Thread: Standard Reference Material for Hanging Hair Test

Originally Posted by Alethephant

However, I am open to the possibility that, once basic good sharpness occurs, then sharpness may no longer correlate with improvements in shaving. Those might relate to other factors. This would also explain the division into camps on SRP. For beginners, attaining initially useful level of sharpness is difficult, so sharpness is the central issue. Thus the HHT interest. For experts, all razors used are very sharp, so very small improvements in sharpness are no longer correlated with significant improvements in shaveability. Something else is going on.

The key question is then: "If two razors differ detectably in sharpness, do they also differ detectably in shaveability, or is this only true for some range of sharpness?"

Good grasp of things, here. I say go for it.

One point to mention, in case it should prove helpful: I found that small changes in the tension of the thread (in my case) resulted in large differences in the amount of force it took to cut it. Instead of hand-winding--if I'm reading that correctly--you really need some way of ensuring that the tension on the line is the same each time. On my crude setup, weighting one end of the line seemed to work fairly well, but there are surely other ways of doing it, too.

EDIT: Just saw your comment at the end of post #15 about "constant deformation rate". I believe that's what I'm getting at.

The tension and deflection rate would show up as increased variation (inconsistency). That doesn't appear to be happening. In fact, right now I don't know if the accuracy of the force gauge isn't the single largest error in the system.

Despite hand winding, hand pressing, off-angle winding, different spools of line and other issues, the standard deviation is still down in the 6-10% range, which is typically about as good as can be expected in these tests.

So, at the moment, I have no real motivation to try to improve the arrangement: It's good enough.

I think the type of filament is important in some of the issues you mention. For example, if the line stretches or doesn't stretch, the amount of tension might be very important.

Revision 2 of the FCT ("Filament Cut Test").

This is a piece of wood 3.5" long, with two 4d nails at 0.5" and 3.0". I also bored two 3/32" holes on the ends for a piece of round toothpick to peg down the line. There is a center 3/16" hole to screw the 10x32 bolt end of my DFI2 force gauge into.

My DFI2 gauge remembers peak force in grams (unfortunately no decimal). It also has a computer interface. It's measurement rate is 400 sps, with the meter reading an average of 40 samples and the peak reading instantaneous. I will connect this up later and get some force-time curves. (If I connect to the motorized test stand, I can get force-deflection curves.)

The filament deflects a cm or so before breaking, so I don't think slack is a big issue.

I carry out the rev. 2 test as follows: Peg the filament down one side. Draw taut up to the first nail and wrap with two loops. Pull taut over to the 2nd nail and do two loops. Pull taut down to the other edge and peg down it down. Zero the meter. Then take the razor and press slowly down perpendicular to the filament at the center of the span. After fracture, record the peak value captured.

With this setup, I obtained the following values:

Black (Fermarud) razor:

After honing, before stropping: 36 g
After stropping: 29, 30 g [previous jig: 28 g]
Hanging hair test ("HHT"): Ok to Good (50-75% success)
Shave test: Poor (snags on chin)
(There is a known honing problem with this razor I haven't fixed yet: It doesn't lie flat at the heel.)

White (Fermarud) razor:

After honing, before stropping: 36 g
After stropping: 21, 23 g [previous jig: 25 g]
Hanging hair test ("HHT"): Very Good (100%)
Shave test: Fair (some snagging over chin)

SuperMax DE blade, new: 14, 16 g [previous jig: 12 g]

The first and second versions of the FCT were done with different jigs and different spools of filament.

Conclusions:

1. Good reproducibility, particularly given hand driven deflection, different spools and different test jigs.

2. Repeatability appears to be a standard deviation of about 1.6 g, or a relative standard deviation of 11% for the lowest value and 6-7% for the highest values. Appears to be a constant error rather than a constant relative error. This should clarify in more extensive testing.

3. Correlates well with my HHT.

4. Correlates well with shaveability.

5. I'm happy with the current jig. It appears to satisfy my needs with a minimum level of effort. It is a little time-consuming reloading filament. I don't have any good ideas on how to speed things up, other than having a better lockdown arrangement.

See pictures attached.