As noted in the CBN thread : http://www.spyderco.com/forums/showthre ... 4-CBN-Rods
I had been using the CBN rods to sharpen various utility blades for tradesmen who had generally positive responses. Being curious I then did an experiment to compare the initial sharpness and edge retention with a common utility knife :
Basic overview :
-edges were all sharpened at 15 dps
-sharpness was measured by slicing light cord under 50 grams of tension
-1/8" ridged cardboard (used) was sliced across the corrugations with a very short draw (2 cm)
-in total two kilometers of cardboard was cut
-results were partially blinded (the individual measuring the sharpness was not aware of how much cardboard was cut)
The results (red is edge retention, yellow is initial sharpness) :
-the CBN and Diamond rods are able to match the performance of the as-boxed edges
-the medium rods alone have high initial sharpness but lower edge retention
-the steeled edge is able to match the initial sharpness but the edge retention is even worse
-the edge reset on a Bester 700 and then finished on the medium rods is superior in both initial sharpness and edge retention
The steeled edge pushes the deformed material on the edge back into place but doesn't actually remove it and thus the edge is left very weak and brittle and it blunts very quickly. The medium rods have a similar effect but since they are abrasive they remove some of the damaged material and have improved edge retention over the steeled edges but are worse than the as-boxed edges in edge retention. The CBN and Diamond rods are abrasive enough to remove the damaged material and thus the edge is on undamaged steel and can match the edge retention of the as-boxed edge.
Now why is the medium rod finish after the edge reset the highest? Two factors :
-all weakened material is removed
-as the draw is very short this is close to a push cut and the ideal finish is likely not the most coarse
A few interesting questions :
-how would the fine rod edge retention be if they were used after the edge reset
-how would the medium rod edge retention be if the edge was first ground with the CBN rods
I would have looked at those as well but after two km of cardboard I ran through all the boxes of that type. I might look at those finishes later.
To clarify this is edge retention on a slice, it can't be extrapolated to edge retention in general. Edge retention on a push cut could very well suffer significantly with the CBN/Diamond rods, I do have some hard plastics to check this on.
As a few points of interest/curiosity :
-these are very cheap blades (less than $0.25 if you buy a large amount of them)
-the medium edge finish (after the B700 reset) could cut 500 feet of used/dirty 1/8" ridged cardboard cut across the corrugations and still wet shave arm hair
-the edges had no chips or visible deformation (except the steeled ones)
Five hundred feet of cardboard is a HUGE pile if the pieces are of any size, it is easily 1000+ pieces, and again the edge would still wet shave and this is a basic steel. Keep that in mind the next time you think that a pile of cardboard is an impressive example of edge retention.