I put a pair of fresh sharpened knives on my small chipper. Problem, it can't shallow anymore the wood if the diameter is about half what it can eat usually (4"). I can help by pushing for the light wood like willow and poplar, but no way for the hard or stringy wood.
Looking at it; some crud was glued on the knives
after the outside bevel (intended for clearance with the wood), contrary to a dull saw chain with the crud beginning just after the worn cutting edge. And the knives were sharp. The bevel itself was very clean. My conclusion, the bevel collides and pushes away the wood instead of letting the cutting edge do its job.
I tried to grind the base of the bevel (maybe on one third) with the angle grinder to gain some clerance. A little better but still not good. The crud is now on the new bevel, the first one being still very clean. So the wood is still rejected.
After some mesurements, it turns out that the guy messed the sharpening on my knives. Nice and sharp, yes, but with a wrong angle. Actually, he set his machine at 26°, the same angle than the inside bevel ( the one breaking and pushing away the chips). The knife's bulk needs to clear the trajectory of the cutting edge (and the limb), the outside bevel should be set at 14° on this chipper !
That's a lot of hardened steel to grind away now. Seeing my first try (about 4 time more steel to grind), I am not too tempted to do it by hand.