As I and others have posted the hard knots in spruce cause waves on the band mill. I have some rather clear 2ft spruce logs from the property next door. The branches were trimmed when they were young as they were growing along the wooded driveway. Figured I may find some metal with them having been along the driveway but with how clear they looked expected nice flat 2x6. Wrong. No metal but even the knots no bigger then half inch caused waves.
What blade are you running. Steve
Dull blade? I continually run my hand over the previous cut to see if there is any wave around knots and such, as that is a first indication of a dull blade... I have a saying, "dull blades cause big problems". I would dare bet that if you throw a new or sharpened blade on, the wave would go away.
I've said before, it's not the knots themselves but the way the grain changes around them in spruce. If you have a knot, the wave will start 4" before the blade even gets to it, and end 4" after. I'd bet that your logs have the same grain change where it grew over the pruned branches, and that's where the waves are. Those grain changes also severely weaken the lumber, even though there aren't any knots.
I think it would be best if possible to cut the trees or tree to logs and saw the same day on the sawmill. Spruce and hemlock knots that are dry or frozen will dull a saw fast.
I started with a 10 degree band that had run 6 or 7 passes on an oak that were perfect. I agree with BB that very slight waves started near the surface above where a knot became more apparent. As soon as I saw that I put a brand new 4 degree band on and measured my tension. Still the waves which did get a bit worse the deeper I went. The feed would slow a bit as the mill reached the waves and then speed right back up when they cleared. I did use a light drip of MM glub glub lube mix so there was no pick up on the band. At that point I resided to just live with it and will run them through the planer after they dry a while. After milling two spruce I went back to oak and back to the 10 degree band I started with. All was well. ( The logs had been sitting since the spring. I log in the winter and mill in the summer. Between the knots/waves which there were not really that many, the band ran through the wood like butter)
I have the best luck with a 4°, 3/4 pitch, shallow gullet , heavy set blade and it still isn't perfect in wide cuts. Does real good in 12" and under. Steve
I'm cutting knotty spruce that my neighbor dropped last winter.As soon as I get a less than smooth cut a fresh blade gets installed. 4° blades, slow feed , high tension and blade guide no more than inch away.
My results are pretty good.
Everything makes a difference when cutting spruce , hook angle, set, gullet dept, and sharpness. You cant cut spruce with a high hook angle deep gullet blade straight even brand new. Steve