Originally Posted by
Bart
In absolute terms, a secondary bevel is stronger, as already mentioned by Kevint. The very edge caries a more obtuse bevel angle, which is stonger. There's no escaping that.
BUT... The edge is most likely also keener. Why, you might wonder? If you use a very fine hone such as the 30K Shapton, to finish a single bevel, the edge will most likely not reach the sharpness limit of the hone. Two principles that deny the hone's full potential:
1. the bevel is so wide that the hone has not enough abrasive power to remove the layer of steel that needs to go in order to reach the limit that the hone can offer. You could try to compensate for this by doing many, many laps. Not sure you'd ever get there. It depends on the hone.
2. There can be suction between the bevel flat and the hone's surface. As a result, the very thin part of the outer edge can start to flutter. This is a hypothesis, based upon an observation I have made time and time again when looking at magnified edges (40x): the bevel looks completely polished, but the very edge has a small region of a less polished nature, that wasn't there when the edge was less sharp. Some might suspect overhoning, and you can call it what you want, but honing on a small secondary bevel has never led me to make such an observation.
So far what happens at a single bevel. On a narrow secondary bevel:
1. On such a narrow strip, the hone can work its full potential. As already said, the edge is made buy nothing else than that hone. You can really reach the hone's physical limit.
2. No suction, no fluttering of the very edge. As said, on a narrow (micro)bevel, I have never noticed the above occurrence that seems to hold back the very edge from further development.
Because of those differences, the secondary bevelled edge is most likely the sharpest you can ever get off the hone used to cut the bevel. How keener the edge, how more fragile it is and prone to quickly loose performance. Since you guys are talking about a Shapton 30K (which I do not own) I can only imagine that you are talking about the sharpest edges around. I don't think you can expect those to last forever and a day. Furthermore, any woodworker will confirm that an initially deteriorating ultra-sharp edge falls back further than a somewhat less sharp edge that holds up. For that reason in certain woodworking appliances, edges are sometimes pre-dulled in a controlled manner. I don't know if any of this applies to shaving whiskers.
Whenever I need to test a hone, cutting a secondary bevel with it, is always one of my test procedures. It tells a lot about how keen an edge that hone can deliver and how skin friendly that edge is. The more tape you use, the quicker the secondary bevel will hit the hone's keenness limit. Yet I like to use as little tape as possible, because I don't like edges with a really obtuse angle.
Great thread. Sorry I have no 30K to add more data.
Best regards,
Bart.