large but clearly visible with the naked eye and rather pronounced under a microscope. The edge had deformed during the test and this was why it suddenly took more force than the Yo.

Now I start to wonder: At what forces does microscopic edge deformation begin. When does this test become unreliable. It is quite clear from this little test that even the slightest burr will give very poor performance on anything but the thinnest thread....so what is the ideal thread thickness, and how do we measure it?

I will go to an arts and crafts store tomorrow and will sample some threads.

Btw. this little setup costs in total about $45 ($40 for a nice digital kitchen scale) and can be assembled in about half and hour and you can do about 6 cuts per minute. The thread is clamped between the regular nut and the wing nut.

Oh, I forgot I tried the thread with and without slack, but the difference was not very pronounced. Have to test that further though.

 
i looked at my straight razor closer and it turns out that I completely ruined the edge. I have to take it to the stone and carefully sharpen out the two notches that the heavier thread made :(.
 
It's amazing how delicate a straight razors edge is, they can be a bear to get honed again too. Ask me how I know ;) I've seen horrible damage done to razors where the user had tesed the sharpness by cutting credit card receipt paper. It's amazing it doesn't get trashed during the shave, but they don't.
 
Actually, if you don't soak your facial hair for 3-5 minutes with warm water before a shave, it does get torn up, but to a lesser degree.

HoB: the best way I've found to calibrate thread is to use a disposable bic plastic handled razor. Yellow handles white heads, just use a thin knife like an opinel and stick it directly on top of the razor blade, and pry, a piece of the head will pop off, then cut the posts holding it in off. A bic razor tests at 30 grams on my scale/thread combo, a really sharp knife at 75, and at 40-50 your looking at crazy sharp... Indiscernible from a razor blade, but definately you can immediately tell its much sharper then the 75 gram knife.
 
louisianacook said:
It's amazing it doesn't get trashed during the shave, but they don't.
Click to expand...

Ever use your razor after your wife/girlfriend uses it to shave her legs? You'd think that a woman's hair is made out of Brillo pads.:D
 
HoB said:
...what is the ideal thread thickness
Click to expand...

The smallest that you can cut and still read with precision on the scale. As the diameter gets larger it stops becoming a test of sharpness and starts to become significantly a test of cutting ability because enough of the edge will "see" the thread for geometry to be a factor.

Note any cutting no matter how light is considered to be "destructive" testing in a materials sense meaning that it effects the edge. The "softest" material you can use minimizes this dulling. I use very light baisting thread which is the best I could find a few years back, there are better materials available :

http://cablespeed.com/~sgelliott/blade_testing/html/testing_sharpness.html

The better the scale you have the more optimal thread you can use because the lower the force and thus the greater the sharpness you can measure. Note this test only measures half of sharpness, there is also slicing aggression. Blades should score high in both.

-Cliff
 
Cliff Stamp said:
The better the scale you have the more optimal thread you can use because the lower the force and thus the greater the sharpness you can measure. Note this test only measures half of sharpness, there is also slicing aggression. Blades should score high in both.
-Cliff
Click to expand...

Sure, but one thing at a time. I have also always been more interested in pushcutting ability than slicing ability. Just a personal preference.

Yes, the scale is really important. The first one that I got was $10 cheaper, also digital and same resolution (1 g), but it read with a delay: You would push on it and a second later the numbers would come up or change. Probably not such a bad thing to get maximum force, but I really didn't like it. I returned it and got the one in the picture and it reads very smoothly in 1 g intervals. I checked accuracy and it is actually good to the rounding of the last digit (+/- 0.5 g) and it reads so smoothly that measurements to +/- 1 g are quite feasable. It also has a tarring (zero function) to take out the weight of the little setup. The response is slightly slower (more mass/inertia), but still as accurate.

Thanks for the link, I will go and look for 40 wt. Rayon thread.

Ghostsquire: I am trying to find those BIG razors (I looked last night and Stop & Shop didn't have any). I am going to take some old style double sided razor blades home with me tonight (we have them in the lab, used to use them for ice shaving). Merkur (German) and Feather (Japanese) brand.

I attached a microscope image (about 150x) of the damage to the razor blade. You can really see how the edge is folded over....I am not looking forward to fixing this :(. I have sharpened this razor before, but never had to fix a defect like that.

 
The loops are annoying to tie while you are doing it, I typically do it while doing something else like watching a movie, I have a set of pegs in my room with a bajillion loops on it so when I want to measure the sharpness I can typically run the measurements in seconds per cut. I use a really cheap scale, it is calibrated in 20 g increments so the accuracy is about 2 grams ideally and 5 grams realistically. I never get edges that consistent anyway. I brought a few into the lab and ran tham on force probes which are much more precise and got the same results. The variance along the edge is way above the repeatibility of even coarse scales, digitals are nicer to read though. I like the ones with the memory and average functions, you can just enter a bunch of values and they even do basic statistics for you.

-Cliff
 
Ghostsquire: I am trying to find those BIG razors (I looked last night and Stop & Shop didn't have any). I am going to take some old style double sided razor blades home with me tonight (we have them in the lab, used to use them for ice shaving). Merkur (German) and Feather (Japanese) brand.
Click to expand...

Yeah, you have to be careful with those Merkurs though, because some parts of the edge will test slightly duller then others due to a burr being there. I've found Bic's to be more reliable. Generally Merkurs test in at 30-40ish for me.
 
Ahh, well, you can say what you want about the Japanese, but they know their blades.

So, on the thinner thread the Merkur razorblades come in at about 13-15 g (similar to the straight razor) on a good section, the Feather razor blades at 5-7....:eek:

Ok, tomorrow I go out and buy some thread that is in between the carpet thread and the other thread I have.
 
Does the amount of tension or slack in the thread cause test variability? Seems like it might.:confused:
 
What exact type of scale are you using? Seems very accurate, I'm using an analog one that at best I can estimate to within 20 grams or so. I tried looking on amazon but I don't know which ones are good or not.
 
Funny you mention Feather razor blades, these blades are the best thing that has ever happened to my morning shave. They make "razor blades" seem dull. Truly amazing.
 
Ghostsquire: I simply went to a couple of stores that are selling kitchen utensils. I found what I wanted at Bed, Bath & Beyond. I was specifically looking for a digital scale with a 1 g resolution, a maximum range of at least 3 pounds and that was somewhat affordable. Only a few years ago those where difficult to find, but the prices have come down quite a bit. Now every larger store has a selection of 2-6 different scales. The ones at BB&B where all made by Salter, a German company. Just as the Japanese with blades, the Germans have a history of making fine scales, the one I bought (which you see in the picture) is of course made somewhere overseas. It's still decent enough though. It's probably best to be able to test it in the store. As I said, the first one I grabbed was also by Salter and $10 cheaper. I left the store, opened the package, inserted the battery, got dismayed at the reading delay, wrapped everything up, when straight back in and bought a different one.

I would think that the tension of the thread should play a part. I haven't fully tested that yet, but my initial impression is, that it plays a smaller role than I would have thought. Currently I pull the thread over to the end of the other wing nut and then clamp that position of the thread between the nut and the wing nut with one turn around the screw. That gives the thread always the same slack equivalent to the "length" of the wing nut.

I just got back from an Arts and Crafts store. Turns out that 40wt Rayon (see Cliff's link) is easy enough to find. On all the other threads they either didn't say what weight they are or the composition is a mistery...or both. So I figure 40 wt Rayon is a good selection. I probably going to use that for my future tests, but I bought some mercerised cotton as well.
 
This is more for entertainment:
Picture of the edge of the Merkur razor blade. What you can not see in this picture, since the field of view is too small, is that it has TWO very distinct microbevels. The one you see, and one a little further back.



The feather has only one microbevel and the transition is not as sharp on the Feather as it is on the Merkur. The microbevel is also wider. The finish seem about the same to me, but the feather had these little "bubbles" (roundish features on the side of the bevel) that you also see in the picture of the straight razor. I don't know where they are from. I am also unsure what that stuff on the edge is. I am pretty sure it is some wax that is applied as protective coating. I did not wipe the edge down, this is straight out of the wax paper. The Merkur shows the same but to a lesser degree.....Actually, looking at the pictures, I would say the finish of the Feather is a bit finer than on the Merkur, but, man, both are pretty rough in contrast to the straight razor.



The last one is a "razorblade" for comparison. We have them by the hundreds in out lab to open packages, scrape of residue etc. They are thinner than utility blade and a bit sharper, too, but you wouldn't want to shave with them. I don't think they are even in the same ballpark as the real razor blades in terms of sharpness (they are a lot thicker, too), but I am taking one home tonight and see what it tests at.

 
Well, so I tried the two threads and it turns out that the Rayon stuff is pretty thin. Very close to the thin thread that I used before.

Rayon:
Dovo: 19
Merkur: 18
Feather: 9
Razor blade: 30
Yojimbo: 35
Pacific: 42

Those numbers are a rough average. No statistical analysis. I was just playing around. Just as Ghostsquire said, the Merkur are pretty inconsistent. I had some sections that were as good as the Feather, and others that were more like 25-30. I discounted both. The generic razor blade did better than I would have thought.

The mercerized cotton seems more like the thread that Ghostsquire is using:

Mercerized Cotton (Coats & Clark T39)

Merkur: 30-35
Feather: 20(??)
Yojimbo: 50
Pacific: 60

I liked the cotton thread better than the Rayon, because it seemed to differentiate better, but then I noticed on the Feather the same degradation that I experienced with the Poly carpet thread. Suddenly the force went waaay up to 70ish and afterward I found a notch in the edge. Thankfully this was only on a disposable razorblade and not again on my Dovo.

It seems that for very high performance edges threads thicker than the 40 wt. Rayon can be a liability. I will continue to play around with both, but right now I am thinking that I will stick with the cotton thread for regular knife edges and for ultra thin edges with no width behind the edge, I will transition to the Rayon stuff.
 
Hmmm... that is interesting. If the results are linear then your Yojimbo is incredibly sharp! However for my sake I hope that poly thread calibrates the same with a razor but is different with an Opinel... Otherwise you have shown me up big time! :)
 
Na, that is really not the point here. I like the idea of having a testing method for my own sake and just as you say, to teach myself sharpening with a realiable double check. It is for example interesting for me that an edge that I would consider passing the Murray Carter test, as far as I have always used it, needs to be <60 on this apparatus. Now I can fine tune that Murray Carter test for myself.

I wouldn't assume that anything is linear here. I assume you use the loop method? May very well be that especially on the thicker knifeblades the friction and decomposed force component into the thread is great enough that it requires substantially more force to cut the thread when it runs parallel to the blade than when it is like here more or less perpendicular to the blade. I can for example not relate to the sharpness description in the Cliff's link even though I am useing the same thread. But then again, not even the Feather razorblade catches my arm hair much more than 1/4" over the skin. Thanks though for your trust in my sharpening abilities, but I fear it is misplaced :D.
 
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