Dave: Really good to hear of someone else messing with this bedeviled stuff. I'm obsessed with it, and probably half-insane with obsessive-compulsive disorder, otherwise, I'd probably just round-file it all.
I really do believe that one does have to be just about crazy to become even adequate at sharpening the stuff--there are so many variables.
The reason that the double-bevel or triangle file cuts faster than the round file really has to do (I believe) more with chip-flow through the cutter than it has to do with a better cutting edge. Because the double-bev removes more material longitudinally (will you ever understand what I mean?) through the chisel, it enables the chisels to exit more wood-chip material than does the round-grind, thereby expediting the overall cutting process. (At least--that's what I've come to believe.)
The bedevilment first comes in when you realize that you could cut a lot straighter with the round-chain--despite the claims of the proponents of square-grind--than you can with the double-bevel filed chain. And of course the reason is obvious: at least in my case, it's extremely difficult to get all the side-plate and top plate angles consistent enough on both the right-facing and the left-facing chisels to result in really straight cutting.
Already, you understand how insane I am to take the effort to respond with this long of a post at all, so I guess, as far as my already nearly worthless reputation is concerned, I really don't have to much to lose, so here goes...
I'll start with the various phases I've gone through in my sharpening of the stuff:
Phase one: Cuss. Curse. Swear, then resume cussing. Finally resort to filing from the inside out again like you would with a round file, and got barely tolerable angles.
Phase two: Manually learned how to actually remove the material correctly. (By filing from the outside in. This resulted in many a--very expensive--file being ruined or compromised from busting off the incredibly hard and brittle file "teeth"/grooves. Finally learned a few tricks like actually loosening the chain up on the bar so as to be able to "rock-back" the chisel with the manual force of the file. (This actually facilitates the removal of the steel off of the cutters) All during this phase I never worried too much about getting the saw to cut straight, because I just wanted to get greedier and greedier with my experimentation with the acuteness of the angle on the top plate so as to produce faster and faster cutting.
Phase three: Realized that phase two results in VERY prematurely dull chain, and nasty curving in bigger bucking cuts, so I determined to try to learn to file as if I were an electric grinder. This is the phase I'm still in, and may not master till I die. Here's the thing: The darned file actually cuts different depending on whether you're doing the left or right hand cutters. If you don't believe me, study the angles of the "teeth" on the file. You'll soon realize that the angles at which the teeth remove material from the chisels is completely different depending on whether you're doing the lefts or the rights. This actually alters the way that the file cuts, and you have to adjust your manual technique accordingly. There's actually--if you can believe this--a lot more that I could tell you about different little phases that I entered in between these three, based on little alterations of filing techniques that I stumbled onto here and there (like tightening the chain up on the bar instead of loosening it, and then sliding a felling wedge between the bar and chain to render it even tighter) but, honestly, even if I were crazy enough--and I could: don't tempt me!--to elaborate on all of it, I'm at least sane enough to realize that absolutely no one would read it.
so...... If I could glean all of this madness down to two bits of advice, they are these:
1) Always clean out the gullets with a round file instead of trying to remove a lot of material from underneath with the double-bevel.
2) Always just barley scrape the tie-strap with the downward (material removing) stroke of the file. This acts like a functioning file gauge for at least one of the three relative angles. If you can eyeball the top-plate angle, than that only leaves one more (the side plate) angle unaccounted for. This--if I could only do it--is, I believe only mastered by the pure muscle-memory "feel" of the dexterity in your hands.
Granted: I'm a HUGE wood-cutting nerd, but, I'm going to say that when you nail all three of these angles perfectly, the pleasure derived from the straightness and aggressive efficiency of the cutting is only exceeded by a few other things in this life.
Best of luck and happy cutting.
