Thread: Standard Reference Material for Hanging Hair Test

See:

ASTM D2256 Thread and Yarn Tensile Strength Test

For the ASTM test method for ultimate strength of thread/yarn. This is a starting point for ideas, with the razor introduced as a new feature.

Note that this test has a typical 10% relative standard deviation of ultimate force (to fracture). Typical deformation rates used are 0.8-4 mm/s.

Wish you'd been around when I was having a go at this!

Originally Posted by Alethephant

If a test gives a 10% standard deviation or so in the peak force to fracture, then it is as good as it is going to get, and the test then becomes useful in a comparative sense. The links given suggest this is easily attainable by simple setups.

I'm certain you can get to 10%, at least.

Originally Posted by Alethephant

Designating this new measurement as the "Filament Cut Test" or "FCT", there are different possible ways of getting a low peak force. One is that the blade edge has an ideal shape with sharp angle geometry. The other is that the blade has an erose edge geometry that concentrates peak force into a small contact area, causing initial faults that rip into the material. These two different mechanisms should be distinguishable by supplemental microphotographs.

Lacking the resources to take microphotographs, this was something I struggled with, at least in thinking about it. The easy out is settling for testing finished blades (w/ideal shape).

Originally Posted by Alethephant

I do have a constant speed screw drive force measurement system

Nice!

Originally Posted by Alethephant

I think the idea of initial slackness is a good one, but may not be if scraping of the edge along the filament is an issue.

It is an issue. In fact, it's the primary reason why a simple system like a postal scale and a loop of thread doesn't work, as a sharp razor will catch such a material at the first point of contact and tend to slice it. Not sure if this would be a problem with a monofilament line.

It's all very fascinating - do keep updating, please. I have the feeling that measuring sharpness can be nailed down, eventually. What will then really be interesting is determining its role in creating the ideal shaving edge (i.e. how vague characteristics like "smoothness" factor in).

Originally Posted by northpaw

What will then really be interesting is determining its role in creating the ideal shaving edge (i.e. how vague characteristics like "smoothness" factor in).

Eureka!!!! I think you got it

Sharp is easy, anybody can do sharp.... Smooth takes talent, and touch...

That an even unreliable HHT to determine "Sharpness" has never been a question, that it has nothing to do with "Shave Readiness" has always been the debate...

Northpaw said on a previous thread:

"Since my thread is of an unknown origin, any numbers would be absolutely meaningless. However, three things about my testing were promising:

1. Consistency -- with the same amount of tension on the thread, it took pretty much the same amount of force to cut it time after time (using the same razor, of course).

2. Matched with honing -- as I honed a blade sharper and sharper, it took less and less force to cut a thread with it.

3. Matched with shaving -- if I used two different blades during a shave and felt that one shaved better, the better-shaving blade invariably required less force to cut the thread in post-shave testing."

The best test arrangement is the one that results in the most consistent results. In ultimate force measurements, this is 10% relative standard deviation or less (usually can't get below 5%).

The ideal FCT would correlate highly with honed sharpness so that it would be useful in identifying the endpoint of honing. One virtue would be the ability to make measurements along the longitudinal axis of the blade to verify uniformity of sharpness. Another would be that it could be used as a qualification test for a "Master Honer" designation, if SRP starts accrediting honers.

Finally, correlation of the FCT with shave-readiness is interesting, because it diminishes the importance of other, unmeasured covariates, such as "smoothiness". There are a number of such covariates possible. One thing I've wondered about is whether there is such a thing as "too sharp". Perhaps a razor that is "too sharp" would dull quickly during shaving (roll the edge), becoming a poorer shaver in the process. Another is erosity of the edge.

Another use of the FCT could be to measure sharpness decline pre- and post-shaving, pre- and post-stropping, after pasted strops, after different whetstones, etc. If the FCT were stable and precise enough, a number of interesting questions could be dealt with quantitatively.

I have two razors (both Fermaruds) that are clearly not shave ready yet, despite my first attempts at bevel setting and polishing. They both tug severely in shaving.

I have two more razors that shave acceptably well (a Fromm 72R and a Damisons).

I also have a new Supermax DE blade held in a hemostat to simulate a razor.

In my version of the HHT using my wife's hair, the DE blade cut hair easily 4 of 5 times, with one time failing. The two acceptable razors cut hairs in most spots along the blade. The two duller razors cut hair only occasionally only at a particular point on the blades. Thus the HHT correlates well with my idea of shaveability. (I strongly advise people who use the HHT to test their procedure on a new DE blade to calibrate what "sharp" means.)

I made my first crude test jig yesterday. All it was was a piece of 1x4" wood with a 2" equilateral "V" cut into the top and two 7/64" holes drilled 3" apart across the "V" on the top edge. One inch down on each edge I drilled a 7/64" hole on either side. The 4x4" chunk stands on its unmodified edge.

I put two small iron nails into the top holes as capstans, and used the side holes to pin in the thread with matchstick shafts.

The thread used was Danville's fly tying monofilament (0.006"), which acts close to what a coarse human hair does with razors. I chose this thread for several reasons: 1) monofilament, with simple surface structure; 2) diameter within the range of human hair, so relevant; 3) cutting force roughly similar to human hair; 4) commonly available and cheap. The only problem with it is that it's clear, so it's hard to see and therefore work with.

I pegged down one end of a piece of filament in one side, looped one full turn on the first capstan about 3/16" up from the wood, crossed the gap taut and did a corresponding loop on the same side of the other capstan, and then down to the other hole and pegged. The filament does not touch the wood even during the test.

I kept the filament taut rather than slack, mostly because it was easier to do.

I placed the block on my 400 x 0.01 g Ohaus digital scale. The Ohaus appears to update about once a second. I tared with the block and then pressed each blade down perpendicular against the filament as slow as I could.

The DE blade averaged about 12 g force. The Fromm about 14 g and the Damisons about 17. The first Fermarud as 25 g and the second 28. The results seemed more consistent at the higher end than the lower, perhaps because of the suddenness of the breakage.

From this simple pilot project I learned several things:

1. The usual HHT is qualitatively useful, particular with the comparison to the DE blade.

2. The FCT quantitatively supports the shaveability and HHT results.

3. My honing skills are not as bad as I thought: The Fromm is close to the DE sharpness.

4. The quantitative measurement of sharpness does not appear to be difficult to carry out.

5. I've got the right filament to use in testing.

The next step is to automate the test so that constant deformation rate occurs, as opposed to trying to maintain this by hand. And to use a force gauge that returns peak force, rather than trying to catch it on the scale.

Well, before you get too lost in the technicalities there is a crucial big picture piece missing.

The fundamental part here is shaving, and I'm not quite sure you are yet at the point of being able to evaluate an edge and be within a standard deviation of a pool by say 10 of really well established honemeisters. That's as close as I think we can get to removing subjectivism from what a well shaving straight razor edge means (and of course you know the number ought to be at least 30, if we want to be scientific).

The technical experiments that you're setting up are nice because they're quantifiable and easy to understand. Yet the important point is not quantification, that's been done on industrial scale for over a century and we have plenty of commercially made razors as a result of that, (which you can study and rate scientifically, and you probably should start with that as a simpler test case).

I find shaving with any straight razor quite different than shaving with shavette, or feather, or DE, so even if you can prove you've gotten the sharpest edge out of a razor with numbers, I am still not willing to accept this as a well honed edge until I have shaved with it.

So, at some point you need to address the big question of correlation of numbers with actual shaving beyond the current simplicity.




BTW you should read that old paper of John Verhoeven et.al. if you haven't already.

But then wanting to quantify something as simple as sharpness is a start. We really have no sharpness measurment other than feel and saying "hey man, that is sharp!".

I think the samuri used to measure a sword in how many human bodies it would slice thru in one cut.

.

Actually this doesn't quantify sharpness, for that you need a really good microscope. This quantifies the ability to break fiber.

Gillette and Schick use polymers to reduce the friction so that the edge cuts easier. Almost all of us use water and soap to change the properties of the hair so that it breaks easier.
When you start breaking down the process of shaving the sharpness of the razor is one of the less important factors.

And as far as quantification goes, if you venture to other forums and ebay you'd be surprised how advanced some people are at quantifying it.

This razor is honed up to 15000/30000/40000/50000/60000 grit, therefore it provides the best possible shave.

Interestingly enough a sentence like this tells me everything I want to know about that honester, so I guess for me it works really well.

Originally Posted by gugi

Interestingly enough a sentence like this tells me everything I want to know about that honester, so I guess for me it works really well.

This razor is honed up to 15000/30000/40000/50000/60000 grit, therefore it provides the best possible shave.

Sharpness tester

Of course, the test media isn't exactly equal to a hair... Not sure about consistency. Reasonably objective, and made for the purpose. Also more than I'd want to spend, considering our practical application goals (for sharpening/tooling/hone comparison).