Cardboard Massacre!!!!

[chad234]

Cliff,
Thank you for the additional info, and I totally agree that the Busse folder will rock, a nuclear tough folder is the stuff my INFI dreams are made of.

I also am amazed with the thickness of modern knives, as steels get stronger it allows for thinner knives, with higher performance grinds and edges. It seems like a waste for a maker to have a strong steel and put a thick saber grind and and an obtuse edge on it, the advantage gained by the stronger steel (that should provide an allowance for higher performance geometery at acceptable levels of durability) is largely wasted.

In regards to applying a secondary edge to a mora knife: There was an interesting thread on the subject as it applied to stainless moras, which are softer than the carbon steel, especially the laminated versions. Basicly I have seen a great increase in edge durability during utility work, like food prep and cutting cardboard, but less of an increase during wood work (I assume because the additional edge retention is due to decreased deformation rather than abrading or microshipping of the edge and because the edge is largely supported bt the wood being worked on it does not roll as much)

Interesting comments on geometery effecting edge retention, the only comments that I would add is that I have seen a strong trend towards non-linear edge retention in very acute edges. A very thin edge (say a SAK reprofiled to a 20 degree included edge at a fine ceramic finish and stropped on a supprted charged leather strop) will deform strongly during the first several cuts (with almost exponential degradation, then the performance levels out and becomed more linear. This is with no burrs or wire edge (by feel and under a 10x loupe).
A thicker edge will not degrade nearly as fast during the first few cuts (each cut being 48 inches through fairly heavy cardboard). (thicker being 40 degrees included which is still a fairly high performance edge relative to what most laymen are using)

Thanks again for the info and I hope all is well in the physics world.

Take care,
Chad

 

[Cliff Stamp]

Chad :

I have seen a strong trend towards non-linear edge retention in very acute edges.

This is a very astute observation. For all blades, you will see performance decline in a logarithmic manner, which means it will be quite rapid at first, but then shallow off. However the rate of fall off will be skewed by the geometry just as you noted. Consider that the total force required to cut through a piece of material is equal to the amount of force for the edge to cut the material (F_edge) plus the amount of force necessary for the blade to mash the material along the sides out of the way (F_sides). Thus :

F_total= F_edge + F_sides.

As you cut through the material and the edge blunts the change in force you see will be due to the change in F_edge, as you don't change the gross geometry of the blade. This means that if you look at the differences then you will see them change at a constant rate no matter what the geometry. However people don't tend to look at differences, they tend to see in percentages, and if you look at the results in this manner you will in fact note exactly what you saw which is that with a thinner bevel, the blade gets blunter faster, which you can deduce from the above equation.

To clarify :

Here is the performance on thread of the Twistmaster from Cold Steel with NIB bevel :

210.0000
252.5000
270.0000
287.5000
342.5000
365.0000
415.0000

These are the forces necessary to push cut light thread after rounds of cutting 3/8" hemp [number of cuts per round was 2^(round)]. In percentage form :


100.0000
83.1683
77.7778
73.0435
61.3139
57.5342
50.6024

Thus after the last round (126 cuts) the performance had degraded to 50% of optimal. After re-profiling to about a 10 degree edge :


142.5000
185.0000
207.5000
220.0000
262.5000
285.0000
360.0000

Note the initial cutting ability is higher, which you would expect. The percentages :

100.0000
77.0270
68.6747
64.7727
54.2857
50.0000
39.5833

Now after that last round blade had degraded further percentage wise to ~40% of optimal which is what gives rise to the popular myth about "oversharp" edges leading to poor edge retention, and over thick edges. This percentage comparison is misleading as the force in the last round was was less in the second case thus the cutting ability was higher. If you instead forget percentages and instead look at the differences (how much force was required in excess of the optimal case) you will see a more meaningful picture :

10 degree bevel :

42.5000
65.0000
77.5000
120.0000
142.5000
217.5000

15 degree bevel :


42.5000
60.0000
77.5000
132.5000
155.0000
205.0000

The degradation is the *same* in each case (to within the variance). Since the 10 degree bevel starts off with a higher cutting ability it will take longer to degrade to the same cutting ability as the 15 degree edge bevel, specific to this case it was 254 cuts, one extra round.

This was testing with a 22 degree micro-bevel at 600 grit DMT. Next I will be looking at various different micro bevels.

[the above numbers are medians of 3-5 runs, they have deviations of about 50-10%, anyone wanting to look at the raw data drop me an email]

-Cliff